Fluctations in the cosmic microwave background arising from low‐frequency gravitational radiation

Abstract

AbstractIntense low‐frequency intergalactic gravitational radiation with wave lengths λ smaller than the HUBBLE distance λH ≌ 3000 (100/H0) Mpc but not exceedingly small compared to λH. generates anisotropies in the microwave background radiation. One contribution results from the local wave field and produces mainly a quadrupole‐type temperature variation on the sky. Available data on large‐scale microwave fluctuations do not exclude appreciable amounts of gravitational background radiation in the Megaparsec wave band. A more sensitive test is provided by a second far‐field contribution, which has a small angular scale. Its amplitude depends strongly on the ratio of the (present) rest mass density to the HUBBLE constant, if a cosmological origin of the blackbody radiation is assumed. In a low‐density universe, pre‐galactic COMPTON scattering of the blackbody radiation is not able to reduce the fluctuations caused by the low‐frequency gravitational wave field. The recent small‐scale data by PARIJSKIJ would allow only small amplitudes of gravitational waves with an energy density significantly below the critical cosmological density. On the other hand, in a high‐density universe, the small angular scale fluctuation in the blackbody radiation is completely damped out, and a gravitational radiation cosmos reaching the critical density is admitted. Independent of the matter density, the data by PARIJSKIJ would confine gravitational background radiation to insignificant amplitudes if a discrete source model for the origin of the microwave background has to be assumed.