In case you had any doubt, the human rights situation worldwide is now the worst it’s been in 50 years. Ruben Bolling offers one possible explanation (click for more):

Don’t laugh, I’ve heard crazier ideas.

Yesterday we wondered out loud whether cosmological evidence for dark matter might actually be pointing to something more profound: a deviation of the behavior of gravity from that predicted by Einstein’s general relativity (GR). Now let’s ask the same question in the context of dark energy and the acceleration of the universe.

We have (at least) two problems to face. First, if you do a back-of-the-envelope estimate of what the vacuum energy (the energy density inherent in empty space, or equivalently Einstein’s cosmological constant) should be, you get an answer that exceeds the observationally allowed value by the ridiculous factor of 120 orders of magnitude (10¹²⁰). That’s bad, as far as agreement between theory and experiment is concerned. But nevertheless the universe is accelerating, which could be explained by a tiny amount of vacuum energy — about 10^-8 ergs per cubic centimeter, if you care. Or perhaps by something else.

For example, maybe general relativity works for ordinary bound systems like stars and galaxies, but breaks down for cosmology, in particular for the expansion rate of the universe. In GR the expansion rate is described by the Friedmann equation, which sets the expansion rate proportional to the square root of the energy density. Ordinarily the density drops as the universe expands, and the expansion rate follows suit; if the density is constant (as with vacuum energy), the expansion rate can stay constant. (We would then interpret that as “accelerating”, oddly enough. A distant galaxy has a recession velocity v=Hd, where d is the distance and the Hubble constant H tells us the expansion rate. If H is constant and d is increasing, we would measure v to be increasing.)

So maybe Friedmann was somehow wrong. For example, maybe we can solve the problem of the mismatch between theory and experiment by saying that the vacuum energy somehow doesn’t make the universe accelerate like ordinary energy does. There are different ways to make this happen, none of which strike you as perfectly compelling. Arkani-Hamed, Dimopoulos, Dvali, and Gabadadze have proposed a “filter” by which smoothly-distributed energy doesn’t affect spacetime in the same way as localized energy; it’s somewhat ad hoc, but definitely interesting. With Laura Mersini I suggested that the pressure of a substance, as well as the energy density, contributes to the expansion rate, in just such a way as to make vacuum energy (which comes with a negative pressure) cancel exactly and leave spacetime unaffected. We were inspired by models with extra dimensions of space, but the particular models in question don’t quite work, so I’ve since spent a lot of time looking for models that are strictly four-dimensional. No luck yet.

Solving the cosmological constant problem is hard, and a popular strategy is to ignore it and try to account for dark energy separately. That is, imagine that the vacuum energy is set to zero by some mysterious mechanism, and something else is responsible for the acceleration of the universe. There are different approaches to this possibility as well. One is to be largely phenomenological, and just write down alternatives to the Friedmann equation to see what might work. This approach (inspired again by extra dimensions, but basically phenomenological) has been pursued by Dvali and Turner, and by Freese and Lewis. One issue here is that ultimately it wouldn’t be possible to distinguish experimentally between a modified Friedmann equation and some model of dark energy; both would only show up in the behavior of the expanding universe.

So the complementary approach is to come up with a whole new theory of gravity that can make the universe accelerate, and see if what that theory has to say about other tests of gravity. I’ve been thinking about this recently, having spent the weekend talking to Mark Trodden. He and I have written a paper with Duvvuri and Turner that explores an especially simple approach to doing this. We just suggest that the curvature of spacetime somehow resists going to zero, and can bounce back to infinity when it becomes small. (There are actually a lot of equations involved, but that’s the basic philosophy.) Then we can compare our model to experiments. Sadly, it fails. The simplest way to see this was pointed out by Chiba, who rewrote our theory in a way that resembles other models, and showed that ordinary tests of GR in the solar system are incompatible with our suggestion. Basically, the orbit of Mars would look different. But that’s okay; it just goes to show that GR is actually quite robust, and even an attempt to change it exclusively in cosmology ends up affecting all sorts of other things.

A more elaborate approach has been suggested by Dvali, Gabadadze, and Porrati (see a recent review by Dvali for more details). They again use an extra dimension of space, imagining that our world is a three-dimensional “brane” embedded in a four-dimensional space (plus one time dimension, as usual). They have invented a scheme whereby gravity can behave differently on and off the brane, become much weaker in the four-dimensional “bulk.” But at very large scales the bulk begins to affect our universe. They claim that, if we choose parameters appropriately (there is always a great deal of unnatural fine-tuning involved in these scenarios), we can straightforwardly explain the accelerated expansion of the universe. Even better, they are not yet (as far as we know) ruled bout by solar-system tests, but improved measurements might be able to detect new affects from the extra dimensions in the orbit of the moon (see also this paper by Lue and Starkman). This theory is not very well understood as yet, and deserves a lot more work to be fully explicated; but it’s interesting and promising, so we’ll have to see what happens as people think about it further. (There is a nice popular-level exposition by Dvali in Scientific American, although you have to pay for the full article online.)

So we don’t have any extremely compelling alternatives to Einstein’s theory, but there are a lot of possibilities and we have our work cut out for us. The good news is that observed phenomena (the dynamics of galaxies and clusters, the cosmic microwave background, the accelerating universe) are pushing us to think of profound new scenarios for gravity and cosmology.

By the way (as it were), John Scalzi links here and suggests that GR is bound to give way pretty soon. I hope that’s not the impression I’m actually giving; I personally think it’s an incredible long shot, but one worth pursuing. Probably the “standard” story of cold dark matter and vacuum energy are exactly right; this is a robust model that makes many more predictions than there are free parameters, and it’s well-motivated (although far from completely understood) in fundamental physics. In contrast, when we start modifying gravity, we’re just flailing around, hoping some good will come of it. But that’s how science works; the flailing always looks a little silly until it smacks into a bit of miraculous insight, which we then clean up and proclaim to be genius. Right now there’s a lot of talk of modifying gravity, because it’s well worth considering; but if you’re going to bet, Einstein gets much better odds even than Smarty Jones.

Gravity is the most obvious of the four forces of nature (gravity, electromagnetism, and the strong and weak nuclear forces). It’s also the first for which we had a sensible physical theory: Newton’s law of universal gravitation. Now we have sensible theories for all four of the forces, and Newton’s theory has been superseded by an even better theory, Einstein’s general relativity (GR).

GR has passed a series of experimental challenges with flying colors: the precession of Mercury, deflection of light, gravitational redshift and time delay, gravitational radiation from the binary pulsar, and the expansion rate of the early universe during the nucleosynthesis era. But it doesn’t quite fit in with the rest of physics, since the other three forces seem to be compatible with quantum mechanics in a way isn’t so obvious for gravity. So very few people really believe that general relativity is the final answer; at some point we’ll have to invent a better model (string theory being the leading candidate) that is intrinsically quantum-mechanical yet reduces to GR in the appropriate regimes.

Usually in field theory, if a model works well in a certain regime, you might expect it to break down at shorter distances or higher energies, but continue to be successful at long distances and lower energies. Nevertheless, people have begun to ask whether general relativity might be okay in the solar system but break down on much larger scales (galaxy- or universe-sized distances). The primary motivation for such suggestions is the fact we need to hypothesize dark matter and dark energy to make sense of our universe if GR is correct. It is very likely that GR is correct, and dark matter and dark energy are both for real, but since we can’t be sure we consider the possibility that our understanding of gravity is to blame.

Of course, it’s easy to say “let’s modify gravity,” much harder to come up with a good model. Indeed, it’s not even obvious what issue you’d like your model to address — the need for dark matter in galaxies, clusters, and large-scale structure; or the perplexingly small value of the cosmological constant; or the acceleration of the universe conventionally attributed to dark energy.

Modifying gravity with the goal of replacing dark matter is a long-standing project that has met with mixed success, most famously pursued by Milgrom and his friends. Milgrom has an idea called “Modified Newtonian Dynamics,” or MOND for short. For some introductions see pages by Greg Bothun or Stacy McGaugh, or this review by Sellwood and Kosowsky. The idea is to slightly increase the Newtonian gravitational acceleration when that acceleration is very small, so that slowly-moving particles feel more force than they ordinarily would, mimicking the presence of unseen matter. This idea works extremely well for individual galaxies; indeed, Milgrom made predictions for the behavior of low-surface-brightness galaxies before they were directly observed, and the predictions were later confirmed very nicely.

Unfortunately, there are problems with the MOND paradigm itself. For one thing, it’s not really a “theory”, it’s just a rule for making predictions in a very specific set of circumstances — slowly-moving particles orbiting around massive bodies. (Just as an observational matter, it doesn’t even seem to work very well for clusters of galaxies, although it does quite well for individual galaxies.) Since it’s not a full-blown theory, it’s hard to make predictions for other tests you might like to do, like deflection of light or cosmology. So people have been trying to invent an actual theory that reduces to MOND in the appropriate circumstances. In a recent proposal, Bekenstein has claimed to succeed; now people are at work putting this idea to the test, to see both if it makes sense and if it agrees with other things we know about cosmology.

In addition to the theoretical difficulties, there is at least one model-independent reason to think that no modification of gravity will ever replace the idea of dark matter: we seem to be accumulating evidence (tentatively at the moment, to be sure) for gravitational forces pointing in directions where there is no ordinary matter. The most basic such clue comes from studies of gravitational lensing of clusters of galaxies, which can be used to reconstruct the distribution of dark matter in the clusters. The upshot is that the dark matter seems to be distributed much more smoothly than the ordinary matter; see this reconstructed cluster image for an example. Less direct evidence is found in the acoustic peak structure of the temperature anisotropies in the cosmic microwave background. (For an intro, see Wayne Hu’s tutorial.) Density fluctuations in the plasma of the early universe lead to sound waves, in which regions become more dense and therefore hot, and then bounce back and become less dense, in a repeating cycle; this leads to peaks in the plot of temperature fluctuation as a function of angular scale. But fluctuations in the dark matter don’t heat up (they don’t interact with light, since they’re dark), so they only increase with time. Consequently, odd-numbered peaks have ordinary matter and dark matter in phase, and even-numbered peaks have them out of phase. The out-of-phase oscillations are suppressed, so we expect dark matter to boost the odd-numbered peaks. This is exactly what appears to happen, as this figure indicates. At least a little bit; the data need to improve before we can be sure. But it’s hard to see how a modified theory of gravity could explain this phenomenon.

Of course, perhaps a modified theory of gravity could predict gravitational forces pointing in directions other than where there is ordinary matter; you’d have to tell me the theory first before we could say for sure. MOND doesn’t, though, and such a theory is even harder to imagine than one that simply fits the galaxy data.

Tomorrow I’ll talk a little about modified gravity and the issues of vacuum energy and the accelerating universe.

I’m thinking of starting a new tradition, declaring Friday to be Narcissism Day here at Preposterous Universe. A day we can take off from thinking about important world events and profound cosmic mysteries, and just think about me.

In this spirit, I offer this pointer to my first-ever (so far as I know) appearance in the society pages of a major newspaper. (To balance things out, here’s a media moment more relevant to my purported expertise. [Update: I found another one. What a media slut, hmm?]) The occasion was the Dinosaur Dinner I attended to benefit Project Exploration. Here’s more proof:

L to R: Sean, State Senator Barack Obama, Shureice Kornegay, Jean Claude Francois, Project Exploration Executive Director (and expectant mother) Gabrielle Lyon, Michelle Obama. I’m the one with the martini, to nobody’s surprise.

I have to admit that I didn’t see any evidence of the stalker that Jack Ryan’s campaign has assigned to follow around Obama twenty-four hours a day. Probably he couldn’t afford a ticket. The Obama campaign has started its own blog, which is worth a visit.

From the Reuters story about our raid on Chalabi’s headquarters:

An opinion poll found only seven percent of Iraqis now viewed U.S. troops as “liberators,” compared to 45 percent six months ago.

The poll was conducted by the Iraq Center for Research and Strategic Studies in April, before pictures of soldiers abusing prisoners drove another wedge between Americans and Iraqis.

Do you think it’s improved since then?

More evidence for an accelerating universe, this time from the Chandra X-ray satellite observatory. They observe X-rays from the hot gas in distant clusters of galaxies. A cluster is just a set of galaxies bound together by their mutual gravitational pull; but in addition to the galaxies themselves, the cluster is full of hot gas between the galaxies, not to mention dark matter. This picture is of the cluster Abell 2029; in blue you see the galaxies (visible in ordinary light) and in red the hot gas (reconstructed from the X-ray image). Knowing the properties of the gas, they can figure out the distances to the clusters. Comparing these with the redshifts (which tell us by how much the universe has expanded since the light we see left the cluster), we can reconstructe the expansion history of the universe.

The answer they get for the acceleration is consistent with our recent consensus model for cosmology, including substantial dark energy that seems to be nearly (or exactly) constant as the universe expands. So, not a dramatic overthrowing of what we already knew, but a nice confirmation. Which is very important, when the thing you’re confirming is as surprising and ill-understood as the acceleration of the universe. Our previous evidence (from distances to supernovae, temperature fluctuations in the cosmic microwave background, and the dynamics of galaxies and large-scale structure) was very good, but every extra piece of evidence bolsters the case for this preposterous universe.

Chandra is named after Subrahmanyan Chandrasekhar, one of the leading theoretical astrophysicists of the 20th century. Also a longtime University of Chicago faculty member, and part of a tradition at NASA of naming its satellite observatories after famous scientists with UofC connections — Chandra was preceded by the Hubble space telescope, named after a prominent alumnus, and the Compton gamma-ray observatory, named after another former faculty member. This tradition ended with the Spitzer infrared observatory, but that’s okay because Lyman Spitzer was my grand-advisor (the Ph.D. advisor of George Field, my adviser). After that things went dramatically downhill, with the successor to Hubble being named after James Webb, a former NASA administrator.

Maybe I will not stop posting happy things until all the evil people go away. That might work, don’t you think? From Circa75 via Atrios, a first-person account of getting a marriage license in my old home town of Cambridge.

A sign poking up from the crowd saying “YAY” caught my eye. Some people had “Toto, we’re not in Kansas, Welcome to Equality” signs for the Phelps folks, but the crowd had grown so large you couldn’t tell if they were still there.

Aaron and I looked at each other, mouths open. Who are all these people? They’re not all here to get married, right? Where do we go? Are these all straight people? This is incredible! We didn’t speak, but we were clearly thinking the same thing.

I thought I caught a glimpse of a couple walking up the stairs before one enormous cheer, but I couldn’t be sure. We edged closer to the steps, and I could suddenly see a line of cops in riot gear leading up to the main entrance. I turned to one of them and asked him if we were too late to get in for a license.

“I don’t know,” he said. “We’re just keeping this area clear. You can’t stand here.”

…

At that point another cop walked up to people standing behind us and told them they had to clear a path. He started towards us, and Aaron grabbed me and pulled me back into the crowd.

“I think we can just walk up here,” I told him. “Come on!”

Aaron grabbed my hand and we walked forward up the steps.

Off to my side someone said, “Look, here goes someone else!”

Suddenly a roar erupted all around us. Things began to move more slowly. I grabbed Aaron’s hand tighter and started running forward up the steps. Everything was a blur. I lost his grip briefly as he stopped close to the entrance to accept a rose from someone in the crowd. I paused at the top of the steps, and turned to wait for him.

I’ve been in front of some large, happy, and cheering crowds before, but only on a stage — never with a throng pressing in from all sides, with clapping hands outstretched, cameras flashing, and a deafening roar.

I stood there facing the crowd as Aaron walked towards me with a sparkle-encrusted yellow rose and a huge grin on his face. As he reached me, I put my hand around his waist and waved to the crowd. I tried to look at all the people, but my eyes couldn’t focus.

Try to read the whole thing without getting choked up, I dare you.

From Matt Stoller by way of The Poor Man, one of the funniest things you’ll ever read: the 2000 Republican Party Platform. Probably it wasn’t so amusing at the time, but age does wonders. Here, just a few choice excerpts:

“The arrogance, inconsistency, and unreliability of the administration’s diplomacy have undermined American alliances, alienated friends, and emboldened our adversaries.”

“Nor should the intelligence community be made the scapegoat for political misjudgments. A Republican administration working with the Congress will respect the needs and quiet sacrifices of these public servants as it strengthens America’s intelligence and counter-intelligence capabilities and reorients them toward the dangers of the future.”

“The current administration has casually sent American armed forces on dozens of missions without clear goals, realizable objectives, favorable rules of engagement, or defined exit strategies. Over the past seven years, a shrunken American military has been run ragged by a deployment tempo that has eroded its military readiness. Many units have seen their operational requirements increased four-fold, wearing out both people and equipment.”

“The rule of law, the very foundation for a free society, has been under assault, not only by criminals from the ground up, but also from the top down. An administration that lives by evasion, coverup, stonewalling, and duplicity has given us a totally discredited Department of Justice.”

“Sending our military on vague, aimless, and endless missions rapidly saps morale. Even the highest morale is eventually undermined by back-to-back deployments, poor pay, shortages of spare parts and equipment, inadequate training, and rapidly declining readiness.”

“Our goal for NATO is a strong political and security fellowship of independent nations in which consultations are mutually respected and defense burdens mutually shared.”

“Inspired by Presidents Reagan and Bush, Republicans hammered into place the framework for today’s prosperity and surpluses.”

Here at Preposterous, we aim to provide entertaining distractions from the relentlessly depressing real world. Good to know that the Republican National Committee is collaborating in the effort.

You wonder why people get confused by science stories in the press? Two studies on the efficacy of Atkins-like low-carb/high-protein diets were recently reported in the Annals of Internal Medicine. If you visited the Google News page devoted to coverage of these stories (here is the page itself, although the content may shift with time), these are the first six headlines you would have seen, without any editing on my part:

• Low-carb not a slimmer diet (USA Today)
• Studies test value of low-carb diets (Minneapolis Star Tribune)
• Studies: Beneficial Low-Carb Diet (CBS News)
• Study casts doubt on benefits of Atkins diet (MSNBC)
• Scientists endorse Atkins diet (BBC)
• Low-Carb Diets Work Better in Short Term – Reports (Reuters)

Remember, these are reports on the same two studies. Scorecard: two positive headlines, two negative, one noncommittal, one ambiguous (“…in short term”).

Sometimes, if the medium is not actually the message, it nevertheless garbles the message so much as to be counterproductive. In particular, the need for a short and punchy headline forces distortion, not just oversimplification, of the story being reported. (Let’s face it, would you click first on the story from the Minneapolis Star Tribune?)

You can’t blame science reporters, who have a tough job and don’t write headlines anyway. A daily newspaper is just not an effective way to teach science. The news cycle demands that results be packaged in both catchy and timely ways, whereas the actual way that science is done is more often characterized by a gradual emergence of consensus. Not that I know what the proper remedy is, other than to teach students to be more aware and science-literate by the time they finish high school, so can they take simple headlines with a grain of salt.

A couple of the many couples getting married in Massachusetts today (Associated Press photos). Julie and Hillary Goodridge, lead plaintiffs in the case to allow same-sex couples to marry, getting their marriage license:

John Mirthes and Rick Reynolds chat with volunteer Sian Robertson as they wait to file their intent to marry:

I’m told that scenes like this are going to be the end of civilization as we know it. But with all the other images we’ve been seeing in the news lately, these made me feel good.

(Related: Career choices explained.)