State of the Universe(s)

October 19, 2009

Last night I had the honor of moderating a fantastic discussion between three leading cosmologists: ASU's Lawrence Krauss (of The Physics of Star Trek fame), University of Michigan's Katie Freese, and Neil Turok, now the director of the Perimeter Institute, which organized the city-wide Quantum 2 Cosmos festival. The Q2C organizers have gone all out with the multimedia: most lectures and panels are available online, streaming live while in progress, along with a Twitter feed.

We covered a lot of ground in 55 minutes, discussing the unprecedented explosion in our scientific knowledge of the universe during the 20th century and looking to the future by exploring the mysteries currently facing cosmologists: dark matter, dark energy, gravitational waves, whether inflationary theory is correct, and what might have existed before the Big Bang? These are deep waters, Watson. Okay, I had to ask Krauss about the whole "red matter" scenario in the latest Star Trek reboot (he liked the movie, had little use for the "science"). But other than that, we stuck to the serious stuff. Mostly.

I always learn something new when I talk to scientists, and this time was no exception, thanks to Katie Freese. She told me about "dark stars": not the precursors of black holes first hypothesized back in the 1700s, but a new kind of star that may have been the first type of star to form in the early universe. Freese and a few colleagues published the seminal paper on dark stars in January 2008 in Physical Review Letters, and she's still uber-excited about the possibilities for the existence of these objects -- as well she should be.

See, if these things turn out to exist, it would significantly change current theoretical models for star formation. Right now, scientists believe the first stars formed inside clouds of dark matter, in which hydrogen and helium cooled down sufficiently to make nuclear fusion possible. The only role dark matter plays in this scenario is to supply the gravity needed for the gases to clump together in the first place.

But if Freese and her colleagues are correct, then the concentrations of dark matter particles would be so high that those particles would collide with each other and annihilate, releasing energy, and keeping the almost-star too hot to collapse down to sufficiently high density for fusion to begin. In short, it's an entirely different fuel source than that which powers "normal" stars. The next step is actually detecting them, and Freese thinks the new James Webb Telescope slated for launch in 2011 will be able to see them, although she cautions that while dark stars may shine, "they will look different than stars that operate by fusion." Emissions of gamma rays, neutrinos or antimatter could all turn out to be "signatures" of dark stars.

Like any new idea, it has its skeptics in the scientific community. Freese et al's model does rely on some necessary assumptions that may turn out to be incorrect. Most notably, their calculations are based on a type of Weakly Interacting Massive Particle (WIMP) called a neutralino -- it's the leading candidate for dark matter particles, but it may not be the right one, or the only one. But it's not an implausible scenario either. Like so much in cutting edge cosmology and astrophysics, the excitement comes from exploring what we don't know, because that inevitably leads to new discoveries.

Anyway, we ran out of time before we could really discuss dark stars in detail during the panel, but we covered lots of other great topics, and we certainly didn't ignore the "dark side of the universe." That's where all the cosmological action is these days. You can watch the whole thing below. (Note: For some reason, the "embed" feature has been giving me grief. If it's not showing up in the post, you can still watch the entire panel discussion here.)

Chamber of Secrets

April 10, 2009

For those who caught last night's episode of Bones, it was the long-awaited episode (by me) I've taken to calling "Murder by Physics": a leading physicist at the fictional Collar Institute (a kind of inter-disciplinary think tank at the frontiers of science) is murdered and Booth and Brennan have to figure not just who, but how.

I won't spoil things for those planning to watch it later thanks to the wonder of DVR, but my spouse and I had the privilege of brainstorming with the writers on the show last fall about the ways physics might be used to commit murder. Sean even polled a few of his colleagues, one of whom, Juan Collar, had apparently given this a great deal of thought -- purely as an intellectual exercise, you understand. (Sometimes Sean's friends, they worry me.) His idea found its way onto the show, and the fictional institute bears his name in thanks.

Collar's own research would not be out of place at that intitute: it looks both to the past and to the future. He is resurrecting the relatively old technology of bubble chambers to search for dark matter. His project is called the Chicagoland Observatory for Underground Particle Physics (COUPP) experiment, located 350 feet underground in a tunnel on the Fermilab site. Bubbles chambers were nearly extinct in high-energy physics labs before Collar hit upon the notion of using them to search for dark matter. (They're great as neutrino detectors, too.) While the basic technology might be old, Collar insists, "This is not your daddy's bubble chamber."

COUPP's "detector" is a glass jar filled with a liter or so of a fire-extinguishing liquid (iodotrifluoromethane) -- a simple bubble chamber. When a weakly interacting massive particle (WIMP) hits a nucleus of one of those atoms, it triggers an evaporation of a small amount of that liquid, producing a tiny bubble. It's initially too tiny to see, but it grows, and that growth can be recorded with digital cameras.

Once the bubbles reach about one millimeter in size, the COUPP scientists can study the images in earnest, looking for telltale statistical variations between photographs. Ideally, this enables them to distinguish whether a bubble resulted from background radiation, or from a dark matter particle.

Next on the agenda for COUPP is to increase the detector's sensitivity by increasing the amount of liquid from one liter to around 30 liters. Last year Collar also installed a new compact neutrino detector (germanium-based) 330 feet below ground in the sewers of Chicago. (The lab is renting this unusual lab space from the city -- apparently Chi-Town has one of the longest systems of tunnels ever built, in its case, to control flooding.) The design has been modified to detect not neutrinos, but WIMPs.

So for all intents and purposes, bubble chambers are back, baby! Talk about a stunning comeback. Still, Collar emphasizes that "there is no perfect dark matter detector out there." Each approach has its own strengths and weaknesses. Which is why it's highly unlikely that one single experiment will conclusively "demonstrate" the first detection of dark matter; in fact, several are in hot pursuit to be the first to achieve that honor. One day lots of lines of data from all the different experiments around the world will cross -- and that will constitute what a criminal lawyer might call a preponderance of evidence verifying direct detection.

In the meantime, all those other rival teams might do well to show Collar a bit of extra respect, now that we all know how fiendishly clever he can be when it comes to the fine art of murder.

Photo: Juan Collar's bubble chamber for detecting WIMPs. Source: Collar lab, University of Chicago.

Heart of Darkness

March 12, 2009

There's big news this week from NASA's Hubble Space Telescope, which peered into the heart of the Perseus galaxy cluster and found new evidence that galaxies are embedded in halos of dark matter. Specifically, the Hubble images revealed that a large number of small galaxies within the Perseus cluster remained intact, while larger ones around them are ripped apart by the gravitational pull from other nearby galaxies. This implies that the intact galaxies are surrounded by a sort of "cushion" of dark matter that counters the gravitational effects from neighboring galaxies and keeps them from being ripped apart.

Dark matter was first proposed a good 80 years ago, in 1933, by Fritz Zwicky, who found that individual galaxies within the Coma cluster were moving so fast that they should be able to escape the gravity of the visible mass. Since the cluster showed no signs of flying apart, he concluded that there must be more to the galactic mass than meets the eye -- "dark matter" -- binding it together. But Zwicky was ahead of his time, and his findings were met with skepticism.

Then, in 1969, a young woman astronomer named Vera Rubin teamed up with her astronomy colleague, Kent Ford, of the Carnegie Institution, to study the spectrum of light coming from stars in different parts of spiral galaxies in order to calculate the orbital speeds of stars in different parts of those galaxies. The prevailing assumption was that because the core region of a spiral galaxy possessed the highest concentration of visible stars, the gravity would also be concentrated at the core, so the farther a star was from the center, the slower its orbital speed.

But that's not what Rubin and Ford found when they analyzed the data. The stars furthest from the core region of the spiral galaxies were moving just as fast as those closer in -- despite the fact that the visible mass wasn't sufficient to hold such rapidly moving stars in their orbits. "What you see in a spiral galaxy is not what you get," Rubin concluded, estimating that at least 90 percent of the mass in galaxies had to be invisble and unidentified. Then she remembered reading about Fritz Zwicky's findings in 1933, and realized she had discovered strong evidence that Zwicky had been correct in assuming the presence of dark matter. 

Rubin's achievements would be remarkable by any measure, but even more so given the fact that she blazed a trail in astronomy at a time when women were rarely to be found in the sciences -- indeed, they were usually unceremoniously pushed out. Rubin wanted to be an astronomer since the age of 10, when she used to gaze longingly at the night sky from her bedroom in Washington, DC. Her father was skeptical, but helped her build her own telescope anyway.

The educational establishment was less supportive. Despite being a stellar student, when she applied for admission to Swarthmore College as a science major, the admissions officer asked if she had any other interests. When she mentioned she liked to paint, he responded, "Have you ever considered a career in which you paint pictures of astronomical objects?" Painting, you see, was so much more seemly a past-time for a young woman. Even her high school physics teacher, upon being told she'd been accepted to Vassar, told her she'd probably do okay academically, "as long as you stay away from science."

Rubin's experience was pretty standard for women interested in pursuing scientific careers at the time. Things have improved dramatically, but women continue to be under-represented in the hard sciences, and while the discrimination has become more subtle, the negative messaging still comes through loud and clear: if you're a woman, why do science? It's a bit like the dark matter in that respect: it's subtle and not so easy to see, but the effects are clearly palpable. It's only in the last 15 years or so that the percentage of women in physics, for example, even reached the double digits.

 "It takes an enormous amount of self-esteem to listen to things like that and not be demolished," Rubin later recalled of her earlier negative experiences. Vera Rubin found those inner reserves of strength and self-esteem, and astronomy and astrophysics has benefited greatly as a result. Plus, she inspired future generations of women astronomers to follow in her footsteps. That's a scientific life well lived.

Photo: Vera Rubin measuring spectra, circa 1970. Source: Vera Rubin; scanned by the American Institute of Physics.

Stop Making Sense

February 24, 2009

I've been noticing a few flurries around the blogosphere about a new book by Michael Brooks, 13 Things That Don't Make Sense, discussing 13 controversial scientific anomalies that may (or may not) turn out to be revolutionary. The buzz caught my interest because I read and reviewed the book late last year for New Scientist; you can find my official take here.

Brooks has a physics background and is a careful, engaging writer, but his biases show pretty evidently in 13 Things: he's an unapologetic champion of the underdog scientist. That's all very well and good -- who doesn't like to root for the underdog now and then? -- but it's awfully tempting to lazily resurrect the tired old specter of rigid Establishment Science refusing to be open to new ideas, when in fact, that constant questioning and testing and slowness to accept new theories and results is part of the necessary rigor of the scientific process.

Chad Orzel over at Uncertain Principles summed up the problem with Brooks' tome quite neatly when he reviewed the book last month:

In the physics and astronomy sections, for example, the dark matter chapter makes a passing reference to the Bullet Cluster observations, and says a bunch of unkind things about how the authors were arrogantly ignoring modified gravity theories. It barely explains what the Bullet Cluster results were, though, and merely asserts that the whole thing can be explained by modified gravity, without explaining how.

As a fellow science writer, I'm sympathetic to the need to pare down the technical details when writing about esoteric theories for a general audience.  Describing just the basics of modified gravity in sufficient detail would take an entire chapter in and of itself and Brooks didn't have that luxury in a pop-sci book. Even the Wikipedia entry would make your eyes glaze over in seconds flat. (Thanks to XKCD, I am now hip to simple.wikipedia.org, which is teh awesome.)

Now, Brooks' fundamental premise is perfectly sound: if you want to find where the most exciting scientific breakthroughs are likely to occur, a good strategy is to look to the anomalies. And he does try to cover his chosen topics with care, while still keeping the prose accessible. But even I had the sense that the omissions in the book were carefully cherry-picked to make controversial theories/results more probable than current scientific consensus would warrant.

Why is that a problem? Well, the vast majority of the American public weren't exactly glued to their seats in fascination when the Bullet Cluster images first debuted, nor do they have more than a passing understanding of what dark matter is supposed to be. And that means they'll miss many subtle distinctions that would be immediately obvious to even a scientifically literate layperson like me. A recent review in the Times (UK) Online illustrates my point perfectly:

One of the great discoveries of 20th-century science was that our universe is expanding. The discovery, however, led straight to another puzzle. The puzzle is, there's nowhere near enough matter to prevent the expanding universe from blowing apart completely into a vast, sterile infinity of lifeless interstellar dust. So how come we live in a lumpy universe, one of the lumps being the planet on which we live? There must be more matter than we can see: the famous dark matter and, to go with it, something even more mysterious - dark energy.

To date, however, there's not a shred of evidence for either, even though teams of scientists have been looking for years. [emphasis mine]

Um, thanks for playing, Times Online, but that's a gross overstatement of the case. A simple Google search would yield countless articles and blog posts about all the experimental evidence accumulated thus far in support of both dark energy and dark matter. In fact, you can find an excellent in-depth but accessible analysis of the Bullet Cluster observations, and the ramifications for one popular modified gravity theory in particular called MOND, in the archives of Cosmic Variance, authored by my physicist spouse back in August 2006.

Sean has done his share of work on modified gravity; he's hardly the stereotypical closed-minded Establishment Scientist, willingly admitting that "in principle, it's absolutely possible that gravity could be modified, and it's worth taking seriously."  He actually thinks modified gravity would be really cool -- but only if it's born out by observation.

Personally, I would prefer to explain cosmological dynamics using modified gravity instead of dark matter and dark energy, just because it would tell us something qualitatively different about how physics works. ... We would all love to out-Einstein Einstein by coming up with a better theory of gravity. But our job isn’t to express preferences, it’s to suggest hypotheses and then go out and test them.

And as the rest of the post makes clear, one of the best tests scientists have devised to date to probe for evidence of dark matter shows very clearly that the stuff exists. Even then, Sean is far too good a scientist to assume that's the end of the matter:

So is this the long-anticipated (in certain circles) end of MOND? What need do we have for modified gravity if there clearly is dark matter? Truth is, it was already very difficult to explain the dynamics of clusters (as opposed to individual galaxies) in terms of MOND without invoking anything but ordinary matter. Even MOND partisans generally agree that some form of dark matter is necessary to account for cluster dynamics and cosmology. It’s certainly conceivable that we are faced with both modified gravity and dark matter. If the dark matter is sufficiently “warm,” it might fail to accumulate in galaxies, but still be important for clusters. Needless to say, the picture begins to become somewhat baroque and unattractive.

But the point is not whether or not MOND remains interesting; after all, someone else might come up with a different theory of modified gravity tomorrow that can fit both galaxies and clusters. The point is that, independently of any specific model of modified gravity, we now know that there definitely is dark matter out there. It will always be possible that some sort of modification of gravity lurks just below our threshold of detection; but now we have established beyond reasonable doubt that we need a substantial amount of dark matter to explain cosmological dynamics.

I hope the Times Online reviewer is taking notes...

Image: XKCD.

Fun with Jelly Beans

January 14, 2009

One of my favorite visual analogies for the distribution of "stuff" (as my cosmologist spouse likes to call it) in the universe is a big old jar of jelly beans:

Note that most of them are black, with just a few colored jelly beans dotted about. Those scattered bits of colorful beans represent every single bit of visible matter in the universe: every star, every planet, every galaxy, every planet, every person. It adds up to about 4%. Another 26% is dark matter: we can't see it, but we know it's there because we can see the gravitational effects from all that invisible (to us) mass. The rest of the "stuff" -- a whopping 70% -- consists of dark energy, and scientists still know very little about what this mysterious energy might actually be. But again, they figure it's got to be there, because we can observe its effects in the accelerating expansion of the universe.

I like the image above because it drives home the point of just how insignificant we beings of ordinary matter are in the grand cosmic scheme of things. (As Sean likes to say, we are merely the olive in the martini.) Which in turn makes it all the more amazing that we can accomplish such feats as "weighing" the stuff in the cosmos in the first place -- or, more mundanely, a simple mathematical trick like guessing the number of jelly beans in a one-liter jar.

An episode of the popular TV series Monk showed the "defective detective," Adrian Monk, at a local carnival. In between dodging his usual phobias, he enters a contest to guess the number of jelly beans in a jar -- and naturally, he wins. Such a task is a snap for someone with OCD. That said, he figures it out in part because he observed a bunch of empty jelly bean bags near the jar, and made an educated guess.

Even without that, he could probably have come pretty close, using one possible calculation I found detailed on the back of brochure being distributed at the AAS meeting by the Chandra X-Ray Observatory:

1. First, it's useful to know that your average jelly bean is roughly 1 centimeter long and 1.5 centimeters wide (diameter). You also need to know the volume of the jar (1000 cubic centimeters).

2. Second, they are irregularly shaped, so they're not going to rightly packed in the jar; assume that about 80% of the volume will be filled.

3. Per the brochure, "The number of jelly beans is the occupied volume of the container (80% of one liter) divided by the volume of a single jelly bean." To figure out the volume of a single jelly bean, figure on the volume of a cylinder measuring 2 cm long and 1.5 cm in diameter. (Can't remember how to do this? That old geometry textbook will tell you. Go ahead, get it, we'll wait...) That's about 3.5 cubic centimeters.

So, the approximate number of jelly beans in a one-liter jar is (.80 x 1000 cubic centimeters. divided by 3.5 cubic centimeters... or around 229 beans. And if we're using the above jar as the sample case, and it accurately represents the distribution of stuff in the universe, we can then figure out how many of those jelly beans are going to be black (dark): 96% of them.

See? Was that so hard? There are supposedly other possible calculations, but most of us just need one winning strategy. Now we have one, courtesy of NASA. I just need to bide my time until I happen upon a "guess the jelly beans" contest....

Photo: Fermilab.

Cluster Clash

August 28, 2008

Strange things can happen when galaxies collide. You'd expect the atoms and molecules to crash into each other with very high energy, but what if there was other types of matter in those galaxies that we couldn't see? What happens when so-called "dark matter" collides?

Thanks to the combined efforts of scientists with both the Hubble Space Telescope and the Chandra X-Ray Observatory, we have a pretty good idea.

What is this dark matter of which I speak? Our current model for how much "stuff" there is in the universe -- and yes, astrophysicists have figured out how to "weigh" it -- shows that only 4% of it is what one might call regular matter: the stuff that makes up everything we see in the visible universe, from galaxies and stars all the way down to quarks and leptons, and everything in between. A mysterious thing called dark energy accounts for around 73 percent of the "stuff." That leaves 23 percent, which is the dark matter.

Scientists aren't quite sure what it is just yet -- massive astrophysical compact halo objects (MACHOs) or weakly interacting massive particles (WIMPs) are two of the primary contenders -- but gosh darn it, they know it's there! And they know because in 2006, Hubble and Chandra joined forces to produce a stunning image of two galaxy clusters that collided and literally swept all the ordinary matter out of the way so scientists could hone in on the elusive dark matter and see it in all its weakly interacting glory. That image is known as the Bullet Cluster (or 1E 0657-56, if you want to be all formal about it).

Well, what we're actually seeing is indirect evidence of dark matter's existence through its gravitational impact. But it's pretty strong evidence, nonetheless, particularly if the dark matter turns out to be WIMPs.

MACHOs, at least, are technically "normal" matter: black holes, neutron stars, brown dwarfs, and other objects too dark for us to "see" via our usual imaging techniques.

But WIMPS? They would be an entirely new type of matter, one that almost never interacts with regular matter, except through gravity and the weak nuclear force. WIMPs are the monosyllabic wallflowers at parties that hardly anyone notices. Those that do, don't hang around too long to chat because frankly, WIMPs aren't ones for small talk.

That's why the Bullet Cluster image was so exciting: it dispersed the crowds that had been drowning out the dark matter so we could finally see it -- a feat scientists accomplished by being very clever. First, they took Hubble images and used them to map out the gravitational fields of the cluster by measuring how much they were distorted by gravitational lensing. Then they used that information to deduce what kind of mass concentration would have created that sort of gravitational lens.

Then it was Chadra's turn. The telescope took x-ray images which enabled scientists to map out where the hot gas was located -- the hot that comprised most of the regular matter in the cluster. Finally, they superimposed the two images and voila! You can see the distribution clear as day!

Now astrophysicists have done the same thing with a galaxy cluster known as MACS J0025.4-1222. It formed when two very large separate clusters -- each a quadrillion times larger than the mass of our sun -- collided at speeds of millions of miles per hour. Just as with the Bullet Cluster, the hot gas collided and slowed down, and the weakly interacting dark matter passed right through. Once again, overlaying the visible-light images from Hubble (measuring gravitational lensing) with the x-ray data from Chandra shows a very clear separation between normal matter and dark matter.

So, if the Bullet Cluster already provided strong evidence for the existence of dark matter, why are astrophysicists so excited over this new image? Because it proves the Bullet Cluster isn't just a fluke; this is actually how dark matter behaves.

Scientists love them some reproducibility, and now MACS J0025 (for short) provides that for the dark matter. Now they just need to come up with a cool nickname to rival "Bullet Cluster": "amorphous galactic blob" just won't cut it.  I'd suggest something hip, with a beat, so you can dance to it -- something like MAC-Daddy-J.

Photos: (top) Demotivational poster using Bullet Cluster image, created by Sean Carroll (Cosmic Variance). Bullet cluster image: NASA/ESA/CXC. (bottom) Image of MACS J0025.4-1222 (a.k.a., "MAC-Daddy-J"). Source: NASA, ESA, CXC, M. Bradac (University of California, Santa Barbara), and S. Allen (Stanford University).