Modified gravityà laGalileon: Late time cosmic acceleration and observational constraints

Abstract

In this paper we examine the cosmological consequences of fourth order Galileon gravity. We carry out detailed investigations of the underlying dynamics and demonstrate the stability of one de Sitter phase. The stable de Sitter phase contains a Galileon field which is an increasing function of time ( _ > 0). Using the required suppression of the fifth force, supernovae, Baryon acoustic oscillations, and CMB data, we constrain parameters of the model. We find that the matter coupling parameter is constrained to small numerical values such that < 0:02. We also show that the parameters of the third and fourth order in the action ðc 3 ; c 4 Þ are not independent and with reasonable assumptions, we obtain constraints on them. We investigate the growth history of the model and find that the subhorizon approximation is not allowed for this model. We demonstrate strong scale dependence of linear perturbations in the fourth order Galileon gravity.

Cosmological implications of a shift symmetric Galileon field

“…Now, the model in [31] was formulated in the first order (tetrad) formalism whereas for cosmological applications a metric formulation is more suitable. It is only natural to inquire how the metric formulation of our model behaves in comparison with the covariant Galileon theories introduced earlier in the literature [19,[23][24][25][26]. The crucial difference corresponds to the fact that our model preserves the full internal shift symmetry even in curved spacetime, while those in the literature do not.…”

“…In recent times a lot of interest has been focused on the dark energy models with Galileon scalar field [23][24][25][26] due to the absence of negative energy instability. However, in the original Galileon scalar field cosmology [23], the Galilean symmetry ∂ µ π → ∂ µ π + b µ is essentially broken when gravity is introduced.…”

Beyond the cosmological standard model

“…The galileons have been used to address cosmic acceleration [516][517][518][519][520][521][522][523][524][525] and the origin of density perturbations in the early universe through inflation [526][527][528][529][530][531][532][533][534][535][536] and inflationary alternatives [537][538][539][540][541][542][543][544][545]. They have also been used to violate the null energy condition [537,[544][545][546][547][548][549][550][551][552][553].…”

Matter bispectrum in cubic Galileon cosmologies

“…Many literature has recently appeared on these models and their generalizations [20][21][22][23][24][25][26][27][28][29][30]. Galileon models have been extensively studied at late-times [31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46], during inflation [47][48][49][50][51][52][53][54][55][56], and a subclass of these models has been already compared by observations [57][58][59].…”