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Big Bang Cosmology - Miscellaneous

1. Are galaxies only moving apart of is there a sideways 'flow' to themas well?

2. Is there 'dark matter' that is really not matter in the universe?

3. Where does the energy to accelerate the expansion of the universecome from?

4. If the universe is expanding into nothingness, isn't 'Nothingness'something?

5. Is the Big Bang expansion really accelerating?

6. Are distant objects actually smaller than they appear?

7. How big can the universe safely become?

8. Will the expansion of the universe ever slow down to zero?

9. Why are there 'blue dwarf galaxies' at 10 billion light years ifeverything is supposed to be red-shifted in color?

10. How can an infinite universe expand?

11. Could some of the 'missing mass' in the universe be in the cosmicbackground radiation itself?

12. How does the discovery of the Cosmological Constant relate tomissing mass and an open universe?

13. How could the writers of the Qur'an 1400 years ago know thatwhen the universe reaches its maximum size we will haveJudgment Day?

14. How can we actually prove that we are not at the center of the bigbang?

15. How can you tell the difference between gravitational and dopplershifts?


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16. Astronomers create maps and models of the universe, but thay arealways of where things were not where they are. How can youcreate a model of the universe from such incomplete data that isnot 'sy

17. When you look into space, what is the black stuff you see betweenthe stars?

18. Is it just a curiosity that the rate at which the moon is receedingfrom the earth is nearly the same as the Hubble Constant?

19. What will happen to our own experience of space if/when theuniverse begins to collapse?

20. Exactly where in Big Bang cosmology does it say that local spacedoes not expand?

21. Is the expansion of the universe accelerating?

22. Is there a fifth force causing the universe to expand more rapidly?

23. Is any time dilation seen in the distant supernova used in studyingthe expansion of the universe?

24. Has any time dilation been detected in the distant supernova usedto measure the expansion of the universe?

25. How much of the cosmic background signal can you see in the'noise' on your TV screen?

26. If nothing physical can be infinite, why isn't the universe finite?

27. If there is no unique frame of reference in the universe, why do we

have a specific speed with respect to the cosmic backgroundradiation?

28. What did people think of the universe in 1897?

29. When the universe collapses, will light continue outward?

30. Does the universe expand into a 4th spatial dimension?

31. If the Big bang happened 15 billion years ago, why did the earthonly form recently?


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32. If we could see the edge of the universe, what would we see now?

33. Why doesn't the universe have a center?

34. Will our universe expand and bump into other universes?

35. What is on the other side of the expanding universe?

36. Why are galaxies colliding if the universe is expanding?

37. What word is used to describe everything outside our universe?

38. Could an external universe affect part of ours by its gravity?

39. If the universe is infinite, how can there be other universes outsideit?

40. Could there be other universes outside of our own?

41. Why is the universe expanding if gravity is an attractive force?

42. Does interstellar dust have anything to do with the cosmologicalredshift?

43. If the Big Bang happened at one point, why are galaxies notexpanding at different speeds?

44. If the universe is open, does it have infinite mass?

45. Does the Oort cloud around every star account for dark matter?

46. If I see two quasars 15 billion light years from us at opposite partsof the sky, how can the universe be only 10 billion years old when

they are 30 billion light years apart?

47. Is there nothingness outside of our visible universe?

48. Why are there so many different estimates for the distances toquasars and the size of the universe?

49. If an observed galaxy at 15 billion light years is actually 30 billionlight years away, does that mean the universe is twice as old?

50. If the cosmic background radiation comes from everywhere inspace does that mean it has no source?


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51. How do astronomers measure the temperature of the cosmicbackground radiation without using a thermometer?

52. Does the cosmological redshift mean that the universe is now

expanding more slowly that it used to long ago?

53. If al the galaxies are flying away from us, are we in the center ofthe universe?

54. How do you calculate the distance to an object at a redshift of 5.0?

55. Will the limits to the visible universe expand indefinitely?

56. What is an 'antipode' in cosmology and does one exist in our

universe?

57. What would an observer outside our visible universe see if theylooked in the direction of the Milky Way?

58. Is there any evidence that the Hubble law is not a linear relationbetween distance and expansion speed?

59. How is Hubble's Constant derived from Newtonian physics?

60. How can we see light from a galaxy 14 billion light years away ifthe universe is 14 billion years old?

61. Where can I get a book that discusses the cosmic microwavebackground?

62. Does Stephen Hawking think the universe is open or closed?

63. If the universe is open and infinite, what is it expanding into?

64. Are redshifts really quantized?

65. If the universe exploded from a small piece of space why atedistant galaxies moving so fast?

66. Does the value of Hubble's Constant depend on the galaxiesoutside our visible universe?

67. Is the universe expanding the same way in all directions?


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68. If the ultimate fate of our universe is so bleak, what then is itspurpose?

69. Could the universe be rotating, and if so, with respect to what?

70. Has the 'old' age estimate of 10-20 billion years been replaced by a'new' age estimate of 8-12 billion years for the age of the universe?

71. Why is the universe expanding if gravity is an attractive force?

72. What will the far future be like?

73. Would Dark matter go away if Newton's Law of Gravity wereincorrect at intergalactic distances?

74. How fast is the universe expanding?

75. Have astronomers found galaxies with no redshifts?

76. How do we really know that we are missing 90 percent of thematter in the universe?

77. How is it possible that, looking out at the universe in any

direction, that this lets us see what happened at the Big Bangwhich was a specific point in space?

78. How big is the universe?

79. If the Big Bang happened in an infinite nothingness, the universemust have an expanding edge, right?

80. Can we find a center to the Big Bang by looking at how distantquasars are moving?

81. Does the universe really have a top and a bottom as was recentlydiscovered?

82. What is the universe expanding into?

83. How can a galaxy be 8 - 10 billion light years away, but still be 100million years old and be detectable today?

84. How fast is the visible universe expanding?

85. Why isn't the night filled with stars as bright as the daytime sky?


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86. What is a 'light horizon' and does this mean there are things in theuniverse permanently hidden from us?

87. What does the 'anisotropy' of the cosmic background radiation tell

astronomers?

88. Do galaxies travel parallel to each other, or in a way that can beused to figure out where the Big Bang happened in space?

89. I recently heard there is a unique direction to space. What doesthis mean?

90. How were the distances to the Hubble Deep Field galaxiesdetermined?

91. If the Hubble Deep Field shows galaxies 14 billion light yearsaway, is the edge of the universe 28 billion light years distant?

92. How do we know the universe can expand faster than light if wecan never see it?

93. How do galaxies get the energy to escape each other according toHubble's Theory?

94. Stephen Hawking says the universe has no boundary, so what is itthat is expanding?

95. If the universe is expanding, it has a boundary, so what is at theboundary?

96. What are the consequences of the universe having a preferredaxis?

97. Why does it make a difference is a neutrino has a rest mass if itcarries energy anyway?

98. What are the speculations about the future of the universe?

99. Is the expansion of the universe slowing down right now?

100. Will astronomers need the cosmological constant to reconcilethe ages of old stars and the universe?

101. How is it possible for the energy in the cosmic backgroundradiation to remain constant as the universe expands?


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102. Why isn't an oscillating universe very likely?

103. What is the universe a part of?

104. Is the total energy of the universe decreasing because of theredshift?

105. If space increased faster than light moments after the Big Bang,why do we see anything near us in space at all?

106. Are galaxies only moving apart of is there a sideways 'flow' tothem as well?

107.Does Stephen Hawking think the universe is open or closed?


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1Are galaxies only moving apart of is there a sideways 'flow' to them aswell?At the largest scales, where the rate of cosmological expansion is

measured in 10s of thousands of kilometers/sec, the randommotions of galaxies inside their local clusters ( 300 - 1000 km/sec)is unimportant, and at these 'cosmological scales' ( 10,000/65 =150 megaparsecs or larger) the motion is expected to be with-the-flow of the expansion. However, there have been a few studies thathave claimed to have detected a 'non-Hubble' flow to the mostdistant galaxies out to 300 megaparsecs. The flow of our localuniverse may not be completely random, but may have asystematic direction towards the so- called Great Attractor. If thisis eventually confirmed, it could indicate that the universe at thelargest scales is not expanding exactly the way a truly uniform,

Big Bang would have predicted. Still, these motions are producedby gravitational forces, and at a current age of something like 15billion years, there has been more than enough time for deviationsfrom Big Bang expansion to have built-up over scales of 1 billionlight years or more! What will settle this is better observations,because the Great Attractor flow is based on the speeds of only afew dozen large clusters of galaxies. Also, computer modeling willbe able to tell us what kind of flow speeds we could expect todetect in a realistic universe model, and at a particular scale. This

kind of work is just beginning, so we will have to wait and see.

2Is there 'dark matter' that is really not matter in the universe?


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Possibly. The kinds of universes that are consistent with the data wehave right now, require that about 60 percent or so of the 'critical

density' is in the form of the cosmological constant...recentlydetected for the first time...and the remaining is in some form of'matter'. But, Big Bang cosmology says that the hydrogen/heliumabundance of the universe is consistent only with familiar mattercontributing no more than about 1 percent of the critical density.This means that 100% - 70% -1% = 29% or so is in some type of'matter' that cannot be made from quarks ( protons or neutrons).This is what astronomers call 'dark matter' but it is recognized notto be matter of the usual type. Candidates include things likeneutrinos and other 'non-quark' particles.

Can black holes be dark matter? Not really. If the black holes formedAFTER the hydrogen/helium ratio was fixed, then the quarks thatwent into them have already been 'counted' in the Big Bang toproduce the 1% limit consistent with the observed abundances.Luminous stars and gas contribute about 0.5% to the known formsof 'baryonic matter' in the universe, so black holes couldcontribute at most the other 0.5 % or so. This would help explainthe dynamics of some galaxies which seem to be spinning faster

than they should given the amount of luminous matter they have.But genuine dark matter cannot be explained using black holes.

You need some other ingredient.

3Where does the energy to accelerate the expansion of the universecome from?

It comes from the vacuum.


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The best explanation we have right now, is that Einstein was rightwhen he proposed that there was a 'Cosmological Constant' effectwhich is a new property of empty space. Physicists have knownsince the 1940's that what we intuitively call empty space is not

really empty at all. There are hidden particles and fields whichconstantly come and go within it which cannot be detecteddirectly, but which can be inferred by studying the motion andproperties of sub-atomic particles. The cosmological constant isthe result of a new kind of field in nature which exists in thevacuum state, and which on the cosmological scales given thevacuum a net energy. This energy, however, is unlike any kind ofenergy we have previously studied. The way it works, say thetheorists, is that it produces a constant pressure in every cubiccentimeter of space no matter if space is expanding. Physical gaspressure, on the other hand, would decrease as you increased thevolume of space. So, this new vacuum pressure causes theexpansion of the universe to accelerate as the universe grows.

This is currently the only theoretical explanation we have for what isgoing on, and this explanation is even older than big bangcosmology itself because Einstein proposed it back in 1915, butbig bang cosmology didn't come onto the scene until the early1920's. Big bang cosmology has always had this effect as part of

its 'general cosmological solution', but the cosmological constanthas come in and out of fashion since then, mainly because no onehas ever convincingly shown that it exists. In 1998, the CosmologySupernova Program gave us this evidence in a very convincingway, for the first time.

4If the universe is expanding into nothingness, isn't 'Nothingness'

something?

Yes and no.


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The universe is expanding, but the only theory that describes howthis works is Einstein's Theory of General Relativity developed in1915. Because this theory has been tested and found to hold upwell, at least for the kinds of data we have access to, we have to

trust what it says about the universe too...at least for now. GR, likeso many of the other working theories we have about differentaspects of nature ( quantum mechanics, special relativity and soon) are very hard for humans to work with because you cant usegood-old human intuition to anticipate if their answers 'makesense' or not.

When general relativity says that 'space stretches', we are left with awhole gaggle of human intuition problems just like the one youposed in this question. Big Bang cosmology is based on GeneralRelativity, and there are two kinds of universes described by it.Closed-finite universes and open-infinite ones. In the closeduniverse ( which ours seems not to be according to the data wehave) 3-d space is finite in volume at every instant, and in the farfuture, the eventual collapse will decrease this volume to zero andthen there will be no more 3-d space in existence. Generalrelativity says that there is no 'external space' in which ouruniverse exists, so human intuition fails miserably. Humanintuition DEMANDS that there exist an external space for our

universe to 'float' in like a soap bubble, which is the physicalanalogy your mind is using anyway to understand the universe. Inan open-infinite universe, 3-d space has ALWAYS been infinite,even at the 'birth' of the universe at the Big Bang. This model isfavored by Inflationary Cosmology, in which our universe is justone of an infinite number of 'patches' of 3-d space that exist insome larger arena. The expansion of out particular patch, however,does not happen at the expense of the compaction of the spacesurrounding it. Again, this is an intuitive paradox that humans

cannot resolve because it seems contradictory...thoughmathematically it derives from a higher logic than we arecommonly familiar with in our limited world.

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Is the Big Bang expansion really accelerating?

It sure looks that way! In fact, the discovery of this effect has beenwidely hailed as the Scientific Story of 1998 by a number ofscience journals.

For several decades now, astronomers have dutifully included the so-called cosmological constant in just about every research paper,as a valid alternate to the uniformly expanding Big Bangcosmological model. For more information on this, see my articlein Sky and Telescope magazine. Of course, Big Bang cosmologyincludes versions of the universe where this constant is zero, or

has a non-zero value. Astronomers who perform studies of theobservational aspects of the universe have placed limits to theHubble Constant, the value of 'Omega' and the so-calleddeceleration parameter 'q', but they are also obliged to include intheir analysis cosmological models that have non-zero values ofthe Cosmological Constant as well.

What has changed in the last 5 years is that, from Hubble SpaceTelescope observations, astronomers studying such things as the

number of gravitational lenses, and the dynamics of the clusteringof galaxies over the last 5-10 billion years, have not ben able tosay that the Cosmological Constant is zero. In fact, the very bestthey have ever been able to say is that, compared to the density ofgravitating 'stuff' in the universe, this constant has a non-zerovalue that COULD be about as large as what visible and darkmatter contribute to 'Omega'. That being the case, a non-zerovalue for this constant must mean that, at some level, the universeis not simply expanding and slowing down, but is expanding and

slowing down more slowly than if purely gravitational influenceswere at work.
http://www2.ari.net/home/odenwald/anthol/fudge.htmlhttp://www2.ari.net/home/odenwald/anthol/fudge.html


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By 1997, the general picture was that the Hubble Constant was near65 kilometers/sec per megaparsec to within 10 percent or so. Thisimplied from the mathematics of Big Bang cosmology a specific'critical density' of gravitating matter and energy in order for the

universe to be a 'critical' universe balanced between collapse andeternal expansion. Now the question was whether all forms ofidentifiable gravitating stuff equalled this critical value or not. Thisinventoring of the matter-energy density of the visible universehas been going on for decades, and the current status of this isthat only 1 percent of the critical density is in the form of luminousmatter. However, in order to make distant clusters of galaxiesstable and not fly apart, there must be some 'non-luminous' stuffout there too. In some clusters, there is so much of this 'darkmatter' present that if it were present in the rest of the universe tooit would amount to 40 percent or more of the critical density of theuniverse.

Now, astronomers have to compare there data, not just with modelsthat have 'dark matter' but also the Cosmological Constantbecause we have no way of ruling out the Cosmological Constantbeforehand. When these models are compared against the data,the most consistent cosmologies that come out are those thathave very little luminous matter ( stars, galaxies etc), lots of dark

matter in two forms ( hot and cold ), and a cosmological constant.Still, although these kinds of studies seemed to require aCosmological Constant, there was never any actual detection ofthe required physical effects of such a constant. According to theEinstein-DeSitter model, whenever such a factor is present in theequations, it will result in a peculiar phenomenon...the rapidacceleration of the distances between two gravitating bodies.Gravity of course caused deceleration between bodies and cancause them to fall towards each other.


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According to a review article about this in the December 18, 1998issue of the journal Science, astronomers have just now found thefirst evidence for the expected acceleration effect. In 1998, twoteams of astronomers who have been studying very distant

supernova with the Hubble Space Telescope calculated the rate ofexpansion of the universe with these very distant supernova ingalaxies located several billion light years from Earth. Althoughthe numerous supernova detected in nearby galaxies did show theusual Hubble Expansion, when these teams independently studiedthe most distant galaxies covered by their supernova, they foundthem to be 10-15 percent dimmer than expected based on theirredshifts and distances estimated from the Hubble Expansion. Asthey found more supernovae at these distances, this effect did notgo away as it would if it had ben a statistical fluke.

What it means is that the distant supernova are farther away than theywould be if the expansion has been a steady one over the last fewbillion years. This means that the universe has been expanding atan accelerating pace, not a steady one. This acceleratedexpansion is exactly what you would get if the Cosmologicalconstant were non-zero, as other studies in the past hadsuggested, but not proved. According to their 'best fits' to the datathey have, if the universe is at its critical density, then all forms of

matter and energy ( luminous, dark, photons, neutrinos etc)amount to about 30 percent of the critical density, and thecosmological constant is about 70 percent of the total. Not only isthe kind of matter we are made from a less than 1 percent'impurity' with respect to the vast reservoirs of 'dark matter', buteven dark matter itself doesn't completely determine the evolutionand destiny of the universe.


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Let me explain how the above logic works. Because Type 1Asupernovae are produced by the detonation of white dwarfs, andbecause the maximum white dwarf mass is 1.4 times the Sun,these supernovae should all produce nearly the same peak

luminosity at the maximum of the detonation. This provides the'standard candle' to gauge distances. Now, the astronomers detectsuch a supernova in a distant galaxy, and measure its velocityspectroscopically. They then compare its apparent brightnessagainst its luminosity to get its distance..say 2 billion light years.So far so good! Now there are three outcomes of this, but youhave to keep in mind that what you are seeing is a snapshot ofwhat the universe was like 2 billion years ago. the first outcome isthat, when you divie the speed by the distance you get exacly thesame value for the Hubble Constant as what we measurelocally...65 km/sec/megaparsecs. This means that 2 billion yearsago, the universe was expanding at the same rate it is now. thesecond possibility is that when you do the division you get anumber that is bigger than 65 km/sec/mpc. This means that 2billion years ago, the universe was expanding a bit faster than it istoday, and so the universe is slowing down in its expansion as itgets OLDER. The third possibility is that you ge t a number that issmaller than 65 km/sec/mpc. This means that the universe wasexpanding slower 2 billion years ago than it is now, in other words

the expansion is speeding up today. Another way of saying this isthat objects sem to be farther away ( dimmer) than we wouldexpect if the universe were expanding at the same rate then as it isnow. It is this third case that SEEMS to be supported by the newdata.

There is a fly in the ointment though. According to Big Bangcosmology, the value of the Cosmological Constant is...aconstant...no matter how old the universe gets. The density of

matter and energy, however, continues to decline with time. Why isit that we happen to live at a time in the history of this universewhen these two quantities have about equal value? Astronomersand physicists do not like these kinds of 'anthropic' coincidencesbecause it singles out the observer, and the time that he makes theobservation in the history of the universe, as being special insome way. Since the Copernican Revolution, we have been verynervous about throwing in the towel on these kinds of issues.


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There is another possibility too. Perhaps the environment aroundthese distant supernova is dustier that the typical environmentaround nearby Type 1A supernovae. Astronomers will have to lookat many more of these distant events to sort out the issue of

reddening from dust absorption, and the cosmological constanteffect.

In the next 2-5 years, there will be far more supernova detected, andnew satellites flown by NASA to measure the CosmologicalConstant and the other physical constants of the universe. If the1998 findings are supported, we will have convincing evidencethat we live in a very curious universe indeed!!

6Are distant objects actually smaller than they appear?

Actually, because of gravitational lensing, distant objects shouldappear larger than nearby ones if they are farther away than 5billion light years or so. This effect is an important geometric testof Big Bang cosmology, and has actually been carried out by radioastronomers. Because of the curvature of space, as you look intothe distance, the angular size of galaxies decreases steadily, thenreaches a minimum size, then begins to increase again as you

pass the 'curve' of the universe. Using the dimensions of specificfeatures in active radio galaxies, such as the size of their radio-emitting regions, and correcting for the change in density of theintergalactic or intracluster medium since the big bang, this effecthas been seen.


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7How big can the universe safely become?Hmm...I think you are taking the 'expanding balloon' analogy for the

universe a bit too seriously. There is nothing about gravity thatsuggests a limit can be reached where the universe willcatastrophically 'rupture'. The universe can expand indefinitely,and safely!

8Will the expansion of the universe ever slow down to zero?

If the universe is truely 'open' it will continue to slow down in itsexpansion from the current 65 km/sec/mpc to near stand-stillspeeds in the single digits, but this will take 100s of billions ofyears. The speed will acheive a value as close to zero as you want

in the mnear-eternity to come, but actual zero expansions willprobably only be obtained in certain local regions where local andcosmological gravitational forces just balance. Even today, thereare regions near the outskirts of many stable clusters of galaxieswhere the expansion speed is in fact zero!


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9Why are there 'blue dwarf galaxies' at 10 billion light years ifeverything is supposed to be red-shifted in color?In this context, 'blue' means that if you were traveling along with the

dwarf galaxy and looking at its stellar populations, you would seethat the stars combine to produce a 'rest frame' color for thegalaxy that is rich in 'blue' or ultraviolet light. This is because thereare many hot, young massive stars in the O-B spectral rangecompared to other stars in the galaxy. This is an indication that a'burst' of massive star formation has been occurring in this galaxyfor at least the last 10 million years or so. Our Milky Way has acolor that is more or less typical of G-K-type stars because theseare the dominant spectral types that contribute to the totalluminosity of the galaxy.

In this instance, the galaxy still has a very large redshift due to theexpansion of the universe, but the intrinsic color is bluer thantypical older galaxies around us. This terminology is equivalent tosaying that I have a blue ball which I have thrown so fast that ithas a sizable doppler redshift, but if I was riding along with the ballI would still see it as blue.

10How can an infinite universe expand?


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It's hard to think of how this can, or cannot, be without a firmunderstanding of what physical infinity means. There are manythings about even mathematical infinity that lead us to marvel at it.For example, you can pack an infinite number of infinite 'things'

into infinity. The subject of 'transfinite numbers' by Cantor is amust to read for anyone that thinks they understand what infinityshould be like, intuitively!

In cosmology, we are guided by what Einstein's mathematical theoryof general relativity has to say about the physical universe. Amongthe 'big bang' cosmological solutions, are the ever-popular OpenUniverse models. These are all characterized, at ANY instant incosmological history, by mathematically- infinite space-likesurfaces ( 3-d space in other words). In these space-like surfaces,there are embedded stars, galaxies and other forms of gravitatingmatter and field, which make up an average density of 'stuff'. In the'classical GR picture' on average each of these infinite numbers ofstars and galaxies look similar to the populations of things we seearound us.

Now, in this model of the universe, everything is expanding in thesame sense that the points on a balloon's surface move away fromall other points as the balloon is inflated. General Relativity says

that for infinite universes, the same kind of expansion occurrs. It ishard to visualize, because humans are not very good at visualizinginfinity.

Inflationary cosmology adds to this by saying that we live in a smallpocket of some vaster spacetime. This pocked emerged from atiny patch in the primordial spacetime and inflated to a vast size.By today, it extends 10^100 or more light years across. Physicalconditions are similar throughout this patch today, but it is

surrounded by other patches in a vaster tapestry where thephysical conditions may be very different. Once the universe getsold enough, we will begin to see distant images from these otherpatches, and in the almost infinite future, we will at last see a verycomplex cosmos.


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11Could some of the 'missing mass' in the universe be in the cosmicbackground radiation itself?Hmmm...good try!

The cosmic background radiation contributes about 400 photons percubic centimeter at a wavelength of 1 centimeter. This means thatthe energy carried by each photon is ( E = h c/wavelength) = 6.6 x10^-27 x 3 x 10^10/1.0 = 2 x 10^-16 ergs. From E = mc^2, this manyergs is equivalent to a mass of 2 x 10^-37 grams. Multiply this by400 photons/cc and you get 9 x 10^-35 grams/cc which is about 1million times less than the critical density of the universe ( Omega= 1) so, no, cosmic background photons contribute very little, andare approximately about 50,000 times less important than whatstars and galaxies provide.

12How does the discovery of the Cosmological Constant relate to

missing mass and an open universe?

In adding up all the different quantities that contribute to the densityfo the universe and the famous 'Omega' factor, you get to add upthe familiar forms of matter in stars, dark matter which isunderluminous but still gravitates, and the cosmological constantwhich 'antigravitates'.


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If you want Omega to equal 1.0, which is what most versions ofinflationary big bang theory require, and what many astronomicalobservations are now pointing to, the sum seems to be about 0.1for matter, 0.7 for the cosmological constant, and 0.3 for 'dark

matter'

With Omega = 1.0, you get an infinite open universe as a result. Youalso solve some details about the age of the universe compared tothe age of the oldest stars we can detect with a cosmologicalconstant that is not zero in a low-density matter universe.

13How could the writers of the Qur'an 1400 years ago know that when

the universe reaches its maximum size we will have JudgmentDay?

Well...the best model we have to account for the data we have is thatthe universe will continue to expand forever. There has never beenevidence that suggests we live in a closed universe slated torecollapse in the future. So, your question is answered by sayingthat ancient writings either got the right answer or the wronganswer. In this instance they 'guessed' incorrectly so there isreally no historical mystery to explain.

14How can we actually prove that we are not at the center of the big

bang?


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This is actually very hard to do depending on what evidence you thinkmakes the most sense. It is in exactly the same league as 'proving'from a simple observation that the earth orbits the sun. The onlyway heliocentrism is 'proven' experimentally is by the very SIMPLE

explanation it provides for the observed phenomenon of stellaraberration. For cosmology, we have to use general relativity as thetheory for large-scale gravity, and it says that in any spacetimewhere you see a uniform distance-redshift relationship, theobserver must be living inside a universe that is, on average,homogeneous and isotropic meaning that matter is uniformlydistributed everywhere. This can only lead to a cosmologicalmodel where no single observer is at the center of the universe,but who will FEEL they are because of the uniformity of theexpansion ( redshift) that they see going on all around them. Everyother observer will se exactly the same thing so there is no center.Now...do you feel that general relativity is compelling? It haspassed every known test we have been able to throw at it so far sowe have no a priori reason to disbelieve it, until some experimentalresult in the future tells us otherwise.

15How do you tell the difference between gravitational and Doppler red

shifts?

This is a very important question in astronomy and cosmology. Bothof these redshifts, along with the cosmological redshift, lookexactly the same in terms of how they shift the spectral lines of anatom. Doppler shifts arise from the relative speed of an objectcompared to the local rest frame; gravitational redshifts do notdepend on relative speeds at all but only on the difference instrength between the gravitational field where the light is emittedcompared to where it is received; cosmological redshifts depend

on the relative stretching of space between the time when the lightwas emitted and the time when it was received.


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To distinguish between the three, you need to know something elseabout the object and its location. Objects inside the Milky Waycannot have a cosmological redshift because, buy definition, theyare not at cosmological distance where this effect comes into play

according to general relativity. Only doppler and gravitationalredshifts are plausible possibilities. Any kind of gravitationalredshift requires a strong gravitational field to produce it such asyou would find near a neutron star or a black hole, and even so theamount of the shift that you could detect would only be equivalentto a few thousand kilometers per second of relative speed. Gasbeing ejected or falling into such an object would show both thedoppler contribution of its infall speed, and the gravitationalredshift of its infall for material very close to the object. Materialfarther away by, say, several million kilometers, would only showthe doppler component because at these distances thegravitational field is very weak and only contributes a fewkilometers/sec of less to the total redshift.

For ordinary stars in the milky way, the doppler shift due to theirrelative motions of up to 500 kilometers/sec relative to the sun isthe largest contributor, because the gravitational field they movewithin due to the rest of the Milky Way is insignificant. Also, thelight they emit is gravitationally redshifted at the stellar surface by

an amount equal to their surface escape speed ( about 20 km/sec)divided by the speed of light which equals 0.006 percent, so thisfactor is unimportant.

Gravitational redshifts are only important over doppler andcosmological redshifts, when you a looking at material close to theblack hole horizon distance on a compact object such as aneutron star or a black hole. For all other systems, it isunimportant compared to doppler and cosmological redshifts. The

latter are important only for objects further away than a few millionparsecs at which point the cosmological redshift ( 65 km/sec)becomes comparable to the typical relative speeds of galaxieswithin their respective clusters ( 300 km/sec).

16


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Astronomers create maps and models of the universe, but they arealways of where things were not where they are. How can youcreate a model of the universe from such incomplete data that isnot 'synchronous'?

This is a very fundamental question in astronomy, and the answerdepends on the particular system that you are trying to map. If youare interested in mapping objects within the Milky Way, the lighttravel time between the most distant stars and nebulae from theearth is no more than 100,000 years because the Milky Way is nomore than about 100,000 light years across. That means thedifference between where things appear to be right now, andwhere they actually are at this instant, is only out of sync by lessthan 100,000 years. For stellar evolution, this is a completely trivialamount of time because nothing of major importance happens insuch a short time for the majority of stars similar in mass to thesun. As for positional differences, the maximum relative speeds ofstars in the Milky Way is about 300 kilometers/second so that in100,000 years, this amounts to a positional shift of 300 km x100,000 yrs x 31,000,000 sec/yr = 100 light years for the mostdistant objects we can detect individually. Now, 100 light years

compared to their maximum distance of 100,000 light years is aerror of about 0.1 percent. Typically, the best distance estimatesfor objects within the Milky Way are uncertain to 10 percent ormore. What this all means is that allowing for where objects areRIGHT NOW, introduces a correction to our maps of the Milky Waywhich is smaller than the uncertainties we already have to live withbecause of the imprecision of our standard distance estimates inthe first place, so the correction is of no practical importance.Also, in terms of dynamics, gravity travels at the speed of light, so

that it it is the locations of where things appear to be right nowthat is important to us, not where they actually are.


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As for maps of the rest of the universe, because the redshifts ofdistant galaxies has to be interpreted via general relativity in orderto obtain a distance and location, the time delay is automaticallytaken into consideration in producing a 'synchronous' model of

the universe for a given 'cosmic time' since the big bang.Practically speaking, however, astronomers are now beginning tomap the locations of galaxies out to several hundred million lightyears. Again, the relative speeds of these galaxies with respect tothe Milky Way is about 2000 km/sec per 100 million light years ofdistance, and so in 100 million years, such galaxies have movedan additional 67,300 light years from where they appear to be rightnow, which is about equal to their own diameter. This is an 'error'of about 67,300/100,000,000 = 0.07 percent and must be comparedto the typical distance estimation techniques which have aprecision of no better than about 1 percent or so at best. Again,accounting for the light travel time position shift is a negligableerror. For dynamical considerations, such as in modeling theevolution of a cluster of galaxies, only the relative positions of themember galaxies within a cluster is important, so the model wecreate for a cluster is based on where we see the galaxies as theyappear now, and not where they actually are right now.

17When you look into space, what is the black stuff you see between the

stars?


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We don't really know..honest! We call it 'space' or 'vacuum' or even'spacetime', but the truth is that the 'stuff' between the stars thatmakes our universe 3 dimensional is a bit of a mystery. Einstein'sgeneral theory of relativity states that what we call 'space' and

'time' are just features of the gravitational field of the universe.They have no independent meaning or existence apart from thegravitational field itself. Physicists have known for decades that'empty space' is far from empty, but contains virtual particles andfields that cause the forces we see and the properties of theparticles we observe. At a deeper level, theorists have proposedthat the geometry of the gravitational field itself provides all of theobserved qualities of particles and fields in the universe much asin Euclidean geometry in a flat 2-dimensional plane, you can havean infinite number of polygons, but in flat 3-d space you only have5 regular solids that are possible. The emptiness you see betweenthe stars, even after you remove all the interstellar gas and dust,still contains within it enough 'information' coded in some way, todefine all of the forces and particles we see in a unique way...andno other.

This subject is very deep and complex, and one of the most activelyexplored subjects in modern physics today.

18Is it just a curiosity that the rate at which the moon is receding from

the earth is nearly the same as the Hubble Constant?

The moon is drifting away from the earth at a rate of about 3.5 centimetersper year. The distance to the moon is about 350,000 kilometers or 1.17x 10^- 14 megaparsecs. The recession speed of the moon is equal to1.1 x 10^-12 kilometers/sec, so that dividing the two you get about 96kilometers/sec/mpc. The current estimate for the Hubble Constant is 65kilometers/sec/mpc. Does this mean that the space between the earthand moon is also expanding the way it is in the big bang? No.


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The Hubble Constant, by simple dimensional analysis has the units of1/time. The time constant for the universe to, say, double its scale bythe cosmological process of the big bang is about 8 billion years or so,so that gives us a Hubble constant of the size we see. For the moon,

the time it takes for its separation from the earth to double is also aboutthe same length as 8 billion years so again you end up with 1/timebeing about what you see in that system. The underlying reasons forwhy these timescales exist is different and not related. It is more than arandom coincidence, but it is less than a direct causal connection,because all we are saying is that the timescales ( H = 1/t) arecomparable in size to within a factor of two.

19What will happen to our own experience of space if/when the universe

begins to collapse?


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We will simply see the universe begin to get more crowded, but it willhappen in a most peculiar way. If the collapse already started, say, 100million years ago, we will look out into space and notice that instead ofsystematically increasing redshifts, the galaxies out to 100 million light

years have systematic blue shifts that are highest locally and diminishto zero as we reach the 100 million light year limit. Galaxies furtheraway than 100 million light years will have redshifts that start small andsteadily grow in size with increasing distance as they do now. In time,this 100 million light years limit steadily increases by 1 light year peryear until after a few billion years after the collapse has started, all ofthe galaxies out to several billion light years show only blue shifts.Powerful telescopes, however, can still detect the faint images of distantgalaxies to 10 ,20 ...40 billion light years and see redshifts in the lightfrom the most distant galaxies because the universe is about 50 billionyears old at the point when the collapse phase begins. Eventually,nearby galaxies start crowding together and colliding into smallnumbers of super galaxies with trillions of stars, then even these hugesystems begin to fall together as space contracts so that in the last fewbillion years of the universe you will be living inside a dense star systemthat extends millions of light years into space...but soon even thesehuge systems coelesce and all of space if filled by stars which steadilyget closer and closer together until even the stars collide and you areleft with a hot plasma filing space which is steadily getting hotter and

hotter.

20Exactly where in Big Bang cosmology does it say that local space

does not expand?


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The main assumption of Big Bang cosmological theory is that the universecan be described by a uniform, constant density whic, on average,describes all ther is to know about the large-scale properties of theuniverse in this theory. The distribution of mass does not depends on its

location in space. Under that assumption, the mathematics define asingle-parameter 'scale factor' whose size changes as a function of onlyone parameter, time.

Clearly, this model does not describe space-time inside a solar system, or agalaxy, or a cluster of galaxies, because in these systems you cannotdescribe all of their essential physical properties just by a single densityparameter. So, for these systems you use general relativity for a set ofpoint particles embedded in a larger spacetime provided by the global'cosmological' solution. Local space-time is then seen as a roughly flat,and unchanging asymptotic system modified by the time-dependentchanges caused by the motion of matter as discrete point masses.

To find the scale at which cosmological expansion has to be confronted,you look for the scale at which you CAN describe the distribution ofmass by a single, average parameter. This scale is larger than a clusterof galaxies, and is also larger than superclusters. Anything smaller thanabout 100 megaparsecs is probably too small because the density andmotion are not location independent. Another way of looking at this is to

find the scale where the average density leads to a prediction forcosmological expansion that is faster than the average random motionwithin the largest system that can be recognized. Clusters of galaxiesoften have member galaxies that are moiving at 500 kilometers/sec orso, and for a Hubble Constant of 65 kilometers/sec/mpc, this meansthat at a scale of 7.7 megaparsecs or 25 million lightyears, you are rightat the threshold where cosmological expansion between the clustercenters makes as much of a contribution to the dynamics of the systemas the random, position-dependent internal velocities of the constituent

particles ( galaxies).

21Is the expansion of the universe accelerating?


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It might be.

In the past whenever I have received this question I have answered that,no, the expansion of the universe is slowing down. Astronomers havebeen working very hard to measure the density of the universe bydetecting the gravitational influences of all forms of matter and energythat we can find in our visible universe. Today we seem to have reacheda kind of impass where counting luminous matter in the form of stars,and other types of 'baryonic' matter ( made from protons and neutrons:quarks) which may be dark, leads to the conclusion that the universehas perhaps only a few percent of the matter it needs to prevent futureunending expansion. This would mean that although the expansion ofthe universe was very fast and violent long ago, today it is slowingdown, and will continue to do so for all time to come. The fly in theointment has been that astronomers have also detected in distantclusters of galaxies a potentially new form of gravitating 'something'which is popularly called dark matter, although it may not be matter inthe forms of baryons at all. But there is potentially a LOT of it, perhapseven 100 times as much of it as familiar baryonic matter. Still, addingeven this to the total mix of the universe still gives you a universewhose expansion is slowing down today, though doing so slightly more

quickly than for the pure-baryon universe.

But that isn't all.

Since Albert Einstein proposed it in 1915, and then retracted it as hisbiggest blunder in 1933, the 'cosmological constant' has come andgone in cosmological models, but astronomers out of fairness to testingtheories, have always formally compared their data with big bangtheories which drop this term, and which include its effects. This is a

new kind of force in nature that derives from a new field in nature whichis in some sense buried in the vacuum of space itself. It acts in a wayopposite to gravity and causes space itself to expand even with nomatter present to produce gravity. It is in essence an anti-gravity force.During the last few decades, and especially in the last 10 years,astronomers have been concerned that the ages for the oldest starscan sometimes seem older than the age of the universe based on thesimplest cosmological models that DO NOT include the cosmologicalconstant. By adding the cosmological constant, however, it is easy tomake the ages agree depending on how large this anti-gravity term is.But does it really exist?


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The most stringent limits I have read about involve counting the number ofgravitational lenses among the faintest galaxies. This was done in theearly 1900's by a 'Key Project' using the Hubble Space Telescope. Theyfound that, compared to the density of matter in the universe, the

amount of cosmological constant that could also be present and notaffect the numbers of lensed systems was less than about 20 percent ofthe critical density. This, however, is more than 10 times the contributionmade by matter to the gravitational field fo the universe. But this wasonly an upper limit and NOT a detection. Other methods for detectingthe cosmological constant have led to mixed results, with no firmdetection.

This seems to have changed in the beginning of 1998 when two teams ofastronomers investigating the expansion of the universe usingsupernova 1 billion light years away, announced that they had detectedthe cosmological constant itself...no upper limit. Their reasoning wasthat the distances they computed for these supernova from thecosmological expansion did not match the peak luminosities expectedfor these supernova given that they are a well- known type ofsupernova. This means that they are slightly farther away than ordinarybig bang cosmology without the cosmological constant would havepredicted. A big bang cosmology WITH such a cosmological constantwould match this result however, but that means that the universe is

expanding faster in the last billion years than predicted. This is exactlythe 'space dilation' effect predicted for the cosmological constant, andimplies that the expansion of the universe is actually speeding up!!

Most astronomers still consider this a preliminary result, especially since itis only based on literally a handful of supernova. If...and this is a BIGif...further independent studies of distant supernova bare this out, theneither there is something incorrect about the assumption that thesesupernova are 'standard candles' or we will have to accept the

cosmological constant as a new ingredient to cosmology. It does notspell the end of big bang cosmology, because big bang cosmologyalready includes this new effect, however, astronomers have neverbeen able to satisfy themselves that it is a real phenomenon rather thanjust an additional term in an equation which they otherwise had thefreedom to dial to zero in our universe.


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22Is there a fifth force causing the universe to expand more rapidly?There may well be!! There have been many proposals for such a new force

in nature, mainly proposed by physicists trying to develop newmathematical theories which combine the known particles and fieldsinto a common explanatory ruberik. Some 'supersymmetry' theorieshave predicted new families of particles, among them new force-carrierswhich would behave in some ways like gravity, but with limited ranges.Experiments have been repeatedly attempted, but no clear indication ofa new force has ever been conclusively detected over scales of metersto kilometers. Astronomers and physicists have searched off and on forindications in the many data sets for hints of a new gravity-like force,but to no avail. Like the infamous 'Planet-X', the 'fifth force' may simplynot be there to find. That said, there is a fly in this ointment.

Since Albert Einstein proposed a 'cosmological constant' to create a staticuniverse, physicists have been increasingly drawn by their theories toworking with such a 'blunder' as a real ingredient to the world. Duringthe Inflationary Era, this anti-gravity force propelled the universe todilating trillions of times in size in literally an instant of time...but then itvanished back into the vacuum as the universe continues to cool.Astronomers have felt compelled whenever comparing theirmeasurements of the expansion of the universe to theoreticalexpectations, to always consider cosmological models that have some

magnitude of a cosmological constant 'term' dialed into it, in addition toclassical big bang cosmologies which do not contain this effect. Noastronomers have ever claimed to have actually detected this new anti-gravity force that lurks in the void, only upper limits have been offeredfor this illusive ingredient even as late as 1996.


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Recently, using distant supernova, several teams of astronomers have nowgone on record as claiming an actual detection of this anti-gravityvacuum force which they claim is causing, or has caused, the universe'sexpansion to accelerate during at least the last few billion years. If this

interpretation is confirmed, it will not only liberate cosmologists toformally include it in all future cosmological theories describing thepresent universe, but it will be the first detection of a new fundamentalphysical force, not by physicists but by astronomers. Although it willsolve some thorny technical issues in observational cosmology, it willalso present us with a new set of challenges. Not only may there betremendous quantities of 'dark matter' in the universe, but there wouldnow be mysterious forces emanating from the vacuum of space itself!

23Is any time dilation seen in the distant supernova used in studying the

expansion of the universe?


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Not that can be easily identified. These supernova are, first of all, notlocated far enough away ( less than a redshift of 1.0) for significantgeneral relativistic distortions to be detectable. To detect time dilation,you must first be able to identify a process in a distant object which has

a well- defined natural 'clock rate'. The light curve of a supernovaseems to be a promissing chronometer because the explosion rises to apeak then declines over the course of several months. The problem isthat this is not really an accurate chronometer because the naturaldispersion in these supernova light curves is wide enough to mask anysimple relativistic effects. Having said this, there is one arena in whichtime dilation has possibly been detected, and this is in certain classes ofgamma-ray bursts. These bursts have a millisecond to minute range intheir 'light curves', and in the last few years a group of astronomers atthe Goddard Space Flight Center and the Naval Research Laboratoryhave discovered that these light curves have durations in time thatreflect significant amounts of time dilation. Since it is now believed thatthese bursts come from collisions between neutron stars producingblast waves traveling at nearly the speed of light, such relativistic effectsseem to be consistent with each other.

24Has any time dilation been detected in the distant supernova used to

measure the expansion of the universe?


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The recent Hubble Space Telescope investigations of distant 'Type IB'supernova have been used to measure the rate at which the expansionof the universe has been slowing down in the last few billion years. Sofar, only one supernova at a distance corresponding to a redshift

greater than 0.5 has been detected in the 'high-z' searches for thesesupernova. But by using its peak luminosity as a standard candle, adistance to this supernova was independently determined. Whencompared to its redshift and 'recession velocity' due to the expansion ofthe universe, it was found that the expansion rate is practically thesame as the 'Hubble Constant' determined by looking at nearbygalaxies. This means that in the last few billion years, the expansion ofthe universe has not slowed down by a measureable amount, and thismeans that the universe looks as though it is destined to expandforever.

No time dilation effect was measured from this observation because thereis no fiducial moment in time against which one can assess whethersuch an effect has occurred. Time dilation can only be detected if youcan identify some independent chronometer that you think is keeping'proper time'.

25How much of the cosmic background signal can you see in the 'noise'

on your TV screen?

I looked up the transmission frequencies of the major TV stations andthey run from 54 megacycles ( channel 2) to 800 megacycles ( channel69). The peak of the cosmic background radiation is at a wavelength of1.1 millimeters and a frequency of 272 gigacycles ( 272,000megacycles). Now, the cosmic background is a black body with atemperature of 2.7 K, so the distribution of its energy follows the so-called black body formula:

3
http://image.gsfc.nasa.gov/poetry/ask/www.bext.com/tvchannel.htmhttp://image.gsfc.nasa.gov/poetry/ask/www.bext.com/tvchannel.htm


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A v

B(T,v) = --------------------

( Bv)

e - 1

where A is a constant, v = frequency and B = h/kT where h = Planck'sConstant, k = Boltzman's Constant so that for v in cycles per second, B= 1.8 x 10^-11. The following table compares the brightness of thecosmic background radiation at several TV transmission frequencies:

Channel Frequency B(T,v)/A

Channel 2 54 MHz 1.6 x 10^26

Channel 26 542 MHz 1.6 x 10^28

Channel 69 800 MHz 3.6 x 10^28

Cosmic background 272,000 MHz 1.6 x 10^32


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What this means is that at the frequency of Channel 20, for example, theintensity of the cosmic background is 3.6 x 10^28/1.6 x 10^32 = 0.0002times as intense as it is at its peak. Now, the NASA COBE satellitemeasured an intensity for the background of 1.15 x 10^-4 ergs per

square centimeter per second per steradian per cm^-1. Your TVantenna would intercept this radiation from at most 1/2 the sky or 2pi ( =6.282 steradians) and so for a 1 megahertz bandpass ( 3.3 x 10^-5cm^-1) you get about 2.3 x 10^-8 ergs per centimeter squared persecond. In terms of watts this is 2.3 x 10^-11 watts/centimeter square.How bright is the signal from a local TV station? Lets assume itbroadcasts at 100,000 watts and is located about 10 miles from your TVset. Then 100,000 watts / 4 x pi x (10 kilometers)^2 represents anintensity of 8 x 10^-5 watts/square meter. So, your local radio stationwould produce a station about 4 million times brighter than the cosmicbackground radiation itself.

This represents a ball park estimate and there are probably several factorsof two floating around, but the point is that most of the 'snow' that youwould see on a channel with no station broadcasting is produced by theelectronics inside your TV set most likely, and the sensitivity of your TVset is the main factor. Now, if you had a satellite dish, it is a completelydifferent story because now your collecting area for faint astronomicalsignals is a thousand times greater than the area of your typical TV

antenna ( especially the built- in kind). The 'snow' on a channel with notransmitting station could actually consist of a significant amount ofcosmic background radiation...but the actual amount would be hard toestimate without knowing the details of the Tv set and its receiversensitivity. Still, tune your satellite TV to the highest frequency it canreceive to maximize the brightness of the cosmic background signal,and some fraction of the 'snow' may be the background itself!!

26If nothing physical can be infinite, why isn't the universe finite?


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Although mathematics seems to be a good guide to codifying andunderstanding how nature operates, there is no evidence that thephysical world slavishly adheres to every mathematical principle thathumans can conceive of. One of these is the 'concept' of infinity. There

are only a few domains where infinity could be expected to become areal, physical property...the infinite divisibility of space and time, theinfinite spatial extent of the universe, the infinite span of time..to name afew. Black holes may also harbor 'infinities' called singularities...but it isalmost universally expected that black hole singularities will disappearonce a correct quantum theory of gravity is developed. This will also doaway with the 'infinite divisibility of space and time' because a quantumlimit will be set to the graininess of spacetime itself called the Planckscale. Time does not seem to have an infinite past extension becausethe Big Bang itself probably started time...and space...going. Thisleaves only one last domain in which 'infinity' could reappear physically,namely, the extent of the volume of space produced by the Big Bang.The question asks whether we cannot simply argue that space must befinite because no other physical infinity is known to exist. The answer isno.

Whether space is infinite or finite is not a question of logic, but a question ofwhether for all small volumes of space, local gravity exceeds themotions of the bodies within it. The issue of the actual geometry of

space is a question answerable by observation. If it so happens that thedensity of the universe..Omega..exceeds the critical value, then gravitywill eventually win out and the universe will re-collapse. If Omega is lessthan critical, the universe will continue to expand indefinitely. But morethan this, general relativity says that such a low-density universe isALREADY infinite from literally the instant it was born since finite thingscannot evolve into infinite things, but more particularly, the geometry ofspacetime will not be a closed geometry at any instant. In this particularinstance, the physical world can be for all measurable intents and

purposes...infinite, but this infinity is hidden from us as thoroughly asthough it were stuffed inside a black hole. What nature seems to tell usis that any conditions that hint at a true physical infinity are permanentlyhidden from direct observation. Why this is so is an interesting topic ingeneral relativity.


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27If there is no unique frame of reference in the universe, why do wehave a specific speed with respect to the cosmic backgroundradiation?

Because the cosmic background radiation has a zero net speed relative tothe local expansion of the universe. In this frame, you are moving 'withthe flow' of the universe which is actually a dilation effect rather thanbulk motion. The Milky Way has a net relative motion with respect to theexpansion ofthe universe because it is subject to the gravitational fieldsof nearby galaxies. This gives it a residual speed of about 300kilometers/sec relative to the local Hubble expansion of the universe,and it is in this reference frame that the cosmological backgroundradiation has zero relative speed. The only 'unique' reference frame inthe universe is that reference frame where the expansion of theuniverse provides the only net 'motion'. This may violate special

relativity, but not general relativity, because 'general' is better.

28What did people think of the universe in 1897?


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It was pretty small! The distances to what we now call galaxies were onlyestablished during the 1920's...and the famous Shapley-Curtis debateat the National Academy of Sciences ended the debate over how faraway they were. During the 1890's our galaxy was only as big as proper

motion surveys let us chart stellar positions...perhaps no more than afew hundred light years. The first model of our universe connected fuzzygalaxies and nebula together into 'forming solar systems'. The starryfirmament and the Milky Way in the sky just showed stars to be a flat'slab' suspended in the darkness.

29When the universe collapses, will light continue outward?

No. The path of light and matter would return to a Big Crunch because

gfravity is so strong in a closed universe that space is bent completelyaround upon itself so nothing escapes from space into a largervoid...assuming there is one.

30Does the universe expand into a 4th spatial dimension?


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Who knows? Why add another dimension to space when we have noexperimental evidence what so ever that nature is this complex on thelargest scales? That is the problem with all of these speculations aboutwhat is outside our universe. They all are unprovable and do violence

with the only testable theory we have to describe such things...generalrelativity. Seems a big price to pay just to make our intuitions workbetter.

31If the Big Bang happened 15 billion years ago, why did the earth onlyform recently?

Just lucky I guess. Seriously, there is no known reason why a planet or astar would form at one particular time after the Big Bang, or another

time. It is all random and there is nothing weird about it. If the Sun hadformed 5 billion years, or 3.78965 billion years after the Big Bang, wewould be wondering about that too.

32If we could see the edge of the universe, what would we see now?The edge of our visible universe is simply a zone located 15 billion light

years from us where light from galaxies is redshifted to longer andlonger wavelengths, and you run out of objects to be seen at thesedistances. All you run into is the redshifted image of the cosmicbackground radiation...an imaged taken by the NASA COBE satellite.


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33Why doesn't the universe have a center?

This is a very good question, and one that we expect has an answerbecause after all, wasn't the Big Bang just like all other explosions weknow about? Unfortunately the answer is no. The Big Bang was not likeany explosion we have ever seen, because the very gravitational forcesthat bound matter and energy together in the detonation, also curvedspace. THAT is the factor that always sneaks in, to render our intuitionsirrelevant. Only general relativity is able to help us see the 'big picture'and that picture presents us with a universe in which all particles flyaway from all other particles as the whole of space itself expands anddilates. There is no one center IN SPACE to this explosion, only aunique CENTER IN TIME for it, 15 billion years ago.

34Will our universe expand and bump into other universes?


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We don't really know what the universe looks like beyond the visiblehorizon we see around us, but all modern theories say there isplenty more space 'out there'. The most interesting prospect isdescribed by Inflationary Big Bang cosmology. If the universe

emerged from a quantum patch of energy in a primordialspacetime,, it inflated until now the limits of our particular 'patch'could be 10^100 light years or more. Beyond this patch arepossibly an infinitude of other 'patches', each with slightlydifferent physical properties. In the remote future, our visibleuniverse will inexorably expand at the speed of light until thesedistant patches come into view. The smooth and uniformconditions we see around us today, will be replaced by verynonuniform conditions as more of these distant patches come intoour horizon. In a truly infinite universe, there will be an infinitenumber of these patchwork universes.

35What is on the other side of the expanding universe?

Nothing. In general relativity, which provides us with a relativistic theory forgravity and the large-scale geometry of space-time, there are twopossibilities for the shape of 3-dimensional space. It is either finite orinfinite. If it is finite, then that means that the expansion of the universe

is proceeding by a dilation of space so that space folds back upon itself.There is no 'edge in space' in this picture, for much the same reasonthat the 2-dimensional space on the surface of a ball has no edge but isnevertheless finite. If space is infinite, then it also has no outerboundary so again the expansion produces no receeding edge tospace. These are the only two physical solutions for the shape of spacewhich are consistent with how gravity operates as a physical field.


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36Why are galaxies colliding if the universe is expanding?Because individual galaxies don't even 'feel' the effect of the expansion of

the universe because they are only influenced by the gravitational fieldsthey feel near them. Within a cluster of galaxies, the gravitational fieldsof nearby neighbors is more important in determining how they willmove that the effects of a far weaker 'cosmological' field. Only at scalesof tens of millions of light years do the weaker influences ofcosmological expansion become dominant. Even so, two close galaxiesare more strongly affected by each other than by the expansion of theuniverse, but from a distance of a billion light years, we observe theeffects of the expansion being more important in the relative motion ofthese two galaxies which we see colliding. This is a prediction by BigBang cosmology which is borne out by direct observation. In fact, itpredicts that because the early universe was more crowded than now,

collisions between galaxies were more frequent billions of years agothan today, and this is what the Hubble Space Telescope shows us isthe case.

37What word is used to describe everything outside our universe?


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There are no such terms because our universe is defined mathematicallyby the single spacetime that was generated by the Big Bang .If otherspacetimes exist, then they can never be known to us by anyobservation, so there is no term to describe them other than

'hypothetical'. They are permanently beyong science to investigate...butnot to speculate upon!!!

38Could an external universe affect part of ours by its gravity?

No, because everything that was a part of the genesis of our space timeconstitutes OUR universe and represents all the locations in spacewhich can ever be observed by us given the remaining age of theuniverse...even if that is eternity. What you are thinking of as 'other

universes' represent hypothetical, separate spacetimes that will neverbe in contact with events in our spacetime (universe) no matter if ouruniverse expands infinitely. These other universes can never beobserved by any experiment within our spacetime and so they arethoroughly beyond our science and our reality.

39If the universe is infinite, how can there be other universes outside it?


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What we know about MATHEMATICAL infinity is that it is so big that aninfinite number of individualy infinite objects can exist within 'infinity'without touching. This is like the fact that there are an infinite number ofirrational numbers between 0 and 1, but there are also an infinite

number of counting numbers from 0, 1, 2 ..... infinity. The subject ofTransfinite mathematics is very interesting to explore. We do not knowwhat, if anything, this has to do with how the physical world is puttogether, so it is better to think in terms of 'patches' of spacetime whichare trillions of times bigger than our visible universe, and that spacetimeis carpeted with 'a lot' of these patches. It is not a scientific questionwhat REALLY exists out side this.

40Could there be other universes outside of our own?

If our universe is infinite, then you can still have an infinite number of otherseparate universes outside it because so far as humans understandinfinity, it can accomodate an infinite number of infinite things.

If the universe is finite...like a ball...then you can think of a bunch ofcoconuts floating in an infinite ocean, and again you end up with thepossibillity of having an infinite number of finite universes embedded insome vaster kind of space.

yes there could be other universes out there, but they would beunobservable no matter how old our universe became...even infinitlyold!! So, such universes have no meaning to science because there isno experiment we can perform to detect them.


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41Why is the universe expanding if gravity is an attractive force?Because the initial 'explosion' was so violent and energetic that it

completely overcame the attractive gravitational forces between theconstituents...especially if there was an Inflationary Era where theenergy of the vacuum itself produced an enormous positive 'anti-gravity'pressure.

42Does interstellar dust have anything to do with the cosmological

redshift?

No. interstellar or even intergalactic reddening just changes the intensity ofthe light at certain wavelengths. It has no effect upon the wavelengthsof spectral lines, and it is this wavelength shift that is called the redshift.

43If the Big Bang happened at one point, why are galaxies not

expanding at different speeds?


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The Big Bang did not happen at one point in space .It happened in everypoint in space at the same time, and according to general relativity, it

created space and time itself. It is not a fireworks display at all, or aconventional explosion, so all the rules we intuitively expect suchevents to follow, are not followed by the Big Bang.

44If the universe is open, does it have infinite mass?Yes, but this infinite mass is distributed over infinite space too.

45Does the Oort cloud around every star account for dark matter?

No .The amount of mass in these comet clouds is already included whenwe weigh the Milky Way. They represent an insignificant amount ofmass compared to individual stars and so would not contribute much atall.


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46If I see two quasars 15 billion light years from us at opposite parts ofthe sky, how can the universe be only 10 billion years old whenthey are 30 billion light years apart?

Because the matter that makes up these distant galaxies did not start fromwhere we are and travel to where they now are. This is a basic featureof Big Bang cosmology provided by general relativity. The mater wenow see in these distant galaxies originated from regions of space thatwere far apart even at the Big bang itself. The stretching of space sincethen now makes them even farther apart. But the matter that makesthem up did not physically travel THROUGH the space that literally'stretched' into existence between them since then.

47Is there nothingness outside of our visible universe?

We don't think so! What exists outside of our visible universe today isprobably more of the same of what we see around us right now. Thelimits to the space that emerged from our Big Bang do not appear untilwe reach infinity in an 'open' cosmology, or about 60-100 billion lightyears when we see the backs of our heads in a 'closed' cosmology.Both space-times MAY be embedded in an even larger 'something' butthose regions are beyond any space that we can ever explore no materwhich cosmology we live in.


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48Why are there so many different estimates for the distances toquasars and the size of the universe?Because each estimate is based on what the particular astronomer

chooses to use as the underlying model for the universe. Each of theseis defined by its particular Hubble Constant, its density, and its value ofthe cosmological constant. Each of these 'constants' is not known tovery high precision and their differences lead to a range of possiblevalues for distance.

49If an observed galaxy at 15 billion light years is actually 30 billion lightyears away, does that mean the universe is twice as old?

No, because the galaxy did not get to where it is from here. It was formed

far away from the gas that emerged from the Big Bang to become theMilky Way. The age of the universe is still only about 12-15 billion years.

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If the cosmic background radiation comes from everywhere in spacedoes that mean it has no source?

No. The cosmic background we see now, was emitted by the fireballradiation in the region of space which is now near the edge of ourvisible universe. Similarly, an observer located 14 billion light years fromus today, will be detecting the fireball radiation which was emitted by thedense matter in the space near our Milky Way, but of course, there wasno Milky Way present then, just dense hot matter and radiation. Everyphoton of the CMBR comes from a source which is the ancient hotmatter in regions of space far away from us today.

51If the cosmic background radiation comes from everywhere in space

does that mean it has no source?

No. The cosmic background we see now, was emitted by the fireballradiation in the region of space which is now near the edge of ourvisible universe. Similarly, an observer located 14 billion light years fromus today, will be detecting the fireball radiation which was emitted by thedense matter in the space near our Milky Way, but of course, there wasno Milky Way present then, just dense hot matter and radiation. Everyphoton of the CMBR comes from a source which is the ancient hot

matter in regions of space far away from us today.

52How do astronomers measure the temperature of the cosmic

background radiation without using a thermometer?


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They measure its intensity at many wavelengths where it is detectable,then fit a Planck 'black body function' to the measurements. Since each

black body curve depends ONLY on temperature, the fit to the dataimmediately gives you the effective temperature of the radiation, whichfor the CMBR is 2.7K

53If all the galaxies are flying away from us, are we in the center of theuniverse?No because that is not the only way in which you can get the same end

result observationally. General relativity provides another interpretation

and that is the one that is favored because it 1) is consistent with lots ofother types of experimental data and observations and 2) it does notkeep alive the philosophical idea that we are somehow in a uniqueplace in the universe. This is what the Copernican Revolution was allabout, and it is no longer demanded that Earth be the center ofanything. So, since general relativity seems to work, we go with itsinterpretation that all observers anywhere in the universe would seethemselves 'at the center' like ants on the surface os an inflatingballoon.

54How do you calculate the distance to an object at a redshift of 5.0?


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I did this exercise over at the Astronomy Cafe in the Ask the AstronomerCosmology questions archive.

55Will the limits to the observable universe expand indefinitely?

If our universe is destined to expand forever, our observable universe willincrease in radius by about 1 light year per year.

56What is an 'antipode' in cosmology and does one exist in ouruniverse?

In any closed cosmological model, every point has a 'mirror' point in the

space-like surface. This point is called the antipode, which is a termancient navigators also used to describe the point farthest away fromthem on the surface of the Earth. For open or infinite universes there isno such unique point. So far as we know, our universe does not behavelike a closed universe and no such point seems to exist for us. Note,every observer will have a different antipode, and you will not be able tosee the universe near your antipode unless the light travel distance forlight emitted at the Big Bang exactly equals the distance in space to thispoint. For the present observable model for the universe, if the universewere in fact closed, our antipode is well outside the boundary of ourobservable universe and will not be visible to us until the universe is 1/2of its total age.
http://image.gsfc.nasa.gov/poetry/ask/www.theastronomycafe.nethttp://image.gsfc.nasa.gov/poetry/ask/www.theastronomycafe.net


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57What would an observer outside our visible universe see if they

looked in the direction of the Milky Way?

They would not see the Milky Way at all, because if the current horizon is14 billion light years away and our Milky Way formed 13 billion yearsafter the Big Bang, they would only see the galaxies we now see thatare 1 billion light years away from us in their direction. These galaxieswould appear as very primitive objects. These observers would be ableto see many galaxies that we cannot see from where we are becausethe light would have arrived at their locations, but not at our location yet.

58Is there any evidence that the Hubble law is not a linear relation

between distance and expansion speed?

For nearby galaxies out to, say, a few billion light years, the expansionspeed is linear based on the best statistical tests and best calibrateddata we have. Beyond this we expect to see, and some claim to do so,a change from a linear expansion to a faster rate because the universewas expanding faster long ago, and we would see this 'acceleration'effect in the expansion rates of the most distant galaxies. A very hardmeasurement, but we should know how big an effect this is in the nextfew decades as astronomers relentlessly accumulate more data ondistant 'Type 1A' supernovae.


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59How is Hubble's Constant derived from Newtonian physics?

Assume that the matter within 'R' parsecs from the Milky Way can beapproximaed as having an average density 'Rho' in space. Then themass inside any spherical volume with this radius is:

3

Mass = 4/3 pi R Rho.

The escape velocity for a body of mass 'm' at the surface of this sphericalvolumn is just:

Gravitational Potential = Kinetic Energy

G m Mass 2

---------- = 1/2 m V

R

but since V = H R by Hubble's Law and we have previously derived the

total mass in terms of an average density Rho, a little algebra gives us:

2


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H = 8 pi G Rho

--------------

3

60How can we see light from a galaxy 14 billion light years away if the

universe is 14 billion years old?

Because these objects were always far away from us even at the Big Bang.The light from them has only now reached us across the everexpanding gulf of space that has widened, at some times, at a ratemany times faster than our local light speed.

61Where can I get a book that discusses the cosmic microwavebackground?

The popular books by NASA COBE scientists George Smoot and JohnMather are highly recommended and have good introductory chapterson the nature of this radiation.


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62Does Stephen Hawking think the universe is open or closed?I believe this is his preference because he likes closed manifolds which are

easier to work with mathematically...and are prettier..than openmanifolds. But I cannot speak for what Hawkings thinks these days.

63If the universe is open and infinite, what is it expanding into?

There is no edge to an infinite object so if the universe is truly infinite, its 3-dimensional space is not bounded. It can expand, but in doing so itdoes not displace other space outside it since there is nothing 'outside'of infinity. This sounds like gobbledygook, but this is what themathematics tells us, and who among us has ANY intuitive idea of whatinfinity is like?

64Are redshifts really quantized?


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Many astronomers since Geoffery Burbidge in 1968 have claimed to detectperiodicities in the redshifts from large numbers of quasars. But arecent review of the evidence pro and con by Douglass Scott at theUniversity of California at Berkeley ( reported in 'The Space Density of

Quasars' ASP vol. 21, p. 264) shows that all of the claims are due tosmall sample sizes and to various well-known biases in assemblingthem. When the same analysis is performed on a much larger, well-calibrated sample of quasars called the Large Bright Quasar Survey'absolutely no evidence for periodicities in redshifts can be seen.

65If the universe exploded from a small piece of space why are distant

galaxies moving so fast?

Well...it didn't explode from a single patch of 3-dimensional space, but fromall the patches of 3-dimensional space that make up the entire 'space'that was born in the Big Bang itself. This is a common misconceptionthat then leads to the second part of your question, which no longerneeds to be answered by the particular starting point you used to frameyour question.

66Does the value of Hubble's Constant depend on the galaxies outside

our visible universe?


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No. By simple physics, the strength of the gravitational field only dependson the amount of mass within a spherical volume of space with thedistant galaxies at its outer, mathematical, surface. If you pick anyspherical volumes in the universe with a radius up to the radius of the

visible universe, add up the mass of the galaxies inside, and divide bythe volume, you get an average density for the matter inside the region.Hubble's Constant is then proportional to the square root of this density.It is not affected by the galaxies outside the region you select, becausethe gravitational forces from all the 'external' galaxies cancils out in thesum over their net force upon the galaxies within the volume youselected.

67Is the universe expanding the same way in all directions?

The measurements by the NASA COBE satellite show that the expansionof the universe has been phenomenally smooth to better than 1/10000in every direction. Local irregularities are caused by the patina ofclusters of galaxies spread through out the local universe.

68If the ultimate fate of our universe is so bleak, what then is its

purpose?


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Why does everything have to have a purpose? There is a lot of opportunityfor us in a un

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