Sunday, March 17, 2013
Wednesday, March 13, 2013
Romik Danial is still waiting for you to join Twitter...
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Wednesday, February 27, 2013
Romik Danial is still waiting for you to join Twitter...
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Tuesday, February 19, 2013
Romik Danial is still waiting for you to join Twitter...
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Monday, February 11, 2013
Romik Danial sent you an invitation
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Monday, September 10, 2012
arXiv: 10 September 2012
Claudia G. Scóccola, Ariel G. Sánchez, J. A. Rubiño-Martin, R. Génova-Santos, R. Rebolo, A. J. Ross, W. J. Percival, M. Manera, D. Bizyaev, J. R. Brownstein, G. Ebelke, E. Malanushenko, V. Malanushenko, D. Oravetz, K. Pan, D. P. Schneider, A. Simmons
arXiv:1209.1394v1
We obtain constraints on the variation of the fundamental constants from the full shape of the redshift-space correlation function of a sample of luminous galaxies drawn from the Data Release 9 of the Baryonic Oscillations Spectroscopic Survey. We combine this information with data from recent CMB, BAO and H_0 measurements. We focus on possible variations of the fine structure constant \alpha and the electron mass m_e in the early universe, and study the degeneracies between these constants and other cosmological parameters, such as the dark energy equation of state parameter w_DE, the massive neutrinos fraction f_\nu, the effective number of relativistic species N_eff, and the primordial helium abundance Y_He. When only one of the fundamental constants is varied, our final bounds are \alpha / \alpha_0 = 0.9957_{-0.0042}^{+0.0041} and m_e /(m_e)_0 = 1.006_{-0.013}^{+0.014}. For their joint variation, our results are \alpha / \alpha_0 = 0.9901_{-0.0054}^{+0.0055} and m_e /(m_e)_0 = 1.028 +/- 0.019. Although when m_e is allowed to vary our constraints on w_DE are consistent with a cosmological constant, when \alpha is treated as a free parameter we find w_DE = -1.20 +/- 0.13; more than 1 \sigma away from its standard value. When f_\nu and \alpha are allowed to vary simultaneously, we find f_\nu < 0.043 (95% CL), implying a limit of \sum m_\nu < 0.46 eV (95% CL), while for m_e variation, we obtain f_nu < 0.086 (95% CL), which implies \sum m_\nu < 1.1 eV (95% CL). When N_eff or Y_He are considered as free parameters, their simultaneous variation with \alpha provides constraints close to their standard values (when the H_0 prior is not included in the analysis), while when m_e is allowed to vary, their preferred values are significantly higher. In all cases, our results are consistent with no variations of \alpha or m_e at the 1 or 2 \sigma level.
A Quantum Gravity Extension of the Inflationary Scenario
Ivan Agullo, Abhay Ashtekar, William Nelson
arXiv:1209.1609v1
Since the standard inflationary paradigm is based on quantum field theory on classical space-times, it excludes the Planck era. Using techniques from loop quantum gravity, the paradigm is extended to a self-consistent theory from the Planck scale to the onset of slow roll inflation, covering some 11 orders of magnitude in energy density and curvature. This pre-inflationary dynamics also opens a small window for novel effects, e.g. a source for non-Gaussianities, which could extend the reach of cosmological observations to the deep Planck regime of the early universe.
Sunday, July 8, 2012
arXiv: 9 July 2012
Physical properties underlying observed kinematics of satellite galaxies
arXiv:1207.1647v1 We study the kinematics of satellites around isolated galaxies selected from the Sloan Digital Sky Survey (SDSS) spectroscopic catalog. Using a model of the phase-space density previously measured for the halos of LCDM dark matter cosmological simulations, we determine the properties of the halo mass distribution and the orbital anisotropy of the satellites as a function of the colour-based morphological type and the stellar mass of the central host galaxy. We place constraints on the halo mass and the concentration parameter of dark matter and the satellite number density profiles. We obtain a concentration-mass relation for galactic dark matter haloes that is consistent with predictions of a standard LCDM cosmological model. The number density profile of the satellites appears to be shallower than of dark matter, with the scale radius typically 1.6 times larger than of dark matter. The orbital anisotropy around red hosts exhibits a mild excess of radial motions, in agreement with the typical anisotropy profiles found in cosmological simulations, whereas blue galaxies are found to be consistent with an isotropic velocity distribution. Our new constraints on the halo masses of galaxies are used to provide analytic approximations of the halo-to-stellar mass relation for red and blue galaxies. |
Saturday, July 7, 2012
arXiv: 6 July 2012
Do stochastic inhomogeneities affect dark-energy precision measurements?
arXiv:1207.1286v1 The effect of a stochastic background of cosmological perturbations on the luminosity-redshift relation is computed to second order through a recently proposed covariant and gauge-invariant light-cone averaging procedure. The resulting expressions are free from both ultraviolet and infrared divergences, implying that such perturbations cannot mimic a sizable fraction of dark energy. Different averages are estimated and depend on the particular function of the luminosity distance being averaged. The energy flux, being minimally affected by perturbations at large z, is proposed as the best choice for precision estimates of dark-energy parameters. Nonetheless, its irreducible (stochastic) variance induces statistical errors on \Omega_{\Lambda}(z) typically lying in the few-percent range. Signatures of Modified Gravity on the 21-cm Power Spectrum at Reionisation
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Tuesday, June 5, 2012
arXiv: 5 June 2012
arXiv: 4 June 2012
Tuesday, May 29, 2012
arXiv: 24 May 2012
arXiv: 23 May 2012
Monday, May 28, 2012
arXiv: 22 May 2012
Thursday, May 17, 2012
arXiv: 17 May 2012
arXiv: 16 May 2012
Monday, May 14, 2012
arXiv: 15 May 2012
An Alternative String Landscape Cosmology: Eliminating Bizarreness
Gamma-Ray Bursts are precise distance indicators similar to Type Ia Supernovae?
A new null diagnostic customized for reconstructing the properties of dark energy from BAO data
An Efficient Parameter Space Search as an Alternative to Markov Chain Monte Carlo
Shaping the galaxy stellar mass function with supernova- and AGN-driven winds
Modified gravity as a common cause for cosmic acceleration and flat galaxy rotation curves
arXiv:1205.3088v1 Flat galaxy rotation curves and the accelerating Universe both imply the existence of a critical acceleration, which is of the same order of magnitude in both the cases, in spite of the galactic and cosmic length scales being vastly different. Yet, it is customary to explain galactic acceleration by invoking gravitationally bound dark matter, and cosmic acceleration by invoking a `repulsive` dark energy. Instead, might it not be the case that the flatness of rotation curves and the acceleration of the Universe have a common cause? In this essay we propose a modified theory of gravity. By applying the theory on galactic scales we demonstrate flat rotation curves without dark matter, and by applying it on cosmological scales we demonstrate cosmic acceleration without dark energy. |