Non-standard structure formation scenarios

Observations on galactic scales seem to be in contradiction with recent high resolution N-body simulations. This so-called cold dark matter (CDM) crisis has been addressed in several ways, ranging from a change in fundamental physics by introducing self-interacting cold dark matter particles to a tuning of complex astrophysical processes such as global and/or local feedback. All these efforts attempt to soften density profiles and reduce the abundance of satellites in simulated galaxy halos. In this contribution we are exploring the differences between a Warm Dark Matter model and a CDM model where the power on a certain scale is reduced by introducing a narrow negative feature (`dip'). This dip is placed in a way so as to mimic the loss of power in the WDM model: both models have the same integrated power out to the scale where the power of the Dip model rises to the level of the unperturbed CDM spectrum again. Using N-body simulations we show that that the new Dip model appears to be a viable alternative to WDM while being based on different physics: where WDM requires the introduction of a new particle species the Dip stems from anon-standard inflationary period. If we are looking for an alternative to the currently challenged standard ΛCDM structure formation scenario, neither the ΛWDM nor the new Dip model can be ruled out with respect to the analysis presented in this contribution. They both make very similar predictions and the degeneracy between them can only be broken with observations yet to come. This revised version was published online in August 2006 with corrections to the Cover Date.     

Structure formation in inhomogeneous dark energy models

We investigate how inhomogeneous quintessence models may have a specific signature even in the linear regime of large-scale structure formation. The dynamics of the collapse of a dark matter halo is governed by the value or the dynamical evolution of the dark energy equation of state, the energy density's initial conditions and its homogeneity nature in the highly non-linear regime. These have a direct impact on the redshift of collapse, altering in consequence the linearly extrapolated density threshold above which structures will end up collapsing. We compute this quantity for minimally coupled and coupled quintessence models, examining two extreme scenarios: first, when the quintessence field does not exhibit fluctuations on cluster scales and below – homogeneous dark energy; and secondly, when the field inside the overdensity collapses along with the dark matter – inhomogeneous dark energy. One shows that inhomogeneous dark energy models present distinct features which may be used to confront them with observational data, for instance, galaxy number counting. Fitting formulae for the linearly extrapolated density threshold above which structures will end up collapsing are provided for models of dark energy with constant equation of state.     

Profiles of dark matter haloes at high redshift

I study the evolution of halo density profiles as a function of time in the SCDM and ΛCDM cosmologies. Following Del Popolo, I calculate the concentration parameter c = r _v / a and study its time evolution. For a given halo mass, I find that c ( z ) ∝ 1/(1+ z ) in both the ΛCDM and SCDM cosmology, in agreement with the analytic model of Bullock et al. and N -body simulations. In both models, a ( z ) is roughly constant. The present model predicts a stronger evolution of c ( z ) with respect to the Navarro, Frenk & White model. Finally I show some consequences of the results on galaxy modelling.     

Cosmological implications and structure formation from a time varying vacuum

We study the dynamics of the Friedmann–Lemaitre–Robertson–Walker (FLRW) flat cosmological models in which the vacuum energy varies with time,  Λ( t )  . In this model, we find that the main cosmological functions such as the scale factor of the universe and the Hubble flow are defined in terms of exponential functions. Applying a joint likelihood analysis of the recent Type Ia supernovae data, the cosmic microwave background shift parameter and the baryonic acoustic oscillations traced by the Sloan Digital Sky Survey (SDSS) galaxies, we place tight constraints on the main cosmological parameters of the  Λ( t )  scenario. Also, we compare the  Λ( t )  model with the traditional Λ cosmology and we find that the former model provides a Hubble expansion which compares well with that of the Λ cosmology. However, the  Λ( t )  scenario predicts stronger small scale dynamics, which implies a faster growth rate of perturbations with respect to the usual Λ cosmology, despite the fact that they share the same equation of state parameter. In this framework, we find that galaxy clusters in the  Λ( t )  model appear to form earlier than in the Λ model.     

Effects of merging histories on angular momentum distribution of dark matter haloes

The effects of merging histories of proto-objects on the angular momentum distributions of the present-time dark matter haloes are analysed. An analytical approach to the analysis of the angular momentum distributions assumes that the haloes are initially homogeneous ellipsoids and that the growth of the angular momentum of the haloes halts at their maximum expansion time. However, the maximum expansion time cannot be determined uniquely, because in the hierarchical clustering scenario each progenitor, or subunit, of the halo has its own maximum expansion time. Therefore the merging history of the halo may be important in estimating its angular momentum. Using the merger tree model by Rodrigues &38; Thomas, which takes into account the spatial correlations of the density fluctuations, we have investigated the effects of the merging histories on the angular momentum distributions of dark matter haloes. It was found that the merger effects, that is, the effects of the inhomogeneity of the maximum expansion times of the progenitors which finally merge together into a halo, do not strongly affect the final angular momentum distributions, so that the homogeneous ellipsoid approximation happens to be good for the estimation of the angular momentum distribution of dark matter haloes. This is because the effect of the different directions of the angular momenta of the progenitors cancels out the effect of the inhomogeneity of the maximum expansion times of the progenitors.   The contribution of the orbital angular momentum to the total angular momentum when two or more pre-existing haloes merge together was also investigated. It is shown that this contribution is more important than that of the angular momentum of diffuse accreting matter to the total angular momentum, especially when the mergers occur many times.     

Baryon oscillations in galaxy and matter power-spectrum covariance matrices

We investigate large-amplitude baryon acoustic oscillations (BAOs) in off-diagonal entries of cosmological power-spectrum covariance matrices. These covariance-matrix BAOs describe the increased attenuation of power-spectrum BAOs caused by upward fluctuations in large-scale power. We derive an analytic approximation to covariance-matrix entries in the BAO regime, and check the analytical predictions using N -body simulations. These BAOs look much stronger than the BAOs in the power spectrum, but seem detectable only at about a 1σ level in gigaparsec-scale galaxy surveys. In estimating cosmological parameters using matter or galaxy power spectra, including the covariance-matrix BAOs can have a several per cent effect on error bar widths for some parameters directly related to the BAOs, such as the baryon fraction. Also, we find that including the numerous galaxies in small haloes in a survey can reduce error bars in these cosmological parameters more than the simple reduction in shot noise might suggest.