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Understanding why the Universal expansion is accelerating

Friday, December 15, 2017

Cover of December 2017 Physics TodayUnderstanding the observed late-time acceleration of the cosmological expansion is one of the most compelling problems facing physics today. The University of Waterloo's new Distinguished Chair in Astrophysics, Will Percival, is leading an effort to understand why the Universal expansion is accelerating. 

Dr. Percival's paper, Baryon acoustic oscillations: A cosmological ruler, is featured on the cover of Physics Today, December 2017.

It cannot be explained in standard models using known physics, and there are many theoretical ideas for explaining this phenomenon, collectively termed "Dark Energy’’. Obtaining more observations of the acceleration is key to its future understanding and Baryon Acoustic Oscillations (BAO) are a key technique able to make robust measurements.

BAO are a relic pattern created by spherical acoustic waves in the early universe, which can be seen in the distribution of galaxies and used as a standard ruler with which to measure cosmological expansion. The BAO feature is on such large scales that it expands with the expansion of the Universe to be approximately 150 Mpc (480 million lightyears) at present day. In a galaxy survey this feature is seen as an excess of pairs of galaxies separated by this scale.

In addition to BAO, spectroscopic galaxy surveys can be use to extract further information, including constraints on early-universe inflationary models, measurements of neutrino masses, observations of galaxy formation and evolution, and measurements of the cosmological structure growth rate. They therefore represent a gold-mine for astrophysical and cosmological information.

As with many fields in science, improvement in experimental measurement is driven by improvement in apparatus. For spectroscopic galaxy surveys the key instrument is a Multi-Object Spectrograph (MOS), which allows simultaneous measurement of spectra, and hence redshifts, for multiple galaxies. Future experiments happening over the next 5 years include the Dark Energy Spectroscopic Instrument (DESI) and the Euclid satellite mission, and will provide an order of magnitude more galaxies than we have currently, leading to exciting measurements of Dark Energy.

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