Stochastic background of gravitational waves from cosmological sources

Gravitational waves (GW) can constitute a unique probe of the primordial universe. In many cases, the characteristic frequency of the emitted GW is directly related to the energy scale at which the GW source is operating in the early universe. Consequently, different GW detectors can probe different energy scales in the evolution of the universe. After a general introduction on the properties of a GW stochastic background of primordial origin, some examples of cosmological sources are presented, which may lead to observable GW signals.Comment: Proceedings of LISA Symposium X, accepted for publication in Journal of Physics: Conference Series. Typos corrected, two references adde

Limits on stochastic magnetic fields: A defense of our paper [1]

In their recent paper ``Faraday rotation of the cosmic microwave background polarization by a stochastic magnetic field'', Kosowsky et al. Phys.Rev. D71, 043006 (2005) have commented about our paper [C.Caprini and R.Durrer, Phys. Rev. D65, 023517 (2002)], in which we derived very strong limits on the amplitude of a primordial magnetic field from gravitational wave production. They argue that our limits are erroneous. In this short comment we defend our result.Comment: 2 pages, no figure

Adding helicity to inflationary magnetogenesis

The most studied mechanism of inflationary magnetogenesis relies on the time-dependence of the coefficient of the gauge kinetic term $F_{\mu\nu}\,{F}^{\mu\nu}$. Unfortunately, only extremely finely tuned versions of the model can consistently generate the cosmological magnetic fields required by observations. We propose a generalization of this model, where also the pseudoscalar invariant $F_{\mu\nu}\,\tilde{F}^{\mu\nu}$ is multiplied by a time dependent function. The new parity violating term allows more freedom in tuning the amplitude of the field at the end of inflation. Moreover, it leads to a helical magnetic field that is amplified at large scales by magnetohydrodynamical processes during the radiation dominated epoch. As a consequence, our model can satisfy the observational lower bounds on fields in the intergalactic medium, while providing a seed for the galactic dynamo, if inflation occurs at an energy scale ranging from $10^5$ to $10^{10}$ GeV. Such energy scale is well below that suggested by the recent BICEP2 result, if the latter is due to primordial tensor modes. However, the gauge field is a source of tensors during inflation and generates a spectrum of gravitational waves that can give a sizable tensor to scalar ratio $r={\cal O}(0.2)$ even if inflation occurs at low energies. This system therefore evades the Lyth bound. For smaller values of $r$, lower values of the inflationary energy scale are required. The model predicts fully helical cosmological magnetic fields and a chiral spectrum of primordial gravitational waves.Comment: 17 pages, 4 figures. Minor changes to match the version accepted for publication in JCA