The Universe is expanding, and abundant evidence now shows that it began in a hot, dense state---the Big Bang. The general theory of relativity explains how the expanding Universe works, but on its own it does not explain what made the Big Bang happen in the first place.
Clues have been found in the relic heat from the Big Bang, the Cosmic Microwave Background (CMB).
Light that has been traveling to us since the Universe was 300,000 years
old. Observations reveal minute temperature fluctuations in the brightness
of the CMB that show that the matter content of our Universe, while remarkably
smooth when the relic heat began its journey to us, had already been imprinted
with perturbations at a much earlier time. These have now grown into the
galaxies of stars illuminating our sky. We are therefore faced with a conundrum:
Why has matter in the Universe clumped into galaxies and clusters of galaxies
spread smoothly throughout space?
|The initial map of the cosmic microwave background obtained by NASA's Wilkinson Microwave Anisotropy Probe (WMAP).
WMAP provides the clearest view yet of the CMB, using the wrinkles imprinted
upon it from the earliest moments of the Big Bang to provide precision measurements
on the expansion and contents of the Universe.|
cosmology" provides one explanation of why the Universe is very smooth, yet
not perfectly so. A still-mysterious form of energy generated a repulsive
force that caused the early Universe to expand at a fantastic rate. This
expansion stretched and smoothed any existing inhomogeneities in spacetime.
But the inflation field, like all energy fields, was subject to quantum
fluctuations. These led to imperfections in the cosmic expansion---the Big
Bang got a slightly bigger kick in some places than in others. The effect
of a single quantum fluctuation was enormously inflated along with the Universe
itself. Sky maps of the CMB show a pattern of fluctuations very much like
that predicted by inflation.
Nevertheless, we are far from certain that the inflationary scenario is
correct. Even if inflation is the right story, the details of the process
remain a mystery. We need new data to help decide whether the early Universe
underwent a period of rapid inflation, and if so, what was the mechanism
responsible for driving it.
We now understand a way to uncover these secrets. Calculations predict
that in addition to its energy field fluctuations, inflation should have
created single "particles of spacetime" called gravitons.
The gravitational waves of longest wavelength (with periods of three billion
years!) should have left a subtle pattern in the polarization of the light of the CMB.
The "Inflation Probe"
will seek this subtle pattern. The strength and details of the pattern will
tell us about the properties of the mysterious inflation field that powered
the Big Bang.