Tensor-scalar cosmological models and their relaxation toward general relativity

Abstract

Cosmological models within general tensor-multiscalar theories of gravity are studied. By isolating an autonomous evolution equation for the scalar fields, one shows that the expansion of the Universe during the matter-dominated era tends to drive the scalar fields toward a minimum of the function a(rp) describing their coupling to matter, i.e. , toward a state where the tensor-scalar theory becomes indistinguishable from general relativity. The two main parameters determining the e%ciency of this natural attractor mechanism toward general relativity are the redshift at the beginning of the matter era (or equivalently the present cosmological matter density) and the curvature of the coupling function a(p). Quantitative estimates for the present level of deviation from general relativity, as measured by the post-Newtonian parameters p -1, P -1, and G/G, are derived, which give greater significance to future improvements of solar-system gravitational tests. Another prediction of many tensor-scalar scenarios (whose consequences, particularly for the formation of structure in the Universe, remain to be studied in detail) is the existence of strong oscillations of the effective Newtonian coupling strength during the first few Hubble time scales of the matter era. PACS number(s): 04.80.+z, 04.50.+h, 98.80.Cq, 98.80.Hw

Dark matter relic density in scalar-tensor gravity revisited

“…Although 1−3w is ≈ 0 throughout most of the early universe, there is an important effect that arises when each of the particle species in the cosmic fluid becomes non-relativistic [27]. 7 The choice V (ϕ * ) = 0 does not give a viable late time cosmology in that it cannot account for the recent accelerated expansion of our universe.…”

“…In addition to the screening mechanisms that help shield the scalar field from astrophysical observations, many scalar-tensor gravity models exhibit an inherent attraction mechanism towards General Relativity [26,27]. Throughout its cosmological evolution, the scalar field is driven towards a state where the coupling A(ϕ * ) remains constant so that the scalar-tensor theory is indistinguishable from GR (see section 4).…”

Moduli entrapment with primordial black holes

“…Their energy density dilutes as ρ h ∼ 1/a 6 for the homogeneous mode or as ρ i ∼ 1/a 4 for inhomogeneities [24], where a is the cosmological scale factor. Some aspects of more phenomenologically motivated, but similar, mechanisms have also been discussed in [25,26,27].…”

“…To leading order, the effects of the interactions of the modulus with the black hole gas ρ BH can be described as an environment-induced mass term, similar to the interactions discussed in [25], and more recently to the chameleon fields [38] (see also [39]). The presence of this new mass term ∝ ρ BH suggests a new method for capturing the scalar in a potential minimum.…”

“…Thus as long as the charges hang around the modulus cannot easily escape to either side. This is akin to the environmentally induced stabilization effects of [25]. In what follows we will only consider the effects of the black hole gas populated with only electric and only magnetic charges.…”