The focus of the dissertation "Aspects of cosmic acceleration" is the study of possible mechanisms responsible for the late-time accelerated expansion of the universe. It has 5 main chapters. In... Show moreThe focus of the dissertation "Aspects of cosmic acceleration" is the study of possible mechanisms responsible for the late-time accelerated expansion of the universe. It has 5 main chapters. In the first chapter I have given an overview of modern cosmology. Particularly, an introduction to cosmological perturbation theory, and a general overview of the cosmological standard model, as well as an overview to beyond-standard-model scenarios is presented. Chapter two discusses cosmological models based on the so-called alpha-attractor framework. Chapter three is dedicated to the study of constraints imposed by gravitational wave observations on the so-called doubly-coupled massive bimetric gravity theory. Chapter four studies the cosmological stability of the so-called massive mimetic gravity theory. Finally, Chapter five studies the effect of the Symmetron gravity on the so-called splashback radius of dark matter halos. Show less
Solomon, A.R.; Vardanyan, V.; Akrami Cheghasiahi, Y. 2019
We study the first cosmological implications of the mimetic theory of massive gravity recently proposed by Chamseddine and Mukhanov. This is a novel theory of ghost-free massive gravity which... Show moreWe study the first cosmological implications of the mimetic theory of massive gravity recently proposed by Chamseddine and Mukhanov. This is a novel theory of ghost-free massive gravity which additionally contains a mimetic dark matter component. In an echo of other modified gravity theories, there are self-accelerating solutions which contain a ghost instability. In the ghost-free region of parameter space, the effect of the graviton mass on the cosmic expansion history amounts to an effective negative cosmological constant, a radiation component, and a negative curvature term. This allows us to place constraints on the model parameters—the graviton mass and the Stückelberg vacuum expectation value—by insisting that the effective radiation and curvature terms be within observational bounds. The late-time acceleration must be accounted for by a separate positive cosmological constant or other dark energy sector. We impose further constraints at the level of perturbations by demanding linear stability. We comment on the possibility of distinguishing this theory from ΛCDM with current and future large-scale structure surveys. Show less
The splashback radius rsp has been identified in cosmological N-body simulations as an important scale associated with gravitational collapse and the phase-space distribution of recently accreted... Show moreThe splashback radius rsp has been identified in cosmological N-body simulations as an important scale associated with gravitational collapse and the phase-space distribution of recently accreted material. We employ a semianalytical approach to study the spherical collapse of dark matter halos in symmetron gravity and provide, for the first time, insights into how the phenomenology of splashback is affected by modified gravity. The symmetron is a scalar-tensor theory which exhibits a screening mechanism whereby higher-density regions are screened from the effects of a fifth force. In this model, we find that, as overdensities grow over cosmic time, the inner region becomes heavily screened. In particular, we identify a sector of the parameter space for which material currently sitting at rsp has followed, during the collapse, the formation of this screened region. As a result, we find that for this part of the parameter space the splashback radius is maximally affected by the symmetron force, and we predict changes in rsp up to around 10% compared to its General Relativity value. Because this margin is within the precision of present splashback experiments, we expect this feature to soon provide constraints for symmetron gravity on previously unexplored scales. Show less
Akrami, Y.; Kallosh, R.; Lind, A.; Vardanyan, V. 2018
We review the advanced version of the KKLT construction and pure de Sitter supergravity, involving a nilpotent multiplet, with regard to various conjectures that de Sitter state cannot exist in... Show moreWe review the advanced version of the KKLT construction and pure de Sitter supergravity, involving a nilpotent multiplet, with regard to various conjectures that de Sitter state cannot exist in string theory. We explain why we consider these conjectures problematic and not well motivated, and why the recently proposed alternative string theory models of dark energy, ignoring vacuum stabilization, are ruled out by cosmological observations at least at the 3σ level, i.e. with more than 99.7% confidence. Show less
Akrami, Cheghasiahi Y.; Kallosh, R.; Linde, A.; Vardanyan, V. 2018
We review the advanced version of the KKLT construction and pure d=4" role="presentation" style="display: inline; line-height: normal; font-size: 13.6px; overflow-wrap: normal; white-space: nowrap;... Show moreWe review the advanced version of the KKLT construction and pure d=4" role="presentation" style="display: inline; line-height: normal; font-size: 13.6px; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border-width: 0px; border-style: initial; color: rgb(0, 0, 0); font-family: arial, verdana, sans-serif; position: relative;">𝑑=4d=4 de Sitter supergravity, involving a nilpotent multiplet, with regard to various conjectures that de Sitter state cannot exist in string theory. We explain why we consider these conjectures problematic and not well motivated, and why the recently proposed alternative string theory models of dark energy, ignoring vacuum stabilization, are ruled out by cosmological observations at least at the 3σ" role="presentation" style="display: inline; line-height: normal; font-size: 13.6px; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border-width: 0px; border-style: initial; color: rgb(0, 0, 0); font-family: arial, verdana, sans-serif; position: relative;">3𝜎3σ level, i.e. with more than 99.7%" role="presentation" style="display: inline; line-height: normal; font-size: 13.6px; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border-width: 0px; border-style: initial; color: rgb(0, 0, 0); font-family: arial, verdana, sans-serif; position: relative;">99.7 .7%confidence. Show less
Akrami Cheghasiahi, Y.; Kallosh, R.; Linde, A.; Vardanyan, V. 2018
We propose a simple, nonlocal modification to general relativity (GR) on large scales, which provides a model of late-time cosmic acceleration in the absence of the cosmological constant and with... Show moreWe propose a simple, nonlocal modification to general relativity (GR) on large scales, which provides a model of late-time cosmic acceleration in the absence of the cosmological constant and with the same number of free parameters as in standard cosmology. The model is motivated by adding to the gravity sector an extra spin-2 field interacting nonlocally with the physical metric coupled to matter. The form of the nonlocal interaction is inspired by the simplest form of the Deser-Woodard (DW) model, α R1/R, with one of the Ricci scalars being replaced by a constant m2, and gravity is therefore modified in the infrared by adding a simple term of the form m21/R to the Einstein-Hilbert term. We study cosmic expansion histories, and demonstrate that the new model can provide background expansions consistent with observations if m is of the order of the Hubble expansion rate today, in contrast to the simple DW model with no viable cosmology. The model is best fit by w0~−1.075 and wa~0.045. We also compare the cosmology of the model to that of Maggiore and Mancarella (MM), m2R1/2R, and demonstrate that the viable cosmic histories follow the standard-model evolution more closely compared to the MM model. We further demonstrate that the proposed model possesses the same number of physical degrees of freedom as in GR. Finally, we discuss the appearance of ghosts in the local formulation of the model, and argue that they are unphysical and harmless to the theory, keeping the physical degrees of freedom healthy. Show less
Akrami Cheghasiahi, Y.; Brax, P.; Davis, A.C.; Vardanyan, V. 2018
