“It took less than an hour to make the atoms, a few hundred million years to make the stars and planets, but five billion years to make man!” –George Gamow
Some people are never satisfied. After I wrote last time on the odds for cosmic inflation, I started noticing a flurry of comments on an older post about alternatives to the big bang. So, might as well go back to the basics, and ask what the odds are that the Big Bang is correct! Let’s start by taking a look at what’s out there in the Universe.
Sure, we’ve got stars surrounding us: hundreds of billions of them in our galaxy alone. But out beyond the stars, as you can easily see with a telescope like Hubble, are a huge number of galaxies strewn about the Universe! We have a number of techniques available to us to measure how far away these galaxies are from us. The simplest one was discovered by Hubble himself: look at individual stars in that galaxy!
Stars do various well-studied things, from going nova to being variable stars of many types to the ever-impressive supernova, but they do them in very understandable ways! So if you see a star doing something, and you know how intrinsically bright the thing it’s doing is, all you need to do is measure its apparent brightness! And just like that, because you understand how light works, you can figure out how far away that object is.
But we can do one better. Thanks to the fact that objects moving towards us have their light shift towards the blue end of the spectrum, while objects moving away from us have their light shift towards the red, we can measure the spectral lines from these galaxies and find whether they’re moving towards us or away from us, and how quickly.
What do we find?
That the farther away a galaxy is from us, the faster it moves away from us! (Or, more accurately, the larger the observed redshift of that galaxy is.) Although Hubble had no way of knowing that it would, this relationship — Hubble’s Law — is now known to hold for galaxies at distances well over a billion light years in all directions.
So what causes this? Why does it appear that the farther away things are from us, the more redshifted their light is? On its own, this observation permits many possible explanations, including that:
- light gets tired, and simply loses energy over time,
- the Universe oscillates, contracting and expanding over time, and we are simply close to an expanding portion of it,
- physical constants, such as the speed of light, or the gravitational constant, have changed over time,
- the Universe grows steadily and evenly, and creates new matter as it expands, and
- that the Universe is rotating rapidly, and that the galaxies that are moving away from us more quickly also have large — unobservable — translational motions.
All of these ideas predict different things that are, in principle, observable, and allow us to distinguish them from one another. But in the 1940s, George Gamow (and his students, Ralph Alpher and Robert Herman) had a different idea than all of these!
He said that the redshift was due to the fact that the Universe was expanding, and that the expansion rate was faster in the past! As the Universe moves forwards in time, it cools, expands, and slows down.
And like the alternatives, Gamow’s theory made some amazing predictions. You see, if you go back in time and allow the Universe to get hotter and hotter, what happens?
Eventually, if you go back far enough, it became too hot to form stable, neutral atoms! In an expanding Universe, the radiation that ionized these atoms should, by now, be cool, uniform radiation that’s redshifted well into the microwave part of the spectrum. And not only that, it should have a very specific spectral shape, known as a blackbody distribution.
But Gamow didn’t stop there, although the observations weren’t nearly ready in the 1940s.
What about individual atomic nuclei? At some point, it must have been so hot that even nuclei must have been blasted apart, and it would have been too hot to form anything more complex than individual protons, neutrons, or electrons!
But remember, the Universe is expanding and cooling, and at some point it gets cool enough that you can take that very first step: combine a proton and a neutron to make deuterium. Once you do that, you can start adding more protons and neutrons to make heavier elements!
And perhaps, that was how many of the elements in the Universe were made!
Well, many of these rival theories were seriously considered for some time; after all, theories are great little tools for predicting what could happen, but you have to look to the data — observations and experiments — to determine which theories are best and most valid!
Well, 1964 was the beginning of the end for the alternatives. Why?
Arno Penzias and Bob Wilson, above, with the Horn Antenna, were working for Bell Labs, researching microwave radiation in the Universe. And what they found was that, while there were interesting microwave emissions from the plane of our galaxy, there was this low-temperature noise everywhere that they simply couldn’t get rid of. (Even after, disgustingly, cleaning an incredible amount of bird droppings out of the antenna!)
To be honest, they were puzzled by it, and weren’t sure what could be causing this. But cosmologists knew what this was! It was Gamow’s leftover glow from the Big Bang. But what would the spectrum of this radiation be? Believe it or not, it wasn’t really accurately verified until the COBE mission in the 1990s! And what did it show?
That the Big Bang got it right, to an unbelievable, indisputable precision!
And that last part? The light elements? We predict that it should make a Universe — by mass — of about 75-77% Hydrogen, 23-25% Helium, a small amount of Deuterium, Helium-3, and a very, very tiny amount of Lithium. What do we actually see?
Damn. I mean, this is just incredible! For a theory that just got its bare bones put together in the 1940s, the agreement with observations is outstanding! And the predictive power… yeesh. We can throw all sorts of exotic things into the Universe according to general relativity: magnetic monopoles, cosmic strings, domain walls, a cosmological constant, neutrinos, dark matter, dark radiation, spatial curvature, and textures, in addition to atoms and photons. They all lead to wildly different observations, ranging from large-scale structure…
to fluctuations in the microwave background.
And what we find is that there’s some dark matter, a cosmological constant, a small amount of neutrinos, and the rest is just atoms and photons. And it’s all consistent with the Big Bang.
Don’t like that the initial conditions of the Big Bang have to be fine-tuned to get a Universe that’s full of this much stuff and hasn’t recollapsed?
But I digress. What we’ve got here is the most successful theory of the Universe ever. All the alternatives fail miserably, including Tired Light, Hoyle’s Steady-State Theory, and Alfven’s Plasma Cosmology. (I see you, there, in the comments!)
If I were to give my odds, I would, conservatively, say that there’s a 99.9% chance that the Big Bang was correct from when the Universe was a tiny fraction of a second old up to the present day. A variable speed of light cosmology might hold some promise, but it isn’t clear those models don’t also require a Big Bang! I’d give that one alternative about a 1-in-2000 shot of panning out, and all the others combined even longer odds. And that’s why we really do think it all started with a bang!