Tag Archives: cosmic inflation

Flexing the BICEP2 Results

The physicsverse has been abuzz this week with news of the BICEP2 experiment’s observations of B-mode polarization in the Cosmic Microwave Background.

There are lots of good sources on this, and it’s not really my field, so I’m just going to give a quick summary before talking about a few aspects I find interesting.

BICEP2 is a telescope in Antarctica that observes the Cosmic Microwave Background, light left over from the first time that the universe was clear enough for light to travel. (If you’re interested in a background on what we know about how the universe began, Of Particular Significance has an article here that should be fairly detailed, and I have a take on some more speculative aspects here.) Earlier experiments that observed the Cosmic Microwave Background discovered a surprising amount of uniformity. This led to the proposal of a concept called inflation: the idea that at some point the early universe expanded exponentially, smearing any non-uniformities across the sky and smoothing everything out. Since the rate the universe expands is a number, if that number is to vary it naturally should be a scalar field, which in this case is called the inflaton.

During inflation, distances themselves get stretched out. Think about inflation like enlarging an image. As you’ve probably noticed (maybe even in early posts on this blog), enlarging an image doesn’t always work out well. The resulting image is often pixelated or distorted. Some of the distortion comes from glitches in the program that enlarges the image, while some of it is just what happens when the pixels of the original image get enlarged to the point that you can see them.

Enlarging the Cosmic Microwave Background

Quantum fluctuations in the inflaton field itself are the glitches in the program, enlarging some areas more than others. The pattern they create in the Cosmic Microwave Background is called E-mode polarization, and several other experiments have been able to detect it.

Much weaker are the effect of the “pixels” of the original image. Since the original image is spacetime itself, the pixels are the quantum fluctuations of spacetime: quantum gravity waves. Inflation enlarged them to the point that they were visible on a large-distance scale, fundamental non-uniformity in the world blown up big enough to affect the distribution of light. The effect this had on light is detectably different: it’s called B-mode polarization, and this is the first experiment to detect it on the right scale for it to be caused by gravity waves.

Measuring this polarization, in particular how strong it is, tells us a lot about how inflation occurred. It’s enough to rule out several models, and lend support to several others. If the results are corroborated this will be real, useful evidence, the sort physicists love to get, and folks are happily crunching numbers on it all over the world.

All that said, this site is called four gravitons and a grad student, and I’m betting that some of you want to ask this grad student: is this evidence for gravitons, or for gravity waves?

Sort of.

We already had good indirect evidence for gravity waves: pairs of neutron stars release gravity waves as they orbit each other, which causes them to slow down. Since we’ve observed them slowing down at the right rates, we were already confident gravity waves exist. And if you’ve got gravity waves, gravitons follow as a natural consequence of quantum mechanics.

The data from BICEP2 is also indirect. The gravity waves “observed” by BICEP2 were present in the early universe. It is their effect on the light that would become the Cosmic Microwave Background that is being observed, not the gravity waves directly. We still have yet to directly detect gravity waves, with a gravity telescope like LIGO.

On the other hand, a “gravity telescope” isn’t exactly direct either. In order to detect gravity waves, LIGO and other gravity telescopes attempt to measure their effect on the distances between objects. How do they do that? By looking at interference patterns of light.

In both cases, we’re looking at light, present in the environment of a gravity wave, and examining its properties. Of course, in a gravity telescope the light is from a nearby environment under tight control, while the Cosmic Microwave Background is light from as far away and long ago as anything within the reach of science today. In both cases, though, it’s not nearly as simple as “observing” an effect. “Seeing” anything in high energy physics or astrophysics is always a matter of interpreting data based on science we already know.

Alright, that’s evidence for gravity waves. Does that mean evidence for gravitons?

I’ve seen a few people describe BICEP2’s results as evidence for quantum gravity/quantum gravity effects. I felt a little uncomfortable with that claim, so I asked Matt Strassler what he thought. I think his perspective on this is the right one. Quantum gravity is just what happens when gravity exists in a quantum world. As I’ve said on this site before, quantum gravity is easy. The hard part is making a theory of quantum gravity that has real predictive power, and that’s something these results don’t shed any light on at all.

That said, I’m a bit conflicted. They really are seeing a quantum effect in gravity, and as far as I’m aware this really is the first time such an effect has been observed. Gravity is so weak, and quantum gravity effects so small, that it takes inflation blowing them up across the sky for them to be visible. Now, I don’t think there was anyone out there who thought gravity didn’t have quantum fluctuations (or at least, anyone with a serious scientific case). But seeing into a new regime, even if it doesn’t tell us much…that’s important, isn’t it? (After writing this, I read Matt Strassler’s more recent post, where he has a paragraph professing similar sentiments).

On yet another hand, I’ve heard it asserted in another context that loop quantum gravity researchers don’t know how to get gravitons. I know nothing about the technical details of loop quantum gravity, so I don’t know if that actually has any relevance here…but it does amuse me.

Anthropic Reasoning, Multiverses, and Eternal Inflation (Part Two of Two)

So suppose you want to argue that, contrary to appearances, the universe isn’t impossible, and you want to use anthropic reasoning to do it. Suppose further that you read my post last week, so you know what anthropic reasoning is. In case you haven’t, anthropic reasoning means recognizing that, while it may be unlikely that the location/planet/solar system/universe you’re in is a nice place for you to live, as long as there is at least one nice place to live you will almost certainly find yourself living there. Applying this to the universe as a whole requires there to be many additional universes, making up a multiverse, at least one of which is a nice place for human life.

Is there actually a multiverse, though? How would that even work?

One of the more plausible proposals for a multiverse is the concept of eternal inflation.

Eternal inflation is idea with many variants (such as chaotic inflation), and rather than give the details of any particular variant, I want to describe the setup in as broad strokes as possible.

The first thing to be aware of is that the universe is expanding, and has been since the Big Bang. Counter-intuitively, this doesn’t mean that the universe was once small, and is now bigger: in all likelihood, the universe was always infinite in size. Instead, it means that things began packed in close together, and have since moved further apart. While various forces (gravity, electromagnetism) hold things together on short scales, the wide open spaces between galaxies are constantly widening, spreading out the map of the universe.

You would expect this process to slow down over time. While it might have started with a burst of energy (aforementioned Big Bang), as the universe gets more and more spread out it should be running out of steam. The thing is, it’s not. The evidence (complicated enough that I’m not going to go into it now) shows that the universe actually sped up dramatically shortly after the Big Bang, and seems to be speeding up again now. This speeding up is called inflation.

So what could make the universe speed up? You might have heard of Einstein’s cosmological constant, a constant added to Einstein’s equations of general relativity that, while originally intended to make the universe stay in a steady state forever, can also be chosen so as to speed up the universe’s expansion. While that works mathematically, it’s not really an explanation, especially if it changes with time.

Enter scalar fields. A scalar is what happens when you let what looks like a constant of nature vary as a quantum field. Scalar fields can vary over space, and they can change over time, making them ideal candidates for explaining inflation. And as a quantum field, the scalar field behind inflation (often called the inflaton) should randomly fluctuate, giving rise to the occasional particle just like the Higgs (another scalar field) does.

Well, not just like the Higgs. See, the Higgs controls mass, and if the mass of some particles increases a bit in a tiny area, it’s weird, but it’s not going to spread. On the other hand, if space in some place is inflating faster than space in another place…

Suppose you have two empty blocks in the middle of intergalactic space, each a cube one foot on each side, with one inflating faster than the other. Twice as fast, let’s say, so that when one cube grows to two feet on a side, the other grows to four feet on a side. Then when the first cube is four feet on a side, the other will be sixteen. When the first has eight foot sides, the other’s will be sixty-four. And so forth. Even a small difference in expansion rates quickly leads to one region dominating the other. And if inflation stops slightly later in one region than in another, that can be a pretty dramatic difference too.

The end result is that if inflation were this sort of scalar field, the universe would just keep expanding forever, faster and faster. Only small pockets would slow down enough that anything could actually stick together. So while most of the universe would just tear itself apart forever, some of it, the parts that tear themselves apart slowly, can contain atoms and stars and well, life. A universe like that is one that is experiencing eternal inflation. It’s eternal because it doesn’t have a beginning or end: what looks to us like the Big Bang, the beginning of our universe, is really just the point at which our part of the universe started expanding slow enough that anything we recognize as matter could exist.

There’s no reason for us to be the only bubble that slowed down, though, and that’s where the multiverse aspect comes in. In eternal inflation there are lots and lots of slow regions, each one like a mini-universe in its own right. What’s more, each region can have totally different constants of nature.

To understand how that works, remember that each region has a different rate of inflation, and thus a different value for the inflaton scalar field. It turns out that many types of scalar fields like to interact with each other. If you recall my post on scalar fields (already linked, not gonna link it again), you’ll remember that for everything that looks like a constant of nature, chances are there’s a scalar field that controls it. So different values for inflation means different values for all of those scalar fields too, which means different physical constants. With so many (possibly infinitely many) regions with different physical constants, there’s bound to be one where we could live.

Now, before you get excited here, there are a few caveats. Well, a lot of caveats.

First, it’s all well and good if the multiverse can produce life, but what if it produces dramatically different life? What sort of life is eternal inflation most likely to produce, and what are the chances it would look at all like us? For that matter, how do you figure out the chances of anything in an infinite, eternally expanding universe? This last is a very difficult problem, and work on it is ongoing.

Beyond that, we don’t even know enough about inflation to know whether eternal inflation would happen or not. We’ve got a pretty good idea that inflation involves scalar fields, but how many and in what combination? We don’t know yet, and the evidence is still coming in. We’re right on the cutting edge of things now, and until we know more it’s tough to say for certain whether any of this is viable. Still, it’s fun to think about.