What X-ray source counts can tell about large-scale matter distribution

Sources generating most of the X-ray background (XRB) are dispersed over a wide range of redshifts. Thus, statistical characteristics of the source distribution carry information on matter distribution on very large scales. We test the possibility of detecting the variation in the X-ray source number counts over the celestial sphere. A large number of Chandra pointings spread over both galactic hemispheres are investigated. We searched for all the point-like sources in the soft band of 0.5 - 2 keV and statistically assessed the population of sources below the detection threshold. A homogeneous sample of the number counts at fluxes above ~10^{-15} erg s^{-1} cm^{-2} was constructed for more than 300 ACIS fields. The sources were counted within a circular area of 15 arcmin diameter. The count correlations between overlapping fields were used to assess the accuracy of the computational methods used in the analysis. The average number of sources in the investigated sample amounts to 46 per field. It is shown that the source number counts vary between fields at a level exceeding the fluctuation amplitude expected for the random (Poissonian) distribution. The excess fluctuations are attributed to the cosmic variance generated by the large-scale structures. The rms variations of the source counts due to the cosmic variance within the 15$ arcmin circle reach 8% of the average number counts. An amplitude of the potential correlations of the source counts on angular scales larger than the size of a single pointing remains below the noise level.Comment: 8 pages, 4 figures; expansion of observational material resulted in substantial changes; accepted for publication in A&

Is space really expanding? A counterexample

In all Friedman models, the cosmological redshift is widely interpreted as a consequence of the general-relativistic phenomenon of EXPANSION OF SPACE. Other commonly believed consequences of this phenomenon are superluminal recession velocities of distant galaxies and the distance to the particle horizon greater than c*t (where t is the age of the Universe), in apparent conflict with special relativity. Here, we study a particular Friedman model: empty universe. This model exhibits both cosmological redshift, superluminal velocities and infinite distance to the horizon. However, we show that the cosmological redshift is there simply a relativistic Doppler shift. Moreover, apparently superluminal velocities and `acausal' distance to the horizon are in fact a direct consequence of special-relativistic phenomenon of time dilation, as well as of the adopted definition of distance in cosmology. There is no conflict with special relativity, whatsoever. In particular, INERTIAL recession velocities are subluminal. Since in the real Universe, sufficiently distant galaxies recede with relativistic velocities, these special-relativistic effects must be at least partly responsible for the cosmological redshift and the aforementioned `superluminalities', commonly attributed to the expansion of space. Let us finish with a question resembling a Buddhism-Zen `koan': in an empty universe, what is expanding?Comment: 12 pages, no figures; added Appendix with a calculation of the cosmological redshift in `private space

Is the Two Micron all Sky Survey Clustering Dipole Convergent?

There is a long-standing controversy about the convergence of the dipole moment of the galaxy angular distribution (the so-called clustering dipole). Is the dipole convergent at all, and if so, what is the scale of the convergence? We study the growth of the clustering dipole of galaxies as a function of the limiting flux of the sample from the Two Micron All Sky Survey (2MASS). Contrary to some earlier claims, we find that the dipole does not converge before the completeness limit of the 2MASS Extended Source Catalog, i.e., up to 13.5 mag in the near-infrared K_s band (equivalent to an effective distance of 300 Mpc h ^(−1)). We compare the observed growth of the dipole with the theoretically expected, conditional one (i.e., given the velocity of the Local Group relative to the cosmic microwave background), for the ΛCDM power spectrum and cosmological parameters constrained by the Wilkinson Microwave Anisotropy Probe. The observed growth turns out to be within 1σ confidence level of its theoretical counterpart once the proper observational window of the 2MASS flux-limited catalog is included. For a contrast, if the adopted window is a top hat, then the predicted dipole grows significantly faster and converges (within the errors) to its final value for a distance of about 300 Mpc h ^(−1). By comparing the observational windows, we show that for a given flux limit and a corresponding distance limit, the 2MASS flux-weighted window passes less large-scale signal than the top-hat one. We conclude that the growth of the 2MASS dipole for effective distances greater than 200 Mpc h^(−1) is only apparent. On the other hand, for a distance of 80 Mpc h^(−1) (mean depth of the 2MASS Redshift Survey) and the ΛCDM power spectrum, the true dipole is expected to reach only ~80% of its final value. Eventually, since for the window function of 2MASS the predicted growth is consistent with the observed one, we can compare the two to evaluate β ≡ Ω^(0.55)_m /b. The result is β = 0.38 ± 0.04, which leads to an estimate of the density parameter Ω_m = 0.20 ± 0.08

Nonlinearity and stochasticity in the density--velocity relation

We present results of the investigations of the statistical properties of a joint density and velocity divergence probability distribution function (PDF) in the mildly non-linear regime. For that purpose we use both perturbation theory results, extended here for a top-hat filter, and numerical simulations. In particular we derive the quantitative (complete as possible up to third order terms) and qualitative predictions for constrained averages and constrained dispersions -- which describe the nonlinearities and the stochasticity properties beyond the linear regime -- and compare them against numerical simulations. We find overall a good agreement for constrained averages; however, the agreement for constrained dispersions is only qualitative. Scaling relations for the Omega-dependence of these quantities are satisfactory reproduced. Guided by our analytical and numerical results, we finally construct a robust phenomenological description of the joint PDF in a closed analytic form. The good agreement of our formula with results of N-body simulations for a number of cosmological parameters provides a sound validation of the presented approach. Our results provide a basis for a potentially powerful tool with which it is possible to analyze galaxy survey data in order to test the gravitational instability paradigm beyond the linear regime and put useful constraints on cosmological parameters. In particular we show how the nonlinearity in the density--velocity relation can be used to break the so-called Omega-bias degeneracy in cosmic density-velocity comparisons.Comment: 12 pages, 11 figures; revised version with minor changes in the presentation, accepted for publication in MNRA

SELECTED GEOCHEMICAL CRITERIA IN MIRE PROFILES OF PORĘBY WOJSŁAWSKIE (SANDOMIERZ BASIN, POLAND SE)

The geochemical analysis of biogenic sediments was used to reconstruct environmental conditions and the impact of human activity from a small mire geo-system in the Sandomierz Basin (Poland SE). Changes in the nature of selected geochemical indicators show a significant impact on the transformation of the mire during the early stages of human activity. It is reflected in the geochemical record by means of a high proportion of heavy metals – cadmium, lead and copper. Evident variability of main geochemical components concentration is also visible in a vertical record of analysed cores. This is a consequence of the sedimentary basin asymmetric configuration and the ensuing variable biogenic sedimentary succession. Analizy geochemiczne stanową jedną z podstawowych metod badań osadów biogenicznych deponowanych w torfowiskach. W pracy wykorzystano analizę geochemiczną osadów biogenicznych z małego geosystemu torfowiskowego in the Sandomierz Basin (Poland SE) do odtworzenia warunków środowiskowych i wpływu działalności człowieka. Zmiany charakteru wybranych wskaźników geochemicznych zaznaczają znaczący wpływ na transformację torfowiska w najmłodszych fazach działalności człowieka. Rejestrowana jest ona w zapisie geochemicznym poprzez wysoki udział metali ciężkich – kadm, ołów i miedź. Zaznacza się także wyraźna zmienność koncentracji głównych składników geochemicznych w zapisie pionowym analizowanych rdzeni. Jest to konsekwencją asymetrycznej konfiguracji zbiornika sedymentacyjnego i wynikającej z niej zmiennej w czasie biogenicznej sukcesji osadowej

Using Perturbative Least Action to Reconstruct Redshift Space Distortions

In this paper, we present a redshift space reconstruction scheme which is analogous to and extends the Perturbative Least Action (PLA) method described by Goldberg & Spergel (2000). We first show that this scheme is effective in reconstructing even nonlinear observations. We then suggest that by varying the cosmology to minimize the quadrupole moment of a reconstructed density field, it may be possible to lower the errorbars on the redshift distortion parameter, $\beta$ as well as to break the degeneracy between the linear bias parameter, $b$, and $\Omega_M$. Finally, we discuss how PLA might be applied to realistic redshift surveys.Comment: 34 Pages LaTeX, including 10 postscript figures. Submitted to Astrophysical Journa

Non-Gaussianity from Self-Ordering Scalar Fields

The Universe may harbor relics of the post-inflationary epoch in the form of a network of self-ordered scalar fields. Such fossils, while consistent with current cosmological data at trace levels, may leave too weak an imprint on the cosmic microwave background and the large-scale distribution of matter to allow for direct detection. The non-Gaussian statistics of the density perturbations induced by these fields, however, permit a direct means to probe for these relics. Here we calculate the bispectrum that arises in models of self-ordered scalar fields. We find a compact analytic expression for the bispectrum, evaluate it numerically, and provide a simple approximation that may be useful for data analysis. The bispectrum is largest for triangles that are aligned (have edges $k_1\simeq 2 k_2 \simeq 2 k_3$) as opposed to the local-model bispectrum, which peaks for squeezed triangles ($k_1\simeq k_2 \gg k_3$), and the equilateral bispectrum, which peaks at $k_1\simeq k_2 \simeq k_3$. We estimate that this non-Gaussianity should be detectable by the Planck satellite if the contribution from self-ordering scalar fields to primordial perturbations is near the current upper limit.Comment: 11 pages, 1 figur