119300055
New Parametrization for the Scale Dependent Growth Function in General Relativity
We study the scale-dependent evolution of the growth function $\ensuremath{\delta}(a,k)$ of cosmological perturbations in dark energy models based on general relativity. This scale dependence is more prominent on cosmological scales of $100{h}^{\ensuremath{-}1}\text{ }\text{ }\mathrm{Mpc}$ or larger. We derive a new scale-dependent parametrization which generalizes the well-known Newtonian approximation result ${f}_{0}(a)\ensuremath{\equiv}\frac{d\mathrm{ln}﻿{\ensuremath{\delta}}_{0}}{d\mathrm{ln}﻿a}=\ensuremath{\Omega}(a{)}^{\ensuremath{\gamma}}$ ($\ensuremath{\gamma}=\frac{6}{11}$ for $\ensuremath{\Lambda}\mathrm{CDM}$) which is a good approximation on scales less than $50{h}^{\ensuremath{-}1}\text{ }\text{ }\mathrm{Mpc}$. Our generalized parametrization is of the form $f(a)=\frac{{f}_{0}(a)}{1+\ensuremath{\xi}(a,k)}$, where $\ensuremath{\xi}(a,k)=(3{H}_{0}^{2}{\ensuremath{\Omega}}_{0\mathrm{m}})/(a{k}^{2})$. We demonstrate that this parametrization fits the exact result of a full general relativistic evaluation of the growth function up to horizon scales for both $\ensuremath{\Lambda}\mathrm{CDM}$ and dynamical dark energy. In contrast, the scale independent parametrization does not provide a good fit on scales beyond 5% of the horizon scale ($k\ensuremath{\simeq}0.01{h}^{\ensuremath{-}1}\text{ }\text{ }\mathrm{Mpc}$).
118378984
Parametrization for the scale dependent growth in modified gravity
We propose a scale dependent analytic approximation to the exact linear growth of density perturbations in Scalar-Tensor (ST) cosmologies. In particular, we show that on large subhorizon scales, in the Newtonian gauge, the usual scale independent subhorizon growth equation does not describe the growth of perturbations accurately, as a result of scale-dependent relativistic corrections to the Poisson equation. A comparison with exact linear numerical analysis indicates that our approximation is a significant improvement over the standard subhorizon scale independent result on large subhorizon scales. A comparison with the corresponding results in the Synchronous gauge demonstrates the validity and consistency of our analysis.
118572883
f(T) gravity mimicking dynamical dark energy. Background and perturbation analysis
We investigate f(T) cosmology in both the background, as well as in the perturbation level, and we present the general formalism for reconstructing the equivalent one-parameter family of f(T) models for any given dynamical dark energy scenario. Despite the completely indistinguishable background behavior, the perturbations break this degeneracy and the growth histories of all these models differ from one another. As an application we reconstruct the f(T) equivalent for quintessence, and we show that the deviation of the matter overdensity evolution is strong for small scales and weak for large scales, while it is negligible for large redshifts.
6643789
Island Cosmology
If the observed dark energy is a cosmological constant, the canonical state of the universe is de Sitter spacetime. In such a spacetime, quantum fluctuations that violate the null energy condition will create islands of matter. If the fluctuation is sufficiently large, the island may resemble our observable universe. Phenomenological approaches to calculating density fluctuations yield a scale invariant spectrum with suitable amplitude. With time, the island of matter that is our observable universe, dilutes and re-enters the cosmological constant sea, but other islands will emerge in the future, leading to an eternal universe.
200205149
On the dangers of using the growth equation on large scales
119192051
Big Bang nucleosynthesis with a stiff fluid
Models that lead to a cosmological stiff fluid component, with a density {rho}{sub S} that scales as a{sup -6}, where a is the scale factor, have been proposed recently in a variety of contexts. We calculate numerically the effect of such a stiff fluid on the primordial element abundances. Because the stiff fluid energy density decreases with the scale factor more rapidly than radiation, it produces a relatively larger change in the primordial helium-4 abundance than in the other element abundances, relative to the changes produced by an additional radiation component. We show that the helium-4 abundance varies linearly with the density of the stiff fluid at a fixed fiducial temperature. Taking {rho}{sub S10} and {rho}{sub R10} to be the stiff fluid energy density and the standard density in relativistic particles, respectively, at T=10 MeV, we find that the change in the primordial helium abundance is well-fit by {Delta}Y{sub p}=0.00024({rho}{sub S10}/{rho}{sub R10}). The changes in the helium-4 abundance produced by additional radiation or by a stiff fluid are identical when these two components have equal density at a 'pivot temperature', T{sub *}, where we find T{sub *}=0.55 MeV. Current estimates of the primordial {sup 4}He abundance give the constraint onmore » a stiff fluid energy density of {rho}{sub S10}/{rho}{sub R10}<30.« less
10579423
Pileup subtraction for jet shapes.
Jets in high energy hadronic collisions often contain the fingerprints of the particles that produced them. Those fingerprints, and thus the nature of the particles that produced the jets, can be read off with the help of quantities known as jet shapes. Jet shapes are, however, severely affected by pileup, the accumulation in the detector of the residues of the many simultaneous collisions taking place in the Large Hadron Collider (LHC). We introduce a method to correct for pileup effects in jet shapes. Relative to earlier, limited approaches, the key advance resides in its full generality, achieved through a numerical determination, for each jet, of a given shape's susceptibility to pileup. The method rescues the possibility of using jet shapes in the high pileup environment of current and future LHC running, as we show with examples of quark-gluon discrimination and top tagging.