Gravitational Waves
Key Concepts:
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Gravitational waves show
a power spectrum with both E and B mode
contributions
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Limits on the gravitational wave contribution to
the temperature anisotropy imply B-modes < a
few tenths of a microKelvin.
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Gravitational waves probe the
physics of inflation but will require a thorough
understanding of foregrounds and secondary effects for their detection.
If there were only gravitational waves and
no density perturbations in the Universe, the CMB temperature, polarization
and temperature-polarization cross power spectra would look like:
Notice that the polarization contains power in both the E and B-modes.
That we do see acoustic peaks in the spectrum indicates that this scenario
cannot actually be true. At most, gravitational waves contribute
a fraction of the power in temperature anisotropies.
Adding back in the density fluctuations, the power spectrum as a function
of the ratio of power in the gravitational wave (tensor, T) versus density
(scalar, S) modes becomes:
For realistic values of this ratio or "T/S",
the power in the B-mode corresponds to a tenth of
a micro Kelvin signal on scales of l~100.
Needless to say, this signal will be very difficult to detect in the presence
of foregrounds and secondary anisotropies that also produce B-modes.
The rewards of detecting it are however great. The amplitude and
spectrum of the gravitational wave contributions are our best probes of
the physics of the inflationary epoch.
