119161068
Decaying dark matter mimicking time-varying dark energy
A {Lambda}CDM model with dark matter that decays into inert relativistic energy on a time scale longer than the Hubble time will produce an expansion history that can be misinterpreted as stable dark matter with time-varying dark energy. We calculate the corresponding spurious equation of state parameter, w-tilde{sub {phi}}, as a function of redshift, and show that the evolution of w-tilde{sub {phi}} depends strongly on the assumed value of the dark-matter density, erroneously taken to scale as a{sup -3}. Depending on the latter, one can obtain models that mimic quintessence (w-tilde{sub {phi}>}-1), phantom models (w-tilde{sub {phi} }-1 at high redshift to w-tilde{sub {phi}<}-1 at low redshift. All of these models generically converge toward w-tilde{sub {phi}{approx_equal}-}1 at the present. The degeneracy between the {Lambda}CDM model with decaying dark matter and the corresponding spurious quintessence model is broken by the growth of density perturbations.
118435821
A new perspective on the relation between dark energy perturbations and the late-time ISW effect
Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235(Dated: December 9, 2008)The eﬀect of quintessence perturbations on the ISW eﬀect is studied for a mixed dynamical scalarﬁeld dark energy (DDE) and pressureless perfect ﬂuid dark matter. A new and general methodologyis developed to track the growth of the perturbations, which uses only the equation of state (EoS)parameter w
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
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 ($k\ensuremath{\simeq}0.01{h}^{\ensuremath{-}1}\text{ }\text{ }\mathrm{Mpc}$).
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.
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.