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Cambridge, United Kingdom

Bonvin C.,Kavli Institute for Cosmology | Bonvin C.,University of Cambridge
Classical and Quantum Gravity | Year: 2014

We present a fully relativistic calculation of the observed galaxy number counts in the linear regime. We show that besides the density fluctuations and redshift-space distortions, various relativistic effects contribute to observations at large scales. These effects all have the same physical origin: they result from the fact that our coordinate system, namely the galaxy redshift and the incoming photons' direction, is distorted by inhomogeneities in our Universe. We then discuss the impact of the relativistic effects on the angular power spectrum and on the two-point correlation function in configuration space. We show that the latter is very well adapted to isolate the relativistic effects since it naturally makes use of the symmetries of the different contributions. In particular, we discuss how the Doppler effect and the gravitational redshift distortions can be isolated by looking for a dipole in the cross-correlation function between a bright and a faint population of galaxies. © 2014 IOP Publishing Ltd.

Efstathiou G.,Kavli Institute for Cosmology
Monthly Notices of the Royal Astronomical Society | Year: 2014

I reanalyse the Riess et al. (hereafter R11) Cepheid data using the revised geometric maser distance to NGC 4258 of Humphreys et al. (hereafter H13). I explore different outlier rejection criteria designed to give a reduced X2 of unity and compare the results with the R11 rejection algorithm, which produces a reduced X2 that is substantially less than unity and, in some cases, leads to underestimates of the errors on parameters. I show that there are sub-luminous low-metallicity Cepheids in the R11 sample that skew the global fits of the period-luminosity relation. This has a small but non-negligible impact on the global fits using NGC 4258 as a distance scale anchor, but adds a poorly constrained source of systematic error when using the Large Magellanic Cloud as an anchor. I also show that the small Milky Way Cepheid sample with accurate parallax measurements leads to a distance to NGC 4258 that is in tension with the maser distance. I conclude that H0 based on the NGC 4258 maser distance is H0 = 70.6 ± 3.3 kms-1 Mpc-1, compatible within 1s with the recent determination from Planck for the base six-parameter cold dark matter cosmology. If the H-band period-luminosity relation is assumed to be independent of metallicity and the three distance anchors are combined, I find H0 = 72.5 ± 2.5 kms-1 Mpc-1, which differs by 1.9σ from the Planck value. The differences between the Planck results and these estimates of H0 are not large enough to provide compelling evidence for new physics at this stage. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Amin M.A.,Kavli Institute for Cosmology
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2013

Oscillons are long-lived, localized, oscillatory scalar field configurations. In this work we derive a condition for the existence of small-amplitude oscillons (and provide solutions) in scalar field theories with noncanonical kinetic terms. While oscillons have been studied extensively in the canonical case, this is the first example of oscillons in scalar field theories with noncanonical kinetic terms. In particular, we demonstrate the existence of oscillons supported solely by the noncanonical kinetic terms, without any need for nonlinear terms in the potential. In the small-amplitude limit, we provide an explicit condition for their stability in d+1 dimensions against long-wavelength perturbations. We show that for d≥3, there exists a long-wavelength instability which can lead to radial collapse of small-amplitude oscillons. © 2013 American Physical Society.

Shaw J.R.,Kavli Institute for Cosmology | Shaw J.R.,Canadian Institute for Theoretical Astrophysics | Lewis A.,University of Sussex
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2012

Primordial magnetic fields could provide an explanation for the galactic magnetic fields observed today; in which case, they may leave interesting signals in the CMB and the small-scale matter power spectrum. We discuss how to approximately calculate the important nonlinear magnetic effects within the guise of linear perturbation theory and calculate the matter and CMB power spectra including the Sunyaev-Zel'dovich contribution. We then use various cosmological data sets to constrain the form of the magnetic field power spectrum. Using solely large-scale CMB data (WMAP7, QUaD, and ACBAR) we find a 95% C.L. on the variance of the magnetic field at 1 Mpc of B λ<6.4nG. When we include South Pole Telescope data to constrain the Sunyaev-Zel'dovich effect, we find a revised limit of B λ<4.1nG. The addition of Sloan Digital Sky Survey Lyman-α data lowers this limit even further, roughly constraining the magnetic field to B λ<1.3nG. © 2012 American Physical Society.

Cantalupo S.,Kavli Institute for Cosmology | Porciani C.,Argelander Institute For Astronomie
Monthly Notices of the Royal Astronomical Society | Year: 2011

We present a new three-dimensional radiative transfer (RT) code, radamesh (Radiative-transfer on ADAptive MESH), based on a ray-tracing, photon-conserving and adaptive (in space and time) scheme. radamesh uses a novel Monte Carlo approach to sample the radiation field within the computational domain on a 'cell-by-cell' basis. Thanks to this algorithm, the computational efforts are now focused where actually needed, i.e. within the Ionization-fronts (I-fronts). This results in an increased accuracy level and, at the same time, a huge gain in computational speed with respect to a 'classical' Monte Carlo RT, especially when combined with an Adaptive Mesh Refinement (AMR) scheme. Among several new features, radamesh is able to adaptively refine the computational mesh in correspondence of the I-fronts, allowing to fully resolve them within large, cosmological boxes. We follow the propagation of ionizing radiation from an arbitrary number of sources and from the recombination radiation produced by H and He. The chemical state of six species (Hi, Hii, Hei, Heii, Heiii, e) and gas temperatures are computed with a time-dependent, non-equilibrium chemistry solver. We present several validating tests of the code, including the standard tests from the RT code comparison project and a new set of tests aimed at substantiating the new characteristics of radamesh. Using our AMR scheme, we show that properly resolving the I-front of a bright quasar during reionization produces a large increase of the predicted gas temperature within the whole Hii region. Also, we discuss how H and He recombination radiation is able to substantially change the ionization state of both species (for the classical Strömgren sphere test) with respect to the widely used 'on-the-spot' approximation. © 2010 The Authors Monthly Notices of the Royal Astronomical Society © 2010 RAS.

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