### Browsing by Subject "Astrophysics - Cosmology and Nongalactic Astrophysics"

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- ItemOpen AccessConstraints on anisotropic cosmic expansion from supernovae(2013) Kalus, B; Schwarz, D J; Seikel, M; Wiegand, AAims. We test the isotropy of the expansion of the Universe by estimating the hemispherical anisotropy of supernova type Ia (SN Ia) Hubble diagrams at low redshifts (z
- ItemOpen AccessHow flat is our Universe really?(2013) Okouma, PM; Fantaye, Y; Bassett, B ADistance measurement provide no constraints on curvature independent of assumptions about the dark energy, raising the question, how flat is our Universe if we make no such assumptions? Allowing for general evolution of the dark energy equation of state with 20 free parameters that are allowed to cross the phantom divide, w(z) = -1, we show that while it is indeed possible to match the first peak in the Cosmic Microwave Background with non-flat models and arbitrary Hubble constant, H_0, the full WMAP7 and supernova data alone imply -0.12 < Omega_k < 0.01 (2sigma). If we add an H_0 prior, this tightens significantly to Omega_k = 0.002 pm 0.009 . These constitute the most conservative and model-independent constraints on curvature available today, and illustrate that the curvature-dynamics degeneracy is broken by current data, with a key role played by the Integrated Sachs Wolfe effect rather than the distance to the surface of last scattering. If one imposes a quintessence prior on the dark energy (-1 leq w(z) leq 1) then just the WMAP7 and supernova data alone force the Universe to near flatness: Omega_k = 0.013 pm 0.012. Finally, allowing for curvature, we find that all datasets are consistent with a Harrison-Zel'dovich spectral index, n_s = 1, at 2sigma, illustrating the interplay between early and late-universe constraints.
- ItemOpen AccessParametrizing the transition to the phantom epoch with supernovae Ia and standard rulers(2014) Leanizbarrutia, Iker; Sáez-Gómez, DiegoThe properties of some particular parametrizations of the dark energy Equation of State (EoS) are studied by using Supernovae Ia data (HST Cluster Supernova Survey) combined with Standard Ruler datasets (Cosmic Microwave Background (CMB) and Baryon Acoustic Oscillations (BAO)). In this sense, we propose some parametrizations that may present a (fast) transition to a phantom dark energy EoS (where $w_{DE}<-1$) and compare the results with the $Lambda$CDM model. The best fit of the models is obtained by using Sne Ia and Standard Ruler datasets, which provides some information about whether the phantom transition may be supported by the observations. In this regard, the crossing of the phantom barrier is allowed statistically but the occurrence of a future singularity seems unlikely. Furthermore, the reconstruction of a (non-)canonical scalar field Lagrangian from the EoS parameter is studied, where shown that EoS parametrizations can be well reconstructed in terms of scalar fields.
- ItemOpen AccessThe Core-Cusp Problem(2009) de Blok, W J GThis paper gives an overview of the attempts to determine the distribution of dark matter in low surface brightness disk and gas-rich dwarf galaxies, both through observations and computer simulations. Observations seem to indicate an approximately constant dark matter density in the inner parts of galaxies, while cosmological computer simulations indicate a steep power-law-like behaviour. This difference has become known as the “core/cusp problem,” and it remains one of the unsolved problems in small-scale cosmology.
- ItemOpen AccessTHINGS about MOND(2011) Gentile, G; Famaey, B; de Blok, W J GWe present an analysis of 12 high-resolution galactic rotation curves from The HI Nearby Galaxy Survey (THINGS) in the context of modified Newtonian dynamics (MOND). These rotation curves were selected to be the most reliable for mass modelling, and they are the highest quality rotation curves currently available for a sample of galaxies spanning a wide range of luminosities. We fit the rotation curves with the "simple" and "standard" interpolating functions of MOND, and we find that the "simple" function yields better results. We also redetermine the value of a(0), and find a median value very close to the one determined in previous studies, a(0) = (1.22 +/- 0.33) x 10(-8) cm s(-2). Leaving the distance as a free parameter within the uncertainty of its best independently determined value leads to excellent quality fits for 75% of the sample. Among the three exceptions, two are also known to give relatively poor fits in Newtonian dynamics plus dark matter. The remaining case (NGC 3198) presents some tension between the observations and the MOND fit, which might, however, be explained by the presence of non-circular motions, by a small distance, or by a value of a(0) at the lower end of our best-fit interval, 0.9 x 10(-8) cm s(-2). The best-fit stellar M/L ratios are generally in remarkable agreement with the predictions of stellar population synthesis models. We also show that the narrow range of gravitational accelerations found to be generated by dark matter in galaxies is consistent with the narrow range of additional gravity predicted by MOND.