7 Conclusions6 Gravitational Wave Tests of 6.4 Speed of gravitational waves

6.5 Other strong-gravity tests 

One of the central difficulties of testing general relativity in the strong-field regime is the possibility of contamination by uncertain or complex physics. In the solar system, weak-field gravitational effects could in most cases be measured cleanly and separately from non-gravitational effects. The remarkable cleanliness of the binary pulsar permitted precise measurements of gravitational phenomena in a strong-field context.

Unfortunately, nature is rarely so kind. Still, under suitable conditions, qualitative and even quantitative strong-field tests of general relativity can be carried out.

One example is in cosmology. From a few seconds after the big bang until the present, the underlying physics of the universe is well understood, although significant uncertainties remain (amount of dark matter, value of the cosmological constant, the number of light neutrino families, etc.). Some alternative theories of gravity that are qualitatively different from GR fail to produce cosmologies that meet even the minimum requirements of agreeing qualitatively with big-bang nucleosynthesis (BBN) or the properties of the cosmic microwave background (TEGP 13.2 [147]). Others, such as Brans-Dicke theory, are sufficiently close to GR (for large enough tex2html_wrap_inline3785) that they conform to all cosmological observations, given the underlying uncertainties. The generalized scalar-tensor theories, however, could have small tex2html_wrap_inline3785 at early times, while evolving through the attractor mechanism to large tex2html_wrap_inline3785 today. One way to test such theories is through big-bang nucleosynthesis, since the abundances of the light elements produced when the temperature of the universe was about 1 MeV are sensitive to the rate of expansion at that epoch, which in turn depends on the strength of interaction between geometry and the scalar field. Because the universe is radiation-dominated at that epoch, uncertainties in the amount of cold dark matter or of the cosmological constant are unimportant. The nuclear reaction rates are reasonably well understood from laboratory experiments and theory, and the number of light neutrino families (3) conforms to evidence from particle accelerators. Thus, within modest uncertainties, one can assess the quantitative difference between the BBN predictions of GR and scalar-tensor gravity under strong-field conditions and compare with observations. The most sophisticated recent analysis [49] places bounds on the parameters tex2html_wrap_inline3777 and tex2html_wrap_inline3779 of the generalized framework of Damour and Esposito-Farèse (see Sec.  5.4 and Fig.  8) that are weaker than solar-system bounds for tex2html_wrap_inline5753, but substantially stronger for tex2html_wrap_inline5755 .

Another example is the exploration of the spacetime near black holes via accreting matter. Observations of low-luminosity binary X-ray sources suggest that a form of accretion known as advection-dominated accretion flow (ADAF) may be important. In this kind of flow, the accreting gas is too thin to radiate its energy efficiently, but instead transports (advects) it inward toward the central object. If the central object is a neutron star, the matter hits the surface and radiates the energy away; if it is a black hole, the matter and its advected energy disappear. Systems in which the accreting object is believed to be a black hole from estimates of its mass are indeed observed to be underluminous, compared to systems where the object is believe to be a neutron star. This has been regarded as the first astrophysical evidence for the existence of black hole event horizons (for a review, see [92]). While supporting one of the critical strong-field predictions of GR, the observations and models are not likely any time soon to be able to distinguish one gravitational theory from another (except for theories that do not predict black holes at all).

Another example involving accretion purports to explore the strong-field region just outside massive black holes in active galactic nuclei. Here, iron in the inner region of a thin accretion disk is irradiated by X-ray-emitting material above or below the disk, and fluoresces in the tex2html_wrap_inline5757 line. The spectral shape of the line depends on relativistic Doppler and curved-spacetime effects as the iron orbits the black hole near the innermost stable circular orbit, and could be used to determine whether the hole is a non-rotating Schwarzschild black hole, or a rotating Kerr black hole. Because of uncertainties in the detailed models, the results are inconclusive to date, but the combination of higher-resolution observations and better modelling could lead to striking tests of strong-field predictions of GR.

7 Conclusions6 Gravitational Wave Tests of 6.4 Speed of gravitational waves

image The Confrontation between General Relativity and Experiment
Clifford M. Will
© Max-Planck-Gesellschaft. ISSN 1433-8351
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