### Browsing by Subject "General Relativity and Quantum Cosmology"

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- ItemOpen AccessAre braneworlds born isotropic?(2004) Dunsby, Peter K S; Goheer, Naureen; Bruni, Marco; Coley, AlanIt has recently been suggested that an isotropic singularity may be a generic feature of brane cosmologies, even in the inhomogeneous case. Using the covariant and gauge-invariant approach we present a detailed analysis of linear perturbations of the isotropic model ${cal F}_b$ which is a past attractor in the phase space of homogeneous Bianchi models on the brane. We find that for matter with an equation of state parameter $gamma > 1$, the dimensionless variables representing generic anisotropic and inhomogeneous perturbations decay as $tto 0$, showing that the model ${cal F}_b$ is asymptotically stable in the past. We conclude that brane universes are born with isotropy naturally built-in, contrary to standard cosmology. The observed large-scale homogeneity and isotropy of the universe can therefore be explained as a consequence of the initial conditions if the brane-world paradigm represents a description of the very early universe. Comment: Changed to match published version
- 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 AccessMulticolour observations, inhomogeneity and evolution(2001) Hellaby, CharlesWe propose a method of testing source evolution theories that is independent of the effects of inhomogeneity, and thus complementary to other studies of evolution. It is suitable for large scale sky surveys, and the new generation of large telescopes. In an earlier paper it was shown that basic cosmological observations -luminosity versus redshift, area distance versus redshift and number counts versus redshift -cannot separate the effects of cosmic inhomogeneity, cosmic evolution and source evolution. We here investigate multicolour observations, and show that by comparing luminosity versus redshift in two or more colours, contraints can be placed on source evolution even if unknown source evolution is present, providing an important test of evolution theories that is complementary to present methods. However, number counts in different colours versus redshift are not useful in separating the effects of source evolution and inhomogeneity.
- 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 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.