921 research outputs found
The properties of the dark matter halo distribution in non-Gaussian scenarios
No full textThe description of halo abundance and clustering for non-Gaussian initial conditions has recently received renewed interest, motivated by the forthcoming large galaxy and cluster surveys, which can potentially detect primordial non-Gaussianity of the local form with a non-Gaussianity parameter |f| of order unity. This is particularly exciting because, while the simplest single-field slow-roll models of inflation predict a primordial |f|≪1, this signal sources extra contributions to the effective f of large-scale structures that are expected to be above the predicted detection threshold [C. Carbone, L. Verde, and S. Matarrese, ApJL 684 (2008) L1]. We present tests on N-body simulations of analytical formulae describing the halo abundance and clustering for non-Gaussian initial conditions. In particular, when we calibrate the analytic non-Gaussian mass function of [S. Matarrese, L. Verde, L. and R. Jimenez, ApJL 541 (2000) 10] and [M. LoVerde, A. Miller, S. Shandera and L. Verde, JCAP 04 (2008) 014] and the analytic description of halo clustering for non-Gaussian initial conditions on N-body simulations, we find excellent agreement between the simulations and the analytic predictions if we make the substitutions δ→δ×q and δ→δ×q where q≃0.75, in the density threshold for gravitational collapse and in the non-Gaussian fractional correction to the halo bias, respectively. We discuss the implications of these corrections on present and forecasted primordial non-Gaussianity constraints. We confirm that the non-Gaussian halo bias offers a robust and highly competitive test of primordial non-Gaussianity
Tracking Extended Quintessence
No full textWe investigate the cosmological role of a tracking field φ in extended quintessence scenarios, where the dynamical vacuum energy driving the acceleration of the universe today possesses an explicit coupling with the Ricci scalar R of the form F(φ)R/2, where F(φ) mimics general relativity today, F(φ0)=1/8πG. We analyze explicit nonminimally coupled (NMC) models where F(φ)=1/8πG+ξ(φ2-φ20), with ξ is the coupling constant and φ0 is the Q value today. Tracker solutions for these NMC models, with inverse power-law potentials, possess an initial enhancement of the scalar field dynamics, named the R-boost, caused by the effective potential generated by the Ricci scalar in the Klein-Gordon equation. During this phase the field performs a ``gravitational'' slow rolling until the true potential becomes important. We give accurate analytic formulas describing the R-boost, showing that the quintessence energy in this phase scales with the redshift z as (1+z)2. When the R-boost ends, the field trajectory matches the tracker solution in minimally coupled theories. We compute perturbations in these tracking extended quintessence models, by integrating the full set of equations for the evolution of linear fluctuations in scalar-tensor theories of gravity, and assuming Gaussian scale-invariant initial perturbations. The integrated Sachs-Wolfe (ISW) effect on the cosmic microwave background (CMB) angular spectrum causes a change δCl/Cl~=6[1-8πGF(φdec)] at l<~10, where ``dec'' stands for decoupling. Similarly, the CMB acoustic peak multipoles shift compared to ordinary tracking quintessence models by roughly an amount δl/l~=[8πGF(φdec)-1]/8. The turnover wave number kturn in the matter power spectrum shifts by an amount δkturn/kturn~=[1-8πGF(φeq)]/2, where ``eq'' stands for matter-radiation equivalence. All these corrections may assume positive as well as negative values, depending on the sign of the NMC parameter ξ. We show that the above effects can be as large as 10-30 % with respect to equivalent cosmological constant and ordinary tracking quintessence models, respecting all the existing experimental constraints on scalar-tensor theories of gravity. These results demonstrate that the playground where the data of the next decade will have their impact includes the nature of the dark energy in the Universe, as well as the structure of the theory of gravity
Large-Scale Bias in the Universe: II Redshift Space Bispectrum
No full textThe determination of the density parameter Omega_0 from the large-scale distribution of galaxies is one of the major goals of modern cosmology. However, if galaxies are biased tracers of the underlying mass distribution, linear perturbation theory leads to a degeneracy between Omega_0 and the linear bias parameter b, and the density parameter cannot be estimated. In Matarrese, Verde & Heavens we developed a method based on second-order perturbation theory to use the bispectrum to lift this degeneracy by measuring the bias parameter in an Omega_0-independent way. The formalism was developed assuming that one has perfect information on the positions of galaxies in three dimensions. In galaxy redshift surveys, the three-dimensional information is imperfect, because of the contaminating effects of peculiar velocities, and the resulting clustering pattern in redshift space is distorted. In this paper we combine second-order perturbation theory with a model for collapsed, virialized structures, to extend the method to redshift space, and demonstrate that the method should be successful in determining with reasonable accuracy the bias parameter from state-of-the-art surveys such as the Anglo-Australian 2 degree Field Survey and the Sloan Digital Sky Survey
Extended Quintessence
No full textWe study quintessence cosmologies in the context of scalar-tensor theories of gravity, where a scalar field φ, assumed to provide most of the cosmic energy density today, is nonminimally coupled to the Ricci curvature scalar R. Such ``extended quintessence'' cosmologies have the appealing feature that the same field causing the time (and space) variation of the cosmological constant is the source of a varying Newton constant in the manner of Jordan-Brans-Dicke. We investigate here two classes of models, where the gravitational sector of the Lagrangian is F(φ)R with F(φ)=ξφ2 [induced gravity (IG)] and F(φ)=1+ξφ2 [nonminimal coupling (NMC)]. As a first application of this idea we consider a specific model, where the quintessence field φ, obeying the simplest inverse power potential, has Ωφ=0.6 today, in the context of the cold dark matter scenario for structure formation in the Universe, with scale-invariant adiabatic initial perturbations. We find that, if ξ<~5×10-4 for IG and ξ<~5×10-3(Gφ0)-1 for NMC (φ0 is the present quintessence value), our quintessence field satisfies the existing solar system experimental constraints. Using linear perturbation theory we then obtain the polarization and temperature anisotropy spectra of the cosmic microwave background (CMB) as well as the matter power spectrum. The perturbation behavior possesses distinctive features, that we name ``QR effects:'' the effective potential arising from the coupling with R adds to the true scalar field potential, altering the cosmic equation of state and enhancing the integrated Sachs-Wolfe effect. As a consequence, part of the CMB anisotropy level on COBE scales is due to the latter effect, and the cosmological perturbation amplitude on smaller scales, including the oscillating region of the CMB spectrum, has reduced power; this effect is evident on CMB polarization and temperature fluctuations, as well as on the matter power-spectrum today. Moreover, the acoustic peaks and the spectrum turnover are displaced to smaller scales, compared to ordinary quintessence models, because of the faster growth of the Hubble length, which, for a fixed value today, delays the horizon crossing of scales larger than the horizon wavelength at matter-radiation equality and slightly decreases the amplitude of the acoustic oscillations. These features could be detected in the upcoming observations on CMB and large-scale structure
Large-scale non-Gaussian mass function and halo bias: tests on N-body simulations
No full textThe description of the abundance and clustering of haloes for non-Gaussian initial conditions has recently received renewed interest,motivated by the forthcoming large galaxy and cluster surveys, which can potentially yield constraints of the order of unity on the non-Gaussianity parameter f(NL). We present tests on N-body simulations of analytical formulae describing the halo abundance and clustering for non-Gaussian initial conditions. We calibrate the analytic non-Gaussian mass function of Matarrese, Verde & Jimenez and LoVerde et al. and the analytic description of clustering of haloes for non-Gaussian initial conditions on N-body simulations. We find an excellent agreement between the simulations and the analytic predictions if we make the corrections delta(c) -> delta(c)root q and delta(c) -> delta(c)q, where q similar or equal to 0.75, in the density threshold for gravitational collapse and in the non-Gaussian fractional correction to the halo bias, respectively. We discuss the implications of this correction on present and forecasted primordial non-Gaussianity constraints. We confirm that the non-Gaussian halo bias offers a robust and highly competitive test of primordial non-Gaussianity
An Estimate of the Primordial Non-Gaussianity Parameter f_NL using the Needlet Bispectrum from WMAP
No full textWe use the full bispectrum of spherical needlets applied to the WMAP data of the cosmic microwave background as an estimator for the primordial non-Gaussianity parameter f NL. We use needlet scales up to ellmax = 1000 and the KQ75 galactic cut and find f NL = 84 ± 40 corrected for point-source bias. We also introduce a set of consistency tests to validate our results against the possible influence of foreground residuals or systematic errors. In particular, fluctuations in the value of f NL obtained from different frequency channels, different masks, and different multipoles are tested against simulated maps. All variations in f NL estimates are found statistically consistent with simulations
Directional Variations of the Non-Gaussianity Parameter f_NL
No full textWe investigate local variations of the primordial non-Gaussianity parameter f NL in the WMAP data, looking for possible influence of foreground contamination in the full-sky estimate of f NL. We first improve the needlet bispectrum estimate in Rudjord et al. on the full sky to f NL = 73 ± 31 using the KQ75 mask on the co-added V + W channel. We find no particular values of f NL estimates close to the galactic plane and conclude that foregrounds are unlikely to affect the estimate of f NL in the V and W bands even for the smaller KQ85 mask. In the Q band, however, we find unexpectedly high values of f NL in local estimates close to the galactic mask, as well as significant discrepancies between Q-band estimates and V/W-band estimates. We therefore conclude that the Q band is too contaminated to be used for non-Gaussianity studies even with the larger KQ75 mask. We further note that the local f NL estimates on the V + W channel are positive on all equatorial bands from the north to the south pole. The probability for this to happen in a universe with f NL = 0 is less than 1%
Cosmological consequences of f(R) gravity
No full textreservedIn this thesis, we are focusing on a theory called f(R) gravity, obtained from modifications of General Relativity (GR) by adding higher powers of the Ricci scalar into the standard GR action. By introducing additional terms to the action, we aim to replicate the observed evolution of the Universe without being hindered by the problems of the existence of dark matter and dark energy and address some other interesting phenomena in the universe such as gravitational waves and cosmological perturbations.In this thesis, we are focusing on a theory called f(R) gravity, obtained from modifications of General Relativity (GR) by adding higher powers of the Ricci scalar into the standard GR action. By introducing additional terms to the action, we aim to replicate the observed evolution of the Universe without being hindered by the problems of the existence of dark matter and dark energy and address some other interesting phenomena in the universe such as gravitational waves and cosmological perturbations
Approaching Lambda without Fine-Tuning
No full textWe address the fine-tuning problem of dark energy cosmologies which arises when the dark energy density needs to initially lie in a narrow range in order for its present value to be consistent with observations. As recently noticed, this problem becomes particularly severe in canonical quintessence scenarios, when trying to reproduce the behavior of a cosmological constant, i.e., when the dark energy equation of state wQ approaches -1: these models may be reconciled with a large basin of attraction only by requiring a rapid evolution of wQ at low redshifts, which is in conflict with the most recent estimates from type Ia Supernovae discovered by Hubble space telescope. Next, we focus on scalar-tensor theories of gravity, discussing the implications of a coupling between the quintessence scalar field and the Ricci scalar (“extended quintessence”). We show that, even if the equation of state today is very close to -1, by virtue of the scalar-tensor coupling the quintessence trajectories still possess the attractive feature which allows to reach the present level of cosmic acceleration starting by a set of initial conditions which covers tens of orders of magnitude; this effect, entirely of gravitational origin, represents a new important consequence of the possible coupling between dark energy and gravity. We illustrate this effect in typical extended quintessence scenarios
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