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Recent cosmological observations indicate the existence of extra light species, i.e., dark radiation. In this paper we show that signatures of the dark radiation are imprinted in the spectrum of inflationary gravitational waves. If the dark…
The gravitational radiation originating from a compact binary system in circular orbit is usually expressed as an infinite sum over radiative multipole moments. In a slow-motion approximation, each multipole moment is then expressed as a…
Gravitational waves are propagating undulations in the spacetime fabric, which interact very weakly with their environment. In cosmology, gravitational-wave distortions are produced by most of the inflationary scenarios and their…
Gravitational waves are ripples in spacetime generated by the acceleration of astrophysical objects. A direct consequence of general relativity, they were first directly observed in 2015 by the twin Laser Interferometer Gravitational-Wave…
In this paper, following the previous study, we evaluate the spectrum of gravitational wave background generated by domain walls which are produced if some discrete symmetry is spontaneously broken in the early universe. We apply two…
The geometric properties of spacetimes representing expanding impulsive gravitational waves, propagating on a flat background and generated by snapped cosmic strings, are studied. The construction of the line element is reviewed, and…
String cosmology models predict a stochastic cosmic background of gravitational waves with a characteristic spectrum. I describe the background, present astrophysical and cosmological bounds on it, and outline how it may be possible to…
We compute the spectrum of relic gravitons in a model of string cosmology. In the low- and in the high-frequency limits we reproduce known results. The full spectrum, however, also displays a series of oscillations which could give a…
In the wide variety of axion cosmologies in which axion strings form, their radiative decay is the dominant mechanism for the production of axions, imposing a tight constraint on the axion mass. Here, we focus on the mechanism by which…
Moduli fields are generally present in superstring--inspired models. They are typically characterized by masses of the order of the supersymmetry breaking scale and by interactions of gravitational strength to ordinary matter. If stable,…
Electromagnetic radiation is known to be associated with certain gravitational waves events, i.e. the collision of binary neutron stars. Establishing this connection is non-trivial. However, if electromagnetic counterparts could be produced…
We consider a scenario with axions/axion-like particles Chern-Simons gravity coupling, such that gravitational waves can be produced directly from axion wave parametric resonance in the early universe after inflation. This axion gravity…
Photons can gravitationally scatter off a cosmic string loop and gain or lose energy. We consider the spectral distortion induced by cosmic string loops placed in an ambient thermal bath of photons. The fractional deviation from a thermal…
The evolution of globular cluster systems in some galaxies can be cause of merging of globulars in the very central regions. This high stellar density favours the growth of a central nucleus via swallowing of surrounding stars. The infall…
Flat directions are a generic feature of the scalar potential in supersymmetric gauge field theories. They can arise, for example, from D-terms associated with an extra abelian gauge symmetry. Even when supersymmetry is broken softly, there…
The retarded Green function of the electromagnetic field in spacetime of a straight thin cosmic string is found. It splits into a geodesic part (corresponding to the propagation along null rays) and to the field scattered on the string.…
We study the simultaneous decay of global string loops into scalar particles (massless and massive modes) and gravitational waves (GWs). Using field theory simulations in flat space-time of isolated loops with initial length $\sim 80-1700$…
We have studied numerically the evolution and decay of axion strings. These global defects decay mainly by axion emission and thus contribute to the cosmological axion density. The relative importance of this source relative to misalignment…
The post-bounce oscillations of newly-born relativistic stars are expected to lead to gravitational-wave emission through the excitation of nonradial oscillation modes. At the same time, the star is oscillating in its radial modes, with a…
We summarise the physics of first-order phase transitions in the early universe, and the possible ways in which they might come about. We then focus on gravitational waves, emphasising general qualitative features of stochastic backgrounds…