Observational constraints on self-accelerating cosmology

Abstract

The DGP brane-world model provides a simple alternative to the standard LCDM cosmology, with the same number of parameters. There is no dark energy -the late universe self-accelerates due to an infrared modification of gravity. We compute the joint constraints on the DGP model from supernovae, the cosmic microwave background shift parameter, and the baryon oscillation peak in the SDSS luminous red galaxy sample. Flat DGP models are within the joint 2 sigma contour, but the LCDM model provides a significantly better fit to the data. These tests are based on the background dynamics of the DGP model, and we comment on further tests that involve structure formation.PACS numbers: 95.36.+x, 98.80.-k, 98.80.Es THE DARK ENERGY PROBLEMThe acceleration of the late-time universe, as implied by observations of supernovae, cosmic microwave background anisotropies and the large-scale structure, poses one of the deepest theoretical problems facing cosmology [1]. Within the framework of general relativity, the acceleration originates from a dark energy field (or effective dark energy) with negative pressure (w ≡ p/ρ < − 1 3 ), such as vacuum energy (w = −1) or a slow-rolling scalar field ("quintessence", w > −1). So far, none of the available models has a natural explanation.For the simplest option of vacuum energy, i.e., the LCDM model, the incredibly small value of the cosmological constantcannot be explained by current particle physics. In addition, the value needs to be fine-tuned,which also has no natural explanation. Quintessence models attempt to address the fine-tuning problem, but do not produce a natural solution -and also cannot address the problem of how Λ is set exactly to 0. Alternatively, it is possible that there is no dark energy, but instead an infrared modification of general relativity, i.e., on very large scales, r > ∼ H −1 0 , that accounts for late-time acceleration. (Note that this does not remove the problem of explaining why the vacuum energy does not gravitate.) Schematically, we are modifying the geometric side of the field equations,rather than the matter side,as in general relativity. It is important that the modification is covariant and incorporates deviations from homogeneity and isotropy, so that one can compute not only the background dynamics, but also the perturbations. DGP MODIFIED GRAVITYOne of the simplest covariant modified-gravity models is based on the Dvali-Gabadadze-Porrati (DGP) braneworld model, as generalized to cosmology by Deffayet [2]. (It is worth noting that the original DGP model with a Minkowski brane was not introduced to explain acceleration -the generalization to a Friedman brane was subsequently found to be self-accelerating.) In this model, gravity leaks off the 4-dimensional brane universe into the 5-dimensional bulk spacetime at large scales. At small scales, gravity is effectively bound to the brane and 4D gravity is recovered to a good approximation, via the lightest modes of the 5D graviton -effectively via an ultralight metastable graviton in the 4D universe. The...

Observational Constraints with Recent Data on the DGP Modified Gravity

“…To constrain the parameters of model we fit our model with the new Gold sample and SNLS supernova data, CMB shift parameter, position of the first and third peaks of power spectrum of temperature fluctuations at the last scattering surface, the Cluster Baryon Gas Mass Fraction, location of baryonic acoustic oscillation peak observed by SDSS and large scale structure formation data by 2dFGRS. The best parameters obtained from fitting with the new Gold sample data are: h = 0.62, Ωm = 0.28 +0.03 −0.02 , Ωr c = 0.13 +0.01 −0.01 and Ω K = −0.002 +0.064 −0.053 at 1σ confidence level with [26,31] showed that large scale structure observations from 2dFGRS experiment had weak effect on confining the acceptance intervals for the free parameters. The observational constraint just by using SNIa+CMB indicated that our universe is spatially open but combining these result with SDSS+gas+LSS showed that our universe in the DGP model is very good agreement with the spatially flat universe.…”

“…For the Likelihood analysis we use some weak priors for the model parameters indicated in Table 1. The best values for the parameters of the model [26] and [31]. In the first reference they report the following values for the model parameters: Ωm = 0.34 +0.07 −0.08 and Ωr c = 0.24 +0.04 −0.04 using old SNIa Gold sample while in the second reference Marteens et.…”

“…In addition to the phenomenological modification of action, the brane cosmology also implies modification for the general relativity on a brane embedded in an extra dimension space. Some brane world models which produce the late time acceleration have been tested using many observational experiments such as local gravity [23,24,25], Supernova Type Ia [22,26,27,28,29,30,31,32], angular size of compact ratio sources [33], the age measurements of high redshift objects [34], the optical gravitational lensing surveys [35], the large scale structures [36], and the X-ray gas mass fraction in galaxy clusters [37,38].…”

Consequences of dark matter-dark energy interaction on cosmological parameters derived from type Ia supernova data

“…The shift parameter encapsulates information contained in the detailed CMB power spectrum and, more importantly, its measured values is weakly dependent of assumptions made about dark energy. It has been used recently to put constraints on models with braneworld cosmologies [36]. The shift parameter R is defined by…”

The acceleration of the universe and the physics behind it

“…The influence of this modification of the growth factor on lensing observables was recently studied in Ref. [75]. From these definitions, we deduce that in the DGP-4D case, the equation of evolution of the growth factor (21) can be rewritten in terms of the growth variable, g ≡ D/a, as…”