Ref HAL: hal-01266923_v1
DOI: 10.1088/1475-7516/2015/10/033
WoS: 000365804000034
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9 Citations
Résumé:

Regular bouncing solutions in the framework of a scalar-tensor gravity model were found in a recent work. We reconsider the problem in the Einstein frame (EF) in the present work. Singularities arising at the limit of physical viability of the model in the Jordan frame (JF) are either of the Big Bang or of the Big Crunch type in the EF. As a result we obtain integrable scalar field cosmological models in general relativity (GR) with inverted double-well potentials unbounded from below which possess solutions regular in the future, tending to a de Sitter space, and starting with a Big Bang. The existence of the two fixed points for the field dynamics at late times found earlier in the JF becomes transparent in the EF.

Ref HAL: hal-01203042_v1
DOI: 10.1088/1475-7516/2015/07/002
WoS: WOS:000359259500002
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36 Citations
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We consider the possibility to produce a bouncing universe in the framework of scalar-tensor gravity models in which the scalar field potential may be negative, and even unbounded from below. We find a set of viable solutions with nonzero measure in the space of initial conditions passing a bounce, even in the presence of a radiation component, and approaching a constant gravitational coupling afterwards. Hence we have a model with a minimal modification of gravity in order to produce a bounce in the early universe with gravity tending dynamically to general relativity (GR) after the bounce.

Ref HAL: hal-00632611_v1
Ref Arxiv: 1110.2525
DOI: 10.1103/PhysRevD.84.104003
WoS: 000296913400009
Ref. & Cit.: NASA ADS
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14 Citations
Résumé:

We derive some basic equations related to the redshift drift and we show how some dark energy (DE) properties can be retrieved from it. We consider in particular three kinds of DE models which exhibit a characteristic signature in their redshift drift while no such signature would be present in their luminosity-distances: a sudden change of the equation of state parameter w_{DE} at low redshifts, oscillating DE and finally an equation of state with spikes at low redshifts. Accurate redshift drift measurements would provide interesting complementary probes for some of these models and for models with varying gravitational coupling. While the redshift drift would efficiently constrain models with a spike at z~1, the signature of the redshift drift for models with large variations at very low redshifts z<0.1 would be unobservable, allowing a large arbitrariness in the present expansion of the universe.

Commentaires: Accepted for publication in Phys. Rev. D; 12 pages, 8 figures

Ref HAL: hal-00632711_v1
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A large number of observations suggest that our universe entered at low redshifts a stage with accelerated expansion rate. Many models, Dark Energy (DE) models, able to explain this departure from conventional cosmology have been proposed. These models are conceptually very different, either introducing some new component with sufficiently negative pressure, or modifying the gravitational interaction on cosmic scales. Some of these DE models are reviewed here. Future high precision observations probing both the background and the perturbations will single out viable models.

Ref HAL: hal-00632710_v1
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Résumé:

The present accelerated expansion of the universe is a major challenge for cosmology. Dark Energy models aim to explain this unconventional expansion. We have at the present time a large variety of models which are conceptually very different. We review here some of them, especially those based on a modification of the laws of gravity. Future high precision observations probing both the background and the perturbations will significantly reduce the class of viable models.

Ref HAL: hal-00632706_v1
Ref Arxiv: 1010.3769
DOI: 10.1103/PhysRevD.82.124006
WoS: 000208475300007
Ref. & Cit.: NASA ADS
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54 Citations
Résumé:

In chameleon dark energy models, local gravity constraints tend to rule out parameters in which observable cosmological signatures can be found. We study viable chameleon potentials consistent with a number of recent observational and experimental bounds. A novel chameleon field potential, motivated by f(R) gravity, is constructed where observable cosmological signatures are present both at the background evolution and in the growth-rate of the perturbations. We study the evolution of matter density perturbations on low redshifts for this potential and show that the growth index today gamma_0 can have significant dispersion on scales relevant for large scale structures. The values of gamma_0 can be even smaller than 0.2 with large variations of gamma on very low redshifts for the model parameters constrained by local gravity tests. This gives a possibility to clearly distinguish these chameleon models from the Lambda-Cold-Dark-Matter model in future high-precision observations.

Commentaires: 16 pages, 8 figures Journal: Phys.Rev.D82:124006,2010

Ref HAL: hal-00632704_v1
Ref Arxiv: 1009.0776
DOI: 10.1103/PhysRevD.82.123534
WoS: 000286749400002
Ref. & Cit.: NASA ADS
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26 Citations
Résumé:

We investigate a Friedmann universe filled with a tachyon scalar field, which behaves as dustlike matter in the past, while it is able to accelerate the expansion rate of the universe at late times. The comparison with type Ia supernovae (SNIa) data allows for evolutions driving the universe into a Big Brake. Some of the evolutions leading to a Big Brake exhibit a large variation of the equation of state parameter at low redshifts which is potentially observable with future data though hardly detectable with present SNIa data. The soft Big Brake singularity occurs at finite values of the scale factor, vanishing energy density and Hubble parameter, but diverging deceleration and infinite pressure. We show that the geodesics can be continued through the Big Brake and that our model universe will recollapse eventually in a Big Crunch. Although the time to the Big Brake strongly depends on the present values of the tachyonic field and of its time derivative, the time from the Big Brake to the Big Crunch represents a kind of invariant timescale for all field parameters allowed by SNIa.

Commentaires: v2: slightly expanded, 14 pages, 5 figures, 3 tables; version to be published in Phys.Rev.D Journal: Phys.Rev.D82:123534,2010