Gravitational wave astronomy

Abstract

Gravitational waves (GW) were predicted by Einstein as a consequence of his theory of general relativity. They are produced when mass and energy distributions change their configuration; they present themselves as wave- like distortions of the space-time metric. Only extreme cosmological and astrophysical events may produce GWs at a detectable leveI. This includes the early universe, supernovae, binary compact star systems and pulsars. GWs from such systems will enable us to observe the Universe in a way complementary to what electromagnetic waves (and traditional astronomy) have offered us thus faro Two ground-based observatories, LIGO and Virgo, aim at detecting these waves directly. LIGO -- led by Caltech and MIT -- and Virgo --Ied by a French-Italian consortium-- use kilometer-scale laser interferometers in order to detect the quadrupolar strain characteristic of these waves. When the incoming wave interacts with the interferometer, such strain gives rise to differential changes of the length of the two arms which is measured through laser interferometry. The two observatories have completed their initial phase (2001-2010) without detecting GWs but only setting upper limits on their flux and possible strength. A second generation of these instruments is currently being installed at the two LIGO sites (one in Livingston, LA and another one in Hanford, WA) and at the Virgo site (near Pisa, Italy). This second generation interferometers, called Advanced LIGO (aLlGO) and Advanced Virgo (AdV), will offer an improvement in sensitivity of an order of magnitude with respect to the first generation instruments. Such an improvement will result to three orders of magnitude increase in the number of accessible astrophysical sources and it is expected to make the first direct detection of gravitational-wave sources. aLlGO and AdV will come online respectively in 2015 and 2016 and continue through commissioning and science running until they reach design sensitivity in 2019.Our proposal is focused in using GWs to study compact objects such as black holes (BH) and neutron stars (NS). Among the sources of detectable GWs, compact star binary systems made up of BH and/or NS are the most promising sources. Using Bayesian parameter estimation and model selection algorithms, we will be able to extract from the detected signals fundamental properties of BH and NS. These measurements will be in some cases complementary to what can be done with traditional astronomy, while in other cases they will enable new research. A Bayesian framework can be used to estimate the physical parameters of GW sources and to rank competing models which may describe the data (an example of such models may be the equation of state of NS). The problem of performing parameter estimation of GWs, which may depend on up to 15 unknown parameters, is non-trivial. Sophisticated stochastic samplers, such as Markov chain Monte Carlo or Nested Sampling, must be used to efficiently explore a highly correlated and multi-modal parameter space. We propose to bring together our data analysis and waveform modeling expertise in order to fully explore the capabilities provided by the gravitational-wave detectors in performing astrophysical measurements and observations. (AU)