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Modified Gravity (modified + gravity)

Distribution by Scientific Domains
Physics and Astronomy100%

Selected Abstracts

Galactic Sun's motion in the cold dark matter, MOdified Newtonian Dynamics and modified gravity scenarios

ASTRONOMISCHE NACHRICHTEN, Issue 8 2009
L. Iorio
Abstract We numerically integrate the equations of motion of the Sun in Galactocentric Cartesian rectangular coordinates for ,4.5 Gyr , t , 0 in Newtonian mechanics with two different models for the Cold Dark Matter (CDM) halo, in MOdified Newtonian Dynamics (MOND) and in MOdified Gravity (MOG) without resorting to CDM. The initial conditions used come from the latest kinematical determination of the 3D Sun's motion in the Milky Way (MW) by assuming for the rotation speed of the Local Standard of Rest (LSR) the recent value ,0 = 268 km s,1 and the IAU recommended value ,0 = 220 km s,1; the Sun is assumed located at 8.5 kpc from the Galactic Center (GC). For ,0 = 268 km s,1 the birth of the Sun, 4.5 Gyr ago, would have occurred at large Galactocentric distances (12,27 kpc depending on the model used), while for ,0 = 220 km s,1 it would have occurred at about 8.8,9.3 kpc for almost all the models used. The integrated trajectories are far from being circular, especially for ,0 = 268 km s,1, and differ each other with the CDM models yielding the widest spatial extensions for the Sun's orbital path (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Constraints on modified gravity from the observed X-ray luminosity function of galaxy clusters

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2009
David Rapetti
ABSTRACT We use measurements of the growth of cosmic structure, as inferred from the observed evolution of the X-ray luminosity function (XLF) of galaxy clusters, to constrain departures from general relativity (GR) on cosmological scales. We employ the popular growth rate parameterization, ,m(z),, for which GR predicts a growth index ,, 0.55. We use observations of the cosmic microwave background (CMB), type Ia supernovae (SNIa) and X-ray cluster gas mass fractions (fgas), to simultaneously constrain the expansion history and energy content of the Universe, as described by the background model parameters: ,m, w and ,k, i.e. the mean matter density, the dark energy equation of state parameter and the mean curvature, respectively. Using conservative allowances for systematic uncertainties, in particular for the evolution of the mass,luminosity scaling relation in the XLF analysis, we find ,= 0.51+0.16,0.15 and ,m= 0.27 ± 0.02 (68.3 per cent confidence limits), for a flat cosmological constant, cold dark matter (,CDM) background model. Allowing w to be a free parameter, we find ,= 0.44+0.17,0.15. Relaxing the flatness prior in the ,CDM model, we obtain ,= 0.51+0.19,0.16. When in addition to the XLF data we use the CMB data to constrain , through the ISW effect, we obtain a combined constraint of ,= 0.45+0.14,0.12 for the flat ,CDM model. Our analysis provides the tightest constraints to date on the growth index. We find no evidence for departures from GR on cosmological scales. [source]

The Bullet Cluster 1E0657-558 evidence shows modified gravity in the absence of dark matter

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2007
J. R. Brownstein
ABSTRACT A detailed analysis of the 2006 November 15 data release X-ray surface density ,-map and the strong and weak gravitational lensing convergence ,-map for the Bullet Cluster 1E0657-558 is performed and the results are compared with the predictions of a modified gravity (MOG) and dark matter. Our surface density ,-model is computed using a King ,-model density, and a mass profile of the main cluster and an isothermal temperature profile are determined by the MOG. We find that the main cluster thermal profile is nearly isothermal. The MOG prediction of the isothermal temperature of the main cluster is T= 15.5 ± 3.9 keV, in good agreement with the experimental value T= 14.8+2.0,1.7 keV. Excellent fits to the 2D convergence ,-map data are obtained without non-baryonic dark matter, accounting for the 8, spatial offset between the ,-map and the ,-map reported in Clowe et al. The MOG prediction for the ,-map results in two baryonic components distributed across the Bullet Cluster 1E0657-558 with averaged mass fraction of 83 per cent intracluster medium (ICM) gas and 17 per cent galaxies. Conversely, the Newtonian dark matter ,-model has on average 76 per cent dark matter (neglecting the indeterminant contribution due to the galaxies) and 24 per cent ICM gas for a baryon to dark matter mass fraction of 0.32, a statistically significant result when compared to the predicted ,-cold dark matter cosmological baryon mass fraction of 0.176+0.019,0.012. [source]

Does ,modified gravity', not dark matter, explain galaxy motion?

ASTRONOMY & GEOPHYSICS, Issue 6 2007
Article first published online: 14 NOV 200
New observations of the Bullet Cluster suggest that a revised version of Newton's gravitational theory can account for the patterns seen, just as well as the conventional explanation of dark matter affecting the mass distribution. [source]