Horizon problem
The horizon problem is a problem with the standard cosmological model of the Big Bang which was identified in the 1970s.
Since information can travel no faster than the speed of light, there is a limit to the region of space that is in causal contact with any particular point in the universe. The extent of this region is a function of how long the universe has existed. The particle horizon is closely related to this idea and is where the problem gets its name from.
According to Big Bang theory of the 1970s, the plasma that generated the cosmic microwave background we currently observe was about <math>10^{88}<math> times the size of any causally connected region at that time, roughly 300,000 years after the Big Bang. Problematically, the near uniformity we observe in the cosmic microwave background suggests that this was not the case and that the entire plasma was causally connected.
Inflationary theory tries to solve the problem (along with several others) by positing a short 10-32second period of exponential expansion (dubbed inflation) within the first minute or so of the universe?s history. During inflation, the universe would have increased in size by a factor of <math>10^{26}<math>—enough to make the particle horizon at the time the cosmic microwave background was created roughly the size of the entire plasma responsible for generating the region we currently observe and thus making it causally connected.
One consequence of cosmic inflation is that the anisotropies in the big bang are reduced but not entirely eliminated. Differences in the temperature of the cosmic background are smoothed by cosmic inflation, but they still exist. The cosmic inflation predicts a spectrum for the anisotropies in the microwave background which is consistent with observations from WMAP and COBE.
What caused the inflation is still unclear. During the 1980s, efforts were made to associate the inflation
with vacuum energy associated with grand unification theory, but these later proved problematic.
The horizon problem in computer programs for playing board games (for example, computer chess) is a problem that arises when search depth is fixed. If one side has many ineffective threats that nonetheless require a response, the consequences of a bad move may be postponed beyond the limit of the search (the "horizon") and so not properly taken into account by the program. There are various approaches to solving the problem, most involving search extension.
