Yashar Akrami

Accelerated expansion from ghost-free bigravity: a statistical analysis with improved generality

A bstractWe study the background cosmology of the ghost-free, bimetric theory of gravity. We perform an extensive statistical analysis of the model using both frequentist and Bayesian frameworks and employ the constraints on the expansion history of the Universe from the observations of supernovae, the cosmic microwave background and the large scale structure to estimate the model’s parameters and test the goodness of the fits. We explore the parameter space of the model with nested sampling to find the best-fit chi-square, obtain the Bayesian evidence, and compute the marginalized posteriors and mean likelihoods. We mainly focus on a class of sub-models with no explicit cosmological constant (or vacuum energy) term to assess the ability of the theory to dynamically cause a late-time accelerated expansion. The model behaves as standard gravity without a cosmological constant at early times, with an emergent extra contribution to the energy density that converges to a cosmological constant in the far future. The model can in most cases yield very good fits and is in perfect agreement with the data. This is because many points in the parameter space of the model exist that give rise to time-evolution equations that are effectively very similar to those of the ΛCDM. This similarity makes the model compatible with observations as in the ΛCDM case, at least at the background level. Even though our results indicate a slightly better fit for the ΛCDM concordance model in terms of the p-value and evidence, none of the models is statistically preferred to the other. However, the parameters of the bigravity model are in general degenerate. A similar but perturbative analysis of the model as well as more data will be required to break the degeneracies and constrain the parameters, in case the model will still be viable compared to the ΛCDM.

Bimetric gravity doubly coupled to matter: theory and cosmological implications

A ghost-free theory of gravity with two dynamical metrics both coupled to matter is shown to be consistent and viable. Its cosmological implications are studied, and the models, in particular in the context of partially massless gravity, are found to explain the cosmic acceleration without resorting to dark energy.

Direct constraints on minimal supersymmetry from Fermi-LAT observations of the dwarf galaxy Segue 1

The dwarf galaxy Segue 1 is one of the most promising targets for the indirect detection of dark matter. Here we examine what constraints 9 months of Fermi-LAT gamma-ray observations of Segue 1 place upon the Constrained Minimal Supersymmetric Standard Model (CMSSM), with the lightest neutralino as the dark matter particle. We use nested sampling to explore the CMSSM parameter space, simultaneously fitting other relevant constraints from accelerator bounds, the relic density, electroweak precision observables, the anomalous magnetic moment of the muon and B-physics. We include spectral and spatial fits to the Fermi observations, a full treatment of the instrumental response and its related uncertainty, and detailed background models. We also perform an extrapolation to 5 years of observations, assuming no signal is observed from Segue 1 in that time. Results marginally disfavour models with low neutralino masses and high annihilation cross-sections. Virtually all of these models are however already disfavoured by existing experimental or relic density constraints.

Improved constraints on the primordial power spectrum at small scales from ultracompact minihalos

For a Gaussian spectrum of primordial density fluctuations, ultracompact minihalos (UCMHs) of dark matter are expected to be produced in much greater abundance than, e.g., primordial black holes. Forming shortly after matter-radiation equality, these objects would develop very dense and spiky dark matter profiles. In the standard scenario where dark matter consists of thermally produced, weakly interacting massive particles, UCMHs could thus appear as highly luminous gamma-ray sources or leave an imprint in the cosmic microwave background by changing the reionization history of the Universe. We derive corresponding limits on the cosmic abundance of UCMHs at different epochs and translate them into constraints on the primordial power spectrum. We find the resulting constraints to be quite severe, especially at length scales much smaller than what can be directly probed by the cosmic microwave background or large-scale structure observations. We use our results to provide an updated compilation of the best available constraints on the power of density fluctuations on all scales, ranging from the present-day horizon to scales more than 20 orders of magnitude smaller.

Stable and unstable cosmological models in bimetric massive gravity

Nonlinear, ghost-free massive gravity has two tensor fields; when both are dynamical, the mass of the graviton can lead to cosmic acceleration that agrees with background data, even in the absence of a cosmological constant. Here the question of the stability of linear perturbations in this bimetric theory is examined. Instabilities are presented for several classes of models, and simple criteria for the cosmological stability of massive bigravity are derived. In this way, we identify a particular self-accelerating bigravity model, infinite-branch bigravity (IBB), which exhibits both viable background evolution and stable linear perturbations. We discuss the modified gravity parameters for IBB, which do not reduce to the standard

A profile likelihood analysis of the constrained MSSM with genetic algorithms

\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}

Linear growth of structure in massive bigravity

result at early times, and compute the combined likelihood from measured growth data and type Ia supernovae. IBB predicts a present matter density

The nature of spacetime in bigravity: Two metrics or none?

{\mathrm{\ensuremath{\Omega}}}_{m0}=0.18

How well will ton-scale dark matter direct detection experiments constrain minimal supersymmetry?

and an equation of state

Cosmological constraints on ghost-free bigravity: background dynamics and late-time acceleration

w(z)=\ensuremath{-}0.79+0.21z/(1+z)