Astronomy 100

Lectures

Table of Contents

Astro 100

Lecture 25
Cosmology and the Big Bang

Outline

The Expanding Universe, Revisited
What the Expansion Implies: a Hot Big Bang!
Other Evidence for the Big Bang

Terms to Know

Hubble's Law
cosmic microwave background
last scattering surface
recombination
elemental abundance ratios
cosmological principle
isotropy and anisotropy

1. The Expanding Universe, Revisited

Remember that Hubble found that all galaxies outside the Milky Way (except the very nearest ones) have redshifted spectra and are therefore receding from the Milky Way, and that the farther they are, the faster they recede. This is Hubble's Law (V_r = H x d), and it has been confirmed many times over with larger and larger samples of more and more distant galaxies. The most distant galaxies known lie about 12 billion light years away from the Milky Way and are receding at over 90% of the speed of light.

Remember that this does NOT mean that we are at the center of the expansion! There is no center -- every galaxy sees the same redshift-distance relation and would see itself as the center of the expansion.

2. What the Expansion Implies: a Hot Big Bang!

The Universe is expanding. Now imagine running the movie backwards: as the Universe gets younger, all galaxies turn around and start to fall closer and closer together, faster and faster, compressing denser and denser until -- BOOM! -- there's a Big Bang as all the matter in the Universe crashes together in an immensely hot soup of elementary particles. The temperature of the soup 100 seconds before (in our backwards movie) the Big Bang (after the BB in real life) would be 1 billion K -- too hot even for atomic nuclei to survive more than a split second before being torn apart by collisions. No galaxies, stars, molecules, or even atoms could exist.

This is what the expansion of the Universe implies: that the Universe as we know it popped into existence via a collossal, hot, dense, "event" about 14 billion years ago, and has been coasting apart ever since.

Q: Where was the center of the Big Bang?

A: There was no center! It happened everywhere at once at the same time. Space itself was created and started expanding in the Big Bang. You could just as well ask, "Where is the center of the surface of an expanding balloon?" The volume has a center, but the surface doesn't.

Be careful: we are at the center of the visible Universe (visible to us), because everywhere we look we see galaxies rushing away from us, and younger and younger galaxies farther and farther away. But this is exactly what would happen if the Universe were uniform, infinite, and expanding -- and every observer would see the same thing.

The Cosmological Principle -- which astronomers assume to be correct -- states that all observers in the Universe see the same thing. (This is true only on large scales -- not on the size of people, or planets, or stars, or galaxies, or even clusters of galaxies, but rather on sizes where the Hubble expansion looks smooth and uniform, larger than 10 Mpc or so.) There is no special, preferred place in the Universe. This is the ultimate form of the Copernican Principle.

If the Cosmological Principle is true, then the Universe should be isotropic on those large scales. This means "looks the same in all directions," and to a very high level of precision, the Universe does indeed appear to be isotropic.

3. Other Evidence for the Big Bang

The expansion of the Universe is not the only thing the Big Bang theory has going for it. Two other important pieces of evidence support it strongly:

All the light elements in the Universe (75% H, 23% He, and trace amounts of Be, Li, B, D) appear in just the relative abundances predicted by the Big Bang theory. During the first few minutes of the Universe, when there was nothing but a hot soup of particles, some recipe of nucleosynthesis cooked the particles together, banging this many protons into that many neutrons, until things cooled enough to stop the processes. The products of that recipe are the elements that make up most of the Universe. No theory besides the Big Bang has properly accounted for the observed elemental abundances.
We still see the afterglow of the Big Bang. For the first 0.1 million years after the Big Bang, the Universe was so hot and dense that photons would bounce off of matter before they got very far. About 100,000 years after the Big Bang, well after atomic nuclei had been formed, the expanding Universe finally cooled enough for those nuclei to capture electrons and form neutral atoms of hydrogen. This is called recombination (although it should really be called "combination," since the atoms were never combined to begin with). At that point, photons were suddenly able to fly free: the Universe became transparent.
As we look far away and back in time towards the Big Bang, then, 14 billion light years away, we can see only so far as the time of recombination, 100,000 years after the Big Bang, since the Universe is opaque beyond (earlier than) that point. It looks like a glowing wall that hides the very early Universe from our view -- like the edge of a cloud that surrounds us almost 14 billions light years away. This is called the last scattering surface , the last time in the Universe when photons were scattered by matter. The wall is glowing with a black body spectrum of 2.73 K, so it peaks at about _peak = 1 mm, in the microwave part of the electromagnetic spectrum. This is called the cosmic microwave background (CMB), and its discovery in the 1960's garnered the Nobel Prize in Physics for Arno Penzias and Robert Wilson.

QUESTION: The last scattering surface had a temperature of about 3000 K, so why doesn't it appear like a black body with _peak = .001 mm, just beyond the visible part of the spectrum?

ANSWER: Because the photons from the CMB are redshifted 1000 times by the expansion of the Universe!

No cosmological theory besides the Big Bang has been able to account properly for the CMB.