Science

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.

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The Eternally Existing, Self-Reproducing, Frequently Puzzling Inflationary UniverseRead 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

http://www.youtube.com/watch?v=vUNtO2r_-eo

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CERN Lectures on Cosmology and Particle PhysicsRead 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? …

Dark Energy FAQRead 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.

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. …

Can Neutrinos Kill Their Own Grandfathers?Read More »