Tag Archives: big bang

Big Science News! Inflation, Gravity, and Gravitational Waves


So, big announcement yesterday. Lots of people have asked if I could try to explain it! People have been asking since yesterday morning, but folks, I’ve got a job! I’ve been writing when I have time while running slow tests in another window, so it’s taken more than a day to get to it.

The announcement is really, really fascinating. A group has been able to observe gravity wave fluctuations in the cosmic microwave background. This is a huge deal! For example, Sean Carroll (no relation) wrote:

other than finding life on other planets or directly detecting dark matter, I can’t think of any other plausible near-term astrophysical discovery more important than this one for improving our understanding of the universe.

Why is this such a big deal?

This is not an easy thing to explain, but I’ll do my best.

We believe that the universe started with the big bang – all of the matter and energy, all of the space in the universe, expanding outwards from a point. There’s all sorts of amazing evidence for the big bang – not least the cosmic microwave background.

But the big-bang theory has some problems. In particular, why is everything the same everywhere?

That sounds like a strange question. Why wouldn’t it be the same everywhere?

Here’s why: because for changes to occur at the same time in different places, we expect there to be some causal connection between those places. If there is no plausible causal connection, then there’s a problem: how could things happen at the same time, in the same way?

That causal connection is a problem. To explain why, first I need to explain the idea of the observable universe.

Right now, there is some part of the universe that we can observe – because light from it has reached us. There’s also some part of the universe that we can’t observe, because light from it hasn’t reached us yet. Every day, every moment, the observable universe gets larger – not because the universe is expanding (it is, but we’re not talking about the size of the universe, but rather of the part of the universe that we can observe). It’s literally getting larger, because there are parts of the universe that are so far away from us, that the first light they emitted after the universe started didn’t reach us until right now. That threshold, of the stuff that couldn’t possible have gotten here yet, is constantly expanding, getting farther and farther away.

There are parts of the universe that are so far away, that the light from them couldn’t reach us until now. But when we look at that light, and use it to see what’s there, it looks exactly like what we see around us.

The problem is, it shouldn’t. If you just take the big bang, and you don’t have a period of inflation, what you would expect is a highly non-uniform universe with a very high spatial curvurature. Places very far away shouldn’t be exactly the same as here, because there is no mechanism which can make them evolve in exactly the same way that they did here! As energy levels from the big bang decrease, local fluctuations should have produced very different outcomes. They shouldn’t have ended up the same as here – because there’s many different ways things could have turned out, and they can’t be causally connected, because there’s no way that information could have gotten from there to here in time for it to have any effect.

Light is the fastest thing in the universe – but light from these places just got here. That means that until now, there couldn’t possibly be any connection between here and there. How could all of the fundamental properties of space – its curvature, the density of matter and energy – be exactly the same as here, if there was never any possible causal contact between us?

The answer to that is an idea called inflation. At some time in the earliest part of the universe – during a tiny fraction of the first second – the universe itself expanded at a speed faster than light. (Note: this doesn’t mean that stuff moved faster than light – it means that space itself expanded, creating space between things so that the distance between them expanded faster than light. Subtle distinction, but important!) So the matter and energy all got “stretched” out, at the same time, in the same way, giving the universe the basic shape and structure that it has now.

Inflation is the process that created the uniform universe. This process, which happened to the entire universe, had tiny but uniform fluctuations because of the basic quantum structure of the universe. Those fluctuations were the same everywhere – because when they happened, they were causally connected! Inflation expanded space, but those fluctuations provided the basic structure on which the stuff we observe in the universe developed. Since that basic underlying structure is the same everywhere, everything built on top it is the same as well.

We’ve seen lots of evidence for inflation, but it hasn’t quite been a universally accepted idea.

The next piece of the puzzle is gravity. Gravity at least appears to be very strange. All of the other forces in our universe behave in a consistent way. In fact, we’ve been able to show that they’re ultimately different aspects of the same underlying phenomena. All of the other forces can be described quantum mechanically, and they operate through exchange particles that transmit force/energy – for example, electromagnetic forces are transmitted by photons. But not gravity: we have no working quantum theory for how gravity works! We strongly suspect that it must, but we don’t know how, and up to now, we never found any actual proof that it does behave quantumly. But if it did, and if inflation happened, that means that those quantum fluctations during expansion, the things that provided the basic lattice on which matter and energy hang, should have created an echo in gravity!

Unfortunately, we can’t see gravity. The combination of inflation and quantum mechanics means that there should be gravitational fluctuations in the universe – waves in the basic field of gravity. We’ve predicted those waves for a long time. But we haven’t been able to actually test that prediction, because we didn’t have a way to see gravitational waves.

So now, I can finally get to this new result.

They believe that they found gravity waves in the cosmic microwave background. They used a brilliant scheme to observe them: if we look at the cosmic microwave background – not at any specific celestial object, but just at the background – gravitational waves would created a very subtle tensor polarization effect. So they created a system that could observe polarization. Then they removed all of the kinds of polarization that could be explained by anything other than gravitational waves. What they were left with was a very clear wave pattern in the polarization – exactly what was predicted by quantum inflation! You can see one of their images of this wave pattern at the top of this post.

If these new observations are confirmed, that means that we have new evidence for two things:

  1. Inflation happened. These gravity waves are an expected residue of inflation. They’re exactly what we would have expected if inflation happened, and we don’t have any other explanation that’s compatible with them.
  2. Gravity is quantum! If gravity wasn’t quantum, then expansion would have completely smoothed out the gravitational effects, and we wouldn’t see gravitational waves. Since we do see waves, it’s strong evidence that gravity really does have a quantum aspect. We still don’t know how it works, but now we have some really compelling evidence that it must!