AAS 207th Meeting, 8-12 January 2006
Session 100 Cosmology, Early Universe, Cosmic Distance Scale I
Oral, Tuesday, 2:00-3:30pm, January 10, 2006, Salon 1

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[100.05] Probing the early universe with gravitational waves

L. A. Boyle, P. J. Steinhardt (Princeton University), N. Turok (Cambridge University)

We assess the prospects for observing primordial gravitational waves, and investigate the information that such observations would provide about the early universe. First, we compute the gravitational-wave spectrum generated by the cyclic model and show that it is unobservably small in all frequency bands (hep-th/0307170). By contrast, the gravitational-wave spectrum generated by inflation is a very promising target. In particular (astro-ph/0507455), we reconsider the predictions of inflation for the spectral index of scalar (energy density) fluctuations (ns) and the tensor/scalar ratio (r) using a discrete, model-independent measure of the degree of fine-tuning required to obtain a given combination of (ns, r). We find that, except for cases with numerous unnecessary degrees of fine-tuning, ns is less than 0.98, measurably different from exact Harrison-Zel'dovich. Furthermore, if ns \gtrsim 0.95, in accord with current measurements, the tensor/scalar ratio satisfies r \gtrsim 10-2, a range that should be detectable in proposed cosmic microwave background (CMB) polarization experiments and direct gravitational wave searches. Finally, it is well known that the inflationary gravitational wave spectrum carries important information about the physics of inflation itself; but we stress that it also carries important information about the “dark age” separating the end of inflation from the beginning of big bang nucleosynthesis, and discuss how this information may be extracted by combining CMB polarization experiments with direct (laser-interferometer) gravitational wave measurements.

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