高能暂现源 · 每日文献综述

The presence of a gravitational wave background (GWB) can be established not only via exquisitely precise pulsar timing array (PTA) measurements, but also via astrometric observations. Indeed, the very same background responsible for the delay in the arrival time of pulse causes an apparent displacement of galactic objects as stars and asteroids. In this work we provide a framework that allows to derive the displacement of sources overcoming the usually adopted "infinite distance" approximation. We also present how this formalism can be used to study the displacements of objects at distances comparable to the GW wavelength, as asteroids, and of objects with a non-trivial three-dimensional distribution, as stars in the Milky Way. Thus, it can be used to probe frequencies beyond PTA experiments, reaching the mHz GWs, also detectable by LISA. We forecast the capability of observing the astrometric deflection induced by a GWB evaluating the harmonic signal-to-noise ratio including correlations between different probes. We find an SNR greater than one for the relevant cases considered and as a consequence a promising Fisher forecast, suggesting a constraining power up to the percent level for a flat background.

The large-scale limits on the relic signals of gravitational radiation complement the bounds coming from the interferometric detectors (in the audio band) and from the pulsar timing arrays (in the nHz range). Within this inclusive perspective, the spectral energy density of the gravitons is sharply suppressed in the aHz region even though the high-frequency signal can be comparatively much larger both in the kHz and GHz domains. For there are no direct tests on the expansion rate prior to the formation of the light nuclei, a modified postinflationary timeline affects the total number of e-folds and additionally suppresses the tensor-to-scalar ratio by making the relic signals effectively invisible in the aHz range. The expansion rate prior to nucleosynthesis is further bounded by the evolution of the hypercharge field and the large-scale magnetism also constrains the decelerated expansion rate. The magnetogenesis requirements are compatible with a potentially detectable spectral energy density of the relic gravitons between the MHz and the THz while the tensor-to-scalar ratio remains suppressed in the aHz region. A maximum of the spectral energy density of the gravitons in the audio domain leads instead to a larger magnetic field when the scale of the gravitational collapse of the protogalaxy (of the order of the Mpc) gets comparable to the Hubble radius before equality. Along a converse viewpoint, the results obtained here imply that a long decelerated stage expanding faster than radiation does not affect the high-frequency range but reduces the effective number of e-folds by so enhancing the tensor-to-scalar ratio, possibly beyond its observational limit.