Avignon Day 2: Cosmological Neutrinos

By Sean Carroll | April 20, 2011 1:00 am

By this point in my life, when I attend a large-ish conference like this one the chances are good that I’m older than the average participant. Certainly true here. It’s a great chance to hear energetic young people tackling the hard problems, and I certainly have the feeling that the field is in very good hands. It’s also a good reminder that we old people need to resist the temptation to fall into a rut, churning out tiny variations on the research we’ve been doing for years now. It’s easy to get left behind!

Still, it’s also nice to hear a talk on a perennial topic, especially when you hear something you didn’t know. Yvonne Wong gave a very nice talk on “hot relics” — particles that were moving close to the speed of light in the early universe. (They may have slowed down by now, or maybe not.) Neutrinos, of course, are the classic example here; they are known to exist, and were certainly relativistic at early times. If the neutrinos have masses of order 10 electron volts, they would contribute enough density to be the dark matter. But that doesn’t quite work in the real world; “hot dark matter” tends to wipe out structure on small scales, in a way that is dramatically incompatible with the world we actually observe. Also, ground-based measurements point to neutrino masses less than 0.1 electron volt — not for sure, since what we directly measure are the differences in mass between different kinds of neutrinos, rather than the masses themselves, but that seems to be the most comfortable possibility.

Of course, we know about three kinds of neutrinos (associated with electrons, muons, and taus), but there could be more. So it’s fun to use cosmology to see if we can constrain that possibility. An extra neutrino species, even if it were very light, would slightly affect the expansion rate of the early universe, which works to damp structure on small scales. This is something you can look for in the cosmic microwave background, and the WMAP team has diligently been doing so. Interestingly — the best fit is for four neutrinos, not for three! Here’s a plot from Komatsu et al.’s analysis of the WMAP seven-year data, showing the likelihood as a function of the effective number of neutrino species. (“Effective” because a massive neutrino counts a little less than a massless one.)

Now, maybe this isn’t worth getting too excited about. There’s a nice discussion of this possibility in a recent paper by Zhen Hou, Ryan Keisler, Lloyd Knox, Marius Millea, and Christian Reichardt. I’m not sure how a new neutrino could affect the CMB in this way without being ruled out by primordial nucleosynthesis, but I haven’t looked at it carefully. Regardless, it’s best not to just trust any one measurement, but do every measurement we can think of and make sure they are consistent. Certainly something worth keeping an eye on as CMB measurements improve.

  • Jason Dick

    In the paper, they do have a fair amount of discussion of how the helium fraction is affected by the extra neutrino species. Basically, it sounds like the constraints from BBN force an extra neutrino species to reduce the abundance of matter slightly in order to keep BBN results the same.

  • http://www.astro.multivax.de:8000/helbig/helbig.html Phillip Helbig

    They point out that the red curve is consistent with the standard value of 3 at “1 sigma”. So, doesn’t look like a big sensation.

    The paper itself is 57 pages and contains a huge amount of information.

  • Doug A

    If you look at additional info besides just WMAP, you can improve the constraints. That includes setting a limit on the sum of the masses of the neutrinos. Check it out: http://adsabs.harvard.edu/abs/2010MNRAS.406.1805M

    Interestingly, the number of neutrino species is consistent with 3 or 4. But we definitely don’t have massive neutrinos!

  • Alex

    Recent He4 data is not only compatible with a fourth neutrino specie but is even suggesting it ! the point is that it can be an extra neutrino, or early dark energy, or anything that behaves like radiation at recombination and BBN.

  • eric gisse

    The analysis of WMAP data is the principle reason I think dark matter could very well take the form of a sterile neutrino.

    Other folks have their money on one of the many WIMPs, I have mine on the sterile neutrino.

    It isn’t as if it is an outlandish possibility given recent observations, and the general fact that there is room in particle physics for new results given that the reason we have matter in the universe instead of photons hasn’t been nailed down yet.

  • http://www.sunclipse.org Blake Stacey

    As a former minion on a neutrino astrophysics project, I enjoy hearing about recent developments in the field.

    Here’s hoping the fit of Komatsu et al. wasn’t just an off-by-one error! :-)

  • Sili

    The Christians are gonna be so disappointed if it turns out there are really four generations of fermions.

    They might have to include Mary in the Quarternity or summat just to stress how much Religion™ has always been in line with science.

    (Strong enough feeling for ya?)


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About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] cosmicvariance.com .


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