The scale of homogeneity in the Las Campanas Redshift Survey

We analyse the Las Campanas Redshift Survey using the integrated conditional density (or density of neighbors) in volume-limited subsamples up to unprecedented scales (200 Mpc/$h$) in order to determine without ambiguity the behavior of the density field. We find that the survey is well described by a fractal up to 20-30 Mpc/$h$, but flattens toward homogeneity at larger scales. Although the data are still insufficient to establish with high significance the expected homogeneous behavior, and therefore to rule out a fractal trend to larger scales, a fit with a CDM-like spectrum with high normalization well represents the data.Comment: 8 pages, 3 figures, accepted on Ap.J. Letter

Clarifying spherical collapse in coupled dark energy cosmologies

The spherical collapse model is often used to follow the evolution of overdensities into the nonlinear regime. We describe the correct approach to be used in coupled dark energy cosmologies, where a fifth force, different from gravity and mediated by the dark energy scalar field, influences the collapse. We reformulate the spherical collapse description by deriving it directly from the set of nonlinear hydrodynamical Navier Stokes equations. By comparing with the corresponding relativistic equations, we show how the fifth force should be taken into account within the spherical collapse picture and clarify the problems arising when an inhomogeneous scalar field is considered within a spherical collapse picture. We then apply our method to the case of coupled quintessence, where the fifth force acts among cold dark matter particles, and to growing neutrino quintessence, where the fifth force acts between neutrinos. Furthermore, we review this method when applied to standard cosmologies and apply our analysis to minimally coupled quintessence and check past results for early dark energy parametrizations.Comment: 16 pages, 13 figures, published in Physical Review D, minor changes and correction

Formalism for the Subhalo Mass Function in the Tidal-limit Approximation

We present a theoretical formalism by which the global and the local mass functions of dark matter substructures (dark subhalos) can be analytically estimated. The global subhalo mass function is defined to give the total number density of dark subhalos in the universe as a function of mass, while the local subhalo mass function counts only those subhalos included in one individual host halo. We develop our formalism by modifying the Press-Schechter theory to incorporate the followings: (i) the internal structure of dark halos; (ii) the correlations between the halos and the subhalos; (iii) the subhalo mass-loss effect driven by the tidal forces. We find that the resulting (cumulative) subhalo mass function is close to a power law with the slope of ~ -1, that the subhalos contribute approximately 10 % of the total mass, and that the tidal stripping effect changes the subhalo mass function self-similarly, all consistent with recent numerical detections.Comment: revised version, accepted by ApJ Letters, estimate of the local subhalo mass function included, 10 pages, 1 figur

Reconstructing the linear power spectrum of cosmological mass fluctuations

We describe an attempt to reconstruct the initial conditions for the formation of cosmological large-scale structure. The power spectrum of the primordial fluctuations is affected by bias, nonlinear evolution and redshift-space distortions, but we show how these effects can be corrected for analytically. Using eight independent datasets, we obtain excellent agreement in the estimated linear power spectra given the following conditions. First, the relative bias factors for Abell clusters, radio galaxies, optical galaxies and IRAS galaxies must be in the ratios 4.5:1.9:1.3:1. Second, the data require redshift-space distortion: \Omega^{0.6}/b_{\ss I} = 1.0 \pm 0.2. Third, low values of $\Omega$ and bias are disfavoured. The shape of the spectrum is extremely well described by a CDM transfer function with an apparent value of the fitting parameter $\Omega h =0.25$. Tilted models predict too little power at 100 Mpc wavelengths.Comment: Edinburgh Astronomy Preprint 26/93. Accepted for publication in Monthly Notices of the RAS. 13 pages of LaTeX, plus 10 PostScript figures. You will need the mn.sty style file (from babbage: get mn.sty). The figure .ps files are in the usual self-unpacking unix scrip