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.
I remember you talking about Dr. Chen over at Preposterous Universe and also making the various papers the two of you had worked on available. Thanks for telling us–makes one feel a part of the “family,” so to speak. And I can use that kind of warm feeling today for it is raining lightly and raw with snow to follow although I look forward to walking home in it later today. Congratulations and happy trails to both of you. It seems to me that she is starting very well. Take care.
Congratulations Jennie! Best of luck for the future.
Yay for Jennie!
Oh! You are my academic sister now too!
Hey Jennie!
Congratulations! Hope the job hunt goes well!
So would this go to understanding the origination of this?
Congratulations, Jennie! I really enjoyed the arrow of time paper. (I met Jennie in NYC a couple of summers ago, where I showed her and three other astronomer/particle theorist types around the city following a conference they had gone to in Princeton.)
Best of luck.
http://math.ucr.edu/home/baez/week223.html
“Maxwell’s equations and their p-form generalization make sense when spacetime is any Lorentzian manifold. However, to get a theory where initial data determine a unique global solution, we want our spacetime to be “globally hyperbolic”, which means that it has a “Cauchy surface”: roughly, a spacelike surface that any sufficiently long timelike curve hits precisely once. To get a good quantum theory of p-form electromagnetism with a Hilbert space of states on which time evolution acts as unitary operators, we need more: our spacetime should be ‘stationary’, meaning that it has time translation symmetry. Otherwise there’s no way to define energy and the vacuum state – which is defined to be the least-energy state.”
Does the preceding paragraph argue for alpha invariant over time by reason of a fundamental mathematical requirement?
Jen, Ph.D., wholehearted congratulations from an old TASI friend of yours. Please post some pictures of e.g. Eugene whiping tears of joy from his face, or yourself running — properly dressed, of course — through the streets of Chicago shouting “HEUREKA! HEUREKA!” 🙂
Celebrate! Have a monumental party. I am truly happy for you,
Your friend,
Bjorn
Hey Bjorn!
Nice to hear from you! Unfortunately I have moved on and is not in Chicago to attend Jennie’s defense and party. But fear not! I will be wiping tears of joy here at Yale!
Heureka! Where is the monkey?
here!
Very very hearty congratulations Jennie!!!!
We met a couple of years ago in Santa Barbara and I’m sure Nick Jones and Amanda Weltman (if they are reading this) will recall Jennie’s little epic with her camera during a climbing trip we all took…
Jeff
Hello everyone,
Thanks for the post, Sean! You always manage to convey ideas so much more eloquently than me, even the topic of my own thesis. 🙂 My friends who ask about my research are all getting directed here. Speaking of friends . . .
It’s really good to hear from some old friends out there. My years in grad school wouldn’t have been nearly as cool without you — thanks. Best of luck to you too!
,
Jennie
Congrats Dr. Chen 🙂 Invite me to your party
Big congratulations, Jennie! Best of luck with whatever is next…
Congrats Jennie !! It’s too cold in Chicago right now to have a real party. You should come down to Lisboa and celebrate with style.
Warmest congratulations Jennie!!! We’re so excited and happy for you!
Way to go Jennie!
All the best in the future!
Woohoo!