Gravitational waves from self-ordering scalar fields

Abstract

Gravitational waves were copiously produced in the early Universe whenever the processes taking place were sufficiently violent. The spectra of several of these gravitational wave backgrounds on subhorizon scales have been extensively studied in the literature. In this paper we analyze the shape and amplitude of the gravitational wave spectrum on scales which are superhorizon at the time of production. Such gravitational waves are expected from the self ordering of randomly oriented scalar fields which can be present during a thermal phase transition or during preheating after hybrid inflation. We find that, if the gravitational wave source acts only during a small fraction of the Hubble time, the gravitational wave spectrum at frequencies lower than the expansion rate at the time of production behaves as Ω GW (f ) ∝ f 3 with an amplitude much too small to be observable by gravitational wave observatories like LIGO, LISA or BBO. On the other hand, if the source is active for a much longer time, until a given mode which is initially superhorizon (kη * ≪ 1), enters the horizon, for kη 1, we find that the gravitational wave energy density is frequency independent, i.e. scale invariant. Moreover, its amplitude for a GUT scale scenario turns out to be within the range and sensitivity of BBO and marginally detectable by LIGO and LISA. This new gravitational wave background can compete with the one generated during inflation, and distinguishing both may require extra information.

Perturbations of ultralight vector field dark matter

“…Finally, the energy density today of gravitational waves with + polarization per energy interval and solid angle unit reads [73,74],…”

Gravitational waves from Abelian gauge fields and cosmic strings at preheating

“…However, if inflation occurs at lower energies, as is the case in many models motivated by high-energy physics, GW from inflation would have an amplitude that is too low to be observable. On the other hand, in case a global phase transition took place after the end of inflation, then a scale-invariant GW background is also generated [17][18][19]. The detailed calculations show that the amplitude of this new GW background is 2 orders of magnitude greater than that expected from inflation for the same energy scale [19], and thus might be detected directly by the future GW detectors or in the B-mode polarization of the CMB [20].…”

“…On the other hand, in case a global phase transition took place after the end of inflation, then a scale-invariant GW background is also generated [17][18][19]. The detailed calculations show that the amplitude of this new GW background is 2 orders of magnitude greater than that expected from inflation for the same energy scale [19], and thus might be detected directly by the future GW detectors or in the B-mode polarization of the CMB [20]. Finally, another source of GW that may be relevant for interferometric experiments and whose study will be our main target here, is provided by the violent period following the end of inflation.…”

Search for an emission line of a gravitational wave background

“…In this case, by replacing Ω gw ðf k ;ΩÞ with Ω gw ðf k Þ and using Eqs. (6), (10), and (14), we have…”

“…At present, our observational information before the big bang nucleosynthesis is severely limited [1]. Therefore, once primordial GWs are detected, they would provide us with invaluable clues to understand inflation and the subsequent reheating era [2][3][4][5][6] and high energy physics [7][8][9][10]. Also, GW backgrounds (GWBs) bring us opportunities to test gravity theories [11][12][13][14].…”