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...es, may produce GW bursts that can only be modeled imperfectly, if at all, and more general analysis techniques are needed. (iii) Continuous, narrow-band waveforms: the continuous wave sources search. An example of a continuous source of GWs with a well-modeled waveform is a spinning NS (e.g. a pulsar) that is not perfectly symmetric about its rotation axis [54]. (iv) Continuous, broadband waveforms: the stochastic gravitational-wave background search. Processes operating in the early universe, for example, could have produced a background of GWs that is continuous but stochastic in character [55]. In the following sections we review the astrophysical results that have been generated in each of these search categories using LIGO data; [56] contains links to all the LIGO observational publications. To date, no GW signal detections have been made, so these results are all upper limits on various GW sources. In those cases where the S5 analysis is not yet complete, we present the most recent published results and also discuss the expected sensitivity, or astrophysical reach, of the search based on the S5 detector performance. 15 Rep. Prog. Phys. 72 (2009) 076901 B P Abbott et al 7.1. Comp...

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...vable. A particularly interesting source of stochastic waves is the dynamics of the early Universe, which could produce an all-sky GW background, similar to the cosmic microwave background; see Refs. =-=[87, 88, 89]-=- for detailed reviews. Stochastic waves can be generated in the early Universe via a variety of mechanisms: amplification of primordial fluctuations in the Universe’s geometry via inflation, phase tra...

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...ms: the stochastic gravitational-wave background search. Processes operating in the early universe, for example, could have produced a background of GWs that is continuous but stochastic in character =-=[55]-=-. In the following sections we review the astrophysical results that have been generated in each of these search categories using LIGO data; reference [56] contains links to all the LIGO observational...

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...er construction or in advanced project. The amplitude and frequency sensitivity of some of these instruments are in the proper range to probe many interesting astrophysical and cosmological processes =-=[1]-=-. The most interesting project from our point of view is LISA (Laser Interferometer Space Antenna) which is scheduled to be launched around the end of this decade and will possibly achieve a sensitivi...

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...linear scale. k (43) 4 Present density of gravitational waves The effective energy density of a stochastic background of gravitational waves, on scales much smaller than the Hubble scale, is given by =-=[33]-=- ρGW = 1 32πG 〈˙ hij ˙ h ij k 〉 = 2 32πGa2 ∫ d(lnk) Ph(k, η) . (44) Note that our second-order tensor modes (22) produced by linear scalar perturbations become constant on sub-Hubble scales in the ear...

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... of their random nature, stochastic GWs look just like noise. Ground-based detectors will measure stochastic backgrounds by comparing data at multiple sites and looking for ‘noise’ that is correlated =-=[88, 92]-=-. For comparing to a detector’s noise, one should construct the characteristic stochastic wave strain, h ∝ f−3/2√gw(f )f , (6.7) where f is the frequency band across which the measurement is made. ...

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...acetime perturbations which were stretched and amplified to classical scales during inflation. Exponential inflation contributes a flat spectrum to Qaw(f) today over a very large range of frequencies =-=[50]-=-. The current best limits on gravitational waves from inflation come from the smallness (10-6) of large scale temperature anisotropies measured in the CMB. Long wavelength gravitational waves create o...