horizon problem One of two conceptual problems with the standard Big Bang model, which is that some regions of the universe which have very similar properties are too far apart to have exchanged information in the age of the universe.
Horizon problem. Our observable patch of the Universe grows from a much smaller initial patch in the inflationary Universe (right) than it does in the standard Friedmann models (left.).
Horizon Problem
(a) A quandary in standard big bang theory, which indicates that few of the particles of the early universe would have had time to be in causal contact with one another at the outset of cosmic expansion.
horizon problem: In cosmology, the circumstance that the primordial background radiation seems much more isotropic than could be explained by the standard big bang theory.
The horizon problem
The horizon problem results from the premise that information cannot travel faster than light, and hence two regions of space which are expanding at faster than the speed of light relative to each other cannot communicate.
The horizon problem is solved by inflation because regions that appear to be isolated from each other were in contact with each other before the inflation period. They came into equilibrium before inflation expanded them far away from each other.
To resolve the horizon problem, astronomers introduced an inflationary period into the Big Bang model (blue region in figure).
8 The horizon problem. The isotropy of the microwave background indicates that regions A and B in the universe were very similar to each other when the radiation we observe left them, ...
Horizon problem
The horizon problem is the problem of determining why the universe appears statistically homogeneous and isotropic in accordance with the cosmological principle.
Among these are the observed flatness of the universe (the flatness problem), its extraordinary homogeneity on large (non-causally-connected) scales (the horizon problem), and its lack of any observed topological defects (the monopole problem), ...
See also: Causal contact and Horizon problem
A light cone defines locations that are in causal contact and those that are not.
 See also: Universe, Horizon, Energy, Gravity, Model
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