Cosmology (Physics) (Physics)

An astrophysical peek into Einstein's static universe

by Abhas Mitra

The Strained State Cosmology

by Angelo Tartaglia

Chapter of the book Aspects of Today´s Cosmology, Antonio Alfonso-Faus (Ed.), ISBN: 978-953-307-626-3, InTech (2011), Available from: http://www.intechopen.com/articles/show/title/the-strained-state-cosmology

The cosmology of a four-dimensional space-time treated as a strained elastic continuum is illustrated. Various... more The cosmology of a four-dimensional space-time treated as a strained elastic continuum is illustrated. Various cosmological tests are performed, with positive results.

A Strained Space-Time to explain the Large Scale Properties of the Universe

by Angelo Tartaglia

To appear on Int. J. Mod. Phys. A

Simulating the impact of HI fluctuations on matched filter search for ionized bubbles in redshifted 21 cm maps

by Suman Majumdar

Authors: Kanan K. Datta , Suman Majumdar , Somnath Bharadwaj , T. Roy Choudhury; Published in --- Monthly Notices of the Royal Astronomical Society, Volume 391, Issue 4, pp. 1900-1912

Extending the formalism of Datta, Bharadwaj & Choudhury for detecting ionized bubbles in redshifted 21-cm maps... more Extending the formalism of Datta, Bharadwaj & Choudhury for detecting ionized bubbles in redshifted 21-cm maps using a matched filtering technique, we use different simulations to
analyse the impact ofHI fluctuations outside the bubble on the detectability of the bubble. In the first three kinds of simulations there is a spherical bubble of comoving radius Rb, the one that we are trying to detect, located at the centre, and the neutral hydrogen (H I) outside the bubble
traces the underlying dark matter distribution. We consider three different possible scenarios of re-ionization, i.e. (i) there is a single bubble (SB) in the field of view (FoV) and the hydrogen neutral fraction is constant outside this bubble, (ii) patchy re-ionization (PR) with many small ionized bubbles in the FoV (PR1) and (iii) many spherical ionized bubbles of the same radius Rb (PR2). The centres of the extra bubbles trace the dark matter distribution. The fourth kind of simulation uses more realistic maps based on seminumeric modelling (SM) of ionized regions.We make predictions for the currently functioning Giant Metrewave Radio Telescope (GMRT) and a forthcoming instrument, the MurchisonWidefield Array (MWA) at a redshift of 6 (corresponding to an observed frequency 203 MHz) for 1000 h observations.We find that for both the SB and PR1 scenarios the fluctuating intergalactic medium restricts bubble detection to size Rb ≤ 6Mpc and Rb ≤ 12 Mpc for the GMRT and the MWA, respectively, however
large be the integration time. These results are well explained by analytical predictions. In the PR2 scenario, we find that bubble detection is almost impossible for neutral fraction
xHI < 0.6 because of large uncertainty due to the HI fluctuations. Applying the matched filter technique to the SMscenario, we find that it works well even when the targeted ionized bubble is non-spherical due to surrounding bubbles and inhomogeneous recombination. We find that determining the size and positions of the bubbles is not limited by the HI fluctuations in the SB and PR1 scenario but limited by the instrument’s angular resolution instead, and this can
be done more precisely for larger bubble. We also find that for bubble detection the GMRT configuration is somewhat superior to the proposed MWA.

The impact of anisotropy from finite light travel time on detecting ionized bubbles in redshifted 21-cm maps

by Suman Majumdar

Authors: Suman Majumdar, Somnath Bharadwaj, Kanan K. Datta, T. Roy Choudhury; Monthly Notices of the Royal Astronomical Society, Volume 413, Issue 2, pp. 1409-1418

The detection of ionized bubbles around quasars in redshifted 21-cm maps is possibly one of the most direct future... more The detection of ionized bubbles around quasars in redshifted 21-cm maps is possibly one of the most direct future probes of reionization. We consider two models for the growth of spherical ionized bubbles to study the apparent shapes of the bubbles in redshifted 21-cm maps, taking into account the finite light travel time (FLTT) across the bubble. We find that the FLTT, whose effect is particularly pronounced for large bubbles, causes the bubble's image to continue to grow well after it's actual growth is over. There are two distinct FLTT distortions in the bubble's image: (i) its apparent center is shifted along the line of sight (LOS) towards the observer from the quasar; (ii) it's shape is anisotropic along the LOS. The bubble initially appears elongated along the LOS. This is reversed in the later stages of growth where the bubble appears compressed. The FLTT distortions are expected to have an impact on matched filter bubble detection where it is most convenient to use a spherical template for the filter. We find that the best matched spherical filter gives a reasonably good estimate of the size and the shift in the center of the anisotropic image. The mismatch between the spherical filter and the anisotropic image causes a 10 - 20% degradation in the SNR relative to that of a spherical bubble. We conclude that a spherical filter is adequate for bubble detection. The FLTT distortions do not effect the lower limits for bubble detection with 1000 hr of GMRT observations. The smallest spherical filter for which a detection is possible has comoving radii 24 Mpc and 33 Mpc for a 3-sigma and 5-sigma detection respectively, assuming a neutral fraction 0.6 at z \sim 8.

Influence of the Local Void on Measurements of the Clustering Dipole

by Maciej Bilicki

Monthly Notices of the Royal Astronomical Society, Volume 406, Issue 2, pp. 1358-1363.
Co-author: Michał Chodorowski.

In measurements of the clustering dipole from all-sky surveys, an important problem is the lack of information about... more In measurements of the clustering dipole from all-sky surveys, an important problem is the lack of information about galaxy distribution in the so-called Zone of Avoidance (ZoA). The existence of the Local Void (LV) has a systematic effect on these measurements. If the ZoA is randomly filled with mock galaxies, then the calculated acceleration of the Local Group of galaxies (LG) has a spurious component, resulting from the lack of real galaxies in the intersection of the LV with the ZoA. This component affects both the misalignment angle between the clustering dipole and the CMB dipole, and the inferred value of mean matter density Omega_m. We calculate the amplitude of the spurious acceleration acting on the LG due to the LV. Its value depends on the geometry and size of the LV, as well as on its density contrast. However, under the simplest assumption of the LV being spherical and completely empty, within the linear theory this amplitude amounts only to about 45 km/s in units of velocity. The resulting change in the misalignment angle is smaller than 1 degree, and the fractional change in the deduced value of Omega_m is about 5%. Accounting for observationally indicated elongation of the LV and maintaining the maximising assumption of a complete lack of galaxies inside increases these numbers only moderately. Specifically, the amplitude of the spurious acceleration rises to about 60 km/s, the misalignment angle remains still smaller than 1 degree, and the fractional change in the deduced value of Omega_m is enhanced to about 7%. Thus, despite the overall importance of the Local Void for the motion of the Local Group, the influence of the intersection of the LV with the ZoA on measurements of the clustering dipole is found to be only a minor systematic effect.

Towards the Optimal Window for the 2MASS Dipole

by Maciej Bilicki

The Velocity-Density Relation in the Spherical Model

by Maciej Bilicki

Monthly Notices of the Royal Astronomical Society, Volume 391, Issue 4, pp. 1796-1805.
Co-author: Michał Chodorowski.

We study the cosmic velocity-density relation using the spherical collapse model (SCM) as a proxy to non-linear... more We study the cosmic velocity-density relation using the spherical collapse model (SCM) as a proxy to non-linear dynamics. Although the dependence of this relation on cosmological parameters is known to be weak, we retain the density parameter Omega_m in SCM equations, in order to study the limit Omega_m -> 0. We show that in this regime the considered relation is strictly linear, for arbitrary values of the density contrast, on the contrary to some claims in the literature. On the other hand, we confirm that for realistic values of Omega_m the exact relation in the SCM is well approximated by the classic formula of Bernardeau (1992), both for voids (delta<0) and for overdensities up to delta ~ 3. Inspired by this fact, we find further analytic approximations to the relation for the whole range delta from -1 to infinity. Our formula for voids accounts for the weak Omega_m-dependence of their maximal rate of expansion, which for Omega_m < 1 is slightly smaller that 3/2. For positive density contrasts, we find a simple relation div v = 3 H_0 (Omega_m)^(0.6) [ (1+delta)^(1/6) - (1+delta)^(1/2) ], that works very well up to the turn-around (i.e. up to delta ~ 13.5 for Omega_m = 0.25 and neglected Omega_Lambda). Having the same second-order expansion as the formula of Bernardeau, it can be regarded as an extension of the latter for higher density contrasts. Moreover, it gives a better fit to results of cosmological numerical simulations.

Interpreting large-scale redshift-space distortion measurements

by Alvise Raccanelli

Co-authored with Lado Samushia, Will J. Percival. Submitted to MNRAS, 2011

The simplest theory describing large-scale redshift-space distortions (RSD), based on linear theory and distant... more The simplest theory describing large-scale redshift-space distortions (RSD), based on linear theory and distant galaxies, depends on the growth of cosmological structure, suggesting that strong tests of General Relativity can be constructed from galaxy surveys. As data sets become larger and the expected constraints more precise, the extent to which the RSD follow the simple theory needs to be assessed in order that we do not introduce systematic errors into the tests by introducing inaccurate simplifying assumptions. We study the impact of the sample geometry, non-linear processes, and biases induced by our lack of understanding of the radial galaxy distribution on RSD measurements. Using LasDamas simulations of the Sloan Digital Sky Survey II (SDSS-II) Luminous Red Galaxy (LRG) data, these effects are shown to be important at the level of 20 per cent. Including them, we can accurately model the recovered clustering in these mock catalogues on scales 30 - 200 Mpc/h. Applying this analysis to robustly measure parameters describing the growth history of the Universe from the SDSS-II data, gives f(z=0.25)\sigma_8(z=0.25)=0.3930 +- 0.0457$ and f(z=0.37)\sigma_8(z=0.37)=0.4328 +- 0.0370 when no prior is imposed on the growth-rate, and the background geometry is assumed to follow a \LambdaCDM model with the WMAP + SNIa priors. The standard WMAP constrained \LambdaCDM model with General Relativity predicts f(z=0.25)\sigma_8(z=0.25)=0.4260 +- 0.0141 and f(z=0.37)\sigma_8(z=0.37)=0.4367 +- 0.0136, which is fully consistent with these measurements.

CMB-Galaxy correlation in Unified Dark Matter Scalar Field Cosmologies

by Alvise Raccanelli

Co-authored with Daniele Bertacca, Oliver F. Piattella, Davide Pietrobon, Nicola Bartolo, Sabino Matarrese, Tommaso Giannantonio. Submitted to JCAP, 2011

We present an analysis of the cross-correlation between the CMB and the large-scale structure (LSS) of the Universe in... more We present an analysis of the cross-correlation between the CMB and the large-scale structure (LSS) of the Universe in Unified Dark Matter (UDM) scalar field cosmologies. We work out the predicted cross-correlation function in UDM models, which depends on the speed of sound of the unified component, and compare it with observations from six galaxy catalogues (NVSS, HEAO, 2MASS, and SDSS main galaxies, luminous red galaxies, and quasars). We sample the value of the speed of sound and perform a likelihood analysis, finding that the UDM model is as likely as the LambdaCDM, and is compatible with observations for a range of values of c_\infinity (the value of the sound speed at late times) on which structure formation depends. In particular, we obtain an upper bound of c_\infinity^2 \leq 0.009 at 95% confidence level, meaning that the LambdaCDM model, for which c_\infinity^2 = 0, is a good fit to the data, while the posterior probability distribution peaks at the value c_\infinity^2=10^(-4) . Finally, we study the time dependence of the deviation from LambdaCDM via a tomographic analysis using a mock redshift distribution and we find that the largest deviation is for low-redshift sources, suggesting that future low-z surveys will be best suited to constrain UDM models.

Simulating Redshift-Space Distortions for Galaxy Pairs with Wide Angular Separation

by Alvise Raccanelli

Co-authored with Lado Samushia, Will J. Percival. Published in MNRAS, 2010

The analysis of Redshift-Space Distortions (RSD) within galaxy surveys provides constraints on the amplitude of... more The analysis of Redshift-Space Distortions (RSD) within galaxy surveys provides constraints on the amplitude of peculiar velocities induced by structure growth, thereby allowing tests of General Relativity on extremely large scales. The next generation of galaxy redshift surveys, such as the Baryon Oscillation Spectroscopic Survey (BOSS), and the Euclid experiment will survey galaxies out to z=2, over 10,000--20,000 sq deg. In such surveys, galaxy pairs with large comoving separation will preferentially have a wide angular separation. In standard plane-parallel theory the displacements of galaxy positions due to RSD are assumed to be parallel for all galaxies, but this assumption will break down for wide-angle pairs. Szapudi 2004 and Papai & Szapudi 2008 provided a methodology, based on tripolar spherical harmonics expansion, for computing the redshift-space correlation function for all angular galaxy pair separations. In this paper we introduce a new procedure for analysing wide-angle effects in numerical simulations. We are able to separate, demonstrate, and fit each of the effects described by the wide-angle RSD theory. Our analysis highlights some of the nuances of dealing with wide-angle pairs, and shows that the effects are not negligible even for relatively small angles. This analysis will help to ensure the full exploitation of future surveys for RSD measurements, which are currently confined to pair separations less than \sim80 Mpc/h out to z\simeq 0.5.

A reassessment of the evidence of the Integrated Sachs-Wolfe effect through the WMAP-NVSS correlation

by Alvise Raccanelli

Co-authored with Anna Bonaldi, Mattia Negrello, Sabino Matarrese, Giuseppe Tormen, Gianfranco De Zotti. Published in MNRAS, 2008

We reassess the estimate of the cross-correlation of the spatial distribution of the NRAO VLA Sky Survey (NVSS) radio... more We reassess the estimate of the cross-correlation of the spatial distribution of the NRAO VLA Sky Survey (NVSS) radio sources with that of Cosmic Microwave Background (CMB) anisotropies from the Wilkinson Microwave Anisotropy Probe (WMAP). This re-analysis is motivated by the fact that most previous studies adopted a redshift distribution of NVSS sources inconsistent with recent data. We find that the constraints on the bias-weighted redshift distribution, b(z)xN(z), of NVSS sources, set by the observed angular correlation function, w(theta), strongly mitigate the effect of the choice of N(z). If such constraints are met, even highly discrepant redshift distributions yield NVSS-WMAP cross-correlation functions consistent with each other within statistical errors. The models favoured by recent data imply a bias factor, b(z), decreasing with increasing z, rather than constant, as assumed by most previous analyses. As a consequence, the function b(z)xN(z) has more weight at z<1, i.e. in the redshift range yielding the maximum contribution to the ISW in a standard LambdaCDM cosmology. On the whole, the NVSS turns out to be better suited for ISW studies than generally believed, even in the absence of an observational determination of the redshift distribution. The NVSS-WMAP cross-correlation function is found to be fully consistent with the prediction of the standard LambdaCDM cosmology.

A model for the cosmological evolution of low frequency radio sources

by Alvise Raccanelli

Co-authored with Marcella Massardi, Anna Bonaldi, Mattia Negrello, Sara Ricciardi, Gianfranco De Zotti. Published in MNRAS, 2010.

We present a new evolutionary model that describes the population properties of radio sources at frequencies <5... more We present a new evolutionary model that describes the population properties of radio sources at frequencies <5 GHz, thus complementing the De Zotti et al. (2005) model, holding at higher frequencies. We find that simple analytic luminosity evolution is still sufficient to fit the wealth of available data on local luminosity functions, multi-frequency source counts, and redshift distributions. However, the fit requires a luminosity-dependent decline of source luminosities at high redshifts, at least for steep-spectrum sources, thus confirming earlier indications of a "downsizing" also for radio sources. The upturn of source counts at sub-mJy levels is accounted for by a straightforward extrapolation, using the empirical far-IR/radio correlation, of evolutionary models matching the far-IR counts and redshift distributions of star-forming galaxies. We also discuss the implications of the new model for the interpretation of data on large-scale clustering of radio sources and on the Integrated Sachs-Wolfe (ISW) effect, and for the investigation of the contribution of discrete sources to the extragalactic background. As for the ISW effect, a new analysis exploiting a very clean CMB map, yields at a substantially higher significance than reported before.

Effects of a Cut, Lorentz-Boosted sky on the Angular Power Spectrum

by Amanda Yoho

co-authors Thiago Pereira, Maik Stuke, and Glenn D. Starkman

    Abstract: The largest fluctuation in the observed CMB temperature field is the dipole, its origin being... more     Abstract: The largest fluctuation in the observed CMB temperature field is the dipole, its origin being usually attributed to the Doppler Effect - the Earth's velocity with respect to the CMB rest frame. The lowest order boost correction to temperature multipolar coefficients appears only as a second order correction in the temperature power spectrum, $C_{\ell}$. Since v/c - 10-3, this effect can be safely ignored when estimating cosmological parameters [4-7]. However, by cutting our galaxy from the CMB sky we induce large-angle anisotropies in the data. In this case, the corrections to the cut-sky $C_{\ell}$s show up already at first order in the boost parameter. In this paper we investigate this issue and argue that this effect might turn out to be important when reconstructing the power spectrum from the cut-sky data.

Degree Scale Anomalies in the CMB: Localizing the dip in the first peak to the north ecliptic pole

by Amanda Yoho

Co-authors Francesc Ferrer, Glenn D. Starkman

Noticeable deviations from the prediction of the fiducial LCDM cosmology are found in the angular power spectrum of... more Noticeable deviations from the prediction of the fiducial LCDM cosmology are found in the angular power spectrum of the CMB. Besides large-angle anomalies, the WMAP 1st year data revealed a dip in the power spectrum at l = 200, which seemed to disappear in the 3rd year and subsequent angular power spectra. Using the WMAP 1st, 3rd, and 5th year data release, we study the intensity and spatial distribution of this feature in order to unveil its origin and its implications for the cosmological parameters. We show that in all WMAP data releases there is a substantial suppression of the first Doppler peak in a region near the north ecliptic pole.