Professor, Department of Astronomy and Astrophysics
University of Chicago

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Gravitational Waves

Key Concepts

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.