Horizon problem

The horizon problem (sometimes called the homogeneity problem) is an observational inconsistency in the interpretation of particle horizons of FLRW models of the Big Bang. It was originally noted in the late 1960s, primarily by Charles Misner.

Two theories that attempt to solve the horizon problem are the theory of cosmic inflation and variable speed of light.

When one looks out into the night sky, distances also correspond to time into the past. A galaxy measured at ten billion light years in distance appears to us as it was ten billion years ago, because the light has taken that long to travel to the viewer. If one were to look at a galaxy ten billion light years away in one direction, say "west", and another in the opposite direction, "east", the total distance between them is twenty billion light years. This means that the light from the first has not yet reached the second, because the approximately 13.8 billion years that the universe has existed is not a long enough time to allow it to occur. In a more general sense, there are portions of the universe that are visible to us, but invisible to each other, outside each other's respective particle horizons.

In accepted relativistic physical theories, no information can travel faster than the speed of light. In this context, "information" means "any sort of physical interaction". For instance, heat will naturally flow from a hotter area to a cooler one, and in physics terms this is one example of information exchange. Given the example above, the two galaxies in question cannot have shared any sort of information; they are not in "causal contact". One would expect, then, that their physical properties would be different, and more generally, that the universe as a whole would have varying properties in different areas.

Contrary to this expectation, the universe is observed to be very close to isotropic, which also implies homogeneity. The cosmic microwave background radiation (CMB), which fills the universe, is nearly the same temperature everywhere in the sky, about 2.728 ± 0.004 K. The differences in temperature are so slight that it has only recently become possible to develop instruments capable of making the required measurements. This presents a serious problem; if the universe had started with even slightly different temperatures in different areas, then there would simply be no way it could have evened itself out to a common temperature by this point in time.

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