Gravitational waves and cosmology, reposted from the Orthosphere

Today, the BICEP2 team announced the detection of what they claim is an imprint of long wavelength gravitational waves in the polarization of the cosmic microwave background.  If this holds up (a big if:  lots of exciting discoveries don’t hold up when some neglected systematic error turns up), it will be the most important discovery in cosmology since the first evidence for dark energy, and for physics in general I would rate it more important than the detection of the Higgs boson.

How far back in time we can see

Because light travels at a finite speed, when you see an object, you actually see it as it was in the past, rather than right now.  For objects in the same room, this delay is only nanoseconds, but for distant objects it can be big.  For example, light from the sun takes 8 minutes to reach us, so we see the sun as it was eight minutes ago; the star Sirius is 8.6 light years away, so we see it as it was 8.6 years ago.  So, looking far away is like having a time machine.  The universe is about 13.8 billion years old, so you might wonder, can we look at a region of the universe so far away that light takes 13.8 billion years to get from there to here, meaning we would see this spot as it was during the big bang?  Can we see the moment of creation itself?  The points of the universe that we should see as they were at the beginning of time form a vast sphere centered on the Earth called the “cosmological horizon” because it divides the part of the universe we can see (at some point in its history) from the rest that we can’t see at all.  (Note that every observer, not just Earth, is the center of its own cosmological horizon; no point is special.  The big bang happened everywhere.)

Unfortunately, we can’t see that far back, because if we go to early enough times, the universe is so hot and dense that light cannot travel freely.  Instead, the oldest light we see comes to us from the moment that space became transparent and light could for the first time travel freely.  This radiation is the cosmic microwave background, and when we look at it, we are seeing the universe as it was when it was about 400,000 years old.  We cannot see farther back, because we are essentially hitting a surface inside of which space is opaque, just as we can’t see deeper into the sun that the photosphere, where light stops scattering and becomes able to travel freely.

Other messengers

Is there a way to get around this limit?  In fact, we are able to test and confirm the big bang model back to about a second after the beginning, because if the process from weak interaction freeze-out at this time to the formation of primordial elements (especially most of the universe’s helium) a few minutes later were much different than believed, the abundances of these elements would not be predicted by the theory correctly as in fact it is.  A lot must have happened in that first second, though, and we’d like to know more.  The origin of the universe is a fascinating problem in itself, of course.  In addition, there’s the fact that as we study times closer and closer to the beginning (“t=0”), densities and temperatures get higher and higher (hypothetically being infinite at t=0).  At sufficiently early times, densities and temperatures were higher than the universe has since seen, even reaching levels inaccessible to particle accelerators.  Thus, the big bang is an opportunity to study the laws of physics, probing very high energies (which for quantum systems corresponds to studying very small scales).

One might ask how we were able to get around this problem for the sun.  How do we know what’s going on in the sun’s center?  The answer is to look at the sun through something other than light, something that is generated in the core and isn’t scattered while traveling through the sun but comes directly to us.  For the sun, that something is neutrino radiation.  Neutrinos interact with matter much more weakly than electromagnetic radiation, which makes them very hard to detect, but it also means that they can travel straight from the center of the sun or of a supernova explosion to our detectors.  There actually should be a (undetected) cosmic neutrino background, coming from when the universe was a little under a second old and became transparent to neutrinos.  However, t=1 second is already pretty well covered, so we need something even better.  And, luckily there is something.

Gravitational waves

Gravitational waves (note:  distinct from gravity waves like the ones you see on a lake) are wavelike distortions of spacetime generated by accelerating massive objects.  Like electromagnetic waves, they travel at the speed of light.  However, they interact with matter extremely weakly–only a black hole can block them.  In fact, we basically don’t have to worry about space ever having been opaque to gravitational waves (at least not until we’re so far back that the description of spacetime we have breaks down), so the only worry is whether or not a strong signal of these waves was ever made.  If it was, then it is still carrying information from these unexplored early times.

By the way, there are big hopes that gravitational waves will soon become a tool for astrophysics as well, once the technical challenges of detecting them are worked out.  Much of my own research is tied to these efforts.  The main sources of gravitational waves in the contemporary universe are various combinations of black holes and neutron stars smashing into each other.  Thus, from my point of view, gravitational waves are a tool for studying black holes and neutron stars.


A priori, there’s no reason to think we’d get lucky and find a big gravitational wave signal from the early universe.  Why should the early universe make one?  In fact, such a signal is predicted by the theory of inflation.  Inflation was originally proposed as a way to explain what seem to be unexplained coincidences in the standard big bang theory.  The basic idea is that at some point very early on, say 10^{-35} second after the beginning (we’re talking a lot earlier than t=1 second now!), the universe’s expansion went through an explosive exponential phase, growing by many orders of magnitude; then this phase ended, and the universe has since expanded as predicted by the regular big bang theory.  Why would this happen, and why would it stop?  Nobody really knows, so instead we’ve given the culprit a name, the “inflaton field”, to label our ignorance.  This field apparently acted somewhat like today’s dark energy (which is driving the universe into another exponential growth phase), albeit at a very different scale.  Then for some reason it went away.  How this idea solves problems for the big bang model is something I won’t go into.  Basically, postulating an enormous blow-up means that before that blow-up, the observable universe was smaller than we would have expected, and was better able to settle itself down in various ways.  What is important is that inflation would take small quantum fluctuations and blow them up to macroscopic scale.  This seeded the universe with small inhomogeneities in density which eventually grew to become the structure (stars, galaxies) in the universe we see today.  There should also be quantum fluctuations of gravitation seeded at the same time, meaning an ancient gravitational wave signal generated very, very shortly after t=0.

These gravitational waves would be tremendously difficult to detect directly. Fortunately, we don’t have to.  As they travel through matter, they distort it in various ways–squeezing in one direction, stretching in another.  This leaves a subtle imprint in the cosmic microwave background, in particular on the spatial pattern of the polarization of the light.  And this is what BICEP2 is claiming to have measured.  Because standard inflation models make distinct predictions about these gravitational waves, there is the hope that the theory of inflation can really be tested–its distinctive predictions rather than just its general post-dictions.

A peak at the highest energies we may ever be able to see

If the result holds up, then the BICEP team have found a signal that comes to us from a form of matter and energy that is utterly inaccessible to any conceivable experiment.  They’re talking about characteristic energies of order 10^{16} GeV.  For comparison, the LHC achieves collision energies of around 10^4 GeV.  The scale 10^{16} GeV has actually long been of interest to high-energy physicists as the proposed “grand unification scale”.  The idea is that, although in ordinary life, the electromagnetic, weak, and strong nuclear forces have very different strengths, these strengths change with energy scale, and if you extrapolate the curves 12 orders of magnitude beyond experiments (yes, I agree, this is absurd, but bear with me for a second), they all cross.  Does this mean anything?  I wouldn’t bet on it, but its been taken as a clue that this is the scale above which these forces cease to be distinct.  10^{16} GeV is also only a few orders of magnitude lower than the Planck energy, at which gravity also has the same strength and presumably unifies with the other forces.

Thus, gravitational waves from inflation would be our only signal from these exotic scales.  As far as we now know, it’s the only one we’ll ever have.

2 Responses

  1. Fascinating stuff.

    I’m curious about where and how Catholicism and cosmology/astrophysics intersect for you. Do they also exclude each other or clash at any point? And in which did you first take serious interest?

  2. There’s no clash, but they’re not always as integrated as I would like. See here:

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