Videos from our Setting Time Aright conference are gradually filtering online, courtesy of the Foundational Questions Institute. Perhaps the very first question that should be asked, of course, is whether the subject of the conference actually exists. So we recruited two well-known partisans on this issue to hash things out. Tim Maudlin is a philosopher of science who has argued forcefully that time is real — and furthermore that the arrow of time is an intrinsic part of reality, not just a byproduct of the low-entropy Big Bang. (Crazy talk.) Julian Barbour is a physicist who is well known for arguing that time doesn’t really exist, we can happily eliminate it from all of our equations of physics. (Even crazier.)

So we asked them to go at it, with a twist: here Tim defends the proposition that time doesn’t exist, while Julian argues that it is real. I was not the only one to conclude that these guys were just as good at arguing this side as the one they actually believed.

Well worth watching — both talks are quite brilliant, in very different ways.

Now that we’ve softened you up by explaining a bit about eternal inflation and its puzzles, we’re very happy to host a guest post by Tom Banks in which he really hits on some of these problems hard.

Tom is a professor at Rutgers and UC Santa Cruz, an extremely accomplished researcher in field theory and string theory, and the author of a textbook on quantum field theory. In collaboration with Fischler, Shenker, and Susskind, he proposed the (M)atrix Theory non-perturbative formulation of string theory. Most recently, he (often working with Willy Fischler) has been exploring the connections between holography and cosmology, developing a detailed model of the evolution of the universe that is compatible with the holographic principle. Here is video of a lecture Tom recently gave on holographic cosmology.

This post is at a more technical level than most of our entries here at CV, and we’re going to try to keep the discussion useful for workers in the field. Sincere questions are welcome, but we’ll be deleting any unproductive philosophical gripes or advertisements for anyone’s personal outsider theories.

——————————————————

Why I Don’t Believe in Eternal Inflation

A lot of research in high energy theory has been devoted to the topic of eternal inflation. More and more, over the last few years, I’ve come to regard this as an enormous waste of intellectual resources and I’ve chosen Cosmic Variance as a very public way to make my objections to this theoretical mistake clear. The theory was developed in the 1980s, when it seemed plausible that quantum field theory in curved space-time was a good approximation to a real theory of quantum gravity whenever the energy densities and curvatures of the background geometry were small in Planck units. This idea is simply wrong. The fact that its falsification came through a back door, the rather philosophical discussion of whether black hole evaporation violates the rules of quantum mechanics, has led to a widespread but unfortunate tendency to ignore this FACT.

There are two other psychological reasons for the widespread interest in Eternal Inflation, which I will discuss below. They have led even the inventors of the resolution of the black hole information paradox through the notion of holography, to try to find a sensible holographic theory which incorporates the notion of EI. While this attempt itself is subject to a number of objections, I will not go into them here. Instead, I’ll concentrate on evidence from the seminal Coleman-De Luccia (CDL) theory of tunneling in quantum gravity, which is one of the two biggest clues to what the theory of quantum gravity really is.

There are, in my opinion, two serious conceptual errors behind the theory of EI. The first is the notion that space-time geometry is a fluctuating quantum variable. The second is that de Sitter (dS) space is a system with an ever increasing number of quantum degrees of freedom. The increase is supposed to take place as the global dS time coordinate, or the time coordinate in flat coordinates, goes to future infinity. I’ll end this post with a brief discussion of the formalism of Holographic Space Time (HST), in which both of these ideas are seen to be false, in a very explicit manner. The fact that the HST formalism is able to give an approximate description of particle physics in a curved space-time background is by itself enough to falsify any claim that the semi-classical ideas that lead to EI are inevitable consequences of ANY sensible theory of quantum gravity. For this purpose, it’s not even necessary that HST be right, only that it have a limit in which it reduces to QFT in curved space-time.

There are two flavors of EI. (more…)

My inaugural column for Discover discussed the lighting-rod topic of the inflationary multiverse. But there’s only so much you can cover in 1500 words, and there are a number of foundational issues regarding inflation that are keeping cosmologists up at night these days. We have a guest post or two coming up that will highlight some of these issues, so I thought it would be useful to lay a little groundwork. (Post title paraphrased from Andrei Linde.)

This summer I helped organize a conference at the Perimeter Institute on Challenges for Early Universe Cosmology. The talks are online here — have a look, there are a number of really good ones, by the established giants of the field as well as by hungry young up-and-comers. There was also one by me, which starts out okay but got a little rushed at the end.

What kinds of challenges for early universe cosmology are we talking about? Paul Steinhardt pointed out an interesting sociological fact: twenty years ago, you had a coterie of theoretical early-universe cosmologists who had come from a particle/field-theory background, almost all of whom thought that the inflationary universe scenario was the right answer to our problems. (For an intro to inflation, see this paper by Alan Guth, or lecture 5 here.) Meanwhile, you had a bunch of working observational astrophysicists, who didn’t see any evidence for a flat universe (as inflation predicts) and weren’t sure there were any other observational predictions, and were consequently extremely skeptical. Nowadays, on the other hand, cosmologists who work closely with data (collecting it or analyzing it) tend to take for granted that inflation is right, and talk about constraining its parameters to ever-higher precision. Among the more abstract theorists, however, doubt has begun to creep in. Inflation, for all its virtues, has some skeletons in the closet. Either we have to exterminate the skeletons, or get a new closet.

(more…)

Many of you may know that Discover is not only a web site that hosts a diverse collection of entertaining blogs, but also publishes a monthly “magazine” printed on paper. Wild, right? Just ask this baby, who can tell you that a magazine is kind of broken when compared to an iPad.

Nevertheless, people read these things like crazy. I have recently started contributing an occasional column to the print magazine, known as “Out There.” (Our blog neighbor Carl Zimmer has been columnizing about the brain for a while now.) My first column appeared in the October issue (which comes out in September), and is now online — check it out.

The issue I’m tackling, under the draconian word count limit of an actual print magazine, is whether it’s scientific to talk about the multiverse. (Spoiler: it is!) Let me know what you think.

Here’s a blast from the somewhat-recent past: a set of five lectures I gave at CERN in 2005. It looks like the quality of the recording is pretty good. The first lecture was an overview at a colloquium level; i.e. meant for physicists, but not necessarily with any knowledge of cosmology. The next four are blackboard talks with a greater focus; they try to bring people up to speed on the basic tools you need to think about modern early-universe cosmology.

Obviously I’m not going to watch all five hours of these, so I’ll just have to hope that I’m relatively coherent throughout. (I do remember being a bit jet-lagged.) But I do notice that, while it was only a few years ago, I do appear relatively young and enthusiastic. Ah, the ravages of Time…

Lecture One: Introduction to Cosmology

(more…)

In honor of the Nobel Prize, here are some questions that are frequently asked about dark energy, or should be.

What is dark energy?

It’s what makes the universe accelerate, if indeed there is a “thing” that does that. (See below.)

So I guess I should be asking… what does it mean to say the universe is “accelerating”?

First, the universe is expanding: as shown by Hubble, distant galaxies are moving away from us with velocities that are roughly proportional to their distance. “Acceleration” means that if you measure the velocity of one such galaxy, and come back a billion years later and measure it again, the recession velocity will be larger. Galaxies are moving away from us at an accelerating rate.

But that’s so down-to-Earth and concrete. Isn’t there a more abstract and scientific-sounding way of putting it?

The relative distance between far-flung galaxies can be summed up in a single quantity called the “scale factor,” often written a(t) or R(t). The scale factor is basically the “size” of the universe, although it’s not really the size because the universe might be infinitely big — more accurately, it’s the relative size of space from moment to moment. The expansion of the universe is the fact that the scale factor is increasing with time. The acceleration of the universe is the fact that it’s increasing at an increasing rate — the second derivative is positive, in calculus-speak.

Does that mean the Hubble constant, which measures the expansion rate, is increasing?

No. The Hubble “constant” (or Hubble “parameter,” if you want to acknowledge that it changes with time) characterizes the expansion rate, but it’s not simply the derivative of the scale factor: it’s the derivative divided by the scale factor itself. Why? Because then it’s a physically measurable quantity, not something we can change by switching conventions. The Hubble constant is basically the answer to the question “how quickly does the scale factor of the universe expand by some multiplicative factor?”

If the universe is decelerating, the Hubble constant is decreasing. If the Hubble constant is increasing, the universe is accelerating. But there’s an intermediate regime in which the universe is accelerating but the Hubble constant is decreasing — and that’s exactly where we think we are. The velocity of individual galaxies is increasing, but it takes longer and longer for the universe to double in size.

Said yet another way: Hubble’s Law relates the velocity v of a galaxy to its distance d via v = H d. The velocity can increase even if the Hubble parameter is decreasing, as long as it’s decreasing more slowly than the distance is increasing.

Did the astronomers really wait a billion years and measure the velocity of galaxies again?

No. You measure the velocity of galaxies that are very far away. Because light travels at a fixed speed (one light year per year), you are looking into the past. Reconstructing the history of how the velocities were different in the past reveals that the universe is accelerating.

How do you measure the distance to galaxies so far away?

It’s not easy. The most robust method is to use a “standard candle” — some object that is bright enough to see from great distance, and whose intrinsic brightness is known ahead of time. Then you can figure out the distance simply by measuring how bright it actually looks: dimmer = further away.

Sadly, there are no standard candles.

Then what did they do? (more…)

Sometimes it’s not that hard to predict the future — everyone paying attention (including me) knew that one of the most Nobel-worthy discoveries out there was the 1998 announcement that our universe is accelerating. Now the achievement has been officially honored, with this year’s Physics Prize going to Saul Perlmutter, Adam Riess, and Brian Schmidt. (Great quotes and coverage at the Guardian.) Congrats to three extremely deserving scientists!

Like regular people with major historical events, most physicists can remember where they were when they first heard that the universe is accelerating. That’s how big this discovery was. It was just the right combination of “startling” — very few people really thought the universe was accelerating, and if they did they certainly weren’t proclaiming that belief very loudly — and “believable” — we all knew it was possible, and as soon as the data came in people realized that it solved a bunch of problems at once. There was a healthy amount of skepticism, but in a very short period of time it became difficult to get a Ph.D. as a cosmologist without working on this problem in one way or another — either verifying the result observationally, or trying to come up with a theoretical explanation.

The leading explanation by far, of course, is the existence of a smooth and persistent source of energy known as dark energy, of which Einstein’s cosmological constant is the simplest and most compelling example. If that’s the right answer, we’re talking about 73% or so of the universe. Something to tell your grandkids that you helped discover, eh? A small sampling of what this discovery has wrought, just taken from this here blog:

• Songs
• Frantic attempts to wriggle out
• Amazing cosmological measurements
• More frantic attempts
• Complaints about fundamentalism
• A much better idea what will happen to the universe in the future
• Plant nutrient formulae
• Conferences
• Task forces
• Plans for future satellites
• Prizes galore
• New theories of gravity
• New scenarios for the origin of the universe
• Better theoretical understanding
• Better observational strategies
• Confused vocabulary
• Confused cosmologists
• Multiple confirmations

Not a bad result, I would say.

You don’t think I’m going to leave this without mentioning that Brian Schmidt was my office mate in grad school, do you? Taught the young man all he knows (about inflation and field theory). Adam Riess was a fellow classmate of ours, both of them studying under Bob Kirshner. I even got to collaborate on a follow-up paper with these upstanding gentlemen. Saul Perlmutter was already at Lawrence Berkeley Labs thinking about supernovae and the expansion of the universe, so I can’t claim to have influenced him, but we did chat on the phone several times about what different observational outcomes would imply for theory. This is the first Nobel Prize where I was friends with all the winners before they won.

In this day and age, of course, much good science is done by teams, not by individuals. This is certainly an example; Brian has already said that he’ll be bringing his team to Stockholm. Congratulations again to everyone involved in this discovery, truly one of the historic events in science.

At 2pm today, in a field not far from downtown Chicago, a final proton will smash into an antiproton. And then the Tevatron, the most powerful particle accelerator for almost three decades, will be shut off after producing over 500 trillion proton-antiproton collisions (over 10 inverse femtobarns). The Tevatron discovered the top quark, the Bc meson, and the tau neutrino. It measured direct CP violation, constrained the possible mass of the Higgs, precisely measured a range of masses and lifetimes, as well as a host of other important scientific contributions. It was a remarkable machine, but it has now been superseded by the Large Hadron Collider.

Science marches on, and after a glorious career, it is time for the Tevatron to go dark. Fermilab is throwing a goodbye party.

You don’t just throw a switch and turn off a particle accelerator. The process is slow and deliberate. The superconducting magnets will take roughly a week to warm back up. And the data will take many months to analyze. But at some point tomorrow afternoon North America will witness its last controlled TeV particle collision.

Building in part on my talk at the time conference, Scott Aaronson has a blog post about entropy and complexity that you should go read right now. It’s similar to one I’ve been contemplating myself, but more clever and original.

Back yet? Scott did foolishly at the end of the post mention the faster-than-light neutrino business. Which of course led to questions, in response to one of which he commented thusly:

Closed timelike curves seem to me to be a different order of strangeness from anything thus far discovered in physics—like maybe 1000 times stranger than relativity, QM, virtual particles, and black holes put together. And I don’t understand how one could have tachyonic neutrinos without getting CTCs as well—would anyone who accepts that possibility be kind enough to explain it to me?

The problem Scott is alluding to is that, in relativity, it’s the speed-of-light barrier that prevents particles (or anything) from zipping around and meeting themselves in the past — a closed loop in spacetime. On a diagram in which time stretches vertically and space horizontally, the possible paths of light from any event define light cones, and physical particles have to stay inside these light cones. “Spacelike” trajectories that leave the light cones simply aren’t allowed in the conventional way of doing things.

What you don’t see in this spacetime diagram is a slice representing “the universe at one fixed time,” because that kind of thing is completely observer-dependent in relativity. In particular, if you could move on a spacelike trajectory, there would be observers who would insist that you are traveling backwards in time. Once you can go faster than light, in other words, you can go back in time and meet yourself in the past. This is Scott’s reason for skepticism about the faster-than-light neutrinos: if you open that door even just a crack, all hell breaks loose.

But rest easy! It doesn’t necessarily follow. Theorists are more than ingenious enough to come up with ways to allow particles to move faster than light without letting them travel along closed curves through spacetime. (more…)

Probably not. But maybe! Or in other words: science as usual.

For the three of you reading this who haven’t yet heard about it, the OPERA experiment in Italy recently announced a genuinely surprising result. They create a beam of muon neutrinos at CERN in Geneva, point them under the Alps (through which they zip largely unimpeded, because that’s what neutrinos do), and then detect a few of them in the Gran Sasso underground laboratory 732 kilometers away. The whole thing is timed by stopwatch (or the modern high-tech version thereof, using GPS-synchronized clocks), and you solve for the velocity by dividing distance by time. And the answer they get is: just a teensy bit faster than the speed of light, by about a factor of 10^-5. Here’s the technical paper, which already lists 20 links to blogs and news reports.

The things you need to know about this result are:

• It’s enormously interesting if it’s right.
• It’s probably not right.

By the latter point I don’t mean to impugn the abilities or honesty of the experimenters, who are by all accounts top-notch people trying to do something very difficult. It’s just a very difficult experiment, and given that the result is so completely contrary to our expectations, it’s much easier at this point to believe there is a hidden glitch than to take it at face value. All that would instantly change, of course, if it were independently verified by another experiment; at that point the gleeful jumping up and down will justifiably commence.

This isn’t one of those annoying “three-sigma” results that sits at the tantalizing boundary of statistical significance. The OPERA folks are claiming a six-sigma deviation from the speed of light. (more…)