4.4 Cosmological constraints

Cosmological observations are more difficult to use in order to set constraints on the time variation of G. In particular, they require to have some ideas about the whole history of G as a function of time but also, as the variation of G reflects an extension of General relativity, it requires to modify all equations describing the evolution (of the universe and of the large scale structure) in a consistent way. We refer to [504, 502, 506] for a discussion of the use of cosmological data to constrain deviations from general relativity.

4.4.1 Cosmic microwave background

A time-dependent gravitational constant will have mainly three effects on the CMB angular power spectrum (see [435Jump To The Next Citation Point] for discussions in the framework of scalar-tensor gravity in which G is considered as a field):

  1. The variation of G modifies the Friedmann equation and therefore the age of the Universe (and, hence, the sound horizon). For instance, if G is larger at earlier time, the age of the Universe is smaller at recombination, so that the peak structure is shifted towards higher angular scales.
  2. The amplitude of the Silk damping is modified. At small scales, viscosity and heat conduction in the photon-baryon fluid produce a damping of the photon perturbations. The damping scale is determined by the photon diffusion length at recombination, and therefore depends on the size of the horizon at this epoch, and hence, depends on any variation of the Newton constant throughout the history of the Universe.
  3. The thickness of the last scattering surface is modified. In the same vein, the duration of recombination is modified by a variation of the Newton constant as the expansion rate is different. It is well known that CMB anisotropies are affected on small scales because the last scattering “surface” has a finite thickness. The net effect is to introduce an extra, roughly exponential, damping term, with the cutoff length being determined by the thickness of the last scattering surface. When translating redshift into time (or length), one has to use the Friedmann equations, which are affected by a variation of the Newton constant. The relevant quantity to consider is the visibility function g. In the limit of an infinitely thin last scattering surface, τ goes from ∞ to 0 at recombination epoch. For standard cosmology, it drops from a large value to a much smaller one, and hence, the visibility function still exhibits a peak, but it is much broader.

In full generality, the variation of G on the CMB temperature anisotropies depends on many factors: (1) modification of the background equations and the evolution of the universe, (2) modification of the perturbation equations, (3) whether the scalar field inducing the time variation of G is negligible or not compared to the other matter components, (4) on the time profile of G that has to be determine to be consistent with the other equations of evolution. This explains why it is very difficult to state a definitive constraint. For instance, in the case of scalar-tensor theories (see below), one has two arbitrary functions that dictate the variation of G. As can be seen, e.g., from [435Jump To The Next Citation Point, 378Jump To The Next Citation Point], the profiles and effects on the CMB can be very different and difficult to compare. Indeed, the effects described above are also degenerate with a variation of the cosmological parameters.

In the case of Brans–Dicke theory, one just has a single constant parameter ωBD characterizing the deviation from general relativity and the time variation of G. Thus, it is easier to compare the different constraints. Chen and Kamionkowski [94] showed that CMB experiments such as WMAP will be able to constrain these theories for ωBD < 100 if all parameters are to be determined by the same CMB experiment, ωBD < 500 if all parameters are fixed but the CMB normalization and ωBD < 800 if one uses the polarization. For the Planck mission these numbers are respectively, 800, 2500 and 3200. [2] concluded from the analysis of WMAP, ACBAR, VSA and CBI, and galaxy power spectrum data from 2dF, that ωBD > 120, in agreement with the former analysis of [378]. An analysis [549] indictates that The ‘WMAP-5yr data’ and the ‘all CMB data’ both favor a slightly non-zero (positive) G˙∕G but with the addition of the SDSS poser spectrum data, the best-fit value is back to zero, concluding that − 0.083 < ΔG ∕G < 0.095 between recombination and today, which corresponds to − 1.75 × 10−12 yr−1 < G˙∕G < 1.05 × 10 −12 yr− 1.

From a more phenomenological prospect, some works modeled the variation of G with time in a purely ad-hoc way, for instance [89] by assuming a linear evolution with time or a step function.

4.4.2 BBN

As explained in detail in Section 3.8.1, changing the value of the gravitational constant affects the freeze-out temperature Tf. A larger value of G corresponds to a higher expansion rate. This rate is determined by the combination G ρ and in the standard case the Friedmann equations imply that G ρt2 is constant. The density ρ is determined by the number N∗ of relativistic particles at the time of nucleosynthesis so that nucleosynthesis allows to put a bound on the number of neutrinos N ν. Equivalently, assuming the number of neutrinos to be three, leads to the conclusion that G has not varied from more than 20% since nucleosynthesis. But, allowing for a change both in G and N ν allows for a wider range of variation. Contrary to the fine structure constant the role of G is less involved.

The effect of a varying G can be described, in its most simple but still useful form, by introducing a speed-up factor, ξ = H ∕HGR, that arises from the modification of the value of the gravitational constant during BBN. Other approaches considered the full dynamics of the problem but restricted themselves to the particular class of Jordan–Fierz–Brans–Dicke theory [1Jump To The Next Citation Point, 16, 26Jump To The Next Citation Point, 84Jump To The Next Citation Point, 102, 128, 441Jump To The Next Citation Point, 551Jump To The Next Citation Point] (Casas et al. [84] concluded from the study of helium and deuterium that ωBD > 380 when N ν = 3 and ωBD > 50 when N ν = 2.), of a massless dilaton with a quadratic coupling [105Jump To The Next Citation Point, 106Jump To The Next Citation Point, 134Jump To The Next Citation Point, 446] or to a general massless dilaton [455]. It should be noted that a combined analysis of BBN and CMB data was investigated in [113, 292]. The former considered G constant during BBN while the latter focused on a nonminimally quadratic coupling and a runaway potential. It was concluded that from the BBN in conjunction with WMAP determination of η set that ΔG ∕G has to be smaller than 20%. However, we stress that the dynamics of the field can modify CMB results (see previous Section 4.4.1) so that one needs to be careful while inferring Ωb from WMAP unless the scalar-tensor theory has converged close to general relativity at the time of decoupling.

In early studies, Barrow [26] assumed that G ∝ t−n and obtained from the helium abundances that − 5.9 × 10−3 < n < 7 × 10 −3, which implies that |G˙∕G| < (2 ± 9.3)h × 10−12 yr−1, assuming a flat universe. This corresponds in terms of the Brans–Dicke parameter to ω > 25 BD. Yang et al. [551] included the deuterium and lithium to improve the constraint to −3 n < 5 × 10, which corresponds to ωBD > 50. It was further improved by Rothman and Matzner [441] to −3 |n | < 3 × 10 implying |G˙∕G | < 1.7 × 10−13 yr−1. Accetta et al. [1] studied the dependence of the abundances of D, 3He, 4He and 7Li upon the variation of G and concluded that − 0.3 < ΔG ∕G < 0.4, which roughly corresponds to |G˙∕G | < 9 × 10 −13 yr− 1. All these investigations assumed that the other constants are kept fixed and that physics is unchanged. Kolb et al. [295Jump To The Next Citation Point] assumed a correlated variation of G, αEM and GF and got a bound on the variation of the radius of the extra dimensions.

Although the uncertainty in the helium-4 abundance has been argued to be significantly larger that what was assumed in the past [401], interesting bounds can still be derived [117Jump To The Next Citation Point]. In particular translating the bound on extra relativistic degress of freedom (− 0.6 < δN ν < 0.82) to a constraint on the speed-up factor (0.949 < ξ < 1.062), it was concluded [117], since ΔG ∕G = ξ2 − 1 = 7δN ν∕43, that

ΔG-- − 0.10 < G < 0.13. (154 )

The relation between the speed-up factor, or an extra number of relativistic degrees of freedom, with a variation of G is only approximate since it assumes that the variation of G affects only the Friedmann equation by a renormalization of G. This is indeed accurate only when the scalar field is slow-rolling. For instance [105Jump To The Next Citation Point], the speed-up factor is given (with the notations of Section 5.1.1) by

A (φ∗)1 + α (φ∗)φ′∗ 1 ξ = -------∘------2′-- ∘------2- A0 1 − φ∗ ∕3 1 + α0
so that
′ 2 ξ2 = -G---(1-+-α-(φ-∗)φ∗)---, (155 ) G0 (1 + α2)(1 − φ2∗′∕3)
so that 2 ΔG ∕G0 = ξ − 1 only if α ≪ 1 (small deviation from general relativity) and ′ φ ∗ ≪ 1 (slow rolling dilaton). The BBN in scalar-tensor theories was investigated [105Jump To The Next Citation Point, 134Jump To The Next Citation Point] in the case of a two-parameter family involving a non-linear scalar field-matter coupling function. They concluded that even in the cases where before BBN the scalar-tensor theory was far from general relativity, BBN enables to set quite tight constraints on the observable deviations from general relativity today. In particular, neglecting the cosmological constant, BBN imposes 2 −6.5 −1 2 −3∕2 α0 < 10 β (Ωmath ∕0.15) when β > 0.5 (with the definitions introduced below Equation (164View Equation)).

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