“As far as I see, such a theory [of the Big Bang] remains entirely outside any metaphysical or religious question. It leaves the materialist free to deny any transcendental Being. He may keep, for the bottom of space-time, the same attitude of mind he has been able to adopt for events occurring in non-singular places in space-time… Science has not to surrender in face of the Universe and when Pascal tries to infer the existence of God from the supposed infinitude of Nature, we may think that he is looking in the wrong direction.” –Georges Lemaître
Recently, David Dilworth has been getting some attention for his assertion that the Big Bang has no universally agreed-upon definition. In particular, the International Astronomical Union, of Pluto-demoting infamy, has no definition of it. He argues that the Big Bang is, perhaps, not even a scientific theory because of this, and quotes — out of context (Update 07/20/2011: not out-of-context; that was the complete statement) — famed cosmologist Jim Peebles.
David, the IAU, and anyone else reading, allow me. (And for those of you wondering about my pedigree, Jim Peebles was the Ph.D. supervisor of this excellent cosmologist, who was in turn my Ph.D. supervisor a generation later.)
We know, from many different observations, that the Universe is expanding. Not like an exploding grenade, mind you, where the pieces of it would fly apart, expanding away through an already-existing space.
Rather, thanks to general relativity, we understand that what’s going on is that our Universe itself is expanding. The space-time that makes up our Universe expands in proportion to the amount of stuff-per-unit-volume, or energy density, of our Universe. And this means something remarkable for the radiation in our Universe.
Because one of the defining properties of all radiation is its wavelength, and the Universe is expanding, this means that the expanding Universe gets cooler over time!
As the image above shows, as space expands over time, the wavelength stretches, or gets longer. This necessitates that the radiation (or light) reaches lower and lower energies as the Universe ages. But it also means that in the past, the Universe was hotter, and at higher energies!
Now, remember the first image I showed you?
This one. This was the original conception of what the “Big Bang” was. Originally given names like the “primeval atom” (in reference to the tininess of space early on), or the “primeval fireball” (in reference to the ultra-hot temperatures), the term “Big Bang” was actually coined in 1949 by the theory’s biggest detractor, Fred Hoyle.
But this description of the Universe — with the aforementioned tie between energy density to expansion — predicts some amazing things. Imagine, if you will, everything you know of in the Universe when it was younger. You know it was hotter, closer together (denser), and was expanding away from everything else at an increased, faster rate than it is today. What would it look like?
Well, you might be tempted to go all the way back in time to what you think of as the very beginning. You might want to know what happens when you extrapolate all the way back to all of the matter and energy in the Universe being at a single, isolated point. I’m not going to lie; it’s very tempting to do. And if you do it, you’ll miss the Big Bang. It is the single greatest source of the public’s misunderstanding of what the Big Bang is, and I am here to try and clear things up. Let’s take a look at what we’d run into if we naïvely extrapolated back in time, to arbitrarily higher and higher temperatures and densities.
Before gravity did what it does, and collapsed matter into stars, planets, galaxies, and clusters of galaxies, the Universe was much hotter, and much more uniform. At some point in the past, it was so hot and dense that there were enough high-energy photons zipping around that it was too hot to form neutral atoms! So instead of atoms, we had a Universe made up of atomic nuclei, electrons, and radiation.
The radiation we see left-over from the time that electrons and nuclei first came together is the famous Cosmic Microwave Background: the observation that confirmed the Big Bang and ruled out the alternatives. We know many more details than this today, but we’ve known about this part of it since the early 1960s.
But, of course, we can go back farther in time, to higher temperatures and more exotic happenings. Back to when it was hot enough that individual nuclei were blasted apart, into lonesome protons and neutrons, by the tiny-wavelengthed, incredibly hot radiation bath.
Back to when there was so much energy density to the Universe that all the particles we make in high-energy accelerators — such as unstable quarks, muons, neutrinos, and high-energy bosons — existed in great abundance.
And although that’s the energy limit of what we’ve studied, we know that there are even more things that had to have happened in our Universe.
We live in a Universe, today, with much more matter than anti-matter; at least 99.9% of the normal matter in the Universe is, well, normal matter and not anti-matter. For the very early Universe, that means that there was a very, very tiny overabundance of matter as compared to antimatter; a difference of only one extra baryon for every 10 billion baryon/anti-baryon pairs.
(What’s a baryon? A proton and a neutron are each examples of baryons. It comes from the Ancient Greek: bareus, which means heavy. If you were an Ancient Greek and wanted to call someone a drunk, you called them oinobareus, which means “heavy with wine.” Protons and neutrons contain nearly all the mass of atoms, and that’s why they call them baryons. Now, you’ll never forget it.)
So you have to have some way of making more baryons than antibaryons. This is an exciting sub-field of cosmology, known as baryogenesis. We have ways of making it work at a bunch of different energy scales, but we know it happened after the Big Bang, and before the physics we’ve presently discovered.
(And it could have happened at the electroweak scale, courtesy of The University of Heidelberg.)
And then there are the unknown things. Were the weak, electromagnetic, and strong forces all unified together at some high energy, after the Big Bang, that our Universe actually reached?
Signs point to “no” right now, but we could be wrong about that. The important thing, in all of this, is that the Big Bang doesn’t go back infinitely far! Why not?
Because the Big Bang describes this hot, dense, matter-and-radiation-filled expanding state. But something happened to set that up.
Inflation! There was a time where the Universe wasn’t full of matter and radiation, where it wasn’t cooling as it expanded, and where the expansion rate didn’t drop at all, and that’s what we call the period of cosmic inflation.
We don’t know what caused/came before inflation, and we don’t know how long inflation lasted (save to say, long enough). But inflation was the thing that happened before the Big Bang, and the Universe could not have been as small as a singular, collapsed point at that moment of transition.
So, with all of that in mind — which is the story every physical cosmologist knows — what can we define as the Big Bang?
The Big Bang is the first moment in the history of the Universe where we can describe it as a hot, dense, expanding state, full of matter, antimatter and radiation. It has a temperature of at least a quadrillion Kelvin (but no more than 1029 Kelvin), and it coincides with the time where inflation ends and the Universe’s expansion rate is dominated by the matter and radiation density.
If you go back to this picture, the GUT era may or may not be something that happened in our Universe since the Big Bang; the Planck era certainly is not. The Big Bang does not include inflation nor anything else that happened before it; it also did not occur at one particular place (although it did occur at one particular time, everywhere in space). It occurred roughly 13.7 billion years ago, and it occurred in a Universe that had the same temperature everywhere (to just a few parts in 105) and was (and still is) spatially flat. In the context of inflationary cosmology, the Big Bang coincides with the end of inflation and the cosmic reheating of the Universe.
And our Universe — everything that’s come since — is a consequence of that one event that started it all: the Big Bang! And that’s how I, in my capacity as a physical cosmologist, define the Big Bang.