Freelance Astrophysicist

This conference has been fairly insane, with very little downtime and spotty internet access. I'm still chewing on some of the talks I saw and the conversations I had; in the meantime, here's the second half of the post from last week...

Figuring out what happened during the first billionth of a billionth of a second after the Big Bang is rather hard, mostly because there’s no light we can see from that time. In fact, there’s no light that we can see from the first 380,000 years after the Big Bang. For most of that time, light was trapped in a plasma, a dense soup of electrically charged particles a lot like the interior of the Sun. Light can’t travel in a straight line in a plasma — it keeps running into electrically charged particles, which deflect the light and exchange energy with it. But as the universe expanded, it cooled down, and once it got cool enough, those charged particles — mostly electrons and atomic nuclei — got together and formed atoms, which don’t deflect light nearly as much because they have no electric charge. At that point, 380,000 years after the Big Bang, light was able to propagate freely for the first time in the history of the universe; the light from that time — the oldest light in the universe — is now the cosmic microwave background radiation (CMB). The CMB is an immensely rich source of information about the very early universe, letting us look back at the universe when it was about 0.003% of the age it is now. But that’s still much, much later than the era we’re interested in. The CMB lets us see the tree when it was a sapling — but we want to know what the pinecone looked like.

Can we do better? Well, there’s no light older than the CMB, but there are plenty of things other than light. Most of the atomic nuclei in the universe --hydrogen and helium, with a little lithium thrown in -- were forged in the first 20 minutes after the Big Bang, when the entire universe was as hot as the center of the Sun.¹  By looking at the relative abundances of those elements in the universe — how much helium there is vs. how much hydrogen there is, for example — we can learn a lot about what the universe was like during that first half-hour. But that’s still not going to get us what we want: in the first three minutes after the Big Bang, the universe was too hot for atomic nuclei to remain stable, so all those nuclei we see today didn’t start forming until then. (Think about that: too hot for nuclei to remain stable. That’s much, much hotter than anything anywhere in nature today² — billions of degrees — and hotter than anything humans have created other than the brief, tiny flashes of intense heat generated in particle accelerators.)

So, we’re trying to learn about a period in the universe’s history so far back that there’s no light left over, and there’s no atoms or atomic nuclei left over either. So how can we learn about that first fraction of a second? The key is that the tiny differences in density I talked about last time come from that first fraction of a second. In the violent and hot early history of the universe, those differences in density were stretched and warped a bit, but they should have persisted. And again, as I mentioned last time, those early differences in density — we call them “primordial density perturbations” — seeded all the structure that we see in the universe today. Along the way, they also affected the cosmic microwave background radiation. So by looking at the structure of the universe today, as well as the cosmic microwave background radiation, we can learn about the primordial density perturbations that came into being a fraction of a second after the Big Bang; and through those perturbations, we can learn something about what the very, very, very early universe was like. Continue reading →

1. This process is called Big Bang nucleosynthesis, and it only created hydrogen, helium, and lithium. Other elements didn't get created until much later, by stars. All the elements on the periodic table between beryllium and iron were created by fusion in the cores of living stars; all the elements heavier than iron -- from the gold in your ring to the tungsten in your lightbulbs to the copper and zinc in your pocket change  -- were created by supernovae as stars died. [↩]
2. Except perhaps at the center of very young neutron stars. [↩]

Sorry I’ve been off the radar for the past two weeks. I’ve been preparing to go to this conference, one of the largest cosmology conferences in the world. Hopefully, I’ll have time to post about a few of the things going on there. For now, though, I’m going to tackle what’s easily the most common question that I get...

The story so far:

Physicists are notorious for oversimplifying things in the name of mathematical modeling. There’s the old joke about spherical cows, radiating milk isotropically, which I’ll spare you here, but the reason we do this is that you can often learn an awful lot about something by simplifying it down to the interesting and easy-to-model parts — which are hopefully the same! The trick is knowing which parts can safely be ignored, but if you do that right, you can get an amazing amount of information about something with a very simple model of it. A powerful example of this is thermodynamics: we can totally ignore the working details of a device — any device of any kind that does anything remotely useful, from a refrigerator to a spaceship to a human brain — and still calculate hard limits on how much stuff we can get the device to do, how long it can do it, and how efficient it can be. Cosmologists do the same thing with the universe as a whole. We ignore small-scale details — anything smaller than a galaxy, as a rule — and find that we can still get a picture of what’s happening to the universe on the largest scales over its entire history. For our most impressive tricks, like calculating the age of the universe, we use two of our most helpful simplifying assumptions: we assume that the universe is homogeneous, meaning that no region of space is particularly special or different from any other; and isotropic, meaning that no direction in space is particularly special or different from any other. Plug these assumptions into the Einstein field equations from general relativity, and out pops a set of equations that describe the entire universe. Four guys came up with these equations independently of one other, so the set of equations is named after all of them: the Friedmann-Lemaître-Robertson-Walker (FLRW) metric. I’m not going to go into a detailed description of the FLRW metric here, at least not now. The point is merely that by making a couple of assumptions about the symmetries of the universe, we get these wonderful equations that give us a story about the history of the Universe which matches remarkably well with all of our data.

Except, of course, that those equations are wrong, because the universe is neither homogeneous nor isotropic. Look around you. I’m going to go out on a limb and guess that you see things, probably made of stuff. If the universe were perfectly homogeneous and isotropic, there wouldn’t be any things, because everything would look the same, no matter where you looked. You wouldn’t exist either — after all, you’re a thing too. The universe would simply be filled with stuff, smeared out evenly and smoothly in all directions, like cream cheese on a cosmic bagel, without a thing in sight. Obviously, we don’t live in a universe like that; even on big scales, the universe is lumpy with galaxies and the vast spaces between them. Yet the FLRW metric does describe a perfectly even and smooth universe, and somehow matches all of our data from this universe remarkably well anyway. What’s going on here?

Continue reading →