16,828 research outputs found
Constraining structure formation using EDGES
No full textThe experiment to detect the global epoch of reionization signature (EDGES) collaboration reported the detection of a line at 78 MHz in the sky-averaged spectrum due to neutral hydrogen (HI) 21-cm hyperfine absorption of cosmic microwave background (CMB) photons at redshift z similar to 17. This requires that the spin temperature of HI be coupled to the kinetic temperature of the gas at this redshift through the scattering of Lyman-alpha photons emitted by massive stars. To explain the experimental result, star formation needs to be sufficiently efficient at z similar to 17 and this can be used to constrain models in which small-scale structure formation is suppressed (DMF models), either due to dark matter free-streaming or non-standard inflationary dynamics. We combine simulations of structure formation with a simple recipe for star formation to investigate whether these models emit enough Lyman-alpha photons to reproduce the experimental signal for reasonable values of the star formation efficiency, f(*). We find that a thermal warm dark matter (WDM) model with mass m(WDM) 4.3 keV is consistent with the timing of the signal for f(*). less than or similar to 2%. The exponential growth of structure around z similar to 17 in such a model naturally generates a sharp onset of the absorption. A warmer model with 1 m(WDM) similar to 3 keV requires a higher star formation efficiency, f(*) similar to 6%, which is a factor of few above predictions of current star formation models and observations of satellites in the Milky Way. However, uncertainties in the process of star formation at these redshifts do not allow to derive strong constrains on such models using 21-cm absorption line. The onset of the 21-cm absorption is generally slower in DMF than observed in cold dark matter (CDM) models, unless some process significantly suppresses star formation in halos with masses below similar to 10(8) h(-1) M-circle dot
TESTING GRAVITY USING THE GROWTH OF LARGE-SCALE STRUCTURE IN THE UNIVERSE
No full textFuture galaxy surveys hope to distinguish between the dark energy and modified gravity scenarios for the accelerating expansion of the universe using the distortion of clustering in redshift space. The aim is to model the form and size of the distortion to infer the rate at which large-scale structure grows. We test this hypothesis and assess the performance of current theoretical models for the redshift space distortion using large volume N-body simulations of the gravitational instability process. We simulate competing cosmological models which have identical expansion histories-one is a quintessence dark energy model with a scalar field and the other is a modified gravity model with a time-varying gravitational constant-and demonstrate that they do indeed produce different redshift space distortions. This is the first time that this approach has been verified using a technique that can follow the growth of structure at the required level of accuracy. Our comparisons show that theoretical models for the redshift space distortion based on linear perturbation theory give a surprisingly poor description of the simulation results. Furthermore, the application of such models can give rise to catastrophic systematic errors leading to incorrect interpretation of the observations. We show that an improved model is able to extract the correct growth rate. Further enhancements to theoretical models of redshift space distortions, calibrated against simulations, are needed to fully exploit the forthcoming high-precision clustering measurements
Modelling redshift space distortions in hierarchical cosmologies
No full textThe anisotropy of clustering in redshift space provides a direct measure of the growth rate of large-scale structure in the Universe. Future galaxy redshift surveys will make high-precision measurements of these distortions, and will potentially allow us to distinguish between different scenarios for the accelerating expansion of the Universe. Accurate predictions are needed in order to distinguish between competing cosmological models. We study the distortions in the redshift space power spectrum in Lambda cold dark matter (ΛCDM) and quintessence dark energy models, using large-volume N-body simulations, and test predictions for the form of the redshift space distortions. We find that the linear perturbation theory prediction is a poor fit to the measured distortions, even on surprisingly large scales k≥ 0.05 h Mpc−1. An improved model for the redshift space power spectrum, including the non-linear velocity divergence power spectrum, is presented and agrees with the power spectra measured from the simulations up to k∼ 0.2 h Mpc−1. We have found a density–velocity relation which is cosmology independent and which relates the non-linear velocity divergence spectrum to the non-linear matter power spectrum. We provide a formula which generates the non-linear velocity divergence P(k) at any redshift, using only the non-linear matter power spectrum and the linear growth factor at the desired redshift. This formula is accurate to better than 5 per cent on scales k < 0.2 h Mpc−1 for all the cosmological models discussed in this paper. Our results will extend the statistical power of future galaxy surveys
Testing dark energy using pairs of galaxies in redshift space
No full textThe distribution of angles subtended between pairs of galaxies and the line of sight, which is uniform in real space, is distorted by their peculiar motions, and has been proposed as a probe of cosmic expansion. We test this idea using N-body simulations of structure formation in a cold dark matter universe with a cosmological constant and in two variant cosmologies with different dark energy models. We find that the distortion of the distribution of angles is sensitive to the nature of dark energy. However, for the first time, our simulations also reveal dependences of the normalization of the distribution on both redshift and cosmology that have been neglected in previous work. This introduces systematics that severely limit the usefulness of the original method. Guided by our simulations, we devise a new, improved test of the nature of dark energy. We demonstrate that this test does not require prior knowledge of the background cosmology and that it can even distinguish between models that have the same baryonic acoustic oscillations and dark matter halo mass functions. Our technique could be applied to the completed BOSS galaxy redshift survey to constrain the expansion history of the Universe to better than 2 per cent. The method will also produce different signals for dark energy and modified gravity cosmologies even when they have identical expansion histories, through the different peculiar velocities induced in these cases
Modified gravity with massive neutrinos as a testable alternative cosmological model
No full textWe show that, in the presence of massive neutrinos, the Galileon gravity model provides a very good fit to the current cosmic microwave background (CMB) temperature, CMB lensing and baryonic acoustic oscillation data. This model, which we dub νGalileon, when assuming its stable attractor background solution, contains the same set of free parameters as lambda cold dark matter (ΛCDM), although it leads to different expansion dynamics and nontrivial gravitational interactions. The data provide compelling evidence (≳6σ) for nonzero neutrino masses, with Σmν≳0.4 eV at the 2σ level. Upcoming precision terrestrial measurements of the absolute neutrino mass scale therefore have the potential to test this model. We show that CMB lensing measurements at multipoles l≲40 will be able to discriminate between the νGalileon and ΛCDM models. Unlike ΛCDM, the νGalileon model is consistent with local determinations of the Hubble parameter. The presence of massive neutrinos lowers the value of σ8 substantially, despite of the enhanced gravitational strength on large scales. Unlike ΛCDM, the νGalileon model predicts a negative ISW effect, which is difficult to reconcile with current observational limits
Linear perturbations in Galileon gravity models
No full textWe study the cosmology of Galileon modified gravity models in the linear perturbation regime. We derive the fully covariant and gauge invariant perturbed field equations using two different methods, which give consistent results, and solve them using a modified version of the CAMB code. We find that, in addition to modifying the background expansion history and therefore shifting the positions of the acoustic peaks in the cosmic microwave background power spectrum, the Galileon field can cluster strongly from early times, and causes the Weyl gravitational potential to grow, rather than decay, at late times. This leaves clear signatures in the low-l cosmic microwave background power spectrum through the modified integrated Sachs-Wolfe effect, strongly enhances the linear growth of matter density perturbations and makes distinctive predictions for other cosmological signals such as weak lensing and the power spectrum of density fluctuations. The quasistatic approximation is shown to work quite well from small to the near-horizon scales. We demonstrate that Galileon models display a rich phenomenology due to the large parameter space and the sensitive dependence of the model predictions on the Galileon parameters. Our results show that some Galileon models are already ruled out by present data and that future higher significance galaxy clustering, integrated Sachs-Wolfe, and lensing measurements will place strong constraints on Galileon gravity
Spherical collapse in Galileon gravity: fifth force solutions, halo mass function and halo bias
No full textWe study spherical collapse in the Quartic and Quintic Covariant Galileon gravity models within the framework of the excursion set formalism. We derive the nonlinear spherically symmetric equations in the quasi-static and weak-field limits, focusing on model parameters that fit current CMB, SNIa and BAO data. We demonstrate that the equations of the Quintic model do not admit physical solutions of the fifth force in high density regions, which prevents the study of structure formation in this model. For the Quartic model, we show that the effective gravitational strength deviates from the standard value at late times (zlesssim1), becoming larger if the density is low, but smaller if the density is high. This shows that the Vainshtein mechanism at high densities is not enough to screen all of the modifications of gravity. This makes halos that collapse at zlesssim1 feel an overall weaker gravity, which suppresses halo formation. However, the matter density in the Quartic model is higher than in standard ΛCDM, which boosts structure formation and dominates over the effect of the weaker gravity. In the Quartic model there is a significant overabundance of high-mass halos relative to ΛCDM. Dark matter halos are also less biased than in ΛCDM, with the difference increasing appreciably with halo mass. However, our results suggest that the bias may not be small enough to fully reconcile the predicted matter power spectrum with LRG clustering data
Dark matter-radiation interactions: the impact on dark matter haloes
No full textInteractions between dark matter (DM) and radiation (photons or neutrinos) in the early Universe suppress density fluctuations on small mass scales. Here, we perform a thorough analysis of structure formation in the fully non-linear regime using N-body simulations for models with DM-radiation interactions and compare the results to a traditional calculation in which DM only interacts gravitationally. Significant differences arise due to the presence of interactions, in terms of the number of low-mass DM haloes and their properties, such as their spin and density profile. These differences are clearly seen even for haloes more massive than the scale on which density fluctuations are suppressed. We also show that semi-analytical descriptions of the matter distribution in the non-linear regime fail to reproduce our numerical results, emphasizing the challenge of predicting structure formation in models with physics beyond collisionless DM
Nonlinear structure formation in nonlocal gravity
No full textWe study the nonlinear growth of structure in nonlocal gravity models with the aid of N-body simulation and the spherical collapse and halo models. We focus on a model in which the inverse-squared of the d'Alembertian operator acts on the Ricci scalar in the action. For fixed cosmological parameters, this model differs from ACDM by having a lower late-time expansion rate and an enhanced and time-dependent gravitational strength (similar to 6% larger today). Compared to ACDM today, in the nonlocal model, massive haloes are slightly more abundant (by similar to 10% at M similar to 10(14) M-circle dot/h) and concentrated (approximate to 8% enhancement over a range of mass scales), but their linear bias remains almost unchanged. We find that the Sheth-Tormen formalism describes the mass function and halo bias very well, with little need for recalibration of free parameters. The fitting of the halo concentrations is however essential to ensure the good performance of the halo model on small scales. For k greater than or similar to 1h/Mpc, the amplitude of the nonlinear matter and velocity divergence power spectra exhibits a modest enhancement of similar to 12% to 15%, compared to ACDM today. This suggests that this model might only be distinguishable from ACDM by future observational missions. We point out that the absence of a screening mechanism may lead to tensions with Solar System tests due to local time variations of the gravitational strength, although this is subject to assumptions about the local time evolution of background averaged quantities
Nonlinear growth of structure in cosmologies with damped matter fluctuations
No full textWe investigate the nonlinear evolution of structure in variants of the standard cosmological model which display damped density fluctuations relative to cold dark matter (e.g. in which cold dark matter is replaced by warm or interacting DM). Using N-body simulations, we address the question of how much information is retained from different scales in the initial linear power spectrum following the nonlinear growth of structure. We run a suite of N-body simulations with different initial linear matter power spectra to show that, once the system undergoes nonlinear evolution, the shape of the linear power spectrum at high wavenumbers does not affect the non-linear power spectrum, while it still matters for the halo mass function. Indeed, we find that linear power spectra which differ from one another only at wavenumbers larger than their half-mode wavenumber give rise to (almost) identical nonlinear power spectra at late times, regardless of the fact that they originate from different models with damped fluctuations. On the other hand, the halo mass function is more sensitive to the form of the linear power spectrum. Exploiting this result, we propose a two parameter model of the transfer function in generic damped scenarios, and show that this parametrisation works as well as the standard three parameter models for the scales on which the linear spectrum is relevant
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