prepublication copy - The Department of Astronomy & Astrophysics ...

FIGURE 2‐4 Left panel: Image of the tiny fluctuations in temperature ‐ roughly ten parts per million ‐ of
the Cosmic Microwave Background as observed by the WMAP satellite. The radiation that is observed was
emitted when the Universe was roughly four hundred thousand years old. The red regions are warmer and
the blue regions colder. A careful analysis of this data shows that there is a preferred angular scale of 1
degree ‐ about the size of the moon ‐ called the first acoustic peak in the dark matter density containing
roughly a hundred thousand galaxies like our Milky Way galaxy. (Credit: NASA/ Wilkinson Microwave
Anisotropy Probe Science Team.) Right panel: The same feature can be seen in the distribution of galaxies
around us today as exhibited by the Sloan Digital Sky Survey in a 2.5 degree thick slice of the northern
equatorial sky where color corresponds to galaxy luminosity. Here it is called a Baryon Acoustic Oscillation.
The expansion of the universe by a factor of a thousand makes the size of the feature about four hundred
million light years. Monitoring the growth of this feature as the universe expands is one of the best
approaches to understanding the behavior of dark energy. (Credit: Michael Blanton and Sloan Digital Sky
Survey (SDSS) Collaboration, http://www.sdss.org.)
considerably over the last ten years. We think that, just after the big bang, the universe was totally
different from today—none of the elementary particles that we know compose the matter of today were
present. The universe was an incredibly dense knot of highly curved spacetime. Then came an era of
cosmic inflation, during which the universe rapidly expanded by a truly enormous factor (at least a factor
of 10 26 in growth 3 ). The laws of quantum mechanics suggest that random fluctuations at the time of
inflation would have produced microscopic density variations from place to place, which expanded with
the universe to became macroscopic variations today. Remarkably, astrophysicists are able to connect the
giant filaments and voids in the great cosmic web of galaxies to the seeds from which they grew.
However, just as the cause of the current acceleration is unknown, the underlying detailed physics of
inflation is still a complete mystery.
About 400,000 years after the big bang, the continued expansion and cooling of the universe had
dropped the temperature to about three thousand degrees, which was cool enough for the first atoms to
form. This is the epoch of “recombination”. A fundamental change in the universe occurred at that time
when the cosmos went from being filled with a plasma that was opaque to light into an atomic gas
through which light could freely pass. It is this freely streaming radiation that we observe at radio
wavelengths as the faint glow known as the Cosmic Microwave Background (CMB). The near
uniformity of the CMB observed across the sky and the nature of the minute brightness fluctuations we
measure in the CMB are just what is expected if inflation occurred. The CMB is therefore a fantastic
signal telling us about the early universe 4 .
3 One hundred trillion trillion.
4 The 1978 and 2006 Nobel Prizes in physics were awarded to Americans for CMB research.
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