Sean Carroll

Author: Sean Carroll

  • Dark Energy FAQ

    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…)

  • Nobel Prize for the Accelerating Universe

    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:

    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.

  • NSF Tries to Make Family/Career Balance Easier

    Among the various difficulties that women experience when they embark on a scientific career, a big one is how to balance the challenges of work with raising a family. (In principle men could face the same challenges; in practice the burden usually falls on women. Individual cases will vary.) Science is extremely competitive, and it’s generally not a 9-to-5 job. The years when you might be at your scientifically most productive can be precisely those years when you want to have kids. I’m not familiar myself, but I understand that raising kids actually takes up some of your time.

    So it’s great to see the National Science Foundation trying to do something to help. The White House just announced a major new initiative aimed at giving parents new flexibility in their careers. As explained in this press release, the general focus is flexibility, which is a great idea anyway: letting grant recipients defer for a year, and cutting down on the demands for investigators to travel to NSF headquarters when applying or renewing. (Via New APPS.)

    These are tiny steps, and there are many other hurdles women face in academia other than the timing of their grants. But every little bit helps, and it’s certainly good to know that someone upstairs is paying attention.

  • Can Neutrinos Kill Their Own Grandfathers?

    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…)

  • Faster-Than-Light Neutrinos?

    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…)

  • Cells Repairing Themselves

    Speaking of self-repair, here’s a fascinating new finding from Malin Hernebring in Sweden. Here’s the technical paper, from a few years ago; it’s part of Hernebring’s Ph.D. thesis work. (Via Richard Dawkins’s site.)

    As we age, our cells gradually decay; the DNA stays relatively intact, but proteins degrade with time. This is a big part of the aging process, leading to wrinkled skin as well as more serious consequences. When you think about it a bit, that raises a puzzle. A newborn baby arises out of the cells of its parents. So if the proteins simply decay without repair, every generation would get handed down a degraded set of proteins. At some point, therefore, there has to be some repair job, so that the baby gets fully functioning proteins.

    If this idea is right, you might guess that the repairs happen at the level of ovum and sperm; maybe when these cells are created, extra effort goes into tuning up their proteins into working order. But the new research says no — it’s actually after conception that the clean-up crew arrives. The newly conceived embryo consists of stem cells that soon begin differentiating themselves into the different kind of mature cells. It turns out that it’s during this differentiation process that proteasomes go to work, breaking down the damaged proteins and generally tuning up the engine. (Maybe this is when the soul is implanted in the embryo?)

    The next obvious question is: why can’t these cellular clean-up crews be active all the time? There are clear implications for studies of (and therapeutic approaches to) aging. Nature wants all the individual animal organisms to die, making room for new generations; but there’s no reason we have to go along with the plan.

  • Guest Post: Lisa Randall on Writing Knocking on Heaven’s Door

    Lisa Randall is a friend and collaborator, as well as a science superstar. She is one of the most highly cited physicists of all time, for a variety of contributions to field theory and particle physics, especially her work with Raman Sundrum on warped extra dimensions. Her first book, Warped Passages, was a major success, which naturally raises the question of what one does next. (Besides writing papers, I mean.)

    So we’re very happy to welcome Lisa aboard to guest blog about her new book, just out today: Knocking on Heaven’s Door: How Physics and Scientific Thinking Illuminate the Universe and the Modern World. (Among other virtues, this book has the single most impressive collection of blurbers of any book ever written, from Bill Clinton to Carlton Cuse.) From personal experience I can verify that writing a book doesn’t just happen; it’s a tremendous commitment over an extended period of time, and once it’s done there’s not much chance to go back and change it. So deciding to write a book at all, and more importantly how exactly to target the writing, is a delicate and critical process.

    While Lisa hasn’t yet become a regular blogger, she is active on Twitter, where you can follow her at @lirarandall.

    —————————————-

    In conjunction with the publication of Knocking on Heaven’s Door, I thought I’d take advantage of Sean’s kind invitation to post on Cosmic Variance to explain my motivations in writing my book. I haven’t done a lot of blogging myself but I am impressed at the care and interest that go into science blogs. They are a way of sharing developments as they happen and an opportunity to have meaningful discussion of results.

    I talk about a lot of science in my book. So I thought rather than summarizing it all—at least in this post—I’d focus on the question of why I wrote this particular book. I waited several years before even considering embarking on a second book project. I certainly didn’t want to simply repeat the content of my previous book, and my own personal goal is always to branch out into new arenas—in this case into new types of writing–while still remaining true to my physics roots. I didn’t know the exact book I was after but I did know some of the topics I considered important and timely.

    These topics fell into several categories. First, I wanted to give an accurate picture of what is happening in particle physics and cosmology today—both with experiments and with theory. Particle physicists know this to be the era of the Large Hadron Collider (LHC), the machine that is colliding together protons at unprecedented energies to test the nature of matter and forces at smaller distances than ever explored. The interactions between theorists and experimenters is more intense than it has been during the time I’ve been actively pursuing physics. That is because everyone realizes this interactions are essential with these challenging experiments to get to the right answers. I wanted to convey the excitement and implications of the research taking place there, so when discoveries are made, anyone interested can understand what was found and what it could mean.

    Cosmologists too find this is an important time and I wanted to share some of the interest in that major topic as well. One arena that both particle physicists and cosmologists are excited about are experimental studies of the nature of dark matter. Many find this topic perplexing, whereas even if difficult to tackle experimentally, the underlying idea really is not. I wanted to explain a bit how I think about dark matter and how experiments are searching for its feeble and elusive effects.

    But I wanted to do more than just summarize the physics. (more…)

  • Biology and Self-Repair

    I’ve been traveling like crazy, then hosting visitors, and now am laid up with a nasty cold. So not much energy for blogging. On the other hand — plenty of time for non-expert reflections on the nature of microscopic complex systems!

    The thing is, I’m pretty sure that my body will eventually overcome this cold virus. That’s one of the great things about living organisms — they can, in a wide variety of circumstances, repair themselves. From fighting off germs to healing broken bones, the body is pretty darn resilient.

    Which brings up something that has always worried me about nanotechnology — the fact that the tiny machines that have been heroically constructed by the scientists working in this field just seem so darn fragile. It’s amazingly impressive what modern nano-engineers can do by way of manipulating matter at the atomic and molecular level, creating new materials and tiny machines and motors. But surely one has to worry about the little buggers breaking down. My macroscopic car is also an impressive feat of engineering, but it’s no good if a crucial component breaks.

    So what you really want is microscopic machinery that is robust enough to repair itself. Fortunately, this problem has already been solved at least once: it’s called “life.” Even at relatively tiny scales, living organisms are sufficiently loose and redundant to be able to fix themselves when something small goes wrong, greatly extending their useful lifespan.

    This is why my utterly underinformed opinion is that the biggest advances will come not from nanotechnology, but from synthetic biology. Once we get to the point that we can truly create new organisms from scratch, not simply modifying existing stock, many of the biggest dreams of nanotech will become much more real.

    Some time ago John von Neumann proposed the idea of self-replicating machines. Not everyone believed that such a thing was possible — after all, the machine would have to include blueprints for another version of itself, including the self-replication mechanism, and how do you fit a copy of a machine into itself? (You might think that living organisms are an obvious counterexample to this argument, but some people used it as an argument against the idea that organisms are “just” machines.) But von Neumann figured it out, and immediately proposed the obvious plan: sending self-replicating spacecraft to seed the galaxy.

    But if the machine breaks, it defeats the whole purpose. So you really want a self-repairing self-replicating machine. Which is awfully close to a working definition of “life.” It might not be human beings who eventually fill up the galaxy, but my suspicion is that it will be life in some form or another.

  • Trusting Experts

    Over on the Google+, Robin Hanson asks a leading question:

    Explain why people shouldn’t try to form their own physics opinions, but instead accept the judgements of expert physicists, but they should try to form their own opinions on economic policy, and not just accept expert opinion there.

    (I suspect the thing he wants me to explain is not something he thinks is actually true.)

    There are two aspects to this question, the hard part and the much-harder part. The hard part is the literal reading, comparing the levels of trust accorded to economists (and presumably also political scientists or sociologists) to the level accorded to physicists (and presumably also chemists or biologists). Why do we — or should we — accept the judgements of natural scientists more readily than those of social scientists?

    Although that’s not an easy question, the basic point is not difficult to figure out: in the public imagination, natural scientists have figured out a lot more reliable and non-obvious things about the world, compared to what non-experts would guess, than social scientists have. The insights of quantum mechanics and relativity are not things that most of us can even think sensibly about without quite a bit of background study. Social scientists, meanwhile, talk about things most people are relatively familiar with. The ratio of “things that have been discovered by this discipline” to “things I could have figured out for myself” just seems much larger in natural science than in social science.

    Then we stir in the matter of consensus. (more…)

  • How to Succeed on the Internet Without Really Trying

    Keen eyes will notice tiny improvements in the look-and-feel of the Discover blogs today, thanks to behind-the-scenes work of our crack website team. One improvement is that the social-media buttons at the bottom of each post are a little more clear and logical. They also let you know how many people have passed along a post via each medium.

    Which leads me to an entirely unoriginal observation: the internet loves Top Ten lists. Perusing our home page, it’s easy to be struck by the giant numbers for the Things Everyone Should Know About Time post. It’s true that I like to think the post was actually interesting. (People seem to be divided between whether #4 or #10 is the most striking entry.)

    But still, I’ll be honest: being at the conference I hadn’t been able to blog much, so I thought it would be good to write something that would be popular but not too hard to write. Thus: a top ten list. Box office!

    So why exactly is that? I’m not disparaging: a good list is a way to convey a substantial amount of information in a well-organized form. But still, would it have been as popular had it been Top Seven? What if each entry were three times as long? What if the exact same words were presented without the numbers and bold-face labels?

    No grand theories here, just idle curiosity. Enjoy the tiny aesthetic upgrade.