LISA sensitivities to gravitational waves from relativistic metric theories of gravity

Abstract

The direct observation of gravitational waves will provide a unique tool for probing the dynamical properties of highly compact astrophysical objects, mapping ultra-relativistic regions of space-time, and testing Einstein's general theory of relativity. LISA (Laser Interferometer Space Antenna), a joint NASA-ESA mission to be launched in the next decade, will perform these scientific tasks by detecting and studying low-frequency cosmic gravitational waves through their influence on the phases of six modulated laser beams exchanged between three remote spacecraft. By directly measuring the polarization components of the waves LISA will detect, we will be able to test Einstein's theory of relativity with good sensitivity. Since a gravitational wave signal predicted by the most general relativistic metric theory of gravity accounts for six polarization modes (the usual two Einstein's tensor polarizations as well as two vector and two scalar wave components), we have derived the LISA Time-Delay Interferometric responses and estimated their sensitivities to vectorand scalar-type waves. We find that (i) at frequencies larger then roughly the inverse of the oneway light time (≈ 6 × 10 −2 Hz.) LISA is more than ten times sensitive to scalar-longitudinal and vector signals than to tensor and scalar-transverse waves, and (ii) in the low part of its frequency band is equally sensitive to tensor and vector waves and somewhat less sensitive to scalar signals.

Pulsar timing sensitivities to gravitational waves from relativistic metric theories of gravity

“…Although the one-way Doppler response to a general GW was first derived by Hellings [10], a simpler and more compact expression for it was recently obtained by us [8], which is identical in form to that first obtained by Wahlquist [11] for GWs with s = ±2.…”

“…In what follows we will focus instead on the single-antenna pulsar timing sensitivity to GWs characterized by the presence of all possible polarization modes. This work is a follow up to our recent derivation of the Laser Interferometer Space Antenna (LISA) sensitivities to GWs predicted by the most general relativistic metric theory of gravity [8]. There we showed, in particular, that the LISA sensitivity to scalar-longitudinal waves is significantly better than that to tensor waves when the wavelength of the gravitational wave signal is shorter then the size of the detector.…”

“…In the case of pulsar timing experiments the resulting amplification to waves with longitudinal components over those purely transverse is rather significant due to their operational frequency bandwidth (10 −9 − 10 −6 Hz) and typical pulsar-Earth distance (≈ 1 kpc) [2]. In the same spirit of [8], here we evaluate the pulsar timing sensitivities following the standard definition used for all other GW detectors: the strength of a sinusoidal gravitational wave signal, averaged over the sky and polarization states, required to achieve a given signal-to-noise ratio (SNR) over a specified integration time, as a function of Fourier frequency.…”

“…II we provide the expression for the TOAR response to GWs with helicity s = 0, ± 1 and ±2. Since the time-derivative of the TOAR is equal to the Doppler response, we do not derive its expression here as this was presented in our earlier publication [8]. In Sec.…”

“…If we denote with y GW (t) the relative frequency changes induced by a gravitational wave signal on the radio link between a pulsar and the Earth, its expression is equal to [8] [12]…”