### Baryon acoustic oscillation

### Physics Update:

Investigating the nature of the “dark” vacuum energy (DE) that's presumed to drive the present acceleration of the cosmic Hubble expansion requires tying the redshifts *z* of distant objects to independent measurements of their distances. One such method involves measuring both *z* and celestial position for large numbers of galaxies, in search of a spatial-correlation feature of known length at different values of *z*. That baryon-acoustic-oscillation (BAO) correlation length is attributed to sound-like waves in the early universe’s hot-plasma epoch. At the epoch’s abrupt end, the plasma waves became density fluctuations of ordinary “baryonic” matter that remain imprinted on the spatial distribution of galaxies (see *Physics Today*, April 2008, page 44). How the BAO correlation length at any particular *z* looks from here and now (*z* = 0) measures our distance from that *z*. The Baryon Oscillation Spectroscopic Survey (BOSS) collaboration has now reported its analysis of more than a million galaxies measured with the Sloan Foundation’s 2.5-meter-aperture survey telescope in New Mexico (shown in the photo). The BOSS sample covers almost a quarter of the sky to a depth of *z* = 0.7, which corresponds to a look-back time of 6 billion years. The analysis measures the effective distance to objects at *z* = 0 .57 with a record precision of 1%, and it yields an improved measurement of the DE’s pressure-to-density ratio *w*. If the DE is simply Einstein’s cosmological constant, then *w* = −1. The BOSS analysis, incorporating complementary cosmological data, yields *w* = −1.03 ± 0.06. (L. Anderson et al., BOSS collaboration, http://arxiv.org/abs/1312.4877.)—Bertram Schwarzschild

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