Sound waves that propagate in the early universe, like spreading ripples in a pond, imprint a characteristic scale on cosmic microwave background fluctuations. These fluctuations have evolved today into the clustering of galaxies. The concept is illustrated here.
SDSS III, BOSS
Perhaps
the most astonishing and revolutionary discovery in cosmology was that
galaxies are moving away from us. Hubble's 1929 paper provides the
underpinning of the big bang picture of creation in which the universe
is expanding, and has been for 13.8 billion years. Astronomers since
then have been steadily working to refine this general picture, and in
1998, two teams (one led by CfA scientists) further astonished the world
with their results showing that the universe would expand forever --
and not only that: it is accelerating outward. They used supernovae to
probe the distant cosmos. These discoveries have led to more
sophisticated questions, with a primary task today being to understand
in detail the expansion history of the universe, that is, how the rate
of expansion of the universe evolved from the time of the big bang to
the way it is today. The answers to this question directly address the
properties of the acceleration mechanism, the nature of dark matter, the
evolution of galaxies in early times, and more.
Precision measurements of the cosmic distance scale are crucial for
probing this behavior, and one particularly powerful method uses what
are called baryon acoustic oscillations (BAO). Baryons refer to
ordinary matter, and acoustic oscillations are sound waves. Sound waves
caused by density fluctuations were bouncing through the cosmos during
its first 400,000 years. Then, once ionized atoms became neutral,
radiation no longer interacted strongly with matter and the cosmic
microwave background was released. The intensity maps of the background
radiation contain a record of these sound waves – the BAO. Astronomers
calculate that at the time the cosmic background was produced, sound
waves (traveling at the speed of sound) could have spread across a
distance of about 500 million light-years, leaving in their wake a
coherent record in the matter distribution that eventually condensed
into galaxies and clusters of galaxies. Because the scale of this
acoustic distortion is so large, many times the size of galaxy clusters,
the BAO signature was only modestly altered subsequently as the
universe evolved; simulations and theory suggest deviations are below
1%. The robustness of the scale of this distinctive clustering
signature allows it to be used as a standard ruler to measure the cosmic
distance scale, and indeed the imprint of the BAOs has been detected in
a variety of observations of the structure of the nearby universe.
CfA astronomers Daniel Eisenstein and Cameron McBride were among a
large team of scientists probing BAOs by using the clustering of
galaxies as seen at a time when the universe was about 8.2 billion years
old. They examined 264,283 galaxies of this general epoch observed by
the Sloan Digital Sky Survey, and measured from their spatial
distribution the signature of the acoustic waves left behind to a
precision of better than 10%. Their conclusions about the big bang are
consistent overall with the picture of cosmic evolution that has emerged
from many other lines of evidence (but add some tantalizing hints of
mystery: their measurement of the current rate of expansion as 67.5 +-
1.7 km/second/megaparsec is actually a tad smaller than the currently
favored value). The amazing power of the technique is that it gives a
snapshot of the universe at this era, and that it relies on completely
different data from those used by other studies that rely on supernova
or cosmic background radiation.
Reference(s):
"The
Clustering of Galaxies in the SDSS-III Baryon Oscillation Spectroscopic
Survey: Measuring DA and H at z = 0.57 from the Baryon Acoustic Peak in
the Data Release 9 Spectroscopic Galaxy Sample," Lauren Anderson et
al., MNRAS 439, 83, 2014.
