The first space-based gravitational-wave detectors

Abstract

Gravitational waves provide a laboratory for general relativity and a window to energetic astrophysical phenomena invisible with electromagnetic radiation. Several terrestrial detectors are currently under construction, and a space-based interferometer is envisioned for launch early next century to detect test-mass motions induced by waves of relatively short wavelength. Very-long-wavelength gravitational waves can be detected using the plasma in the early Universe as test masses; the motion induced in the plasma by a wave is imprinted onto the cosmic microwave background ͑CMB͒. While the signature of gravitational waves on the CMB temperature fluctuations is not unique, the polarization pattern can be used to unambiguously detect gravitational radiation. Thus, forthcoming CMB polarization experiments, such as the Microwave Anisotropy Probe and Planck, will be the first space-based gravitational-wave detectors. ͓S0556-2821͑99͒01002-4͔ PACS number͑s͒: 95.55. Ym, 04.80.Nn, 98.70.Vc One of the most spectacular predictions of general relativity is the existence of gravitational waves. A gravitational wave conveys information about the motions of mass and ripples in curvature-the shape of spacetime. Detection of gravitational radiation would allow us to probe ''invisible'' astrophysical phenomena hidden from view by absorption of electromagnetic radiation. Observations of the binary pulsar PSR1913ϩ16, which confirm the orbital-inspiral rate due to the emission of gravitational waves ͓1͔, bring us tantalizingly close to this goal. However, we would still like to detect gravitational radiation directly. Thus, a variety of efforts are now under way to detect gravitational waves ͓2͔. Here, we show that forthcoming maps of the polarization of the cosmic microwave background ͑CMB͒ can be used to detect very-long-wavelength gravitational radiation.Gravitational waves are detected by observing the motion they induce in test masses ͓3,4͔. High-frequency (1 -10 4 Hz) gravitational waves, produced by the inspiral and catastrophic collision of astrophysical objects, may be detectable by terrestrial laser interferometers ͓e.g., the Laser Interferometric Gravitational Wave Observatory ͑LIGO͒ ͓5͔͔ or resonant-mass antennae currently under construction. Low-frequency (10 Ϫ4 Ϫ10 Ϫ1 Hz) gravitational waves, produced by the orbital motion of binaries, could be detected by the Laser Interferometer Space Antenna ͑LISA͒ ͓6͔, a spacebased interferometer targeted for launch circa 2015.How can one detect ultra-low-frequency (10 Ϫ15-10 Ϫ18 Hz) gravitational radiation, with wavelengths comparable to the size of the observable Universe? The photonbaryon fluid in the early Universe acts as a set of test masses for such waves. A gravitational wave in this frequency range alternately squeezes and stretches the primordial plasma. Just as a resonant-mass detector is equipped with electronics to monitor the oscillation modes of the test body, the CMB photons are the electromagnetic signal upon which these plasma motions are imprinted.One might des...

Space missions to detect the cosmic gravitational-wave background

“…To measure the CGB spectrum below 10 −8 Hz requires a return, full circle, to the world of CMB physics. Detailed polarization measurements of the CMB can be used to infer [21] the CGB power spectrum for frequencies in the range 10 −18 → 10 −16 Hz.…”

CMB polarization in theories of gravitation with massive gravitons

“…The two scalar fields, plus the two components of the transverse vector field and the six modes of the tensor field give exactly the required ten degrees of freedom. The mass lagrangian for this model can be constructed analogously as in (22), that is, it appears as a quadratic term in the metric (45). The full action is then obtained by adding up this contribution to the usual Einstein-Hilbert one, and the Einstein equations can be derived using the standard tools.…”

The Quest for B Modes from Inflationary Gravitational Waves

“…We return to these equations later, but for now we show in Figure 4b the resulting CMB temperature-polarization pattern induced by one GW propagating in theẑ direction (Caldwell et al 1999). The quadrupolar variation (i.e., the cos 2φ dependence) of the temperaturepolarization pattern can be seen as one travels along a curve of constant latitude, and the wave-like pattern can be seen as one moves along a line of constant longitude.…”

“…by complementing the CMB measurement, which probes GWs with ∼10 −17 Hz frequencies, with measurements at far higher frequencies (Chongchitnan & Efstathiou 2006;Smith et al 2006aSmith et al , 2008. The idea to seek the inflationary background with GW detectors was considered by Liddle (1994), Bar-Kana (1994), Turner (1997), Smith et al (2006aSmith et al ( , 2008, and Caldwell et al (1999), and has motivated mission concept studies for space-based GW observatories like the Big Bang Observer (Phinney et al 2004, Crowder & Cornish 2005 and DECIGO (DECi-hertz Interferometer Gravitational wave Observatory; Seto et al 2001).…”