Miscellany

NASA reports that the Martian rover Opportunity is sitting on an ancient shoreline. The texture of ripples on the rocks indicates that they used to be under water; it must have been at least a couple of inches deep, in order to create the observed patterns. (Did you know that Opportunity has her own blog? Two of them, actually.)

Let’s indulge ourselves in thinking just about the scientific implications, instead of the icky politics. We instantly jump to speculations about life on Mars; the evidence is thin, but the temptation is irresistible.

I am by no means an expert on exobiology in general or Mars in particular, but it’s clear that sorting this out is going to be both complicated and fascinating. We don’t know all that much about the origin of life, to be honest. The famous Miller-Urey experiment showed that amino acids could be spontaneously generated inside a test tube filled with methane, ammonia, hydrogen, and water, if it was continuously zapped with electrical shocks (to simulate lightning). Amino acids are the building blocks of proteins, so this is certainly a step in the right direction.

But these days scientists think that the atmosphere of Earth long ago didn’t actually have the right compounds. Never fear, though; it seems as if the conditions for making amino acids happen naturally in outer space! Comets in particular seem to be thick with organic materials, and meteorites that have fallen to earth turn out to occasionally have actual amino acids in them. You might worry that the delicate organic materials would get destroyed when objects crashed into the Earth, but there’s some experimental evidence that they actually survive intact. In other words, it’s quite plausible that interplanetary chemistry played an important role in the first steps toward the development of life here on Earth.

I bring this up because 1) it’s intrinsically amazing, and 2) it’s going to make it very hard to sort out the life-on-Mars story. We might find all sorts of organic molecules on Mars, not because they developed there by themselves, but because they were brought by comets. We might even find evidence of Earth-like life, again not because it arose by itself, but because it was carried from Earth by our own spacecraft, or perhaps by rocks ejected from volcanoes.

None of this makes the effort to understand the status of life on Mars any less interesting; all of the possibilities are fascinating, for different reasons. But it will be a long time before we can say anything with confidence. Unless there is an entire civilization hiding underneath the Martian soil, waiting for the right moment to spring out and attack. Someone should make a movie about that.

I’m spending a few days in Cambridge, Mass, site of my old haunts from grad school and my first postdoc. One of the few places in the world where running into string theorists on the street (as I did today) is not too surprising.

And I enjoyed a treat I hadn’t had in years: Toscannini’s burnt caramel ice cream. I’ve always loved ice cream, but this is the flavor that made me a fanatic. After moving to California for my second postdoc, and making a desultory appraisal of the ice-cream situation, I was moved to buy my own ice-cream maker and churn out the burnt caramel myself. Some good comes out of every hardship.

Cambridge has an absolutely unique charm, although it’s not for everybody. I love the brick sidewalks, the scattered cafes and bookstores, the predominant scholarly aesthetic. I’ve only bought one book so far, but there are a couple of days left.

In the last post we talked about inflationary cosmology, the density perturbations it predicts, and how these perturbations show up in the cosmic microwave background (not to mention in the large-scale structure of galaxies today).

If this story is true, properties of the universe during the inflationary era are reflected (albeit faintly) in properties of the perturbations. For example, the overall amount of perturbation A (which is one part in 100,000) is related to the energy density of the universe during inflation; unfortunately it’s also related to another number, the “slow-roll parameter” ε (epsilon), which describes how the rate of inflation gradually slows with time. (Have to be a bit technical here, sorry.) Roughly, the energy scale E_I of inflation (the number which, when raised to the fourth power, is the energy density during inflation) obeys

E_I = A^1/2 ε^1/4 E_P.

Here, E_P is the Planck scale, the magical energy at which quantum gravity is supposed to become important: E_P=10¹⁸ billion electron volts. (A billion electron volts is one GeV, “G” for “giga-“. The rest energy in a single proton, via E=mc², is about 1 GeV.) For comparison, the highest energy yet probed by particle accelerators here on Earth is about a trillion electron volts, smaller than the Planck scale by 10^-15. Accessible experiments are not going to tell us anything directly about energies as high as the Planck scale; cosmology might be our best hope for learning something empirical about quantum gravity. For more details about inflation and perturbations you can look at a technical introduction to inflation by Andrew Liddle.

Let’s plug in numbers. The perturbation amplitude A is 10^-5. The slow-roll parameter ε is supposed to be small, perhaps 10^-2; but we’re taking the fourth root, so we’ll end up with something of order unity. This means that the energy scale of inflation E_I appears to be of order 10¹⁵-10¹⁶ billion electron volts. This is intriguing, since this scale is right where we expect to have grand unification — the coming-together of the three major forces in the universe other than gravity (electromagnetism, the strong nuclear force, and the weak nuclear force).

Could this be a coincidence? Sure. We certainly haven’t been very precise, to say the least. But an optimist would see hints of a consistent picture forming, in which the physics of grand unification is somehow behind the phenomenon of inflation. It’s by no means a complete theory, but an encouraging tidbit that is worth pursuing. Theorists will look for specific models of inflation, while experimentalists will look for new ways to test its predictions. Unfortunately, there aren’t that many predictions. One is that the overall geometry of space is very close to flat; this has been spectacularly confirmed by observations of the microwave background and elsewhere. Another is that the fluctuations in density should be accompanied by independent fluctuations of the gravitational field (gravitational waves), which leave a distinctive signature on the polarization of the microwave background. Looking for such a signal is a big goal of cosmologists right now.

Meanwhile, the fact that the energy scale of inflation seems to be tantalizingly close to the Planck scale leads people to wonder whether we can’t see explicit effects of quantum gravity in the CMB. It’s hard to give a definitive answer to this question, just because we don’t really know what the explicit effects might be. They might, for example, cause the perturbations to deviate from perfectly uniform behavior on all scales, perhaps by imprinting a tiny oscillating variation. But right now there’s little consensus about this quantum gravity/inflation connection; we have a ways to go before making it into something concrete.

Peter Woit has some questions for cosmologists. Too many good ones to answer in a brief comment, or even a single post, but let’s tackle the first big one, which is fun to talk about in its own right.

The general question is “What can observations of the cosmic microwave background tell us about physics at very high energies?” Since there’s no reason why non-experts shouldn’t follow the discussion, I should explain briefly what the microwave background is and why “high energies” are interesting. It will take some time, but it’s not useless for me, since I’m supposed to be writing some public-education web pages for the Kavli Institute for Cosmological Physics (formerly “Center for Cosmological Physics”) here at Chicago, and I’ll use this as a rough draft.

The cosmic microwave background (CMB) is the leftover radiation from the Big Bang. When the universe was much smaller it was much hotter, emitting blackbody radiation just like anything else, and we can detect that radiation today. The very early universe was so hot that it was opaque, since the electrons were ripped off of individual atoms and photons kept bumping into them. At 370,000 years after the Big Bang, the temperature had dipped below about 3,500 Kelvin, cool enough for electrons to recombine with nuclei to make atoms, and the universe suddenly became transparent. The radiation subsequently cooled to about 2.7 Kelvin, which is what we see today; it provides a snapshot of what the universe looked like when it was 370,000 years old (it’s 13.7 billion years old right now, or about 100 billion dog years).

What it looked like was something extremely smooth; fluctuations in density from place to place were only about one part in 100,000. But we can detect these fluctuations; the image reproduced here is the famous map from the Wilkinson Microwave Anisotropy Probe satellite. Blue regions are slightly colder than average, red regions are slightly hotter. (The WMAP team actually knows physics much better than this color scheme would lead you to believe.)

There is a treasure trove of information contained in these fluctuations. In particular, statistical properties of the fluctuations depend on two things: the primordial perturbations from which they presumably arose, and the recipe of ingredients in our universe that controls the subsequent evolution of the perturbations between early times and now. Remarkably, an extremely simple specification of primordial perturbations works very well — simply imagining that the perturbations are (on average) of equal strength at all distance scales. From this guess, and the observed fluctuations in the CMB sky, we can derive very tight constraints on interesting cosmological parameters, such as the amount of ordinary matter and dark matter in the universe. See Wayne Hu’s tutorial for details.

But the simple guess for the form of the primordial perturbations is actually better than a guess — it’s a prediction of the inflationary universe scenario. Inflation is the idea that the extremely early universe underwent a period of accelerated expansion that stretched a tiny portion of space to the size of our entire observable universe. It was originally invented by Alan Guth and others to help explain precisely why the universe looks so smooth on large scales. See this intro by Ned Wright for more details.

Inflation came with an unexpected bonus. Try as it might, inflation can’t make the universe perfectly smooth, simply due to the strictures of quantum mechanics. Heisenberg’s uncertainty principle tells us that we can’t specify the state of a system with perfect precision; there is always an irreducible jiggliness when we look at, for example, the position of an electron. But the principle holds as true for the entire universe as it does for an electron. So inflation makes the universe as smooth as it can (imagine removing the wrinkles form a sheet by stretching it at the edges), but there is some amount of fluctuation left over — which, of course, precisely describes the universe we see. All of the stars, galaxies, and large-scale structure in our universe may have started as tiny quantum fluctuations in the primordial soup.

Okay, this has gone on a while already. Next time I’ll be more quantitative about the perturbations, and talk about how they might reveal something about physics at very high energies.

From Tbogg, a link to an article at Axis of Logic on the status of 9/11 as a political football, and in particular who is to “blame” for the incredible intelligence failure. This is a topic where it’s hard to find level-headed discussion, but this article is sufficiently well-documented as to be quite persuasive. Not to give you the wrong impression — it’s completely partisan, but defensibly so.

Astronauts who have risked their lives to explore space are joining the chorus to save Hubble. Let’s keep it up, hopeless though it may seem at the moment.

Peter Woit has a new blog. He’s a mathematician who argues that string theory (our attempt to derive a consistent quantum theory of gravity from a theory of extended objects, rather than ordinary point particles) is bad news for physics. I don’t agree, but it’s worth listening. I think string theory is fantastically promising and quite remarkable, but plenty of scientists (including particle physicists and gravitational physicists) disagree, which is an interesting state of affairs. Some time in the future our current generation of string theorists will either be viewed as visionary pioneers soldiering forward despite overwhelming odds, or misguided crazies who derailed progress in physics with their hopeless detachment from experiment. Maybe we’ll live to see which one.

Out here in the desert today we were worrying about dark energy. Seventy percent of the energy in the universe is some mysterious stuff whose density, as far as we can tell, remains constant as the universe expands. (In contrast to, say, ordinary matter, which dilutes away as a result of the expansion.)

The leading candidate for dark energy is vacuum energy, or the cosmological constant, which is exactly constant throughout space and time. But it’s not an especially attractive candidate, so we’re looking for alternatives anywhere we can — some persistent but nevertheless dynamical field, or even a modification of gravity on large scales.

To tell if the dark energy is vacuum energy, we try to see whether it’s changing or absolutely constant. The parameter we go out and measure is w; its value is -1 for pure vacuum energy. We know that w is pretty close to -1; if it’s a little bit greater (like -0.9) the dark energy is gradually diminishing, while if it’s less than -1 the density is actually increasing. (The amount of dark energy per cubic centimeter is going up everywhere in the universe.) That seems crazy, which was what my talk this morning was about. It is crazy, but it can’t be completely ruled out, so we should keep an open mind.

The first direct evidence for dark energy came from using supernovae as standard candles (objects whose intrinsic brightness is known, so their distance can be inferred from their aparent brightness). Many of the talks today were devoted to current and future supernova searches. There was much discussion between the observers, who wanted to know just what kind of deviation from w=-1 we should expect, and theorists (like myself) who kept admitting that we have no idea. I don’t think they believed us. Unfortunately it’s a question we have to keep asking, since it costs money to do these observations; money is tight, and we have to decide which experiments are most deserving of our efforts.

The enigmatic title isn’t anything I can explain right now; it is the title of the talk I’m supposed to give at the conference in Tuscon at which I just arrived. A collection of cosmologists at a golf resort in the desert; who thinks of these things? The talk is tomorrow morning, and it’s not written yet, so no substantive posting today.

Instead, read Michael Bérubé’s ideas about conferences. Spit-takes and electric shocks are involved, so you won’t be wasting your time.

Tomorrow (Weds) I’ll be a guest on Odyssey, a syndicated program from WBEZ, Chicago’s public radio station. We’re supposed to be talking about the “early universe”; of course, what’s early to one person may be pretty late to another. I think we’ll be covering a lot of ground, from 10^(-35) seconds (inflation) through one minute (nucleosynthesis) and 400,000 years (the cosmic microwave background) up to 500 million years (the earliest galaxies). The other guest will be Bob Kirshner from Harvard, a supernova expert and one of the co-discoverers of dark energy. Bob is an engaging speaker and a great scientist; I reviewed his popular-level book for Nature, and we managed to remain friends.

If your local public radio station doesn’t get Odyssey, you can easily listen on the web. (I’ve been on a few times before, the shows are available in the archive.) But even better, you should call your local station and demand they get the program. It’s an hour-long discussion, typically with two or three guests, about every sort of topic you can think of, with a decided emphasis on high-level (but accessible) intelligent discourse. The host, the glamorous and charming Gretchen Helfrich, does an amazing job of keeping the dialogue lucid and amusing no matter what the topic is.

[Update: here’s the audio. The metaphor of the moment was that of a movie in which the first reel is mostly missing except for a few frames. Personally, given that our universe is pretty clever, but prone to violence and self-indulgence, I’m thinking it’s a Tarantino film.]

Grading papers right now for our Moments in Atheism class. I figured that I would learn a lot from reading these (at least the good ones), and it’s true. One of the students (Nicholas Boterf) found a wonderful quote in Juliette, a 1797 novel by the Marquis de Sade. Juliette is somewhat scandalized by the implications of what her “tutor,” Madame Delbene, has been telling her:

“But … if there be neither God nor religion, what is it runs the universe?”

“My dear,” Madame Delbene replied, “the universe runs itself, and the eternal laws inherent in Nature suffice, without any first cause or prime mover, to produce all that is and all that we know.”

I tend not to go along with some of the Marquis’ ethical deductions from the godlessness of the world, but he got it exactly right with that quote.

For me, one of the rewards from looking at the history of these ideas was a better understanding of the change in perspective between Aristotelian mechanics and Galileo/Newton. Only now does it make sense why anyone would think the “first cause” arguments (Aristotle, Aquinas, etc.) held any weight at all. Aristotle thought that, to keep an object moving in a straight line, you had to keep pushing it. This seems silly to us post-Newtonians, but in fact it’s pretty straightforward. Take a chair sitting on the floor and give it a push — once you stop pushing, it will stop moving. “Aha,” you say, “but that’s only because of friction. If we ignore the friction, objects continue to move in straight lines unless forces act upon them.” True, but highly non-intuitive. Why should we ignore friction, when it is ubiquitous in the real world abound us? Aristotle wasn’t making a mistake, he was accurately describing the world he saw. If we take his description seriously, it’s not so crazy to argue all the way to God. Lots of things in the world are moving, and moving objects require something to keep them moving, and ultimately that thing will be God.

Galileo’s insight — that the way to describe dynamics is to ignore friction and air resistance, find a simple model for the resulting motions, and then re-introduce friction afterwards — was one of the most important moments in the history of science, and indirectly of religion as well. After he and Newton figured out conservation of momentum and the laws of motion, the Aristotle/Aquinas line of argument suddenly makes no sense. We don’t need a “cause” or “mover” to explain why things are moving; that’s the natural thing for them to do. This Newtonian revolution was, at a purely intellectual level, just as important as the Darwinian revolution for taking the philosophical wind out of religion’s sails. After Newton, the primary justification for God shifted from cosmological arguments about first causes, to design arguments. Then Darwin made those seem silly (although Hume had done a pretty convincing job years before).

Have we discovered a new planet in the outer regions of the solar system? NASA seems to think so. Two orbiting telescopes — the Hubble Space Telescope, about which we’ve been ranting previously, and the Spitzer Space Telescope, a relatively new infrared observatory named after my Ph.D. advisor’s Ph.D. advisor — have both seen evidence for a relatively large new object. It might be as big as Pluto. Deciding that the Roman pantheon has become politically incorrect, the new object has been named “Sedna,” after the Inuit goddess of the ocean.

This is fun, but not an earth-shaking (as it were) discovery, to be honest. There’s likely to be all sorts of medium-sized rocky objects lurking in the far-flung regions of the Sun’s orbit. And the debate about whether Pluto is really a planet was boring and silly. Probably there is an advanced civilization floating deep in the atmosphere of Jupiter, that spends coffee breaks arguing whether Mercury, Venus, Earth and Mars should be classified as planets. (Okay, not “probably.”) But the relentless series of new discoveries has to make the Hubble-killers uncomfortable.