Monday, 12 October 2015

CosPA 2015 Liveblog: Day One, Session One

It's my first conference in Korea, and I've got off to a good start by sleeping in.  I don't even have the excuse of jet lag, but I do have the excuse of not having slept well for the last month due to the heat and humidity.  Though it looks like they are already running late, we seem to still be in the first talk which should have finished fifteen minutes ago!  The conference webpage is here, but annoyingly it seems that the plenary talks are not online (yet?).
09:15 am: Status of Neutrino Astronomy, Carsten Rott

Based on what little I saw of this talk, it was about IceCube.  The main experimental result seems to have been the PeV neutrinos discovered a year or two ago.

10:00 am: Baryon Acoustic Oscillations: A Robust and Precise Route to the Cosmological Distance Scale, Daniel Eisenstein

Director of SDSS, so that's an obvious focus.  Daniel gave a talk at Pheno this year, and this does seem to be essentially the same talk with a new title.  Certainly, the animations are the same.  My previous thoughts were here.

BAO is, as the title says, a cosmological distance scale.  It is thus closely related to the expansion history of the Universe, and thus to Dark Energy.  Can we discriminate between different candidate DE theories?  BAO are sound waves.  Distinctive feature in galaxy clustering: about 500 million light years is most probable distance between two galaxies.  Shows up in CMB as characteristic size of blobs; peaks in power spectrum.

Acoustic oscillations from stabilisation of density perturbations pre-recombination.  Excesses in galactic densities modified by photon pressure, till gas and light decouple.  Oscillation scale is simple calculation: just based on time till recombination, itself a function of matter and photon densities.  (Undergrad problem!)

Measurement of distances different along and transverse to the line of sight.  This makes redshift surveys useful: Redshift measures distances along los, and angular sizes measure transverse.  Additional complication comes from non-linear effects from gravity that smears out peak, though overall position largely unchanged.  Use N-body simulations to extract the relevant information.

All this motivates the experimental searches for the BAO scale.  At low redshift this is done by looking for, measuring and counting galaxies.  SDSS started in 1990, took first data in 1998 and is still running; largest compilation of astronomical spectroscopy.  BOSS, subproject in SDSS-3 that took data from 2008 to 2014, searched one-quarter of sky.  Looking for 1% distance measurements.  Data released in January 2015, online at

Unambiguous measurement (10σ) of BAO in galaxy data.  Also using quasar spectra and Lyman-α forest to measure hydrogen density profile along line of site, measuring to higher redshift because of quasar brightness.  BAO now detected in that data at the 5σ level.  Sensitive to redshifts up to 2.4.  Percent-level measurements agree with ΛCDM prediction from CMB at 1σ level.

Strong agreement with flat cosmological constant.

Next: Measurements to fill gap in redshift 0.7 < z < 2.  Also future plans for surveying galaxies out to as high as z = 3.5 (DESI).  eBOSS at SDSS-IV (running now) will push galaxies to z = 1 and aiming for a large quasar sample.

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