Correlation between galactic HI and the Cosmic Microwave Background

We revisit the issue of a correlation between the atomic hydrogen gas in our local Galaxy and the Cosmic Microwave Background (CMB), a detection of which has been claimed in some literature. We cross-correlate the 21-cm emission of Galactic atomic hydrogen as traced by the Leiden/Argentine/Bonn Galactic HI survey with the 3-year CMB data from the Wilkinson Microwave Anisotropy Probe. We consider a number of angular scales, masks, and HI velocity slices and find no statistically significant correlation.Comment: 4 pages, 4 figures, accepted in PRD brief repor

Detecting neutrino mass difference with cosmology

Cosmological parameter estimation exercises usually make the approximation that the three standard neutrinos have degenerate mass, which is at odds with recent terrestrial measurements of the difference in the square of neutrino masses. In this paper we examine whether the use of this approximation is justified for the cosmic microwave background (CMB) spectrum, matter power spectrum and the CMB lensing potential power spectrum. We find that, assuming m^2_{23} ~ 2.5x10^{-3}$eV^2 in agreement with recent Earth based measurements of atmospheric neutrino oscillations, the correction due to non-degeneracy is of the order of precision of present numerical codes and undetectable for the foreseeable future for the CMB and matter power spectra. An ambitious experiment that could reconstruct the lensing potential power spectrum to the cosmic variance limit up to l~1000 will have to take the effect into account in order to avoid biases. The degeneracies with other parameters, however, will make the detection of the neutrino mass difference impossible. We also show that relaxing the bound on the neutrino mass difference will also increase the error-bar on the sum of neutrino masses by a factor of up to a few. For exotic models with significantly non-degenerate neutrinos the corrections due to non-degeneracy could become important for all the cosmological probes discussed here.Comment: 5 pages, 4 figures, v2: replaced with version accepted to the PRD: added fisher matrix analysis, conclusions somewhat chage

Did Boomerang hit MOND?

Purely baryonic dark matter dominated models like MOND based on modification of Newtonian gravity have been successfully in reproducing some dynamical properties of galaxies. More recently, a relativistic formulation of MOND proposed by Bekenstein seems to agree with cosmological large scale structure formation. In this work, we revise the agreement of MOND with observations in light of the new results on the Cosmic Microwave Anisotropies provided by the 2003 flight of Boomerang. The measurements of the height of the third acoustic peak, provided by several small scale CMB experiments have reached enough sensitivity to severely constrain models without cold dark matter. Assuming that acoustic peak structure in the CMB is unchanged and that local measurements of the Hubble constant can be applied, we find that the cold dark matter is strongly favoured with Bayesian probability ratio of about one in two hundred.Comment: 5 pages, 2 figures; v2 minor modifications to match version published as "Test of modified newtonian dynamics with recent Boomerang data." in PRD rapid com

Pairwise velocities in the Halo Model: Luminosity and Scale Dependence

We investigate the properties of the pairwise velocity dispersion as a function of galaxy luminosity in the context of a halo model. We derive the distribution of velocities of pairs at a given separation taking into account both one-halo and two-halo contributions. We show that pairwise velocity distribution in real space is a complicated mixture of host-satellite, satellite-satellite and two-halo pairs. The peak value is reached at around 1$h^{-1}$Mpc and does not reflect the velocity dispersion of a typical halo hosting these galaxies, but is instead dominated by the satellite-satellite pairs in high mass clusters. This is true even for cross-correlations between bins separated in luminosity. As a consequence the velocity dispersion at a given separation can decrease with luminosity, even if the underlying typical halo host mass is increasing, in agreement with recent observations. We compare our findings to numerical simulations and find a good agreement. Numerical simulations also suggest a luminosity dependent velocity bias, which depends on the subhalo mass. We develop models of the auto- and cross-correlation function of luminosity subsamples of galaxies in the observable r_\proj - \pi space and calculate the inferred velocity dispersion as a function of wave vector if dispersion model is fit to the redshift space power spectrum. We find that so derived pairwise velocity dispersion also exhibits a bump at $k\sim 1 h/{\rm Mpc}$.Comment: 11 pages, 12 figures; v2: major revision matching version accepted by MNRA

Constraints on local primordial non-Gaussianity from large scale structure

Recent work has shown that the local non-Gaussianity parameter f_NL induces a scale-dependent bias, whose amplitude is growing with scale. Here we first rederive this result within the context of peak-background split formalism and show that it only depends on the assumption of universality of mass function, assuming halo bias only depends on mass. We then use extended Press-Schechter formalism to argue that this assumption may be violated and the scale dependent bias will depend on other properties, such as merging history of halos. In particular, in the limit of recent mergers we find the effect is suppressed. Next we use these predictions in conjunction with a compendium of large scale data to put a limit on the value of f_NL. When combining all data assuming that halo occupation depends only on halo mass, we get a limit of -29 ~ (-65)< f_NL < +70 ~(+93) at 95% (99.7%) confidence. While we use a wide range of datasets, our combined result is dominated by the signal from the SDSS photometric quasar sample. If the latter are modeled as recent mergers then the limits weaken to -31 ~(-96) < f_NL < +70 ~ (+96) . These limits are comparable to the strongest current limits from the WMAP 5 year analysis, with no evidence of a positive signal in f_NL. While the method needs to be thoroughly tested against large scale structure simulations with realistic quasar and galaxy formation models, our results indicate that this is a competitive method relative to CMB and should be further pursued both observationally and theoretically.Comment: 18 pages, 5 figures; v2 matches version accepted by JCAP, several small changes in the text, added refs and fixed typo

Cross-correlation studies as a probe of reionization physics

The process of reionization is now believed to have proceeded in an orchestrated manner beginning with UV photons emitted by high redshift galaxies containing a large fraction of Population III stars carving out ionised regions around them. The physics during this era can be studied with a combination of redshifted 21-cm spin-flip transition tracing neutral hydrogen gas, IR emission from massive primordial stars that trace the global star-formation rate during reionization, and the imprint of hot-electrons in first supernovae remnants Compton-cooling off of cosmic microwave background (CMB) radiation through the Sunyaev-Zel'dovich effect. While these individual effects and their observable signatures have been advocated as probes of reionization history, here we show how cross-correlation studies between these signals can be used to further understand physics during reionization. Cross-correlation studies are advantageous since the measurable statistics do not suffer in the same manner from foregrounds and systematic effects as is the case of auto-correlation function measurements. We discuss the prospects for detecting various cross-correlation statistics using present and next generation experiments and the information related to reionization captured by them.Comment: Revised version of a paper submitted to MNRAS in Dec 06; v2: updated references, matches version accepted by MNRA

Reconstructing large-scale structure with neutral hydrogen surveys

Upcoming 21-cm intensity surveys will use the hyperfine transition in emission to map out neutral hydrogen in large volumes of the universe. Unfortunately, large spatial scales are completely contaminated with spectrally smooth astrophysical foregrounds which are orders of magnitude brighter than the signal. This contamination also leaks into smaller radial and angular modes to form a foreground wedge, further limiting the usefulness of 21-cm observations for different science cases, especially cross-correlations with tracers that have wide kernels in the radial direction. In this paper, we investigate reconstructing these modes within a forward modeling framework. Starting with an initial density field, a suitable bias parameterization and non-linear dynamics to model the observed 21-cm field, our reconstruction proceeds by combining the likelihood of a forward simulation to match the observations (under given modeling error and a data noise model) with the Gaussian prior on initial conditions and maximizing the obtained posterior. For redshifts $z=2$ and $4$, we are able to reconstruct 21cm field with cross correlation, $r_c > 0.8$ on all scales for both our optimistic and pessimistic assumptions about foreground contamination and for different levels of thermal noise. The performance deteriorates slightly at $z=6$. The large-scale line-of-sight modes are reconstructed almost perfectly. We demonstrate how our method also reconstructs baryon acoustic oscillations, outperforming standard methods on all scales. We also describe how our reconstructed field can provide superb clustering redshift estimation at high redshifts, where it is otherwise extremely difficult to obtain dense spectroscopic samples, as well as open up cross-correlation opportunities with projected fields (e.g. lensing) which are restricted to modes transverse to the line of sight.Comment: 30 pages, 12 figures. Updated text to make discussion more robus

Inflation and WMAP three year data: Features have a Future!

The new three year WMAP data seem to confirm the presence of non-standard large scale features in the Cosmic Microwave Anisotropies power spectrum. While these features may hint at uncorrected experimental systematics, it is also possible to generate, in a cosmological way, oscillations on large angular scales by introducing a sharp step in the inflaton potential. Using current cosmological data, we derive constraints on the position, magnitude and gradient of a possible step. We show that a step in the inflaton potential, while strongly constrained by current data, is still allowed and may provide an interesting explanation to the currently measured deviations from the standard featureless spectrum. Moreover, we show that inflationary oscillations in the primordial power spectrum can significantly bias parameter estimates from standard ruler methods involving measurements of baryon oscillations.Comment: 8 pages, 11 figures; final version published in Phys. Rev.