Unfortunately, nature is rarely so kind. Still, under suitable conditions, qualitative and even quantitative strong-field tests of GR could 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, in terms of a Standard Model of a nearly spatially flat universe,
old, dominated by dark matter and dark energy. 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 [281]). 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
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. For recent analyses,
see [234, 84, 58, 60].
Another example is the exploration of the spacetime near black holes and neutron stars via accreting matter. Studies of certain kinds of accretion known as advection-dominated accretion flow (ADAF) in low-luminosity binary X-ray sources may yield the signature of the black hole event horizon [185]. The spectrum of frequencies of quasi-periodic oscillations (QPO) from galactic black hole binaries may permit measurement of the spins of the black holes [218]. Aspects of strong-field gravity and frame-dragging may be revealed in spectral shapes of iron fluorescence lines from the inner regions of accretion disks [225, 224]. Because of uncertainties in the detailed models, the results to date of studies like these are suggestive at best, 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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