“Some scientists claim that hydrogen, because it is so plentiful, is the basic building block of the universe. I dispute that. I say there is more stupidity than hydrogen, and that is the basic building block of the universe.” –Frank Zappa
Oh yeah, Zappa? Well, let me show you what hydrogen can do!
With just one proton and one electron, hydrogen is not only the simplest of all the atoms in the Universe, it’s also the most abundant: over 90% of all the atoms in the Universe are hydrogen atoms!
How could such a small thing possibly have anything interesting to tell us about the entire Universe?
Well, for starters, hydrogen has very specific energy levels where its electron is allowed to live. Hit it with just the right amount of energy, and it will absorb it, and the electron will jump up to a higher energy level.
Or, if you let it sit there in a higher energy state, it’ll look at those lower — and more stable — energy levels, and spontaneously jump down there.
And when they do, they emit radiation! If you jump down to the first energy level, you emit ultraviolet light and belong to the Lyman series; if you jump down to the second energy level, you emit (mostly) visible light and belong to the Balmer series; while if you jump down to the third, you give off infrared light and belong to the Paschen series.
In fact, the jump from the third to the second — the Balmer alpha line — is so strong that if you look through a telescope at a galaxy that’s forming stars:
it’s that line that causes the galaxy to glow red! In the case of the Whirlpool Galaxy, above, you can see exactly where along its great spiral arms it’s presently forming stars, just from this red glow of the hydrogen!
But most of the hydrogen in the Universe isn’t in some exciting, star-forming region. Most of it’s in the cool, boring depths of space, sitting around in its lowest energy state.
And if you’re hydrogen in its ground state, you’re just waiting for some light of just the right energy to come along and — ever so briefly — to give you a ride up to the next exciting energy level!
What’s hydrogen’s best friend in this case? Ultraviolet light of a wavelength 1216 Angstroms, known as the Lyman-alpha line, or the right amount of energy to kick it up from the ground state to the first excited state! Of course, there are other excited states, and they make absorption lines too, but the Lyman-alpha line is the strongest one.
So why should I care? And moreover, how can something as mundane as this super-simple atom emitting and absorbing light teach me anything about dark matter?!
Well, we know where light comes from in the Universe: from stars and galaxies!
Well, my Universe isn’t just empty; there ought to be clumps of this hydrogen gas all over the place! And wherever my light from these distant galaxies passes through these clumps of gas, that neutral hydrogen will leave its mark by absorbing that 1216 Angstrom light.
But because of the expansion of the Universe, this light gets redshifted! In other words, you place cold, neutral hydrogen gas at different distances away from us, in between us and a distant galaxy, and it will leave absorption lines at different wavelengths!
What we basically do is take a spectrum of distant galaxies using a super-powerful telescope (like Hubble), and see where we have clumps of hydrogen gas along the way.
If you look at something nearby, you’ll only have a few clumps of gas in between you and the object you’re observing. But if you look at something very far away, you’re likely to get a whole slew of absorption lines! For very distant objects, there are so many clumps of gas that the lines we see are known as the Lyman-alpha forest!
Now, here’s where it gets interesting! Because when we look at things that are farther away, we’re also looking back in time! And if we want to get these big, deep absorption lines happening far away, we need to have dense, collapsed clouds of gas.
Guess what? That tells you something about your dark matter! Because if you want to make something that’s dense and collapsed, it can’t be moving too quickly. In astrophysics, if you’re moving quickly, we call you hot, and if you’re moving slowly, we call you cold.
For dark matter, the cosmic microwave background doesn’t care whether you’re hot or cold. But structure does, and the Lyman-alpha forest is very sensitive to it! If dark matter were hot (or even if it were too warm), the forest would be too shallow; in other words, hot dark matter makes it too hard to form small-scale structure at early times.
But we see the evidence of this small-scale structure directly in the Lyman-alpha forest! What does this tell us?
It tells us that dark matter can be WIMPs (like from supersymmetry), because they’re too massive to move quickly, or they can be particles that are born cold, like axions or (some) sterile neutrinos, because they started off moving slowly. But they can’t be regular neutrinos or hot sterile neutrinos, among others, because this small-scale structure — and hence the hydrogen lines that we see — would get washed out at early times!
So just like that, from looking at hydrogen, we can tell how cold our Universe’s dark matter has to be. And that’s how hydrogen teaches us the temperature of dark matter!