Defining the Universe

A simple definition for the universe is: “all that is”. The universe is the sum of all energy, matter, space and time.

But there’s a difference between the universe we see (the observable universe) and the universe as it really exists.

This gets complicated….

Olber’s Paradox

Why is it dark at night? There should be stars in every direction, like trees in a deep woods.

Olber’s Paradox

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The Universe by Poe

“The more distant stars are so far away that light from them has not reached Earth yet.”

You can only look as far back into time as long as the universe has been in existence,.

The night sky is dark because the universe had a beginning.

Hubble’s Law - 1929

Hubble plotted galaxy redshifts (speeds) vs. distances to discover that the universe is expanding.

Edwin Hubble discovered a relationship between a galaxy’s redshift (an indicator of speed) and its distance.

Like runners that begin at the crack of a gunshot, the fastest move the farthest along the track.

Unlike runners, these galaxies aren’t really moving on their own…the observed redshift is called cosmological redshift and is due to the expansion of space.

(But this idea came along after Hubble’s pioneering discovery.)

A Doppler Effect review

An object moving away from an observer shows an apparent shift to longer wavelengths, while an object approaching shows a shift to shorter wavelengths. This principle applies to any sort of wave: electromagnetic or mechanical. This manifests itself in spectra by a shift in the absorption lines of a star or galaxy towards the red or blue end.The shift covers only one component of motion of the star or galaxy, not its entire velocity.

Three kinds of redshift

Doppler Shift is due to the relative motion of an object away from an observer.

Cosmological Redshift is caused by the expansion of space.

Gravitational Redshift is caused by gravitational fields, such as when a photon of light is near a black hole.

All three are present among objects in the universe, although it is assumed that cosmological redshift is very large compared to the other two when looking at far away galaxies.

An example of cosmological redshift

Observations from the rubber band demonstration

Galaxies in the universe are not moving primarily due to their own velocities, but because space is expanding.

Galaxies initially closer together maintain the same relative spacing during expansion.

Although space expands, objects like atoms and galaxies do not enlarge, because local gravitational andelectromagnetic forces supersede the expansion of space.

The “Fireworks” Theory

Georges Lemaitre published a model in 1927 of a non-static universe that had an energetic beginning:

“The present universe is the "ashes and smoke of bright but very rapid fireworks.”

“Ylem”: Cosmology enters the Lab

A 1949 picture with Robert Herman on the left, Ralph Alpher on the right, and George Gamow in the center, as the genie coming out of the bottle of "Ylem," the initial cosmic mixture of protons, neutrons, and electrons from which the elements supposedly were formed.

In the first hour of time, energy and matter “decoupled” to form discrete entities.

The universe had expanded and cooled to the point where nuclear fusion ceased and only the elements hydrogen and helium had been made. The composition of matter in the universe was about 75% hydrogen and 25% helium, with a tiny amount of lithium.

According to Ylem…

The “Big Bang”

Fred Hoyle was an advocate of the Steady State model of the universe. He coined the term “Big Bang” while speaking on BBC radio to describe the opposition’s model.

Hoyle is best known for his work with advanced nucleosynthesis. He derived how elements heavier than iron could be produced by supernova explosions.

The Big Bang Theory

The term ”Big Bang” is a misnomer. There was no bang, or classical explosion of any sort. And there was no center point where it all originated. Space and time were created when the universe came into existence, so it arose everywhere at once.

The Big Bang theory but does not actually describe the moment the universe came into existence. That moment is called creation. In creation, something comes out of nothing, and that is what seems to have happened.

Penzias and Wilson vs. the Pigeons

Penzias and Wilson had detected the cosmic microwave background (CMB)—radiation from the Big Bang. They each received 1/4 of the 1978 Nobel Prize in physics for their discovery.

The Cosmic Background Radiation

Why microwaves?

Microwaves were redshifted from the shorter wavelengths of heat and light due to the expansion of the universe.

In January 1990, the COBE satellite measurementsconfirmed that the CMB is blackbody radiation, with an apparent temperature of 2.725 +/- 0.002 K.– This is in good agreement with theoretical predictions.

The Cosmic Microwave Background

WMAP: the Wilkinson Microwave Anisotropy Probe - 2010

The more recent map produced by WMAP shows the universe as it existed only 380,000 years after the Big Bang.

The results led astronomers to derive that the universe contains 4 percent normal matter, 23 percent dark matter, and 73 percent dark energy.

WMAP Cosmic Microwave Background Analysis

What is dark matter?Dark matter does not give off observable energy in any electromagnetic wavelength, but can be detected by watching the behavior of space objects. A few examples are:

The stars in the outer reaches of a spiral galaxy orbit with the same speed as those much closer in. This implies that the galaxy is surrounded by a massive spherical distribution of unseen matter, which adds to its total mass.

The gravitational centers of galaxy clusters do not lie at their expected place, but are offset. This implies that there is much more unseen matter present.

Gravitational lenses are often found with no apparent visible matter in the foreground to act as the lens. The lensing agent is usually a very large concentration of dark matter.

Types of dark matter

WIMPs: Weakly-Interacting Massive Particles. Dark, tiny particles fill the universe, but they are so tiny and weakly-interacting that they are difficult to detect by gravity studies, such as certain types of neutrinos and axions, if they exist. This is currently the favored idea.

MACHOs: Massive Compact Halo Objects. Dark, ultra dense matter surrounds galaxies and resides within and beyond their observed halos. It also may exist between galaxies in a cluster or even in the voids of the bubble structure of the universe. This dark matter could take the form of neutron stars and black holes, or other types of objects made of normal baryonic matter.

What is dark energy?

Dark energy may be interpreted as a sort of “antigravity” repulsive force. (But cosmologists will tell you it is NOT a force.) If something in the universe is counteracting gravity, then there does not need to be as much matter (mass) factored into calculations for the Big Bang/ Inflationary models.

Einstein first proposed that there was something like this in 1917. He called it the cosmological constant, but later recanted it, calling it his “greatest blunder”. Maybe he was right all along.

Pros:• The Big Bang explained the composition of the universe.• It implies there was a creation event.• It provides a mechanism for an expanding universe.• It explains the observed Cosmic Microwave Background.• It predicts the observed “bubble and void” structure of the

universe.Cons:• The universe “banged” too smoothly and appears to be flat.• Most of the matter of the universe is unaccounted for- visible

matter only comprises 4% of the needed critical mass of the universe. Factoring in dark matter, we gain another 23% but are still missing about 73% of the universe!

Big Bang Theory - Pros and Cons

Big Bang/ Inflation Theory

Alan Guth proposed a solution to two flaws (the flatness problem and the smoothness problem) in the Big Bang theory in 1979 by introducing an inflation period:

When the universe was only 10-35 seconds old, it was the size of a grape, but in about 10-32 seconds it expanded by a factor of 1050.

That’s about a billion times the speed of light, but there was no motion, since only space was expanding.

Chaotic Inflation Theory - 1983

Guth’s theory had some flaws, which were subsequently overcome by a newer version of the theory by Andrei Linde called chaotic inflation. (This was subsequently modified in small ways by lots of others)

Chaotic inflation proposes a “meeting ground” for two major fields of physics, quantum mechanics and relativity, that are usually in conflict.

It also suggested a way to explain the tiny quantum fluctuations seen in CMB maps, and it suggested gravitational waves were a mechanism for this process.

In March of 2014, Linde’s theory was validated by observational proof- thedetection of gravitational waves by the BICEP2 radio telescope in Antarctica.

Chaotic Inflation verified - 2014

Astronomers using BICEP2 produced a map of tiny quantum fluctuations in the early universe produced by gravity waves. These same fluctuations are the ones invoked in multiverse models.

These large-scale structure of the universe has been mapped by various surveys.

Galaxies form clusters, clusters form superclusters, and superclusters form sheets and walls that appear as if they are draped on the surface of giant invisible bubbles.

Dark energy is thought to have some role in the formation of this bubble and void structure.

The Observed Large Scale Structure

Superclusters

The Shapley Supercluster is currently the largest known, having a diameter of more than 400 million light years.

Great Walls

Geller and Huchra in 1989 announced the discovery of the first “great wall” - a clustering of thousands of galaxies that formed a “stick man” structure.

Bubble and Void structure

The Hubble Constant and expansion

The Hubble Constant is the slope of the graph of Hubble’s Law and is used to determine the rate of expansion of the universe.

Present values (2014) for the Hubble Constant range from 67.3±1.2 (Planck Mission) to 72.6±2.9 (Sloan Digital Sky Survey) to 73.8±2.4 (HST). All units are in km/sec/Mpc.

The rate of expansion vs. the speed of lightAssume an average value for the Hubble Constant is 71km/sec/Mpc.

A megaparsec (Mpc) is a unit of length equal to about 3.26 million light years, and is an important part of this rate. It means that a galaxy that is one million parsecs away (1 Mpc) from us is receding from us at only 71 km/sec because of the expansion of the universe.

A galaxy that is 2 Mpc away would be receding from us at twice that velocity; 144 km/sec

But consider a galaxy that is over 4225 Mpc away from us. It is receding faster than the speed of light - we can never receive light from it. The light would be moving toward us slower than space is expanding. 4225 X 71 = 300000 km/sec, slightly faster than light speed.

Hubble’s Law and the shape of the universe

The shape of the universe indicates its matter and energy content

The curvature of the universe as a whole depends on how the combined average mass density ρ0 compares to a critical density Ω

The size of the irregularities in the cosmic background radiation mapped by the WMAP

satellite show that the observations fit the flat universe model well.

CMB Fluctuations and the Curvature of Space-Time

The acceleration of the universe was first discovered in 1998 when astronomers found that supernovae a few billion light-years away were slightly fainter than expected.

But then, they found even more distant supernovae were a bit brighter than expected.

This means that sometime about 6 billion years ago, the universe shifted gears from deceleration to acceleration!

What’s going on?

Accelerating Universe

Supernova studies by Conley et al (2011) support a model for the universe that shows an accelerated rate of expansion due to a dark energy influence.

Observations of the distances to galaxy clusters made by the Chandra X-Ray Observatory confirm that the universe expansion initially slowed down—but shifted gears about 6 billion years ago and is now accelerating.

This is an independent piece of evidence that agrees with the observations of supernovae.

Dark energy is the only current explanation for how the universe could have shifted gears from slowing down to speeding up. The chart below from the recent Baryon Oscillation Spectroscopic (BOSS) Survey illustrates these 2014 results:

Blame Dark Energy

Two variables that are important to making an accurate prediction of our ultimate fate are the amounts of dark matter and dark energy in the universe. Another factor is if the “energy” of dark energy changes over time (the so-called phantom energy factor). Recent studies from the Chandra X-Ray Observatory suggest that dark energy appears to be constant across space, with a strength that never changes with distance or time.

The evolution of the universe, then, is determined by the interplay of the rate of its expansion caused by the Big Bang, and the pull or push of gravity. Traditional gravity would attempt to counteract expansion, while dark energy, which acts as a sort of “gravity that pushes”, would aid expansion.

Dark Factors

If the rate of acceleration does not change with time, our flat universe will expand forever. Stars will eventually die and black holes evaporate, leaving a uniform, cold, motionless universe. This is called the Heat Death, or the Big Chill.

This is the currently favored model based upon observations.

The Fate of the Universe I. - Big Chill

If the rate of acceleration continues to speed up, our universe will expand in an uncontrollable fashion. Expansion will overwhelm gravity and even the nuclear forces, shredding all matter, even at the atomic level.

The Fate of the Universe II. - The Big Rip.

If acceleration stops at some point and the universe falls back to a small, hot dense state, it will end in what is described as the Big Crunch.

Current observations and models do not predict this as a likely outcome.

The Fate of the Universe III. - Big Crunch

The universe will continue to recycle itself in crunches and bangs, unless in one of its attempts, the rate of inflation is too rapid or slow. If too rapid, it will die a heat death. If too slow, it will fail to inflate, perhaps remaining in a compactified cosmic string condition.

The Fate of the Universe IV. - Big Bounce

A quasar with a redshift of Z = 6 would have taken about 12.7 billion years to send its light to us. The time it takes the light to travel to Earth is called the look-back time and the distance (12.7 billion LY) is termed its light travel distance (LTD).

But in that time, space has expanded more and the quasar is now at a distance of 27 billion LY. The distance to the quasar now is known as its comoving distance.

Distances in the Universe

Estimated number of stars: 1022 - 1024

Estimated number of galaxies: 100 billionExpansion speed (WMAP): 33.8 km/sec/MpcAge: 13.798±.037 billion yearsEstimated mass: 1.46 x 1053 kgCritical density (Planck): 8.5 x 10-26 kg/m3, or about 5 H atoms per cubic meterDensity parameter (WMAP): Ω0 = 1.02 +/- 0.02Most distant object: UDFj-39546284 (infrared from HST),

redshift = 11.9, 13.42 billion light years away (LTD)Temperature of the CMB: 2.726 K Shape: flat Center: there isn’t one. Edge:

none

Universe Statistics

The “Edge” of the Universe

Assuming the universe is flat, there is no edge.

(Euclidean) flat shapes do not have to resemble a sheet of paper. They could take the form of a cylinder, torus, Mobius strip, dodecahedron or even a coffee mug.

Using this scenario, expansion is infinite, and there is no edge. The universe will not come to an end, but will eventually suffer heat death.

The Cosmic “Event Horizon”

Although there is no edge to the universe, there’s a limit to what we see now, and what we will be able to see in the future. It is called the Cosmic Event Horizon.

The distance to this horizon changes over time because of the expansion of space. Right now it is about 16 billion light years away, meaning that a signal from an event happening at present would eventually be able to reach us in the future if the event was less than 16 billion light years away, but the signal would never reach us if the event was more than 16 billion light years away.

Consequences of a Cosmic Event Horizon

Though in principle more galaxies will become observable in the future, in practice an increasing number of galaxies will become extremely redshifted due to ongoing expansion, so much so that eventually they will seem to disappear from view and become invisible.

At some point in the far future, most of the observable universe will disappear.

This gives rise to the adage: :Be a cosmologist today, because in the future, you won’t have anything out there to observe!

Does having a cosmic event horizonmean that the Big Bang was like a giant black hole?

No. The Big Bang was a singularity extending through all space at a single instant, while a black hole is a singularity extending through all time at a single point.

An idea to explain some of the problems of the Big Bang theory and the flatness of the universe is related to string theory.

String theory dictates that even the tiniest particles of matter are in fact made up of incredibly small strings of energy.

String Theory and Cosmology

String Theory can be used as a way to show how the Big Bang might have happened.

In this model, two interacting “branes” (short for the word membrane) collide and create the universe we observe today. Some refer to this idea as the “Big Splat”.

String Theory and Cosmology

When Branes Collide

Multiple Universes: Multiverses

The dimensional universe idea was first proposed as a thought experiment that our entire universe might be as small as “an atom in a giant’s cup of tea”.

Multiple Universes: Multiverses

Versions of string theory predict up to 26 dimensions in a space-time foam. (We are only using three in our universe.)

Perhaps universes exist in other dimensions that we cannot mutually detect. These dimensions are “compactified” to us but they still may affect our universe by affecting the forces in our universe, such as gravity.

A Cyclic Universe?

Perhaps our universe recycles itself as branes interact.

WebsitesAn atlas of the universe: http://www.atlasoftheuniverse.com

Scale of the universe: http://scaleofuniverse.com/

Most frequently asked questions in cosmology: http://www.astro.ucla.edu/~wright/cosmology_faq.html

Big Bang Central: http://www.bigbangcentral.com/index.html

The formation of the universe in 10 steps:http://www.space.com/13320-big-bang-universe-10-steps-explainer.html

VideosMinute Physics series: https://www.youtube.com/playlist?list=PLED25F943F8D6081C

Illustris- a 7 minute computer simulation of the evolution of the universe: http://www.youtube.com/watch?v=T5h7NmyTytE#t=76

Most distant quasar: http://www.youtube.com/watch?v=JGpe-jLmSJE

Scale of the universe video: https://www.youtube.com/watch?v=lFZbllbMJes

LHC rap song: https://www.youtube.com/watch?v=j50ZssEojtM