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Cosmic Variance

Dark Matter is Just Messing With Us Now

by Sean

The state of play in dark matter searches just refuses to settle down. Just a few weeks ago, the XENON100 experiment released the best-yet limits on WIMP dark matter (a two-dimensional parameter space, “mass of the dark matter particle” and “cross section with ordinary matter”). These limits seemed to firmly exclude the hints of a signal that had been trickling in from other experiments. But… the story isn’t over yet.

Remember that XENON, like CDMS and other experiments, tries to find dark matter by making a very quiet experiment and picking out individual events where a dark matter particle bumps into a nucleus inside the detector. There is a complementary strategy, looking for annual modulations in the dark matter signal: rather than being very picky about what event is and is not a DM interaction, just take lots of events and look for tiny changes in the rate as the Earth moves around the Sun. Dark matter is like an atmosphere through which we are moving; when we’re moving into a headwind, the rate of interactions should be slightly higher than when our relative speed through the ambient dark matter is smaller. The DAMA experiment was designed to look for such a modulation, and it certainly sees one. The problem is that lots of things modulate on a one-year timescale; as Juan Collar explained in a guest post here, there were many questions about whether what DAMA is detecting is really dark matter.

Now one of Juan’s own experiments, CoGeNT, has seen (very tentative) hints of an annual modulation itself! CoGeNT had already teased us with a hint of a dark matter signal, which (like DAMA) seemed to imply lower masses (about 10 GeV, where 1 GeV is the mass of a proton) rather than the usual masses for weakly-interacting dark matter favored by theorists (hundreds of GeV). But the competitor experiment CDMS, and later of course XENON, seemed to put the kabosh on those claims. The CDMS result was especially hurtful to CoGeNT’s claims, as both experiments use germanium as their detector material. Theorists are very clever at inventing models in which dark matter interacts with one substance but not some other substance (see e.g.), but it’s harder to invent models where dark matter interacts with one substance in one experiment but not the same substance in some other experiment.

Yesterday Juan Collar gave a talk at the April Meeting of the APS, where he revealed something about CoGeNT’s latest findings. (I don’t think there’s a paper yet, but it’s supposed to come very soon, and they are promising to share their data with anyone who asks.) Now, unlike for their earlier results, they are explicitly looking for annual modulation. And … they see it. Maybe. Well, not really enough to take it seriously, but enough to be intrigued. Or, in science-speak: it’s a 2.8 sigma result. It doesn’t seem to have hit the news very hard, but there are writeups by Valerie Jamieson and David Harris. The CoGeNT folks have 442 days of data, with a rate of about three events per day.

Ordinarily, a tasteful physicist would claim that a 2.8 sigma result doesn’t even rise to the level of “intriguing”; you need three sigma to count as “evidence,” and five sigma for “discovery,” by the accepted standards of the field. The reason this is even blogworthy (a low bar indeed) is that it’s the first attempt to check DAMA by looking for an annual modulation signal, and the result matches the phase of DAMA’s oscillation, and is claimed to be consistent with its amplitude (the experiments use different materials, so it’s hard to do a direct comparison). Also, of course, because the team was looking to bury DAMA, not to praise it: “We tried like everyone else to shut down DAMA, but what happened was slightly different.” On the other hand, what you would need to explain this purported signal is at first glance still very much incompatible with XENON’s limits.

In the end: probably still nothing to get too excited about. But at least it will keep the pot boiling a while longer. Don’t fear; the experiments are getting better and better, and temporary confusions eventually evaporate. Or are swept away by the dark matter wind.
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May 3rd, 2011 10:49 AM
in Science, Top Posts | 22 Comments | RSS feed | Trackback >
The World Changes, We Stay Largely the Same

by Sean

One thing is pretty much guaranteed, in the wake of a big-time news event: people are going to make it about themselves.

When Osama bin Laden is killed in a raid in Pakistan, politically-inclined folks in the U.S. are immediately going to wonder how this impacts the 2012 elections. Obama supporters are going to celebrate a bit more readily than they would have if the same thing had happened when George W. Bush was in office. Obama’s opponents are going to be a bit more skeptical, likewise. (From Free Republic: “We got him in spite of Obama, he’s more interested in getting our military Homosexualized than he is about any war on terror.”) Or they will use the opportunity to make some sort of political statement amidst the crowd outside the White House.

People from NYC and DC and elsewhere who lost friends and family on 9/11 might attain a bit of closure. Pakistanis will both worry about and celebrate how the operation went down. In China, some will mourn the loss of a strong anti-American presence, while others will lump bin Laden in with their own Politburo as forces of evil in the world. People who think about social media will focus on the way the news bypassed traditional channels. Wolf Blitzer will make sure a national TV audience understands that this was big enough news to drag him from home into the studio.

All that is okay. When news hits, we don’t immediately leap from receiving new information to having a fully developed and highly nuanced set of reactions. If people naturally interact with the news in terms of their pre-existing feelings and interests, let them. Some people are going to celebrate the death of a terrorist, while others will recoil at celebrating the death of anybody. It should be fine either way; let people have their moments.

I have no idea what the ramifications of the raid on bin Laden’s compound are going to be for international relations. Generally I lean toward the side that we focused on one guy because it’s useful to personalize the enemy in wartime, not because bin Laden himself was the real problem. But what do I know? It could be that he served a crucial symbolic or even operational role, and that this will really diminish the scope of al-Qaeda terrorism. Or maybe it will serve as a rallying cry, and things will get worse. I suspect that going through security at airports is going to be even more intrusive than usual for the next few months.

The social-media cognoscenti certainly do have something to talk about. In the soon-to-be-immortal words of Bill the Lizard, “I heard about 9/11 on the radio, bin Laden’s death on Twitter.” Me too. We did actually turn on the TV when it became clear that big news was coming. What a contrast; the internet was interesting and lively, while the TV pundits swerved between ponderous and clueless.

And, naturally, the attack itself was live-tweeted. Read the rest of this entry »
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May 2nd, 2011 12:07 PM
in Humanity, Top Posts, World | 22 Comments | RSS feed | Trackback >
Because April is Poetry Month

by Julianne

“Quick Black Hole Spin“, by Edward Sanders

I don’t like it—

two massive Black Holes
each twirling at the core of
two merging galaxies

get close enough
to fuse together

then quick as a wink
just as they are melting into a New Black Hole Blob

they undergo something called a "spin-flip"

they change the axes of their spins
and the fused-together Black Hole Blob
gets its own

quick as a cricket’s foot

Don’t like it at all

And then the new Black Hole Blob sometimes
bounces back and forth inside
its mergèd Galaxy

till it settles at the center

but sometimes a "newly" up-sized Black Hole

leaves its Galaxy
to sail out munchingly on its own
into the Universal It

I don’t like it

Nothing about it
in the Bhagavad Gita
the Book of Revelation
Shakespeare, Sappho, or Allen Ginsberg

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April 29th, 2011 10:09 AM
in Arts, Words | 10 Comments | RSS feed | Trackback >
Dark Matters

by Sean

Jorge Cham, creator of the celebrated PhD Comics, sits down to talk with Daniel Whiteson and Jonathan Feng about dark matter (and visible matter!). But rather than a dry and boring video of the encounter, he cleverly illustrates the whole conversation.

Dark Matters from PHD Comics on Vimeo.

I think it’s an exaggeration to say we have “no idea” about dark energy — physicists like to say this to impress upon people how weird DE is, but it gives the wrong impression because we actually do know something about it. But not much!
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April 28th, 2011 10:46 AM
in Humor, Science | 20 Comments | RSS feed | Trackback >
Does Time Run Faster When You’re Terrified?

by Sean

Neuroscientists have all the fun. When we physicists think about the fundamental nature of time, it largely involves standing hopefully in front of a blackboard and writing the occasional equation, or at best sending clocks on strange journeys. All in the service of very good ideas, of course. But when I give talks about these wonderful ideas, I learn that what people care more about are down-to-earth questions about aging and memory. So not only do neuroscientists get to tackle those questions directly, but they do so by dropping people from tall buildings. How cool is that?

David Eagleman is an interesting guy, as a recent New Yorker profile reveals. Mild-mannered neuroscientist by day, in his spare time he manages to write fiction as well as iPad-based superbooks. But his research focuses on how the mind works, in particular how we perceive time.

I’ve written previously about how, as far as the brain is concerned, remembering the past is like imagining the future. Eagleman studies a different neurological feature of time: how we perceive it passing under a variety of different conditions. You might be familiar with the feeling that “time slows down” when you are frightened or in some extreme environment. The problem is, how to test this hypothesis? It’s hard to come up with experimental protocols that frighten the crap out of human subjects while remaining consistent with all sorts of bothersome regulations.

So Eagleman and collaborators did the obvious thing: they tied subjects very carefully into harnesses, and threw them from a very tall platform. The non-obvious thing is that they invented a gizmo that flashed numbers as they fell, so that they could determine whether the brain really did speed up (perceiving a larger number of subjective moments per objective second) during this period of fear.

Answer: no, not really. There is a perceptual effect that kicks in after the event, giving the subject the impression that time moved more slowly; but in fact they didn’t perceive any more moments than a non-terrified person would have. Still, incredibly interesting results; for example, when you’re afraid, the brain lays down memories differently than when you’re in a normal state.

Obviously, of course, these findings need to be replicated. If you’ll excuse me, I’m off to find some grad students and a tall building.
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April 26th, 2011 11:10 AM
in Science, Time, Top Posts | 26 Comments | RSS feed | Trackback >
Hell

by Sean

Is Gandhi in hell? It’s a question that should puncture religious chauvinism and unsettle fundamentalists of every stripe. But there’s a question that should be asked in turn: Is Tony Soprano really in heaven?

A couple of rhetorical questions posed by Ross Douthat, who does us all the favor of reminding us how certain ideas that would otherwise be too ugly and despicable to be shared among polite society become perfectly respectable under the rubric of religion. (Via Steve Mirsky on the twitters.) In this case, the idea is: certain people are just bad, and the appropriate response is to subject them to torment for all time, without hope of reprieve. Now that’s the kind of morality I want my society to be based on.

The quote is extremely telling. Note that the first question is never actually answered — is Gandhi in hell? And there’s a good reason it’s never answered, because the answer would probably be “yes.” Hell is an imaginary place invented by people who think that eternal torture for people they disapprove of would be a good idea. And it’s the rare religion that says “we approve of all good people, whether or not they share our religious beliefs.” Much more commonly, Hell is brought up to scare people away from deviating from a particular religious path. Here’s the Catechism of the Catholic Church:

Jesus often speaks of “Gehenna” of “the unquenchable fire” reserved for those who to the end of their lives refuse to believe and be converted, where both soul and body can be lost. Jesus solemnly proclaims that he “will send his angels, and they will gather . . . all evil doers, and throw them into the furnace of fire”, and that he will pronounce the condemnation: “Depart from me, you cursed, into the eternal fire!”

Do you think that, at the end of his life, Gandhi decided to believe in Jesus and converted?

The second question is equally telling, because even Douthat can’t bring himself to use a non-fictional person as an example of someone who deserves Hell. He’s trying to make the point that “we are defined by the decisions we make,” and if there is no way to make bad decisions then making good decisions is devalued. Which is a fine point to make, and many atheists would be happy to agree. The difference is that we don’t think that people who make bad decisions deserve to be tortured for all of eternity.

This enthusiastic stumping for the reality of Hell betrays not only a shriveled sense of human decency and a repulsive interest in pain inflicted on others, but a deplorable lack of imagination. People have a hard time taking eternity seriously. I don’t know of any theological descriptions of Hell that involve some version of parole hearings at regular intervals. The usual assumption is that it’s an eternal sentence. For all the pious musings about the centrality of human choice, few of Hell’s advocates allow for some version of that choice to persist after death. Seventy years or so on Earth, with unclear instructions and bad advice; infinity years in Hell for making the wrong decisions.

Hell isn’t an essential ingredient in humanity’s freedom of agency; it’s a horrible of invention by despicable people who can’t rise above their own petty bloody-mindedness. The thought of condemning millions of people to an eternity of torment makes Ross Douthat feel good about himself and gives him a chance to indulge in some saucy contrarianism. I tend to take issue with religion on the grounds that it’s factually wrong, not morally reprehensible; but if you want evidence for the latter, here you go.
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April 25th, 2011 8:29 AM
in Religion, Top Posts | 135 Comments | RSS feed | Trackback >
Avignon Day 4: Dark Matter

by Sean

Yesterday’s talks were devoted to the idea of dark matter, which as you know is the hottest topic in cosmology these days, both theoretically and experimentally.

Eric Armengaud and Lars Bergstrom gave updates on the state of direct searches and indirect searches for dark matter, respectively. John March-Russell gave a theory talk about possible connections between dark matter and the baryon asymmetry. The density of dark matter and ordinary matter in the universe is the same, to within an order of magnitude, even though we usually think of them as arising from completely different mechanisms. That’s a coincidence that bugs some people, and the last couple of years have seen a boomlet of papers proposing models in which the two phenomena are actually connected. Tracy Slatyer gave an update on proposals for a new dark force coupled to dark matter, which could give rise to interesting signatures in both direct and indirect detection experiments.

This is science at its most intense. A big, looming mystery, a bounty of clever theoretical ideas, not nearly enough data to pinpoint the correct answer, but more than enough data to exclude or tightly constrain most of the ideas you might have. It wouldn’t be at all surprising if we finally discover the dark matter in the next few years; unfortunately, it wouldn’t really be surprising if it eluded detection for a very long time. If we knew the answers ahead of time, it wouldn’t be science (or nearly as much fun).

Today is our last day in Avignon, devoted to cosmic acceleration. My own talk later today is on “White and Dark Smokes in Cosmology.” (The title wasn’t my idea, but I couldn’t have done better, given the context.) It’s the last talk of the conference, so I’ll try to take a big-picture perspective and not sweat the technical details, but (following tradition) I will admit that it’s an excuse to talk about my own recent papers and ideas I think are interesting but haven’t written papers about. At least it should be short, which I understand is the primary criterion for a successful talk of this type.

Also, few people have strong feelings about non-gaussianities or neutrinos, but many people have strong feelings about reductionism. Quelle surprise!
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April 22nd, 2011 12:40 AM
in Science, Travel | 14 Comments | RSS feed | Trackback >
Avignon Day 3: Reductionism

by Sean

Every academic who attends conferences knows that the best parts are not the formal presentations, but the informal interactions in between. Roughly speaking, the perfect conference would consist of about 10% talks and 90% coffee breaks; an explanation for why the ratio is reversed for almost every real conference is left as an exercise for the reader.

Yesterday’s talks here in Avignon constituted a great overview of issues in cosmological structure formation. But my favorite part was the conversation at our table at the conference banquet, fueled by a pretty darn good Côtes du Rhône. After a long day of hardcore data-driven science, our attention wandered to deep issues about fundamental physics: is the entire history of the universe determined by the exact physical state at any one moment in time?

The answer, by the way, is “yes.” At least I think so. This certainly would be the case is classical Newtonian physics, and it’s also the case in the many-worlds interpretation of quantum mechanics, which is how we got onto the topic. In MWI, the entirety of dynamics is encapsulated in the Schrodinger equation, a first-order differential equation that uniquely determines the quantum state in the past and future from the state at the present time. If you believe that wave functions really collapse, determinism is obviously lost; prediction is necessarily probabilistic, and retrodiction is effectively impossible.

But there was a contingent of physicists at our table who were willing to believe in MWI, but nevertheless didn’t believe that the laws of microscopic quantum mechanics were sufficient to describe the evolution of the universe. They were taking an anti-reductionist line: complex systems like people and proteins and planets couldn’t be described simply by the Standard Model of particle physics applied to a large number of particles, but instead called for some sort of autonomous description appropriate at macroscopic scales.

No one denies that in practice we can never describe human beings as collections of electrons, protons, and neutrons obeying the Schrodinger equation. But many of us think that this is clearly an issue of practice vs. principle; the ability of our finite minds to collect the relevant data and solve the relevant equations shouldn’t be taken as evidence that the universe isn’t fully capable of doing so.

Yet, that is what they were arguing — that there was no useful sense in which something as complicated as a person could, even in principle, be described as a collection of elementary particles obeying the laws of microscopic physics. This is an extremely dramatic ontological claim, and I have almost no doubt whatsoever that it’s incorrect — but I have to admit that I can’t put my objections into a compact and persuasive form. I’m trying to rise above responding with a blank stare and “you can’t be serious.”

So, that’s a shortcoming on my part, and I need to clean up my act. Why shouldn’t we expect truly new laws of behavior at different scales? (Note: not just that we can’t derive the higher-level laws from the lower-level ones, but that the higher-level laws aren’t even necessarily consistent with the lower-level ones.) My best argument is simply that: (1) that’s an incredibly complicated and inelegant way to run a universe, and (2) there’s absolutely no evidence for it. (Either argument separately wouldn’t be that persuasive, but together they carry some weight.) Of course it’s difficult to describe people using Schrodinger’s equation, but that’s not evidence that our behavior is actually incompatible with a reductionist description. To believe otherwise you have to believe that somewhere along the progression from particles to atoms to molecules to proteins to cells to organisms, physical systems begin to violate the microscopic laws of physics. At what point is that supposed to happen? And what evidence is there supposed to be?

But I don’t think my incredulity will suffice to sway the opinion of anyone who is otherwise inclined, so I have to polish up the justification for my side of the argument. My banquet table was full of particle physicists and cosmologists — pretty much the most sympathetic audience for reductionism one can possibly imagine. If I can’t convince them, there’s not much hope for the rest of the world.
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April 21st, 2011 3:40 AM
in Science, Travel | 91 Comments | RSS feed | Trackback >
Avignon Day 2: Cosmological Neutrinos

by Sean

By this point in my life, when I attend a large-ish conference like this one the chances are good that I’m older than the average participant. Certainly true here. It’s a great chance to hear energetic young people tackling the hard problems, and I certainly have the feeling that the field is in very good hands. It’s also a good reminder that we old people need to resist the temptation to fall into a rut, churning out tiny variations on the research we’ve been doing for years now. It’s easy to get left behind!

Still, it’s also nice to hear a talk on a perennial topic, especially when you hear something you didn’t know. Yvonne Wong gave a very nice talk on “hot relics” — particles that were moving close to the speed of light in the early universe. (They may have slowed down by now, or maybe not.) Neutrinos, of course, are the classic example here; they are known to exist, and were certainly relativistic at early times. If the neutrinos have masses of order 10 electron volts, they would contribute enough density to be the dark matter. But that doesn’t quite work in the real world; “hot dark matter” tends to wipe out structure on small scales, in a way that is dramatically incompatible with the world we actually observe. Also, ground-based measurements point to neutrino masses less than 0.1 electron volt — not for sure, since what we directly measure are the differences in mass between different kinds of neutrinos, rather than the masses themselves, but that seems to be the most comfortable possibility.

Of course, we know about three kinds of neutrinos (associated with electrons, muons, and taus), but there could be more. So it’s fun to use cosmology to see if we can constrain that possibility. An extra neutrino species, even if it were very light, would slightly affect the expansion rate of the early universe, which works to damp structure on small scales. This is something you can look for in the cosmic microwave background, and the WMAP team has diligently been doing so. Interestingly — the best fit is for four neutrinos, not for three! Here’s a plot from Komatsu et al.’s analysis of the WMAP seven-year data, showing the likelihood as a function of the effective number of neutrino species. (“Effective” because a massive neutrino counts a little less than a massless one.)

Now, maybe this isn’t worth getting too excited about. There’s a nice discussion of this possibility in a recent paper by Zhen Hou, Ryan Keisler, Lloyd Knox, Marius Millea, and Christian Reichardt. I’m not sure how a new neutrino could affect the CMB in this way without being ruled out by primordial nucleosynthesis, but I haven’t looked at it carefully. Regardless, it’s best not to just trust any one measurement, but do every measurement we can think of and make sure they are consistent. Certainly something worth keeping an eye on as CMB measurements improve.
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April 20th, 2011 1:00 AM
in Science, Travel | 7 Comments | RSS feed | Trackback >
Avignon Day 1: Calculating Non-Gaussianities

by Sean

Greetings from Avignon, where I’m attending a conference on “Progress on Old and New Themes” in cosmology. (Name chosen to create a clever acronym.) We’re gathering every day at the Popes’ Palace, or at least what was the Pope’s palace back in the days of the Babylonian Captivity.

This is one of those dawn-to-dusk conferences with no time off, so there won’t be much blogging. But if possible I’ll write in to report briefly on just one interesting idea that was discussed each day.

On the first day (yesterday, by now), my favorite talk was by Leonardo Senatore on the effective field theory of inflation. This idea goes back a couple of years to a paper by Clifford Cheung, Paolo Creminelli, Liam Fitzpatrick, Jared Kaplan, and Senatore; there’s a nice technical-level post by Jacques Distler that explains some of the basic ideas. An effective field theory is a way of using symmetries to sum up the effects of many unknown high-energy effects in a relatively simple low-energy description. The classic example is chiral perturbation theory, which replaces the quarks and gluons of quantum chromodynamics with the pions and nucleons of the low-energy world.

In the effective field theory of inflation, you try to characterize the behavior of inflationary perturbations in as general a way as possible. It’s tricky, because you are in a time-dependent background with a preferred (non-Lorentz-invariant) frame provided by the expanding universe. But it can be done, and Leonardo did a great job of explaining the virtues of the approach. In particular, it provides a very nice way of calculating non-gaussianities. Read the rest of this entry »
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April 19th, 2011 2:23 AM
in Science, Travel | 12 Comments | RSS feed | Trackback >
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Cosmic Variance Cosmic Variance is a group blog by people who, coincidentally or not, all happen to be physicists and astrophysicists:
Daniel Holz
JoAnne Hewett
John Conway
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Sean Carroll
Our day (and night) jobs notwithstanding, the blog is about whatever we find interesting — science, to be sure, but also arts, politics, culture, technology, academia, and miscellaneous trivia. We have similar outlooks on many things, widely disparate opinions about others, and will do our best to keep the discourse reasonably elevated.
Recent Posts
Dark Matter is Just Messing With Us Now
The World Changes, We Stay Largely the Same
Because April is Poetry Month
Dark Matters
Does Time Run Faster When You’re Terrified?
Hell
Avignon Day 4: Dark Matter
Avignon Day 3: Reductionism
Avignon Day 2: Cosmological Neutrinos
Avignon Day 1: Calculating Non-Gaussianities
Here and There
No Dark Matter Seen by XENON
Guest Post: Jim Kakalios on the Quantum Mechanics of Source Code
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Science is Hard
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