1,721,005 research outputs found
A new smooth-k space filter approach to calculate halo abundances
Full text linkWe propose a new filter, a smooth-k space filter, to use in the Press-Schechter approach to model the dark matter halo mass function which overcomes shortcomings of other filters. We test this against the mass function measured in N-body simulations. We find that the commonly used sharp-k filter fails to reproduce the behaviour of the halo mass function at low masses measured from simulations of models with a sharp truncation in the linear power spectrum. We show that the predictions with our new filter agree with the simulation results over a wider range of halo masses for both damped and undamped power spectra than is the case with the sharp-k and real-space top-hat filters
Nonlinear structure formation in the cubic Galileon gravity model
No full textWe model the linear and nonlinear growth of large scale structure in the Cubic Galileon gravity model, by running a suite of N-body cosmological simulations using the ECOSMOG code. Our simulations include the Vainshtein screening effect, which reconciles the Cubic Galileon model with local tests of gravity. In the linear regime, the amplitude of the matter power spectrum increases by similar to 20% with respect to the standard ACDM model today. The modified expansion rate accounts for similar to 15% of this enhancement, while the fifth force is responsible for only similar to 5%. This is because the effective unscreened gravitational strength deviates from standard gravity only at late times, even though it can be twice as large today. In the nonlinear regime (k greater than or similar to 0.1hMpc(-1)), the fifth force leads to only a modest increase (less than or similar to 8%) in the clustering power on all scales due to the very efficient operation of the Vainshtein mechanism. Such a strong effect is typically not seen in other models with the same screening mechanism. The screening also results in the fifth force increasing the number density of halos by less than 10%, on all mass scales. Our results show that the screening does not ruin the validity of linear theory on large scales which anticipates very strong constraints from galaxy clustering data. We also show that, whilst the model gives an excellent match to CMB data on small angular scales (1 greater than or similar to 50), the predicted integrated Sachs-Wolfe effect is in tension with Planck/WMAP results
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
Full text linkWe 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
Constraining structure formation using EDGES
Full text linkThe 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
The effect of thermal velocities on structure formation in N-body simulations of warm dark matter
Full text linkWe investigate the impact of thermal velocities in N-body simulations of structure formation in warm dark matter models. Adopting the commonly used approach of adding thermal velocities, randomly selected from a Fermi-Dirac distribution, to the gravitationally-induced velocities of the simulation particles, we compare the matter and velocity power spectra measured from CDM and WDM simulations, in the latter case with and without thermal velocities. This prescription for adding thermal velocities introduces numerical noise into the initial conditions, which influences structure formation. At early times, the noise affects dramatically the power spectra measured from simulations with thermal velocities, with deviations of the order of similar to O (10) (in the matter power spectra) and of the order of similar to O (10(2)) (in the velocity power spectra) compared to those extracted from simulations without thermal velocities. At late times, these effects are less pronounced with deviations of less than a few percent. Increasing the resolution of the N-body simulation shifts these discrepancies to higher wavenumbers. We also find that spurious haloes start to appear in simulations which include thermal velocities at a mass that is similar to 3 times larger than in simulations without thermal velocities
Parameter space in Galileon gravity models
No full textWe present the first constraints on the full parameter space of the Galileon modified gravity model, considering both the cosmological parameters and the coefficients which specify the additional terms in the Lagrangian due to the Galileon field, which we call the Galileon parameters. We use the latest cosmic microwave background measurements, along with distance measurements from supernovae and baryonic acoustic oscillations, performing a Monte Carlo Markov Chain exploration of the nine-dimensional parameter space. The integrated Sachs-Wolfe signal can be very different in Galileon models compared to standard gravity, making it essential to use the full cosmic microwave background data rather than the cosmic microwave background distance priors. We demonstrate that meaningful constraints are only possible in the Galileon parameter space after taking advantage of a scaling degeneracy. We find that the Galileon model can fit the Wilkinson microwave anisotropy probe 9-year results better than the standard Lambda-cold dark matter model, but gives a slightly worse fit overall once lower redshift distance measurements are included. The best-fitting cosmological parameters (e.g., matter density, scalar spectral index, fluctuation amplitude) can differ by more than 2 sigma in the Galileon model compared with Lambda CDM. We highlight other potential constraints of the Galileon model using galaxy clustering and weak lensing measurements
N-body simulations of structure formation in thermal inflation cosmologies
Full text linkThermal inflation models (which feature two inflationary stages) can display damped primordial curvature power spectra on small scales which generate damped matter fluctuations. For a reasonable choice of parameters, thermal inflation models naturally predict a suppression of the matter power spectrum on galactic and sub-galactic scales, mimicking the effect of warm or interacting dark matter. Matter power spectra in these models are also characterised by an excess of power (with respect to the standard Lambda CDM power spectrum) just below the suppression scale. By running a suite of N-body simulations we investigate the non-linear growth of structure in models of thermal inflation. We measure the non-linear matter power spectrum and extract halo statistics, such as the halo mass function, and compare these quantities with those predicted in the standard Lambda CDM model and in other models with damped matter fluctuations. We find that the thermal inflation models considered here produce measurable differences in the matter power spectrum from Lambda CDM at redshifts z > 5 for wavenumbers k is an element of [2, 64] h Mpc(-1), while the halo mass functions are appreciably different at all redshifts in the halo mass range M-halo is an element of [10(9), 10(12)]h(-1) M-circle dot resolved by our simulations. The halo mass function at z = 0 for thermal inflation displays an enhancement of around similar to 20% with respect to Lambda CDM and a damping at lower halo masses, with the position of the enhancement depending on the value of the free parameter in the model. The enhancement in the halo mass function (with respect to Lambda CDM) increases with redshift, reaching similar to 40% at z = 5. We also study the accuracy of the analytical Press-Schechter approach, using different filters to smooth the density field, to predict halo statistics for thermal inflation. We find that the predictions with the smooth-k filter we proposed in a separate paper agree with the simulation results over a wider range of halo masses than is the case with other filters commonly used in the literature
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
- …
