|The centre of the galaxy and a sliver of the CMB anisotropies|
I was asked recently how I know that the Big Bang definitely happened. This post will be my attempt to answer that question. I will focus on something called the Cosmic Microwave Background (CMB for the rest of this post). The CMB is as close to smoking gun evidence of the Big Bang as you can get. In fact, it's such good evidence that it is better than a smoking gun. The figure of speech should no longer be “smoking gun evidence”. It should be “smoking CMB evidence”.
The other reason I am writing about the CMB is that humanity's prediction of its existence and our subsequent measurements of it and its properties are jaw-droppingly stunning pieces of detective work. If you are ever feeling down about human nature and our propensity to do kind of stupid things then just remember what you're about to read and reassure yourself that at times we can be incredible.
What is “The Big Bang”? (A very brief review)
Everywhere we look things are moving away from us and the further away we look the faster things are moving. This means that the distance between any two unbound objects in the observed universe is increasing. Or, in other words, the universe is expanding. As time goes on things will get further apart, the total density of the universe will decrease and the temperature of outer space will go down.
But what happens if we run the clock backwards? Well, naturally, things will get closer together, the total density of the universe will increase and the temperature of outer space will go up. This suggests that at one point far back in time the universe was in a very hot, very dense state that was rapidly expanding. This, and nothing else, is the essence of what the Big Bang model of the universe is.
Of course, we understand how the matter in the universe behaves at the current, low, temperatures. Also, thanks to results from particle accelerators and other experiments we even know how the matter in the universe behaves at quite high temperatures. This means that we can make very definite statements about what the universe should have looked like when it was below those temperatures. But, although we can and should speculate about what the universe might have looked like above these temperatures, we can't yet say anything about those times with certainty.
So, if you let me repeat myself for emphasis, at its heart the Big Bang model is nothing more and nothing less than the idea that the universe was at some point in the past very hot, very dense and rapidly expanding. To work out whether this is what the universe was actually like or not we need to know what the present day consequences of this might be. To answer that question we should take a closer look at what the universe we see now would actually look like at these higher temperatures.
The smoking CMB.
Most of the stuff we can see is hydrogen, about 75% of it in fact. So, what happens when we heat hydrogen up? Well, not much at first. Hydrogen is made up of one proton and one electron. Positive charges attract negative charges and this keeps those electrons bound to those protons. However, if you give enough energy to a hydrogen atom, than that electron will have enough energy to escape the pull of the proton. When this happens, a gas of hydrogen will undergo a transition and become a hydrogen plasma. This is kind of similar to how ice undergoes a transition and turns into water when you heat it.
The most important aspect of this hot, hydrogen plasma is that the charged particles in it (electrons and protons) are free to move around. Light interacts with charged particles, so any light shone on or in a hydrogen plasma will immediately be absorbed by the plasma. Hydrogen gas however, is completely transparent (because the charged particles are bound tightly to each other in neutral atoms).
What does this mean for our universe (remember 75% of the stuff we can see is hydrogen)? Well, at some point the universe was hot enough that hydrogen was a plasma. At this point, the entire universe was opaque and any light emitted, was immediately re-absorbed by the plasma. However if you wait a little while then the universe will expand and cool. Eventually it will cool enough that this all pervading hydrogen plasma will undergo the transition into neutral hydrogen gas and just as suddenly the entire universe will become transparent. Any light emitted at that precise moment will then travel freely for the rest of the universe's history, not interacting with anything.
But we know from the lab here on Earth that this transition will give off some light. If the Big Bang is the correct description of our early universe then this light should still be there, travelling through space, now. The only thing (well, almost only) that has happened while it has been travelling all of this time is that the universe has cooled and expanded. So it should look exactly like it did when it was emitted, only colder.
This is an extremely specific prediction. That is, that as well as all the light we can see coming from galaxies, quasars, clusters and stars there should be some extra, very cold, light permeating the entire universe. The temperature of this light in all different directions should also be incredibly uniform. It was all made in the same way and has then spent exactly the same amount of time cooling. Any tiny imperfections in it based on the slightly different paths it has taken to reach us will need to be very, very small.
If you didn't know of the Big Bang and the physics of hydrogen, this is not the sort of thing that you would naively expect to be just sitting out there in space ready to be measured.
But it is there alright and it has been measured. And it is what we now call the Cosmic Microwave Background radiation.
It was first detected by Arno Penzias and Robert Wilson. The amusing story of its discovery can be found here. I would show you a picture of it, but it is so incredibly uniform that any “picture” of it looks like this. Instead I've given you a picture of the people and satellite that first saw it.
The markings on the bullet that came out of the smoking gun of the Big Bang
The simple existence and discovery of the CMB is extremely strong evidence that the Big Bang happened. Remember, there was no other reason at the time to suspect that this light should be there – but the Big Bang requires it.
However we're clever detectives, us humans. The Big Bang says more about the CMB than just that it exists.
The hydrogen plasma that made the CMB was made up of protons, electrons and a lot of thermal radiation. These various components of the plasma were all interacting with each other. If the Big Bang had been going on for long enough then this plasma would have been in thermal equilibrium - “thermal equilibrium” basically means that no region of component of the plasma is either hotter or colder than any other region. Now we know from laboratory physics that the light emitted from anything in thermal equilibrium has some very specific properties. Most importantly, we know that the intensity of the light, as a function of its frequency (its spectrum) has a very distinctive shape (intensity basically means brightness and frequency basically means colour).
Even before we measured the CMB there was reasonable evidence to suggest that the Big Bang would have been going on for a while by the time that the CMB was made. So the Big Bang predicted very clearly that the spectrum of the CMB needed to have this distinctive shape.
Unfortunately physics had to wait a while before technology was good enough to make this measurement. But finally, in the 1990s NASA put a satellite in space (the COBE satellite) with the intention of measuring the spectrum of the CMB. The figure below is the result. It is one of the greatest achievements in scientific history (the Nobel committee agrees). The solid line is the distinctive shape predicted by the Big Bang. The data points are on the figure too, but they all lie precisely on this line. So you can't see them. The CMB doesn't just exist, it looks exactly like the Big Bang model said it would.
I want you to take a step back right now and consider what I've just told you. Not only did the Big Bang predict that this all pervading field of light should be out there it also said that if we measure what is effectively the brightness of this light field as a function of its colour we should get one very specifically shaped curve. Without the Big Bang, not only would we have no reason to expect this light in the first place, but even if it did exist, this curve could have any shape it wanted.
With that in mind, remember that we have now seen this light and measured this curve. The curve matches the prediction precisely. By measuring this ambient light field today we are directly learning properties of the universe 13.7 billion years ago. We're a pretty clever species at times.
Deeper into the rabbit hole
I have already written a rather long post (do let me know in the comments if you don't think this is too long and wanted more) so I need to wrap up. However I've had to leave out the most subtle and thus most incredible things we've measured in the CMB. These are the tiny, tiny, fluctuations in its temperature that are 0.001% of the average. While the temperature of our favourite primordial hydrogen plasma was more or less completely uniform, the density of the plasma had tiny imperfections in it. The CMB inherited these tiny imperfections and we have now measured them.
What is incredible about this measurement (and it is for this that humanity deserves the greatest detective ever award) is that through these CMB fluctuations we have been able to see the effects of sound waves that were rippling through the primordial hydrogen plasma. When I step back from my own research to write about this like I am now I am blown away by how ingenious the cosmologists who came before me were. We've measured the lingering effects of the sound of the big bang. That's amazing.
I will go into detail how we did it, later... but if you want a teaser, it's all in this figure.
[Edit: I have now gone deeper into the rabbit hole in this post]
[Edit: I have now gone deeper into the rabbit hole in this post]