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Register a new userThe Promise of Low-Frequency Gravitational Wave Astronomy
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T. A. Prince
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This Astro2010 science white paper provides an overview of the opportunities in low-frequency gravitational-wave astronomy, a new field that is poised to make significant advances. While discussing the broad context of gravitational-wave astronomy, this paper concentrates on the low-frequency region (10^(-5) to 1 Hz), a frequency range abundantly populated in strong sources of gravitational waves including massive black hole mergers, ultra-compact stellar-mass galactic binaries, and capture of compact objects by massive black holes in the nuclei of galaxies.
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The focus of this Chapter is on describing the prospective sources of the gravitational wave universe accessible to present and future observations, from kHz, to mHz down to nano-Hz frequencies. The multi-frequency gravitational wave universe gives a deep view into the cosmos, inaccessible otherwise. It has as main actors core-collapsing massive stars, neutron stars, coalescing compact object binaries of different flavours and stellar origin, coalescing massive black hole binaries, extreme mass ratio inspirals, and possibly the very early universe itself. Here, we highlight the science aims and describe the gravitational wave signals expected from the sources and the information gathered in it. We show that the observation of gravitational wave sources will play a transformative role in our understanding of the processes ruling the formation and evolution of stars and black holes, galaxy clustering and evolution, the nature of the strong forces in neutron star interiors, and the most mysterious interaction of Nature: gravity. The discovery, by the LIGO Scientific Collaboration and Virgo Collaboration, of the first source of gravitational waves from the cosmos GW150914, and the superb technological achievement of the space mission LISA Pathfinder herald the beginning of the new phase of exploration of the universe.
The new field of gravitational wave astrophysics requires a growing pool of students and researchers with unique, interdisciplinary skill sets. It also offers an opportunity to build a diverse, inclusive astronomy community from the ground up. We describe the efforts used by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) NSF Physics Frontiers Center to foster such growth by involving students at all levels in low-frequency gravitational wave astrophysics with pulsar timing arrays (PTAs) and establishing collaboration policies that ensure broad participation by diverse groups. We describe and illustrate the impact of these techniques on our collaboration as a case study for other distributed collaborations.
On a time scale of years to decades, gravitational wave (GW) astronomy will become a reality. Low frequency (nanoHz) GWs are detectable through long-term timing observations of the most stable pulsars. Radio observatories worldwide are currently carrying out observing programmes to detect GWs, with data sets being shared through the International Pulsar Timing Array project. One of the most likely sources of low frequency GWs are supermassive black hole binaries (SMBHBs), detectable as a background due to a large number of binaries, or as continuous or burst emission from individual sources. No GW signal has yet been detected, but stringent constraints are already being placed on galaxy evolution models. The SKA will bring this research to fruition. In this chapter, we describe how timing observations using SKA1 will contribute to detecting GWs, or can confirm a detection if a first signal already has been identified when SKA1 commences observations. We describe how SKA observations will identify the source(s) of a GW signal, search for anisotropies in the background, improve models of galaxy evolution, test theories of gravity, and characterise the early inspiral phase of a SMBHB system. We describe the impact of the large number of millisecond pulsars to be discovered by the SKA; and the observing cadence, observation durations, and instrumentation required to reach the necessary sensitivity. We describe the noise processes that will influence the achievable precision with the SKA. We assume a long-term timing programme using the SKA1-MID array and consider the implications of modifications to the current design. We describe the possible benefits from observations using SKA1-LOW. Finally, we describe GW detection prospects with SKA1 and SKA2, and end with a description of the expectations of GW astronomy.
In this paper, we systematically study gravitational waves (GWs) produced by remote compact astrophysical sources. To describe such GWs properly, we introduce three scales, $lambda, ; L_c$ and $L$, denoting, respectively, the typical wavelength of GWs, the scale of the cosmological perturbations, and the size of the observable universe. For GWs to be detected by the current and foreseeable detectors, the condition $lambda ll L_c ll L$ holds, and such GWs can be well approximated as high-frequency GWs. In order for the backreaction of the GWs to the background to be negligible, we must assume that $left|h_{mu u}right| ll 1$, in addition to the condition $epsilon ll 1$, which are also the conditions for the linearized Einstein field equations for $h_{mu u}$ to be valid, where $g_{mu u} = gamma_{mu u} + epsilon h_{mu u}$, and $gamma_{mu u}$ denotes the background. To simplify the field equations, we show that the spatial, traceless, and Lorentz gauge conditions can be imposed simultaneously, even when the background is not vacuum, as long as the high-frequency GW approximation is valid. However, to develop the formulas that can be applicable to as many cases as possible, we first write down explicitly the linearized Einstein field equations by imposing only the spatial gauge. Applying the general formulas together with the geometrical optics approximation to such GWs, we find that they still move along null geodesics and its polarization bi-vector is parallel-transported, even when both the cosmological scalar and tensor perturbations are present. In addition, we also calculate the gravitational integrated Sachs-Wolfe effects, whereby the dependences of the amplitude, phase and luminosity distance of the GWs on these two kinds of perturbations are read out explicitly.
In this paper we present the results of the first low frequency all-sky search of continuous gravitational wave signals conducted on Virgo VSR2 and VSR4 data. The search covered the full sky, a frequency range between 20 Hz and 128 Hz with a range of spin-down between $-1.0 times 10^{-10}$ Hz/s and $+1.5 times 10^{-11}$ Hz/s, and was based on a hierarchical approach. The starting point was a set of short Fast Fourier Transforms (FFT), of length 8192 seconds, built from the calibrated strain data. Aggressive data cleaning, both in the time and frequency domains, has been done in order to remove, as much as possible, the effect of disturbances of instrumental origin. On each dataset a number of candidates has been selected, using the FrequencyHough transform in an incoherent step. Only coincident candidates among VSR2 and VSR4 have been examined in order to strongly reduce the false alarm probability, and the most significant candidates have been selected. Selected candidates have been subject to a follow-up by constructing a new set of longer FFTs followed by a further incoherent analysis, still based on the FrequencyHough transform. No evidence for continuous gravitational wave signals was found, therefore we have set a population-based joint VSR2-VSR4 90$%$ confidence level upper limit on the dimensionless gravitational wave strain in the frequency range between 20 Hz and 128 Hz. This is the first all-sky search for continuous gravitational waves conducted, on data of ground-based interferometric detectors, at frequencies below 50 Hz. We set upper limits in the range between about $10^{-24}$ and $2times 10^{-23}$ at most frequencies. Our upper limits on signal strain show an improvement of up to a factor of $sim$2 with respect to the results of previous all-sky searches at frequencies below $80~mathrm{Hz}$.
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