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
) that they conform to all cosmological observations, given the
underlying uncertainties. The generalized scalar-tensor theories,
however, could have small
at early times, while evolving through the attractor mechanism
to large
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
and
of the generalized framework of Damour and Esposito-Farèse (see
Sec.
5.4
and Fig.
8) that are weaker than solar-system bounds for
, but substantially stronger for
.
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
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.
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The Confrontation between General Relativity and
Experiment
Clifford M. Will http://www.livingreviews.org/lrr-2001-4 © Max-Planck-Gesellschaft. ISSN 1433-8351 Problems/Comments to livrev@aei-potsdam.mpg.de |