Stochastic Gravitational Wave Background: Upper Limits in the 10−6to 10−3Hz Band

Abstract

We have used precision Doppler tracking of the Cassini spacecraft during its 2001-2002 solar opposition to derive improved observational limits to an isotropic background of low-frequency gravitational waves. Using the Cassini multilink radio system and an advanced tropospheric calibration system, the effects of heretofore leading noises—plasma and tropospheric scintillation—were, respectively, removed and calibrated to levels lower than other noises. The resulting data were used to construct upper limits to the strength of an isotropic background in the 10^-6 to 10^-3 Hz band. Our results are summarized as limits on the strain spectrum S_h( f), the characteristic strain (h_c = the square root of the product of the frequency and the one-sided spectrum of strain at that frequency), and the energy density (Ω = energy density in bandwidth equal to center frequency assuming a locally white energy density spectrum, divided by the critical density). Our best limits are S_h( f) < 6 × 10^-27 Hz^-1 at several frequencies in the millihertz band, h_c < 2 × 10^-15 at about 0.3 mHz, and Ω < 0.025 × h, where h₇₅ is the Hubble constant in units of 75 km s^-1 Mpc^-1, at 1.2 × 10^-6 Hz. These are the best observational limits in the low-frequency band, the bound on Ω, for example, being about 3 orders of magnitude better than previous constraints from Doppler tracking.