Ref HAL: in2p3-00115601_v1
Ref Arxiv: astro-ph/0605532
DOI: 10.1063/1.2399692
Ref. & Cit.: NASA ADS
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Résumé:

The late-time accelerated expansion of the universe is a major challenge for cosmology. It may well be that a solution to this problem will require a theory of gravitation beyond General Relativity. It is emphasized that precision cosmology will strongly constrain the possibilities by using observational data probing the background as well as the inhomogeneities.

Commentaires: Invited talk at Albert Einstein's Century international conference, 18-22 July 2005, Paris, France

Ref HAL: in2p3-00115597_v1
Ref Arxiv: astro-ph/0605384
DOI: 10.1142/S0218271807010936
WoS: 000252849500002
Ref. & Cit.: NASA ADS
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84 Citations
Résumé:

In the recent paper astro-ph/0603703 we have shown that $f(R)=R+\mu R^n$ modified gravity dark energy models are not cosmologically viable because during the matter era that precedes the accelerated stage the cosmic expansion is $a ~ t^{1/2}$ rather than $t^{2/3}$. In this short note we wish to comment on the recent paper astro-ph/0604431 which criticised our results. We show here that the $R^n$ models presented in astro-ph/0604431 cannot generate a stage with $a ~ t^{2/3}$ preceding a stage of accelerated expansion. Hence, though acceptable $f(R)$ dark energy models might exist, the $R^n$ models presented in astro-ph/0604431 are not viable, confirming our previous results.

Commentaires: 3 pages

Ref HAL: in2p3-00149208_v1
DOI: 10.1002/andp.200510192
WoS: 000237317400009
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11 Citations
Résumé:

The observed recent accelerated expansion of the Universe constitutes a dramatic departure from traditional views. The mechanism behind this accelerated expansion is still mysterious and is a major challenge to present-day cosmology. Though it can be due to a new perfect fluid with negative pressure coined Dark Energy, it could also originate from modifications of gravity. We present some Dark Energy models in the context of General Relativity and review in some details the properties of Dark Energy models in scalar-tensor gravity. We emphasize that observations probing both the background and the perturbations will constrain the proposed model most effectively.

Ref HAL: in2p3-00115618_v1
PMID 17501182
Ref Arxiv: astro-ph/0603703
DOI: 10.1103/PhysRevLett.98.131302
WoS: 000245331500015
Ref. & Cit.: NASA ADS
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420 Citations
Résumé:

We show that all f(R) modified gravity theories are conformally identical to models of quintessence in which matter is coupled to dark energy with a large coupling. This strong coupling induces a cosmological evolution radically different from standard cosmology. We find that in all f(R) theories that behave as a power of R at large or small R (which include all those proposed so far in the literature) the scale factor during the matter phase grows as t^{1/2} instead of the standard law t^{2/3}. This behaviour is grossly inconsistent with cosmological observations (e.g. WMAP), thereby ruling out these models even if they pass the supernovae test and can escape the local gravity constraints.

Commentaires: 4 pages

Ref HAL: hal-00288524_v1
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Ref HAL: in2p3-00025705_v1
Ref Arxiv: gr-qc/0501021
DOI: 10.1142/S0217732305016622
WoS: 000228162600003
Ref. & Cit.: NASA ADS
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3 Citations
Résumé:

Finding signatures of quantum gravity in cosmological observations is now actively pursued both from the theoretical and the experimental side. Recent work has concentrated on finding signatures of light-cone fluctuations in the CMB. Because in inflationary scenarios a Gravitational Wave Background (GWB) is always emitted much before the CMB, we can ask, in the hypothesis where this GWB could be observed, what is the imprint of light cone fluctuations on this GWB. We show that due to the flat nature of the GWB spectrum, the effect of lightcone fluctuations are negligible.

Commentaires: 10 pages, references added

Ref HAL: in2p3-00025048_v1
Ref Arxiv: astro-ph/0507290
DOI: 10.1016/j.physletb.2005.09.008
WoS: 000232955700001
Ref. & Cit.: NASA ADS
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37 Citations
Résumé:

The formalism in order to obtain the Dark Energy equation of state is extended to non-flat universes and we consider the inequalities that must be satisfied by Phantom Dark Energy in this case. We show that due to a non-vanishing spatial curvature satisfying the observational bounds, the uncertainty on the determination of the Dark Energy equation of state parameter $w$, when it is taken constant, can be significant and that it is minimal for some redshift $z_{cr}\sim 3$. We consider the potential of future measurements of the gravitational waves emitted by binaries at high redshifts $z>z_{cr}$ to reduce this uncertainty. Results obtained here should also be relevant for a weakly varying equation of state with $w\approx -1$.