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waynehu

Professor, Department of Astronomy and Astrophysics
University of Chicago

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CMB Introduction '96   Intermediate '01   Polarization Intro '01   Cosmic Symphony '04   Polarization Primer '97   Review '02   Power Animations   Lensing   Power Prehistory   Legacy Material '96   PhD Thesis '95 Baryon Acoustic Oscillations Cosmic Shear Clusters
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Cosmology [321] Current Topics [282] Galaxies and Universe [242] Radiative Processes [305] Research Preparation [307] GR Perturbation Theory [408] Advanced CMB [448]

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.