Difference between revisions of "Proper horizons"
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| − | The following five problems are based on work by F. Melia | + | The following five problems are based on work by F. Melia ([http://arxiv.org/abs/0711.4181 F. Melia. The cosmic horizon. MNRAS 382 (4), 1917--1921 (2007)]; [http://arxiv.org/abs/0907.5394 F. Melia and M. Abdelqader, The Cosmological Spacetime, Int. J. Mod. Phys. D 18, 1889 (2009)]). |
Standard cosmology is based on the FLRW metric for a spatially homogeneous and isotropic three-dimensional space, expanding or contracting with time. In the coordinates used for this metric, $t$ is the cosmic time, measured by a comoving observer (and is the same everywhere), $a(t)$ is the expansion factor, and $r$ is an appropriately scaled radial coordinate in the comoving frame. | Standard cosmology is based on the FLRW metric for a spatially homogeneous and isotropic three-dimensional space, expanding or contracting with time. In the coordinates used for this metric, $t$ is the cosmic time, measured by a comoving observer (and is the same everywhere), $a(t)$ is the expansion factor, and $r$ is an appropriately scaled radial coordinate in the comoving frame. | ||
Latest revision as of 10:39, 4 February 2014
The following five problems are based on work by F. Melia (F. Melia. The cosmic horizon. MNRAS 382 (4), 1917--1921 (2007); F. Melia and M. Abdelqader, The Cosmological Spacetime, Int. J. Mod. Phys. D 18, 1889 (2009)).
Standard cosmology is based on the FLRW metric for a spatially homogeneous and isotropic three-dimensional space, expanding or contracting with time. In the coordinates used for this metric, $t$ is the cosmic time, measured by a comoving observer (and is the same everywhere), $a(t)$ is the expansion factor, and $r$ is an appropriately scaled radial coordinate in the comoving frame.
F. Melia demonstrated the usefulness of expressing the FRLW metric in terms of an observer-dependent coordinate $R=a(t)r$, which explicitly reveals the dependence of the observed intervals of distance, $dR$, and time on the curvature induced by the mass-energy content between the observer and $R$; in the metric, this effect is represented by the proximity of the physical radius $R$ to the cosmic horizon $R_{h}$, defined by the relation \[R_{h}=2G\,M(R_h).\] In this expression, $M(R_h)$ is the mass enclosed within $R_h$ (which terns out to be the Hubble sphere). This is the radius at which a sphere encloses sufficient mass-energy to create divergent time dilation for an observer at the surface relative to the origin of the coordinates.
