Research
Transparency of the Universe
The Universe is extremely transparent. Despite considerable column depths of dark matter, plasma, gas, and dust, the line-of-sight to the majority of extragalactic sources is far less opaque than the shutter of a typical astronomical camera. In the radio, the cosmic background radiation comes from a redshift ~ 1100 and is a near-perfect blackbody, as confirmed by the COBE DMR experiment to the percent level. In the optical the constraints on the transparency of the Universe are much less stringent. Sources of attenuating material can be both clustered and unclustered with matter. In the following two papers, my collaborators and I performed a very precise measurement of the transparency of galaxy clusters by statistically comparing the spectra of early-type galaxies behind and not-behind clusters of galaxies (an angular difference measurement). We also performed a radial difference measurement in order to constrain unclustered sources of photon-attenuation, by testing the distance duality relation (or Etherington relation).
Cosmic transparency: A test with the baryon acoustic feature and type Ia supernovae
Surhud More, Jo Bovy, & David W. Hogg
Conservation of the phase-space density of photons plus Lorentz invariance requires that the cosmological luminosity distance be larger than the angular diameter distance by a factor of $(1+z)^2$, where $z$ is the redshift. Because this is a fundamental symmetry, this prediction--known sometimes as the "Etherington relation" or the "Tolman test"--is independent of world model, or even the assumptions of homogeneity and isotropy. It depends, however, on Lorentz invariance and transparency. Transparency can be affected by intergalactic dust or interactions between photons and the dark sector. Baryon acoustic feature and type Ia supernovae measures of the expansion history are differently sensitive to the angular diameter and luminosity distances and can therefore be used in conjunction to limit cosmic transparency. At the present day, the comparison only limits the change $\Delta\tau$ in the optical depth from redshift 0.20 to 0.35 at visible wavelengths to $\Delta\tau < 0.13$ at 95-percent confidence. In a model with a constant comoving number density $n$ of scatterers of constant proper cross-section $\sigma$, this limit implies $n \sigma< 2\times10^{-4} h \Mpc^{-1}$. These limits depend weakly on cosmological world model. Within the next few years, the limits could extend to redshifts $z\approx2.5$ and improve to $n \sigma<1.1 \times10^{-5} h \Mpc^{-1}$. Cosmic variance will eventually limit the sensitivity of any test using the baryon acoustic feature at the $n \sigma\sim 4\times10^{-7} h \Mpc^{-1}$ level. Comparison with other measures of the transparency are provided; no other measure in the visible is as free of astrophysical assumptions.
The Transparency of Galaxy Clusters
Jo Bovy, David W. Hogg, & John Moustakas,
Astrophys. J. 688, 198 (2008)
If galaxy clusters contain intracluster dust, the spectra of galaxies lying behind clusters should show attenuation by dust absorption. We compare the optical (3500 - 7200 \AA) spectra of 60,267 luminous, early-type galaxies selected from the Sloan Digital Sky Survey to search for the signatures of intracluster dust in z ~ 0.05 clusters. We select massive, quiescent (i.e., non-star-forming) galaxies using an EW(Halpha) <= 2 \AA cut and consider galaxies in three bins of velocity dispersion, ranging from 150 to 300 km s^{-1}. The uniformity of early-type galaxy spectra in the optical allows us to construct inverse-variance-weighted composite spectra with high signal-to-noise ratio (ranging from 10^2-10^3). We compare the composite spectra of galaxies that lie behind and adjacent to galaxy clusters and find no convincing evidence of dust attenuation on scales ~ 0.15-2 Mpc; we derive a generic limit of E(B-V) < 3 x 10^{-3} mag on scales ~ 1-2 Mpc at the 99% confidence level, using conservative jackknife error bars, corresponding to a dust mass <~ 10^8 $M_{\odot}$. On scales smaller than 1 Mpc this limit is slightly weaker, E(B-V) < 8 x 10^{-3} mag.
Non-gravitational forces in the dark sector
About 80% of all the matter in the Universe is in the form of dark matter, whose presence we mainly know about because of its gravitational interaction with ordinary, baryonic matter. Given the complexity of the 20% of matter that we can see — the SU(3)xSU(2)xU(1) standard model, with three generations of quarks and leptons and a bunch of force-mediating gauge particles — there is no reason to think that the dark sector might not be arbitrarily complex. With Glennys Farrar I have looked into the consequences of a Yukawa-type fifth force between dark matter particles. Such an equivalence-principle-violating force could help clear up some problems with CDM. For instance, it would tend to clear out the voids and would explain the lower than expected number of satellite galaxies around the Milky Way, since an additional large-scale attractive force in the dark sector would accelerate the growth of structure. However, fifth forces in the visible sector are heavily constrained, and as we showed in the following paper, if there is a coupling between the dark and the visible sector, these constraints lead to another constraint on the combination of the coupling between the dark and the visible sector (think ''direct detection'') and the strength of the fifth force in the dark sector.
Connection between a possible fifth force and the direct detection of Dark Matter
Jo Bovy & Glennys R. Farrar
If there is a fifth force in the dark sector and dark sector particles interact non-gravitationally with ordinary matter, quantum corrections generically lead to a fifth force in the visible sector. We show how the strong experimental limits on fifth forces in the visible sector constrain the direct detection cross section, and the strength of the fifth force in the dark sector. If the latter is comparable to gravity, the spin-independent direct detection cross section must typically be <~ 10^{-55} cm^2. The anomalous acceleration of ordinary matter falling towards dark matter is also constrained: \eta_{OM-DM} <~ 10^{-8}.
contact info
email: jb2777 [at] nyu [dot] edu
address:
Center for Cosmology and Particle Physics
Department of Physics
New York University
4 Washington Place
New York, NY 10003
