Personal

Man, I go away for a couple of days and all my co-bloggers choose to take a siesta. I’m going to have to give them a good talking-to, I tell you.

Now I’m stuck in Philadelphia for one night more than planned, due to an unforeseen outbreak of weather back in Chicago. The City of Brotherly Love has greatly come up in the world since I grew up in the suburbs a couple of decades ago — the Rittenhouse Square neighborhood, where I’m staying, is a really lively and engaging downtown environment.

Nothing of substance to report, so I’ll point you to this takedown of astrology by Phil Plait of Bad Astronomy fame, which is worthy of some contemplation. We all know that astrology is nonsense, but it’s worth the exercise to try to explain to people who aren’t well-versed in science why we know that astrology can’t work even without doing elaborate double-blind tests. Phil’s argument is the same one that I’ve given before: we really do know something about the forces of nature, and there is absolutely no room to fit paranormal phenomena into what we know. There’s much we don’t know, and much we do; sometimes we even have a pretty good idea of where the boundary is.

This is the time of year when a lot of undergraduate students are filling out applications to graduate school. So it’s nice to be reminded that all that effort occasionally pays off. Join me in congratulating brand-new Ph.D. Jennifer Chen, who successfully defended her thesis yesterday!

Jennie’s previous work with me was on spontaneous inflation and the arrow of time, in which we tried (and even succeeded, I might claim) to answer a century-old question: why does the early universe have such low entropy? This work was briefly deemed press-worthy, and was the basis for our second-place winning essay in the Gravity Research Foundation essay competition.

For her thesis work, Jennie looked at experimental constraints on light scalar fields in the universe. We’ve never detected a fundamental scalar field, for the sensible reason that they tend to be very massive. But one possible candidate for dark energy is an extremely light scalar field (a mass about 10^-40 times the mass of the electron), known as “quintessence.” Some time back I explored how you might detect a quintessence field directly through its couplings to matter, rather than indirectly through the expansion of the universe, in my paper Quintessence and the Rest of the World. Basically there are two ways to do it: looking for very weak long-range forces via 5th-force experiments (light fields always give rise to long-range forces), and looking for gradual evolution of the “constants” of nature such as the fine-structure constant.

Jennie took this idea and did a thorough job of exploring what the current data are telling us. For the 5th-force experiments, this meant exploring what the “charge” for different test masses would be, especially from the complicated effects of quarks and gluons. As particle physicists know but rarely admit, most of the mass in ordinary matter comes not from the fundamental masses of elementary particles themselves, but from the chromodynamic binding energy of quarks confined into protons and neutrons. Jennie showed that couplings to gluons and quarks would be the most significant contributor to the 5th-force effects from light scalars.

The other idea, that coupling constants could evolve over the history of the universe due to the gradual evolution of a light scalar field, has received a lot of attention recently due to claims that the fine structure constant α (characterizing the strength of the electromagnetic interaction) actually does vary. This work looks at the spacing of spectral lines in systems at high redshift, and purportedly provides evidence that α has varied by about 10^-5 between today and a redshift of a few. Other studies, it should be mentioned, claim that α actually does not vary at all, and place an upper limit.

Here is Jennie’s plot of the data, with some theoretical curves (click for larger version).

This is the inferred value of α as a function of cosmological redshift. The points with the big error bars that lie below zero are from the group claiming to see a variation in α (the data have been binned for easier viewing). The points above those, consistent with zero, are from other groups looking at quasar spectra. The two points near the top left are interesting; the leftmost one is from the Oklo natural reactor, and the next one uses data from abundances of radioactive isotopes in meteors.

The moral is simple enough: trying to fit the data with a simple quintessence model doesn’t readily accomodate the Oklo and meteor points, much less the new quasar data. Probably α is not changing, and if it is, it’s not doing so in a way we would expect in a simple model. That’s what complicated models are for, of course. But I wouldn’t bet a lot of money on this one.

Another young scientist joins the ranks of credentialed scholars. Congratulations to Vikram Duvvuri, who just defended his Ph.D. thesis on “Modified Gravity as an Alternative to Dark Energy”!

This was not an easy one, through no fault of Vikram’s — a certain member of his thesis defense committee got stuck on the East Coast, and had to phone in, after numerous delays. But Vikram kept his cool throughout all of the tense drama, and made it through the defense itself unscathed.

The thesis was based on two papers on which I was a collaborator: Is Cosmic Speed-Up Due to New Gravitational Physics? with Mark Trodden and Michael Turner, and The Cosmology of Generalized Modified Gravity Models with the same authors plus Antonio De Felice and Damien Easson. The idea is to explain the observed acceleration of the universe by modifying gravity rather than introducing dark energy. That is to say, we look out into the universe and see that distant galaxies are accelerating away from us. In the context of Einstein’s general relativity, that can’t happen in a universe consisting of ordinary matter and radiation — we need some form of energy density that persists, rather than dissipating away, as the universe expands. So we fit the data by imagining that about 70% of the universe is some exotic dark energy, perhaps a cosmological constant, that is smoothly distributed through space and nearly-constant in time.

But the other possibility is that Einstein was wrong, and we need to modify general relativity on cosmological scales. There are various ways to do this; one that seems potentially viable is a brane-world construction by Dvali, Gabadadze, and Porrati. Our approach was to ask for the simplest possible modified-gravity model that would make the universe accelerate. So we stuck with four dimensions, no new fields, and just played with the dynamics of the spacetime metric.

Einstein’s equation for general relativity can be derived by minimizing an action, where the action is simply the integral over spacetime of the curvature scalar R. We wanted a new action that looked like Einstein’s when R was large, but looked different when R was small, as in the late universe. So we did the obvious thing: replaced R with R+1/R in the action. Vikram’s thesis was an examination of this model and some more complicated variations on the same theme.

Sadly the original model doesn’t quite work; as noted by Chiba, it is ruled out by tests of gravity in the Solar System. That’s basically because our theory introduces a new degree of freedom. In the weak-field limit, general relativity is a theory of a massless spin-2 particle, the graviton. Our modified action turns on a new degree of freedom, which is a massive (tachyonic) spin-0 particle. This turns out to be fairly generic; if you mess with Einstein’s theory by adding new terms to the action, it almost always happens. Thus, a lesson is learned: general relativity is hard to mess with without running into conflict with experiment. But such messing can nevertheless lead to useful outcomes, like a new doctorate!