Tag Archives: cosmology

Love It or Hate It, Don’t Fear the Multiverse

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“In an infinite universe, anything is possible.”

A nice maxim for science fiction, perhaps. But it probably doesn’t sound like productive science.

A growing number of high profile scientists and science popularizers have come out in favor of the idea that there may exist a “multiverse” of multiple universes, and that this might explain some of the unusual properties of our universe. If there are multiple universes, each with different physical laws, then we must exist in one of the universes with laws capable of supporting us, no matter how rare or unlikely such a universe is. This sort of argument is called anthropic reasoning.

(If you’re picky about definitions and don’t like the idea of more than one universe, think instead of a large universe with many different regions, each one separated from the others. There are some decent physics-based reasons to suppose we live in such a universe.)

Not to mention continuity reasons.

Why is anyone in favor of this idea? It all goes back to the Higgs.

The Higgs field interacts with other particles, giving them mass. What most people don’t mention is that the effect, in some sense, goes both ways. Because the Higgs interacts with other particles, the mass of the Higgs is also altered. This alteration is large, much larger than the observed mass of the Higgs. (In fact, in a sense it’s infinite!)

In order for the Higgs to have the mass we observe, then, something has to cancel out these large corrections. That cancellation can either be a coincidence, or there can be a reason for it.

The trouble is, we’re running out of good reasons. One of the best was supersymmetry, the idea that each particle has a partner with tightly related properties. But if supersymmetry was going to save the day, we probably would have detected some of those partners at the Large Hadron Collider by now. More generally, it can be argued that almost all possible “good reasons” require some new particle to be found at the LHC.

If there are no good reasons, then we’re stuck with a coincidence. (This is often referred to as the Naturalness Problem in particle physics.) And it’s this uncomfortable coincidence that has driven prominent physicists to the arms of the multiverse.

There’s a substantial backlash, though. Many people view the multiverse as a cop-out. Some believe it to be even more toxic than that: if there’s a near-infinite number of possible universes then in principle any unusual feature of our universe could be explained by anthropic reasoning, which sounds like it could lead to the end of physics as we know it.

You can disdain the multiverse as a cop-out, but, as I’ll argue here, you shouldn’t fear it. Those who think the multiverse will destroy physics are fundamentally misunderstanding the way physics research works.

The key thing to keep in mind is that almost nobody out there prefers the multiverse. When a prominent physicist supports the multiverse, that doesn’t mean they’re putting aside productive work on other solutions to the problem. In general, it means they don’t have other solutions to the problem. Supporting the multiverse isn’t going to stop them from having ideas they wouldn’t have had to begin with.

And indeed, many of these people are quite supportive of alternatives to the multiverse. I’ve seen Nima Arkani-Hamed talk about the multiverse, and he generally lists a number of other approaches (some quite esoteric!) that he has worked (and failed to make progress) on, and encourages the audience to look into them.

Physics isn’t a zero-sum game, nor is it ruled by a few prominent people. If a young person has a good idea about how to explain something without the multiverse, they’re going to have all the support and recognition that such an idea deserves.

What the multiverse adds is another track, another potentially worthwhile line of research. Surprising as it may seem, the multiverse doesn’t automatically answer every question. It might not even answer the question of the mass of the Higgs! All that the existence of a multiverse tells us is that we should exist somewhere where intelligent life could exist…but if intelligent life is more likely to exist in a universe very different from ours, then we’re back to square one. There’s a lot of research involved in figuring out just what the multiverse implies, research by people who wouldn’t have been working on this sort of problem if the idea of the multiverse hadn’t been proposed.

That’s the key take-away message here. The multiverse may be wrong, but just considering it isn’t going to destroy physics. Rather, it’s opened up new avenues of research, widening the community of those trying to solve the Naturalness Problem. It may well be a cop-out for individuals, but science as a whole doesn’t have cop-outs: there’s always room for someone with a good idea to sweep away the cobwebs and move things forward.

(Interstellar) Dust In The Wind…

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The news has hit the blogosphere: the team behind the Planck satellite has released new dust measurements, and they seem to be a nail in the coffin of BICEP2’s observation of primordial gravitational waves.

Some background for those who haven’t been following the story:

BICEP2, a telescope in Antarctica, is set up to observe the Cosmic Microwave Background, light left over from the very early universe. Back in March, they announced that they had seen characteristic ripples in that light, ripples that they believed were caused by gravitational waves in the early universe. By comparing the size of these gravitational waves to their (quantum-small) size when they were created, they could make statements about the exponential expansion of the early universe (called inflation). This amounted to better (and more specific) evidence about inflation than anyone else had ever found, so naturally people were very excited about it.

However, doubt was rather quickly cast on these exciting results. Like all experimental science, BICEP2 needed to estimate the chance that their observations could be caused by something more mundane. In particular, interstellar dust can cause similar “ripples” to those they observed. They argued that dust would have contributed a much smaller effect, so their “ripples” must be the real deal…but to make this argument, they needed an estimate of how much dust they should have seen. They had several estimates, but one in particular was based on data “scraped” off of a slide from a talk by the Planck collaboration.

Unfortunately, it seems that the BICEP2 team misinterpreted this “scraped” data. Now, Planck have released the actual data, and it seems like dust could account for BICEP2’s entire signal.

I say “could” because more information is needed before we know for sure. The BICEP2 and Planck teams are working together now, trying to tease out whether BICEP2’s observations are entirely dust, or whether there might still be something left.

I know I’m not the only person who wishes that this sort of collaboration could have happened before BICEP2 announced their discovery to the world. If Planck had freely shared their early data with BICEP2, they would have had accurate dust estimates to begin with, and they wouldn’t have announced all of this prematurely.

Of course, expecting groups to freely share data when Nobel prizes and billion-dollar experiments are on the line is pretty absurdly naive. I just wish we lived in a world where none of this was at issue, where careers didn’t ride on “who got there first”.

I’ve got no idea how to bring about such a world, of course. Any suggestions?

Insert Muscle Joke Here

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I’m graduating this week, so I probably shouldn’t spend too much time writing this post. I ought to mention, though, that there has been some doubt about the recent discovery by the BICEP2 telescope of evidence for gravitational waves in the cosmic microwave background caused by the early inflation of the universe. Résonaances got to the story first and Of Particular Significance has some good coverage that should be understandable to a wide audience.

In brief, the worry is that the signal detected by BICEP2 might not be caused by inflation, but instead by interstellar dust. While the BICEP2 team used several models of dust to show that it should be negligible, the controversy centers around one of these models in particular, one taken from another, similar experiment called PLANCK.

The problem is, BICEP2 didn’t get PLANCK’s information on dust directly. Instead, it appears they took the data from a slide in a talk by the PLANCK team. This process, known as “data scraping”, involves taking published copies of the slides and reading information off of the charts presented. If BICEP2 misinterpreted the slide, they might have miscalculated the contribution by interstellar dust.

If you’re like me, the whole idea of data scraping seems completely ludicrous. The idea of professional scientists sneaking information off of a presentation, rather than simply asking the other team for data like reasonable human beings, feels almost cartoonishly wrong-headed.

It’s a bit more understandable, though, when you think about the culture behind these big experiments. The PLANCK and BICEP2 teams are colleagues, but they are also competitors. There is an enormous amount of glory in finding evidence for something like cosmic inflation first, and an equally enormous amount of shame in screwing up and announcing something that turns out to be wrong. As such, these experiments are quite protective of their data. Not only might someone with early access to the data preempt them on an important discovery, they might rush to publish a conclusion that is wrong. That’s why most of these big experiments spend a large amount of time checking and re-checking the data, communicating amongst themselves and settling on an interpretation before they feel comfortable releasing it to the wider community. It’s why BICEP2 couldn’t just ask PLANCK for their data.

From BICEP2’s perspective, they can expect that plots presented at a talk by PLANCK should be accurate, digital plots. Unlike Fox News, scientists have an obligation to present their data in a way that isn’t misleading. And while relying on such a dubious source seems like a bad idea, by all accounts that’s not what the BICEP2 team did. PLANCK’s data was just one dust model used by the team, kept in part because it agreed well with other, non-“data-scraped” models.

It’s a shame that these experiments are so large and prestigious that they need to guard their data in such a potentially destructive way. My sub-field is generally much nicer about this sort of thing: the stakes are lower, and the groups are smaller and have less media attention, so we’re able to share data when we need to. In fact, my most recent paper got a significant boost from some data shared by folks at the Perimeter Institute.

Only time will tell whether the BICEP2 result wins out, or whether it was a fluke caused by caustic data-sharing practices. A number of other experiments are coming online within the next year, and one of them may confirm or deny what BICEP2 has showed.

Flexing the BICEP2 Results

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The physics–verse 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.

Braaains…Boltzmann Braaaains…

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In honor of Halloween yesterday, let me tell you a scary physics story:

Sarah was an ordinary college student, in an ordinary dorm room, ordinary bean bag chairs strewn around an ordinary bed with ordinary pink sheets. If she concentrated, she could imagine her ordinary parents back home in ordinary Minnesota. In her ordinary physics textbook on her ordinary desk, ordinary laws of physics were written, described as the result of centuries of experimentation.

Unbeknownst to Sarah, the universe was much more chaotic and random than she realized, and also much more vast. Arbitrary collections of matter formed and dissipated, and over the universe’s long history, any imaginable combination might come to be.

Combinations like Sarah.

You see, Sarah too was a random combination, a chance arrangement of particles formed only a bare few moments ago. In truth, she had no ordinary parents, nor was she surrounded by an ordinary college, and the laws of physics that her textbook asserted were discovered through centuries of experimentation were just a moment’s distribution of ink on a page.

And as she got up to open the door into the vast dark of the outside, her world dissipated, and she ceased to exist.

That’s the life of a Boltzmann Brain. If a universe is random and old enough, it is inevitable that such minds exist. They might have memories of an extended, orderly world, but these would just be illusions, chance arrangements of their momentary neurons. What’s more, they may think they know the laws of physics through careful experiment and reasoning, but such knowledge would be illusory as well. And most frightening of all, if the universe is truly ancient and unimaginably vast, there would be many orders of magnitude more Boltzmann Brains than real humans…so many, that it would almost certainly be the case that you are in fact a Boltzmann Brain right now!

This is legitimately worrying to some physicists. The situation gets a bit more interesting when you remember that, as a Boltzmann Brain, anything you know about physics may well be a lie, since the history of research you think exists might not have. The problem is, if you manage to prove that you are probably a Boltzmann Brain, you had to use physics to do it. But your physics is probably wrong!

This, as Sean Carroll argues is why the concept of a Boltzmann Brain is self-defeating. It is, in a way, a logical impossibility. And if a universe of Boltzmann Brains is logically impossible, then any physics that makes Boltzmann Brains more likely than normal humans must similarly be wrong. That’s Carroll’s argument, one that he uses to argue for specific physical conclusions about the real world, namely a proposal about the properties of the Higgs boson.

It might seem philosophically illegitimate to use such a paradox to argue about the real world. However, philosophers have a similar argument when it comes to such “reality is a lie” scenarios. In general, modern philosophers point out that any argument that proves that all of our knowledge is false or meaningless by necessity also proves itself false or meaningless. This is what allows analytical philosophy to carry forward and make progress, even if it can’t reject the idea that reality is an illusion by more objective means.

With that said, there seems to be a difference between simply rejecting arguments that “show” that the world is an illusion or that we are all Boltzmann Brains, and using those arguments to draw conclusions about other parts of the world. I would be curious if there are similar arguments to Carroll’s in philosophy, arguments that draw conclusions more specific than “we exist and can know things”. Any philosopher readers should feel welcome to chime in in the comments!

And for the rest of you, you probably aren’t a Boltzmann Brain. But if the outside world looks a little too dark tonight…

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

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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.

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

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You and I are very very lucky. Human life is very delicate, and the conditions under which it can thrive are not in the majority. Going by random chance, neither of us should exist.

I am referring, of course, to the fact that the Earth’s surface is about 70 percent ocean. Just think how lucky you are not to have been born there: you would have drowned! Let alone if you were born beneath the Earth’s crust!

If you understand why the above is ridiculous, congratulations: you’ve just discovered anthropic reasoning.

There are some situations we find ourselves in because they are common. Most (all) of the Earth is in orbit around the Sun, so if you find yourself in orbit around the Sun you should hardly be surprised. Some situations, on the other hand, keep happening not because they are common in the universe in general, but because they are the part of the universe in which we can exist. Recognizing those situations is anthropic reasoning.

It’s not weird that you were born on land, even though land is rarer than water, because land, and not water, is where people live. As long as there was any land on the earth at all, you would expect people to be born on it (or on ships, I suppose) rather than on the ocean.

The same sort of reasoning explains why we evolved on Earth to begin with. There are eight planets in the solar system (yes, Pluto is not a planet, get over it), and only one of them is in the right place for life like us. We aren’t “lucky” that we ended up on Earth rather than another planet, nor is it something “unlikely” that needs to be explained: we’re on Earth because the universe is big enough that there happens to be a planet that has the right conditions for life, and Earth is that planet.

What anthropic reasoning has a harder time explaining (but what some people are working very hard to make it explain) is the question of why our whole universe is the way it is. Our universe is a pretty good place for life to evolve. Granted, that’s probably just a side effect of it being a good place for stars to evolve, but let’s put that aside for a second. Suppose the universe really is a particularly nice place for life, even improbably nice. Can anthropic reasoning explain that?

Probably. But it takes some work.

See, the difficulty is that in order for anthropic reasoning to work, you need to be certain that some place hospitable to life actually is likely to exist. Earthlike planets may be rare, but there are enough planets in the universe that some of them are bound to be like Earth. If universes like ours are rare, though, then how can there be enough universes to guarantee one like ours? How can there be more than one universe at all?

That’s why you need a multiverse.

A multiverse, in simple terms, is a collection of universes. If you object that a universe is, by definition, all that exists, and thus there can’t possibly be more than one, then you can use an alternate definition: a multiverse is a vast universe in which there are many smaller universe-like regions. These sub-universes don’t have much (or any) contact with eachother, and (in order for anthropic reasoning to work) must have different properties.

Does a multiverse exist, though? How would one work?

There are several possibilities, of varying degrees of plausibility. Some people have argued that quantum mechanics leads to many parallel universes, while others posit that each universe could be like a membrane in some higher dimensional space. The multiple universes could be separated in ordinary space, or even in time.

In the next post, I will discuss one of the more plausible (if still controversial) possibilities, called eternal inflation, in which new universes are continually birthed in a vast sea of exponentially expanding space. If you have no idea what the heck I meant by that, great! Tune in next time to find out!