arXiv subjects

Vincent Comeau

Publications and source records attributed to Vincent Comeau.

7 recordsLinked to original sources

arXiv2026

Evolving Dark Energy from the Back-Reaction of Cosmological Perturbations

We argue that the back-reaction of inhomogeneities, when combined with a pure cosmological constant, produces an effective dark energy whose equation of state can evolve with time. Our analytical computation agrees with recent numerical simulations, which have shown that the back-reaction of inhomogeneities can impact dark energy around the present time. Here, its equation of state is derived from an average version of the Friedmann equations, for which the average is taken over a fixed volume of matter particles (not necessarily global). When treating the inhomogeneities as small perturbations around a homogeneous background, dark energy acquires a time dependence which reduces to a boundary term, at least at quadratic order in the scalar modes of the perturbations. Thus, it becomes negligible for a global average. On the other hand, when treating instead the spatial derivatives of the inhomogeneities perturbatively in a gradient expansion, dark energy gets the same time dependence, which no longer reduces to a boundary term. This time dependence can be significant even for a global average or when the constant spatial curvature induced by the inhomogeneities is negligible. It also keeps the same sign around the present time, thus causing dark energy to remain either always phantom or never phantom, not allowing for any crossing from one type of behaviour to the other.

gr-qc↗

arXiv2023

Gauge-Invariant Scalar-Induced Gravitational Waves from Physical Observables

This paper discusses the gauge issue touching the gravitational waves induced at the second order by the scalar modes of cosmological perturbations. These waves are known to depend on the gauge used for their calculation. In this paper, we propose a simple method of obtaining physically meaningful expressions for such scalar-induced gravitational waves at the leading order. The method is centred on well-defined observables, such as the magnetic part of the Weyl tensor, or the Cotton tensor of a slicing of spacetime, which vanish in the background and do not depend linearly on the scalar perturbations. Generalizing the Stewart-Walker lemma, it is shown that the gravitational waves contributing to such observables at the second order are automatically gauge-invariant, even when the observable itself does not vanish at the first order. In each case, the scalar-induced gravitational waves are related to the ones computed in the Newtonian gauge, first for a general background, and then for the particular case of a spacetime dominated by either radiation or cold matter.

gr-qc↗

arXiv2023

Perturbative Correction to the Average Expansion Rate of Spacetimes with Perfect Fluids

This paper discusses the leading-order correction induced by cosmological perturbations on the average expansion rate of an expanding spacetime, containing one or many perfect fluids. The calculation is carried out up to the second order in the perturbations, and is kept as general as possible. In particular, no approximation such as a long-wavelength or a short-wavelength limit is invoked, and all three types of perturbations (scalar, vector, and tensor) are considered. First, the average value of the expansion rate is computed over a three-dimensional space-like surface where the total density of the fluids is constant. Then, a formula is derived relating that average value to the one over any other surface, on which a different scalar property of the fluids is constant. Moreover, the general formulas giving the correction to the average expansion rate are applied, in particular, to the case of a spacetime containing a single fluid with a constant equation of state. The sign and the effective equation of state of the corresponding back-reaction effect in the first Friedmann equation are examined.

gr-qc↗

arXiv2023

Back-Reaction of Long-Wavelength Cosmological Fluctuations as Measured by a Clock Field

We consider the back-reaction of cosmological fluctuations on the local expansion rate averaged over a space-like hypersurface of constant value of a clock field. We show that in the infrared limit, the fluctuations lead to a decrease in the average expansion rate, measured at a fixed value of the clock field, compared to what would be obtained in a homogeneous universe. We work in the context of Einstein gravity coupled to perfect fluid matter.

gr-qc↗

arXiv2022

Non-linear stability of $α'$-corrected Friedmann equations

We study the non-linear stability of fixed-point solutions to the $α'$-exact equations from O$(d,d)$ invariant cosmology, with and without matter perturbations. Previous non-linear analysis in the literature is revisited, and its compatibility with known linear perturbation results is shown. Some formal aspects of cosmological perturbations in duality invariant cosmology are discussed, and we show the existence of time-reparameterization invariant variables for perturbations.

hep-th↗

arXiv2022

Clock Fields and Logarithmic Decay of Dark Energy

We investigate the physical measurability of the infrared instability of a de Sitter phase in the formalism recently proposed by Kitamoto et al.. We find that the logarithmic decay of the effective cosmological constant is only measurable if an additional clock field is introduced.

hep-th↗

arXiv2020

Further Results on a Function Relevant for Conformal Blocks

We present further mathematical results on a function appearing in the conformal blocks of four-point correlation functions with arbitrary quasi-primary operators. The $H$-function was introduced in a previous article and it has several interesting properties. We prove explicitly the recurrence relation as well as the $D_6$-invariance presented previously. We also demonstrate the proper action of the differential operator used to construct the $H$-function.

hep-th↗