the expansion of the universe, and the big...

The expansion of the Universe, and the big bang

Q: What is Hubble’s law?A. The larger the galaxy, the faster it is moving way

from us.B. The farther away the galaxy, the faster it is

moving away from us.C. The larger the galaxy, the slower it is moving away

from us.D. The farther away the galaxy, the slower it is

moving away from us.

Review: the expansion of the universe

By measuring the distances to other galaxies, Edwin Hubble found that their recessional velocity is proportional to their distance

Hubble’s law:v=Ho×dwhere Ho is Hubble’s constantHo=22/km/s/Mly

Review: the expansion of the universe

This is what we mean by the expansion of the universe: we see that everything is flying away from everything else

(unless gravity is strong enough to overcome the expansion, then the objects will reverse course and start moving towards each other…)

The expansion of the universe

A common misconception:

If everything is moving away from us, doesn’t that meant that we’re at the center of the universe?

The expansion of the universe


If everything is moving away from us, doesn’t that meant that we’re at the center of the universe?

Why this is a misconception:

Even if you were in a different galaxy, you would still see everything moving away from you

If you were sitting on any point on the surface of this balloon, you would see everything moving away from you. And in fact the recessional velocity would be proportional to the distance, just like in Hubble’s law!

Wednesday, June 25, 14

Wednesday, June 25, 14

No matter where you are, you are going to see galaxies moving away from you. So there is no “center” of the Universe.

The expansion of the Universe

Why we use the word expansion:

In the context of Einstein’s theory of General Relativity, we tend not to think of objects as moving through space as they recede from one another. Instead we think of space itself as expanding… kind of like with the balloon analogy.


Another (somewhat less) common misconception:

Due to the expansion of space, you are actually getting taller, Brooklyn is getter larger, and in general everything is getting bigger.


Another (somewhat less) common misconception:

Due to the expansion of space, you are actually getting taller, Brooklyn is getter larger, and in general everything is getting bigger.


Objects that are bound together (for instance by gravitational or electromagnetic forces) don’t get bigger.

The big bangRight now we see everything flying apart. But if you were to run the clock backwards, then everything would be flying toward one another, until everything in the observable universe is compacted into a single point. We call that moment the big bang.

The big bang


The big bang happened at one place, which can be called the center of the Universe.

The big bang


The big bang happened at one place, which can be called the center of the Universe.


The big bang happened everywhere at once — or at least, everywhere that we can conceive of!

The big bang

• Georges Lemaître was the first person to discover the expansion of the Universe, although this is usually attributed to Edwin Hubble.

• Soon afterwards he proposed that the Universe began in a single moment of creation. It began in an extremely hot and dense state which he called the “primeval atom” or a “cosmic egg.”

(1894-1966)

Using our knowledge of physics we can get a pretty good idea of what happened all the way back to just a tiny fraction of a second after the big bang.

The history of the Universe

Using our knowledge of physics we can get a pretty good idea of what happened all the way back to just a tiny fraction of a second after the big bang. - First we can ask, when did the big bang occur? That’s easy: we know how fast two galaxies are moving away from each other using Hubble’s law, so let just calculate how long it took for them to get there.



Using our knowledge of physics we can get a pretty good idea of what happened all the way back to just a tiny fraction of a second after the big bang. - First we can ask, when did the big bang occur? That’s easy: we know how fast two galaxies are moving away from each other using Hubble’s law, so let just calculate how long it took for them to get there.➡ T=14 billion years


• T=10-43 seconds: the Universe was compacted to the smallest quantum scales; this means that matter and energy fluctuated randomly due to quantum fluctuations. The four fundamental forces were indistinguishable, and at the end of this era gravity became a distinct force.

• T=10-38 seconds: the strong force became distinct. This is thought to have released a tremendous amount of energy causing a period of inflation: in 10-36 seconds a piece of the universe a size of an atomic nucleus expanded to the size of the solar system!


• T=10-10 seconds: the electromagnetic force became distinct. The temperature of the Universe was still 100 million times hotter than in the core of the Sun, and photons spontaneously turned into matter and vice-versa. In particle accelerators we can reach comparable amounts of energy, and experiments have confirmed our theories.

• T=10-3 seconds: the Universe had expanded and cooled enough that photons could no longer spontaneously turn into matter (and anti-matter!), and vice-versa.


• T=5 minutes: Up until this point the Universe was so hot and dense that nuclear reactions could take place, just like in the core of a star. So hydrogen could fuse into helium, and trace amounts of lithium. At the end of this period, the Universe consisted of 75% hydrogen and 25% helium.

• T=380,000 years: up until this point the Universe was so hot that the atoms were ionized. Photons could not stream freely through the Universe. But now the electrons become bound to nuclei, and the Universe becomes transparent. This is when the cosmic microwave background radiation comes from.


• At this time the Universe was still just a hot soup of particles and photons that cooled as it expanded. Gravity began to make the densest regions of the Universe collapse.

• Eventually stars and galaxies began to form.

Now, is all of this actually true?

It didn’t take long after the discovery that the Universe was expanding for many scientists to conclude that the Universe must have had a beginning, and that something like the big bang really did happen.

But there was one prominent alternative theory: the steady state universe.


• In the steady state universe theory, the universe is expanding but is also eternally unchanging.

• This is because new galaxies are formed in the empty space that is created by the expansion.

Fred Hoyle(1915-2001)

Fred Hoyle is actually the one who coined the term “big bang.” He meant it ironically!


The case was closed in favor of the big bang theory with the discovery of the cosmic microwave background (CMB) radiation in 1964.


• Recall that when the Universe was less than 380,000 years old it was so hot that all atoms were ionized. This meant that photons could not travel very far.

• But once the Universe cooled to 3000 Kelvin the free electrons became bound to nuclei, and suddenly the Universe became transparent.

• The free-streaming photons left over from this period that are reaching us today constitute the CMB.

This is the light (heat) that is left over from 380,000 years after the big bang. But we’ll never be able to directly see earlier than that… at least not using light.


The case was closed in favor of the big bang theory with the discovery of the cosmic microwave background (CMB) radiation in 1964.

Robert WilsonArno Penzias


We have a few extremely good reasons to believe that the big bang theory is basically correct:

1. Detection of the CMB2. The spectrum of the CMB:

- originally this light followed a thermal spectrum of 3000 Kelvin, but has been redshifted by a factor of 1100; so now we observe a spectrum with 2.73 Kelvin

- this agrees perfectly with the temperature that was required in the very early Universe to fuse 25% of the hydrogen into helium.


More reasons that we have become so confident about this picture of the big bang:

3. The size of the temperature fluctuations in the CMB agrees perfectly with the sizes that we predict based on our knowledge of how large structures (galaxies, clusters of galaxies) must have formed.

hot spots

cold spots


More reasons that we have become so confident about this picture of the big bang:

4. The big bang seems to have occurred about 14 billion years ago. The ages of the very oldest stars that we can find are also estimated to be about 14 billions years.


But this isn’t what we see!

Olber’s paradox: if the Universe if infinite, then in every single direction that you can look all you should be able to see is stars.



What we see is more like this.

There are basically two ways out of this paradox:

1. The Universe does not have an infinite number of stars.




There are basically two ways out of this paradox:

1. The Universe does not have an infinite number of stars.

2. The Universe had a beginning: since when we look at distant objects we are looking backwards in time, the beginning of the Universe marks the farthest we can ever see.


The limits of scientific knowledge

What caused the big bang?• We have no idea… but scientific thought tends to

describe the big bang itself as a kind of quantum fluctuation that got a little out of hand.

What happened before the big bang?• We tend to describe “the Universe” as everything

that we can know and observe. So if the Universe was created in the big bang, there is no “before.” But perhaps there are an infinite number of other universes that we can never know about.

the expansion of the universe, and the big...

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