arXiv subjects

Radosław Wojtak

Publications and source records attributed to Radosław Wojtak.

22 records · Page 2Linked to original sources

arXiv2016

Voids in cosmological simulations over cosmic time

We study evolution of voids in cosmological simulations using a new method for tracing voids over cosmic time. The method is based on tracking watershed basins (contiguous regions around density minima) of well developed voids at low redshift, on a regular grid of density field. It enables us to construct a robust and continuous mapping between voids at different redshifts, from initial conditions to the present time. We discuss how the new approach eliminates strong spurious effects of numerical origin when voids evolution is traced by matching voids between successive snapshots (by analogy to halo merger trees). We apply the new method to a cosmological simulation of a standard LambdaCDM cosmological model and study evolution of basic properties of typical voids (with effective radii between 6Mpc/h and 20Mpc/h at redshift z=0) such as volumes, shapes, matter density distributions and relative alignments. The final voids at low redshifts appear to retain a significant part of the configuration acquired in initial conditions. Shapes of voids evolve in a collective way which barely modifies the overall distribution of the axial ratios. The evolution appears to have a weak impact on mutual alignments of voids implying that the present state is in large part set up by the primordial density field. We present evolution of dark matter density profiles computed on iso-density surfaces which comply with the actual shapes of voids. Unlike spherical density profiles, this approach enables us to demonstrate development of theoretically predicted bucket-like shape of the final density profiles indicating a wide flat core and a sharp transition to high-density void walls.

astro-ph.CO↗

arXiv2016

Testing the mapping between redshift and cosmic scale factor

The canonical redshift-scale factor relation, 1/a=1+z, is a key element in the standard LambdaCDM model of the big bang cosmology. Despite its fundamental role, this relation has not yet undergone any observational tests since Lemaitre and Hubble established the expansion of the Universe. It is strictly based on the assumption of the Friedmann-Lemaitre-Robertson-Walker metric describing a locally homogeneous and isotropic universe and that photons move on null geodesics of the metric. Thus any violation of this assumption, within general relativity or modified gravity, can yield a different mapping between the model redshift z=1/a-1 and the actually observed redshift z_obs, i.e. z_obs neq z. Here we perform a simple test of consistency for the standard redshift-scale factor relation by determining simultaneous observational constraints on the concordance LambdaCDM cosmological parameters and a generalized redshift mapping z=f(z_obs). Using current baryon acoustic oscillations (BAO) and Type Ia supernova (SN) data we demonstrate that the generalized redshift mapping is strongly degenerated with dark energy. Marginalization over a class of monotonic functions f(z_obs) changes substantially degeneracy between matter and dark energy density: the density parameters become anti correlated with nearly vertical axis of degeneracy. Furthermore, we show that current SN and BAO data, analysed in a framework with the generalized redshift mapping, do not constrain dark energy unless the BAO data include the measurements from the Ly-alpha forest of high-redshift quasars.

astro-ph.CO↗

arXiv2015

Non-universality of dark-matter halos: cusps, cores, and the central potential

Dark-matter halos grown in cosmological simulations appear to have central NFW-like density cusps with mean values of $d\logρ/d\log r \approx -1$, and some dispersion, which is generally parametrized by the varying index $α$ in the Einasto density profile fitting function. Non-universality in profile shapes is also seen in observed galaxy clusters and possibly dwarf galaxies. Here we show that non-universality, at any given mass scale, is an intrinsic property of DARKexp, a theoretically derived model for collisionless self-gravitating systems. We demonstrate that DARKexp - which has only one shape parameter, $ϕ_0$ - fits the dispersion in profile shapes of massive simulated halos as well as observed clusters very well. DARKexp also allows for cored dark-matter profiles, such as those found for dwarf spheroidal galaxies. We provide approximate analytical relations between DARKexp $ϕ_0$, Einasto $α$, or the central logarithmic slope in the Dehnen-Tremaine analytical $γ$-models. The range in halo parameters reflects a substantial variation in the binding energies per unit mass of dark-matter halos.

astro-ph.CO↗

arXiv2014

Cosmological Parameters From Supernovae Associated With Gamma-ray Bursts

We report estimates of the cosmological parameters $Ω_m$ and $Ω_Λ$ obtained using supernovae (SNe) associated with gamma-ray bursts (GRBs) at redshifts up to 0.606. Eight high-fidelity GRB-SNe with well-sampled light curves across the peak are used. We correct their peak magnitudes for a luminosity-decline rate relation to turn them into accurate standard candles with dispersion $σ= 0.18$ mag. We also estimate the peculiar velocity of the low-redshift host galaxy of SN 1998bw, using constrained cosmological simulations. In a flat universe, the resulting Hubble diagram leads to best-fit cosmological parameters of $(Ω_m, Ω_Λ) = (0.58^{+0.22}_{-0.25},0.42 ^{+0.25}_{-0.22})$. This exploratory study suggests that GRB-SNe can potentially be used as standardizable candles to high redshifts to measure distances in the universe and constrain cosmological parameters.

astro-ph.CO↗