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.