Blogs / Cosmic Variance

As has been discussed on CV before, Californians have an opportunity to vote on a ballot initiative to invalidate gay marriage in the state. While we’re being all endorse-y today, I’m throwing my two cents in with a link to a piece by a friend of mine:

We used to joke about it, or sort of joke, whenever we drove up to Vancouver to visit friends. “We’re married now!” We’d cry, after crossing through Customs and handing over all our papers and the Mermaid Girl’s birth certificate with both our names on it. And then, on the way home, as we passed the Peace Arch: “Not married any more! Hey, girlfriend!”

It wasn’t that funny, though, to tell the truth.

Read the whole thing, and then come back and tell me how giving this family legal protection has damaged the fabric of Canadian society.

I’m sure our blog audience includes a wide swath of undecided voters, a/k/a “Joe the Reader of Blogs.” (Okay, perhaps not.) So, to help along the decision-making process, here are some endorsements from respected sources.

If you are a reader of sciencey blogs, you have undoubtedly heard that Seed has endorsed Barack Obama. This is consistent with newspapers across the country, who have gone for Obama at the rate of 234 to 105 — a healthy difference with 2004, when Kerry squeaked by Bush, 213 to 205. The Economist has endorsed Obama; we’ve already mentioned the Financial Times. Even prominent conservative Stephen Colbert, not wanting to feel left out, has endorsed Obama. When McCain’s “supporters” offer up helpful testimony like this, who is to blame him?

Most interesting to me is that Nature has endorsed Obama for President. (Thanks to Alex Witze.) It’s interesting because Nature has been around a long time as one of the world’s premier scientific journals, and has never before endorsed a candidate for the U.S. Presidency. And their reasons sound pretty similar to mine:

But science is bound by, and committed to, a set of normative values — values that have application to political questions. Placing a disinterested view of the world as it is ahead of our views of how it should be; recognizing that ideas should be tested in as systematic a way as possible; appreciating that there are experts whose views and criticisms need to be taken seriously: these are all attributes of good science that can be usefully applied when making decisions about the world of which science is but a part. Writ larger, the core values of science are those of open debate within a free society that have come down to us from the Enlightenment in many forms, not the least of which is the constitution of the United States.

On a range of topics, science included, Obama has surrounded himself with a wider and more able cadre of advisers than McCain. This is not a panacea. Some of the policies Obama supports — continued subsidies for corn ethanol, for example — seem misguided. The advice of experts is all the more valuable when it is diverse: ‘groupthink’ is a problem in any job. Obama seems to understands this. He tends to seek a range of opinions and analyses to ensure that his own opinion, when reached, has been well considered and exposed to alternatives. He also exhibits pragmatism — for example in his proposals for health-care reform — that suggests a keen sense for the tests reality can bring to bear on policy.

Obama is very far away from being an infallible political savior, and if he wins I’m sure there will be times when he does the wrong thing. But, to reiterate something I said at American Airspace, he thinks like an academic in the best sense of the word. He listens, and considers what he hears critically and analytically, and then comes to a conclusion and deals with the consequences. Even if I don’t always agree with the conclusions, it will be an unambiguous blessing to at long last have a President with that cast of mind.

We can close with some words from the guy who invented quarks.

It’s humbling to think that ordinary matter, including all of the elementary particles we’ve ever detected in laboratory experiments, only makes up about 5% of the energy density of the universe. The rest, of course, comes in the form of a dark sector: some form of energy density that can be reliably inferred through the gravitational fields it creates, but which we haven’t been able to make or touch directly ourselves.

It’s irresistible to imagine that the dark sector might be interesting. In other words, thinking like a physicist, it’s natural to wonder whether the dark sector might be complicated, with a rich phenomenology all its own. And in fact there is something interesting going on: over the last 15 years we’ve established that the dark sector comes in at least two different pieces! There is dark matter, 25% of the universe, which we know is like “matter” because it behaves that way — in particular, it clumps together under the force of gravity, and its energy density dilutes away as the universe expands. And then there is dark energy, 70% of the universe, which seems to be eerily uniform — smoothly distributed through space, and persistent (non-diluting) through time. So, there is at least that much structure in the dark sector.

But so far, there’s no evidence of anything interesting beyond that. Indeed, the individual components of dark matter and dark energy seem relatively vanilla and featureless; more precisely, taking them to be “minimal” provides an extremely good fit to the data. For dark matter, “minimal” means that the particles are cold (slowly moving) and basically non-interacting with each other. For dark energy, “minimal” means that it is perfectly constant throughout space and time — a pure vacuum energy, rather than something more lively.

Still — all we have are upper limits, not firm conclusions. It’s certainly possible that there is a bushel of interesting physics going on in the dark sector, but it’s just too subtle for us to have noticed yet. So it’s important for we theorists to propose specific, testable models of non-minimal dark sectors, so that observers have targets to shoot for when we try to constrain just how interesting the darkness really is.

Along those lines, Lotty Ackerman, Matt Buckley, Marc Kamionkowski and I have just submitted a paper that explores what I think is a particularly provocative possibility: that, just like ordinary matter couples to a long-range force known as “electromagnetism” mediated by particles called “photons,” dark matter couples to a new long-range force known (henceforth) as “dark electromagnetism,” mediated by particles known (from now on) as “dark photons.”

Dark Matter and Dark Radiation
Authors: Lotty Ackerman, Matthew R. Buckley, Sean M. Carroll, Marc Kamionkowski

We explore the feasibility and astrophysical consequences of a new long-range U(1) gauge field (”dark electromagnetism”) that couples only to dark matter, not to the Standard Model. The dark matter consists of an equal number of positive and negative charges under the new force, but annihilations are suppressed if the dark matter mass is sufficiently high and the dark fine-structure constant $\hat\alpha$ is sufficiently small. The correct relic abundance can be obtained if the dark matter also couples to the conventional weak interactions, and we verify that this is consistent with particle-physics constraints. The primary limit on $\hat\alpha$ comes from the demand that the dark matter be effectively collisionless in galactic dynamics, which implies $\hat\alpha \lesssim 10^{-4}$ for TeV-scale dark matter. These values are easily compatible with constraints from structure formation and primordial nucleosynthesis. We raise the prospect of interesting new plasma effects in dark matter dynamics, which remain to be explored.

Just to translate that a bit, here is the idea. We’re imagining there is a completely new kind of photon, which couples to dark matter but not to ordinary matter. So there can be dark electric fields, dark magnetic fields, dark radiation, etc. The dark matter itself consists half of particles with dark charge +1, and half with antiparticles with dark charge -1. Now you might say to yourself, “Why don’t the particles and antiparticles all just annihilate into dark photons?” That kind of thinking is probably why ideas like this weren’t explored twenty years ago (as far as we know). But if you think about it, there is clearly a range of possibilities for which the dark matter doesn’t annihilate very efficiently; for example, if the mass of the individual dark matter particles was sufficiently large, their density would be very low, and they just wouldn’t ever bump into each other. Alternatively, if the strength of the new force was extremely weak, it just wouldn’t be that effective in bringing particles and antiparticles together.

None of that is surprising; the interesting bit is that when you run the numbers, they turn out to be pretty darn reasonable, as far as particle physics is concerned. For DM particles weighing several hundred times the mass of the proton, there should be about one DM particle per coffee-cup-sized volume of space. The strength of the dark electromagnetic force is characterized, naturally, by the dark fine-structure constant; remember that ordinary electromagnetism is characterized by the ordinary fine-structure constant α = 1/137. It turns out that the upper limit on the dark fine-structure constant required to stop the dark matter particles from annihilating away is — about the same! I was expecting it to be 10^-15 or something like that, and it was remarkable that such large values were allowed.

However, we know a little more about the dark matter than “it doesn’t annhilate.” We also know that it is close to collisionless — dark matter particles don’t bump into each other very often. If they did, all sorts of things would happen to the shape of galaxies and clusters that we don’t actually observe. So there is another limit on the strength of dark electromagnetism: interactions should be sufficiently weak that dark matter particles don’t “cool off” by interacting with each other in galaxies and clusters. That turns into a more stringent bound on the dark fine-structure constant: about two orders of magnitude smaller, at Still, not so bad.

More interestingly, we can’t say with perfect confidence that the dark matter really is effectively non-interacting. If a model like ours is right, and the strength of dark electromagnetism is near the upper bound of its allowed value, there might be very important consequences for the evolution of large-scale structure. At the moment, it’s a little bit hard to figure out what those consequences actually are, for mundane calculational reasons. What we are proposing is that the dark matter is really a plasma, and to understand how structure forms, one needs to consider dark magnetohydrodynamics. That’s a non-trivial task, but we’re hoping it will keep a generation of graduate students cheerfully occupied.

The idea of new forces acting on dark matter is by no means new; I’ve worked on it recently myself, and so have certain co-bloggers. (Strong, silent types who are too proud to blog about their own papers.) What’s exciting about dark photons is that they are much more natural from a particle-physics perspective. Typical models of quintessence and long-range fifth forces invoke scalar fields, which are easy and fun to work with, but which by all rights should have huge masses, and therefore not be very long-range at all. The dark photon comes from a gauge symmetry, just like the ordinary photon, and its masslessness is therefore completely natural.

Even the dark photon is not new. In a recent paper, Feng, Tu, and Yu proposed not just dark photons, but a barrelful of new dark fields and interactions:

Thermal Relics in Hidden Sectors
Authors: Jonathan L. Feng, Huitzu Tu, Hai-Bo Yu

Dark matter may be hidden, with no standard model gauge interactions. At the same time, in WIMPless models with hidden matter masses proportional to hidden gauge couplings squared, the hidden dark matter’s thermal relic density may naturally be in the right range, preserving the key quantitative virtue of WIMPs. We consider this possibility in detail. We first determine model-independent constraints on hidden sectors from Big Bang nucleosynthesis and the cosmic microwave background. Contrary to conventional wisdom, large hidden sectors are easily accommodated…

They show that these models manage to evade all sorts of limits you might be worried about, from getting the right relic abundance to fitting in with constraints from primordial nucleosynthesis and the cosmic microwave background.

Our model is actually simpler, because we have a different flavor of fish to fry: the possible impacts of this new long-range force in the dark sector on observable cosmological dynamics. We’re not sure yet what all of those impacts are, but they are fun to contemplate. And of course, another difference between dark electromagnetism and a boring scalar force is that electromagnetism has both positive and negative charges — thus, both attractive and repulsive forces. (Scalar forces tend to be simply attractive, and get all mixed up with gravity.) So we can imagine much more than a single species of dark matter; what if you had two different types of stable particles that carried dark charge? Then we’d be able to make dark atoms, and could start writing papers on dark chemistry.

You know that dark biology is not far behind. Someday perhaps we’ll be exchanging signals with the dark internet.

The Laser Interferometer Gravitational-Wave Observatory (LIGO) is one of the most amazing instruments ever built. It was constructed (and is now being upgraded) to search for gravitational waves. I’ll wax poetic about it soon enough. In the interim, readers can whet their appetites with Einstein’s cosmic messengers, a collaboration between Andrea Centazzo (a multimedia artist) and Michele Vallisneri (a physicist at the Jet Propulsion Laboratory). They will be performing a world premiere of their work on the Caltech campus tomorrow evening (Oct. 30); the event is free, and open to the public. The concert will be preceded by public talks by Kip Thorne and Jay Marx, two of the most knowledgeable people alive when it comes to gravitational-wave theory and observation. The evening promises to be an interesting melding of science and art. Centazzo will perform the music live, synchronized with the video. The concert attempts to capture the grandeur of LIGO, as well as shed light (and sound) on the nature of gravitational waves. For those of us poor souls unfortunate enough not to live near Pasadena, we will have to satisfy ourselves with video:

For those of you that are able to make it to the concert, please let us know your thoughts on the event!

It’s not only socialists and Muslims who get trotted out when the masses need to be scared into voting Republican. Here’s Elizabeth Dole’s latest ad for her North Carolina Senate race. Via.

Note that, via Balloon Juice, Dole’s opponent Kay Hagan is a Sunday School teacher. What would Dole say if an actual atheist were running?

Time is running out! October is careening its way toward Halloween, at which point the month devoted to the Donors Choose Blogger Challenge will be over. As of this typing, we’ve received $6,110 worth of donations, which, I must admit, is extremely awesome. Even better, out of 23 proposals we chose for support, 13 have been fully funded! Still, it falls a bit short of our $10,000 goal. And this despite the fact that we’ve been fortunate enough to receive boosts from the following awesome blogs and quasi-blogs:

• Shores of the Dirac Sea
• Hot Chicks Dig Smart Men
• Morning Coffee Physics
• 3 Quarks Daily
• Optimus Waffle
• BioCurious
• Abstruse Goose
• High Energy Mayhem
• Peculiar Velocity
• Ron Hsu
• Twisted Physics
• Saturday Morning Breakfast Cereal
• Cocktail Party Physics
• US/LHC Blogs (unofficially)

And what is more galling, despite this groundswell of support, Uncertain Principles has pulled ahead! And he’s only one blogger (plus a dog). Are you going to stand for that?

It’s a great program, and you feel great after you donate. It’s the swank $200 donations that get all the glory (and we’re very grateful for them, don’t get me wrong), but — following the lead of the Obama campaign — we’re running a people-powered donation drive here. For the starving students out there, consider throwing in $10. Contributions that size would really add up if everyone chipped in. A small price to make the world a better place.

But hey, I know how it is. Money’s tight, and in times like this you have to look out for yourself. We understand that, and we won’t be bugging you any more. I just wanted to point out to you this little missive on the subject of charitable giving.

You see Flavor Flav there? That’s you. You are Flavor Flav. Hey, it’s your choice.

The sometimes crazy, sometimes razor-sharp Christopher Hitchens lets loose in Slate about Sarah Palin’s war on science. He hits most of the high points - fruit flies, grizzlies, and creationism (but misses the overhead projector nonsense). In any case, it’s worth a read, and I’ll just leave you with his concluding paragraph, to set the scene

This is what the Republican Party has done to us this year: It has placed within reach of the Oval Office a woman who is a religious fanatic and a proud, boastful ignoramus. Those who despise science and learning are not anti-elitist. They are morally and intellectually slothful people who are secretly envious of the educated and the cultured. And those who prate of spiritual warfare and demons are not just “people of faith” but theocratic bullies. On Nov. 4, anyone who cares for the Constitution has a clear duty to repudiate this wickedness and stupidity.

Hitchens is such an elitist, and clearly not from Real America^©.

If you were watching the third Presidential debate, you may have noticed that John McCain had hit on a new line of attack: Barack Obama wants to “redistribute wealth.” To those of us who interpret phrases by attaching meanings to the individual words within them, this comes off as pretty weak sauce. Of course Barack Obama wants the government to redistribute wealth; so does John McCain. That’s one of the things that government does. Every time the government takes money in the form of taxes or fees, or spends money on social services or public works or anything else at all, it redistributes wealth. Most obviously, we have a progressive tax system: people with higher incomes (supposedly) pay a higher percentage of their income in taxes. This is nothing new, and no mainstream candidate for national office proposes to do away with it.

Admittedly, as a country we are not very good at redistributing wealth. The gap between rich and poor in the U.S. is larger than in any other developed country. And progressive taxation isn’t nearly what it appears at first blush:

Even in the United States, the rich pay a disproportionate share of the federal income tax, which mildly reduces inequality. Other taxes, however, like Social Security, are regressive: the rich pay a lesser share. Thus, the upper tenth of households pay 70 percent of the income tax, but only 52 percent of all federal taxes. State sales taxes make the system even more regressive, because poorer people spend a higher share of their total income on them. Kevin Hassett, of the American Enterprise Institute, estimates that a family of four earning $50,000 pays exactly the same share of its income (30 percent) on taxes as one earning $150,000.

There’s little question that Obama’s policies would be slightly more redistributive than the status quo. Most obviously, he wants to raise taxes on the upper few percent of earners, and cut taxes to the middle class; he also proposes to expand health care coverage quite a bit. These are concrete policy proposals that are squarely in the mainstream of popular debate — his health care proposal was notably less ambitious than those of Hillary Clinton or John Edwards — but are certainly arguable; a freeze on health-care spending and a giant tax cut for the wealthiest Americans is also squarely within the mainstream of popular debate. Here is the graph of the impact that Obama’s and McCain’s tax proposals would have on different income groups:

Obama’s plan would hit the upper 1%, who benefited the most from Bush’s tax cuts, and it would lighten the burden on the lower 80%; McCain’s help is targeted at the top 20%, and (by virtue of not raising taxes on anyone) would cost an extra trillion dollars over ten years. Given what passes for a mainstream consensus in contemporary U.S. politics, the choice between these two options is considered to be a close one. So there is nothing crazy or desperate about criticizing Obama’s proposals on the merits.

But McCain and his supporters aren’t fretting over graphs of the growth of American inequality, or even over the distribution of tax rates. They are fretting over this, the histogram of likely electoral-college outcomes from fivethirtyeight.com:

As a response to this stark reality, they have decided to seize upon “redistribute wealth” not in terms of the actual meaning of its actual words, but as a slogan of SECRET SOCIALISM. For whatever reasons — this is a matter for future psychohistorians, not for humble physicist/bloggers — a substantial segment of right-wing punditry refuses to believe that Barack Obama is what he says he is, or what he has actually acted like his entire adult life: a thoughtful center-left politician. They have no doubt that he is the most radical figure ever to come this close to the Presidency.

Obama’s entire campaign is built on class warfare and human envy. The “change” he peddles is not new. We’ve seen it before. It is change that diminishes individual liberty for the soft authoritarianism of socialism… Unlike past Democrat presidential candidates, Obama is a hardened ideologue. He’s not interested in playing around the edges. He seeks “fundamental change,” i.e., to remake society.

To these folks, “redistribute wealth” isn’t a straightforward description of how the government operates under the present system. Rather, it’s a slip of the tongue, revealing the dictatorship-of-the-proletariat leanings hidden behind the nonthreatening exterior. And here is the revealing moment to which McCain was referring in that debate, when Obama explains to Joe the Plumber how his plans will remake Amerikkka as a socialist utopia:
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The week before last, I spent several delightful days at the Causality, Analyticity, and Superluminal Propagation Workshop at the Michigan Center for Theoretical Physics in Ann Arbor. This very general title could, in principle, cover all of the many reasons that physicists have considered the possibility of faster than light travel and its implications during the century since the development of special relativity. However, in reality, the workshop was designed to be predominantly focused on a specific subset of such questions.

Over the past decade or so, the quest to understand the class of models that can drive the accelerated expansion of the early universe - inflation - has been joined by the equally, if not more complicated challenge of explaining our observed epoch of late time acceleration. Approaches to these two problems vary from phenomenological proposals for new mass energy sources to drive acceleration (the inflaton, dark energy) and the analogous attempts to modify the Einstein-Hilbert action for general relativity to allow for self-acceleration (modified gravity) to searches for an origin of either phenomenon within a complete theory of matter and gravity, such as string theory.
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Brad DeLong, in re Quantum Hyperion, wonders whether photons are really responsible for the decoherence of Saturn’s moon:

But gravity works–presumably, at some level–by massive objects constantly bombarding each other with gravitons, so we are also averaging over all the possible states of gravitons that we are not keeping track of, aren’t we? That should cause decoherence too, shouldn’t it?

This is an annoyingly good question. In fact, I’m probably not giving anything away if I reveal that my esteemed co-blogger Daniel and I once tried to figure out whether or not dark matter, if it truly interacts with ordinary matter only through gravity, would be in a coherent quantum state. Still don’t know the answer (although I strongly suspect it is “no,” I’m just not sure how to prove it).

The force due to gravity on Hyperion is much larger than the force due to electromagnetism on Hyperion. All else being equal, gravity is a much weaker force, but it has the helpful quality of adding up rather than canceling out, which is why it tends to dominate over astrophysical distances.

However — it’s not always useful to think of the gravitational force on a planet as due to the exchange of gravitons. You can think of the static force between two objects as arising from the exchange of virtual particles, whether you are talking about gravity or electromagnetism. But it is also true that, in the limit where the bodies giving rise to the gravitational force are perfectly static, those gravitons add up to define a unique quantum state. (The Sun, Saturn, and Titan are not static, but probably good enough for these purposes.) So the state of Hyperion becomes entangled with the quantum states of the individual gravitational fields of those celestial bodies, not with a jillion separate gravitons from each source. When we ignore the quantum states of all the gravitons reflected off of Hyperion, we are ignoring a lot more than when we ignore the quantum states of the gravitational fields of the Sun, Saturn, and Titan.

So I think it’s the photons, not the gravitons, that are primarily responsible for the decoherence, by a wide margin. But I wouldn’t bet my reputation on it. Maybe Daniel’s reputation.