The spectrum of Primordial Gravitational Waves

Details The spectrum of Primordial Gravitational Waves The spectrum of Primordial Gravitational Waves

Aug 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006iaujd...7e..37s&link_type=abstract

The Universe at z > 6, 26th meeting of the IAU, Joint Discussion 7, 17-18 August 2006, Prague, Czech Republic, JD07, #37

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Scientific paper

Primordial gravitational waves are the oldest relics ever conceived in the scientific ambit for the early Universe. Such relics must constitute a cosmological background of gravitational radiation resulting from quantum perturbations occurred in the space-time at energy scales of the order of 10^15 GeV. In the present work, the origin and evolution of the cosmological gravitational waves is computed using the general relativity and the theory of cosmological perturbations. The temporal evolution of tensor perturbations is classically governed by a differential equation that can be interpreted as the one of an excited oscillator, so that the power spectrum depends on the evolution of the scale factor of the Universe. The initial conditions of the problem are determined during the inflationary era (de Sitter model), at z >= 10 ^25, when the quantum regime is established. Both classical and quantum regimes are treated and, by means of a semi-analytic method, the system is evolved up to the present. The shape of the spectrum depends on the initial conditions and on the scale factor at the moments that the modes are in the threshold of the horizon; the amplitude depends also on the energy scale of inflation. Since the physical nature of the fluid that leads the accelerated expansion observed in the recent Universe (known as dark energy) is still uncertain, four categories of models were analyzed, according to the type of dark energy: cosmological constant, X-fluid (phantom or not), generalized Chaplygin gas and (a parametrical form of) quintessence. The results are conclusive with respect to the insensitivity of the gravitational waves to the dark energy, due to the fact that its phase of domain is recent (z ~ 1). The empirical counterpart of those theoretical forecasts is still inexistent, for the noise levels and the frequencies of operation of the experiments already built are inadequate to detect this primordial gravitational radiation. The perspectives are more promising for the space detectors, which are planned to be sensitive to amplitudes of the order of 10^-23 in frequencies of 10^-3-1 Hz. On the other hand, the cosmic microwave background radiation provides a concrete alternative of obtaining that information indirectly. Both observational perspectives are also discussed in this work.

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