arXiv subjects

Roman Juszkiewicz

Publications and source records attributed to Roman Juszkiewicz.

16 recordsLinked to original sources

arXiv2014

DM haloes in the fifth-force cosmology

We investigate how long-range scalar interactions affect the properties of dark matter haloes. For doing so we employ the ReBEL model which implements an additional interaction between dark matter particles. On the phenomenological level this is equivalent to a modification of gravity. We analyse the differences between five ReBEL models and $Λ$CDM using a series of high resolution cosmological simulations. Emphasis is placed on investigating how halo properties change in the presence of a fifth force. We report that the density profile of ReBEL haloes is well described by the NFW profile but with mean concentrations from $5\%$ to a few times higher than the standard $Λ$CDM value. We also find a slight increase of the halo spin for haloes more massive than $5\times10^{11}\M_{\odot}$, reflecting a higher rotational support of those haloes due to scalar forces. In addition, the dark matter haloes in our models are more spherical than their counterparts in $Λ$CDM. The ReBEL haloes are also more virialised, with a large difference from $Λ$CDM for strong fifth forces and a much smaller change for weak scalar interactions.

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arXiv2013

Hierarchical clustering and the BAO signature

In this contribution we present the preliminary results regarding the non-linear BAO signal in higher-order statistics of the cosmic density field. We use ensembles of N-body simulations to show that the non-linear evolution changes the amplitudes of the BAO signal, but has a negligible effect on the scale of the BAO feature. The latter observation accompanied by the fact that the BAO feature amplitude roughly doubles as one moves to higher orders, suggests that the higher-order correlation amplitudes can be used as probe of the BAO signal.

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arXiv2012

Skewness as a probe of Baryon Acoustic Oscillations

In this study we show that the skewness S_3 of the cosmic density field contains a significant and potentially detectable and clean imprint of Baryonic Acoustic Oscillations. Although the BAO signal in the skewness has a lower amplitude than second order measures like the two-point correlation function and power spectrum, it has the advantage of a considerably lower sensitivity to systematic influences. Because it lacks a direct dependence on bias if this concerns simple linear bias, skewness will be considerably less beset by uncertainties due to galaxy bias. Also, it has a weaker sensitivity to redshift distortion effects. We use perturbation theory to evaluate the magnitude of the effect on the volume-average skewness, for various cosmological models. One important finding of our analysis is that the skewness BAO signal occurs at smaller scales than that in second order statistics. For an LCDM spectrum with WMAP7 normalization, the BAO feature has a maximum wiggle amplitude of ~3% and appears at a scale of ~82Mpc/h. We conclude that the detection of BAO wiggles in future extensive galaxy surveys via the skewness of the observed galaxy distribution may provide us with a useful, and potentially advantageous, measure of the nature of Dark Energy.

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arXiv2010

Hierarchy of N-point functions in the LCDM and ReBEL cosmologies

In this work we investigate higher order statistics for the $\lcdm$ and ReBEL scalar-interacting dark matter models by analyzing $180\hmpc$ dark matter N-body simulation ensembles. The N-point correlation functions and the related hierarchical amplitudes, such as skewness and kurtosis, are computed using the Count-In-Cells method. Our studies demonstrate that the hierarchical amplitudes $S_n$ of the scalar-interacting dark matter model significantly deviate from the values in the $\lcdm$ cosmology on scales comparable and smaller then the screening length $r_s$ of a given scalar-interacting model. The corresponding additional forces that enhance the total attractive force exerted on dark matter particles at galaxy scales lowers the values of the hierarchical amplitudes $S_n$. We conclude that hypothetical additional exotic interactions in the dark matter sector should leave detectable markers in the higher-order correlation statistics of the density field. We focussed in detail on the redshift evolution of the dark matter field's skewness and kurtosis. From this investigation we find that the deviations from the canonical $\lcdm$ model introduced by the presence of the ``fifth'' force attain a maximum value at redshifts $0.5<z<2$. We therefore conclude that moderate redshift data are better suited for setting observational constraints on the investigated ReBEL models.

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arXiv2010

Nonlinear Effects in the Amplitude of Cosmological Density Fluctuations

The amplitude of cosmological density fluctuations, sigma_8, has been studied and estimated by analysing many cosmological observations. The values of the estimates vary considerably between the various probes. However, different estimators probe the value of sigma_8 in different cosmological scales and do not take into account the nonlinear evolution of the parameter at late times. We show that estimates of the amplitude of cosmological density fluctuations derived from cosmic flows are systematically higher than those inferred at early epochs from the CMB because of nonlinear evolution at later times. We discuss the past and future evolution of linear and nonlinear perturbations, derive corrections to the value of sigma_8 and compare amplitudes after accounting for these differences.

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arXiv2009

Dark Matter Gravitational Clustering With a Long-Range Scalar Interaction

We explore the possibility of improving the $Λ$CDM model at megaparsec scales by introducing a scalar interaction which increases the mutual gravitational attraction of dark matter particles. Using N-body simulations, we study the spatial distribution of dark matter particles and halos. We measure the effect of modifications in the Newton's gravity on properties of the two-point correlation function, the dark matter power spectrum, the cumulative halo mass function and density probability distribution functions. The results look promising: the scalar interactions produce desirable features at megaparsec scales without spoiling the $Λ$CDM successes at larger scales.

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arXiv2008

A new method of measuring the cluster peculiar velocity power spectrum

We propose to use spatial correlations of the kinetic Sunyaev-Zeldovich (KSZ) flux as an estimator of the peculiar velocity power spectrum. In contrast with conventional techniques, our new method does not require measurements of the thermal SZ signal or the X-ray temperature. Moreover, this method has the special advantage that the expected systematic errors are always sub-dominant to statistical errors on all scales and redshifts of interest. We show that future large sky coverage KSZ surveys may allow a peculiar velocity power spectrum estimates of an accuracy reaching ~10%.

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arXiv2004

A new method of measuring the peculiar velocity power spectrum

We show that by directly correlating the cluster kinetic Sunyaev Zeldovich (KSZ) flux, the cluster peculiar velocity power spectrum can be measured to $\sim 10%$ accuracy by future large sky coverage KSZ surveys. This method is almost free of systemics entangled in the usual velocity inversion method. The direct correlation brings extra information of density and velocity clustering. We utilize these information to construct two indicators of the Hubble constant and comoving angular distance and propose a novel method to constrain cosmology.

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arXiv2003

An estimate of Ω_m without priors

Using mean relative peculiar velocity measurements for pairs of galaxies, we estimate the cosmological density parameter $Ω_m$ and the amplitude of density fluctuations $σ_8$. Our results suggest that our statistic is a robust and reproducible measure of the mean pairwise velocity and thereby the $Ω_m$ parameter. We get $Ω_m = 0.30^{+0.17}_{-0.07}$ and $σ_8 = 1.13^{+0.22}_{-0.23}$. These estimates do not depend on prior assumptions on the adiabaticity of the initial density fluctuations, the ionization history, or the values of other cosmological parameters.

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arXiv2001

The transition to nonlinearity and new constraints on biasing

We present two new dynamical tests of the biasing hypothesis. The first is based on the amplitude and the shape of the galaxy-galaxy correlation function, $ξ_g(r)$, where $r$ is the separation of the galaxy pair. The second test uses the mean relative peculiar velocity for galaxy pairs, $\vs(r)$. This quantity is a measure of the rate of growth of clustering and it is related to the two-point correlation function for the matter density fluctuations, $ξ(r)$. Under the assumption that galaxies trace the mass ($ξ_g = ξ$), the expected relative velocity can be calculated directly from the observed galaxy clustering. The above assumption can be tested by confronting the expected $\vs$ with direct measurements from velocity-distance surveys. Both our methods are checked against N-body experiments and then compared with the $ξ_g(r)$ and $\vs$ estimated from the {\sc APM} galaxy survey and the Mark III catalogue, respectively. Our results suggest that cosmological density parameter is low, $Ω_m \approx 0.3$, and that the {\sc APM} galaxies trace the mass at separations $r \ga 5 \Mlu$, where $h$ is the Hubble constant in units of 100 km s$^{-1}$Mpc. The present results agree with earlier studies, based on comparing higher order correlations in the {\sc APM} with weakly non-linear perturbation theory. Both approaches constrain the linear bias factor to be within 20% of unity. If the existence of the feature we identified in the {\sc APM} $ξ_g(r)$ -- the inflection point near $ξ_g = 1$ -- is confirmed by more accurate surveys, we may have discovered gravity's smoking gun: the long awaited ``shoulder'' in $ξ$, generated by gravitational dynamics and predicted by Gott and Rees 25 years ago.

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arXiv2000

Skewness as a probe of non-Gaussian initial conditions

We compute the skewness of the matter distribution arising from non-linear evolution and from non-Gaussian initial perturbations. We apply our result to a very generic class of models with non-Gaussian initial conditions and we estimate analytically the ratio between the skewness due to non-linear clustering and the part due to the intrinsic non-Gaussianity of the models. We finally extend our estimates to higher moments.

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arXiv1999

A Simple Method for Computing the Non-Linear Mass Correlation Function with Implications for Stable Clustering

We propose a simple and accurate method for computing analytically the mass correlation function for cold dark matter and scale-free models that fits N-body simulations over a range that extends from the linear to the strongly non-linear regime. The method, based on the dynamical evolution of the pair conservation equation, relies on a universal relation between the pair-wise velocity and the smoothed correlation function valid for high and low density models, as derived empirically from N-body simulations. An intriguing alternative relation, based on the stable-clustering hypothesis, predicts a power-law behavior of the mass correlation function that disagrees with N-body simulations but conforms well to the observed galaxy correlation function if negligible bias is assumed. The method is a useful tool for rapidly exploring a wide span of models and, at the same time, raises new questions about large scale structure formation.

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arXiv1998

Dynamics of pairwise motions

We derive a simple closed-form expression, relating $\vs(r)$ -- the mean relative velocity of pairs of galaxies at fixed separation $r$ -- to the two-point correlation function of mass density fluctuations, $ξ(r)$. We compare our analytic model for $\vs(r)$ with N-body simulations, and find excellent agreement in the entire dynamical range probed by the simulations ($0.1 \lsim ξ\lsim 1000$). Our results can be used to estimate the cosmological density parameter, $\Om$, directly from redshift-distance surveys, like Mark III.

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arXiv1998

Skewed exponential pairwise velocities from Gaussian initial conditions

Using an Eulerian perturbative calculation, we show that the distribution of relative pairwise velocities which arises from gravitational instability of Gaussian density fluctuations has asymmetric (skewed) exponential tails. The negative skewness is induced by the negative mean streaming velocity of pairs (the infall prevails over expansion), while the exponential tails arise because the relative pairwise velocity is a number, not volume weighted statistic. The derived probability distribution is compared with N-body simulations and shown to provide a reasonable fit.

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arXiv1996

Previrialization: Perturbative and N-Body Results

We present a series of N-body experiments which confirm the reality of the previrialization effect. We also use weakly nonlinear perturbative approach to study the phenomenon. These two approaches agree when the rms density contrast, $σ$, is small; more surprisingly, they remain in agreement when $σ\approx 1$. When the slope of the initial power spectrum is $n>-1$, nonlinear tidal interactions slow down the growth of density fluctuations and the magnitude of the suppression increases when $n$ (i.e. the relative amount of small scale power) is increased. For $n<-1$ we see an opposite effect: the fluctuations grow more rapidly than in linear theory. The transition occurs at $n=-1$ when the weakly nonlinear correction to $σ$ is close to zero and the growth rate is close to linear. Our results resolve recent controversy between two N-body studies of previrialization. Peebles (1990) assumed $n=0$ and found strong evidence in support of previrialization, while Evrard \& Crone (1992), who assumed $n=-1$, reached opposite conclusions. As we show here, the initial conditions with $n=-1$ are rather special because the nonlinear effects nearly cancel out for that particular spectrum. In addition to our calculations for scale-free initial spectra, we show results for a more realistic spectrum of Peacock \& Dodds (1994). Its slope near the scale usually adopted for normalization is close to $-1$, so $σ$ is close to linear. Our results retroactively justify linear normalization at 8$h^{-1}$ Mpc, while also demonstrating the danger and limitations of this practice.

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arXiv1994

Kurtosis of Large-Scale Cosmic Fields

An attractive and simple hypothesis for the formation of large-scale structure is that it developed by gravitational instability from primordial fluctuations with an initially Gaussian probability distribution. Non-linear gravitational evolution drives the distribution away from the Gaussian form, generating measurable skewness and kurtosis even when the variance of the fluctuations is much smaller than unity. We use perturbation theory to compute the kurtosis of the mass density field and the velocity divergence field that arises during the weakly non-linear evolution of initially Gaussian fluctuations. We adopt an Einstein--de~Sitter universe for the perturbative calculations, and we discuss the generalization to a universe of arbitrary $Ω$. We obtain semi-analytic results for the case of scale-free, power-law spectra of the initial fluctuations and final smoothing of cosmic fields with a Gaussian filter. We also give an exact analytical formula for the dependence of the skewness of these fields on the power spectrum index. We show that the kurtosis decreases with the power spectrum index, and we compare our more accurate results for the kurtosis to previous estimates from Monte Carlo integrations. We also compare our results to values obtained from cosmological N-body simulations with power-law initial spectra. Measurements of the skewness and kurtosis parameters can be used to test the hypothesis that structure in the universe formed by gravitational instability from Gaussian initial conditions.

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