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A second example is de Sitter space which contains a particle horizon.
Indeed, it seems likely that the universe is larger than the observable particle horizon.
How the particle horizon changes with time depends on the nature of the expansion of the universe.
The criterion for determining whether a particle horizon for the universe exists is as follows.
In other words, the particle horizon recedes constantly as time passes, and the observed fraction of the universe always increases.
The particle horizon of the observable universe is the boundary that represents the maximum distance at which events can currently be observed.
The boundary past which events cannot ever be observed is an event horizon, and it represents the maximum extent of the particle horizon.
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 terms of comoving distance, the particle horizon is equal to the conformal time that has passed since the Big Bang, times the speed of light .
It is as if the visible universe were enclosed by a spherical boundary, the "particle horizon" or "event horizon," moving outward by one light year each year.
Because it is necessarily a large fraction of the signal, workers must be very careful in interpreting the statistical significance of measurements on scales close to the particle horizon.
So the observable universe (the so-called particle horizon of the universe) is the result of processes that follow some general physical laws, including quantum mechanics and general relativity.
The Fischler-Susskind mechanism, first proposed by Willy Fischler and Leonard Susskind in 1998, is a holographic prescription based on the particle horizon.
An extremely important concept in the theory of structure formation is the notion of the Hubble radius, often called simply the horizon as it is closely related to the particle horizon.
In an expanding universe, an observer may find that some regions of the past cannot be observed ("particle horizon"), and some regions of the future cannot be influenced (event horizon).
The particle horizon (also called the cosmological horizon, the light horizon, or the cosmic light horizon) is the maximum distance from which particles could have traveled to the observer in the age of the universe.
Telescopes cannot see events earlier than about 380,000 years after the Big Bang, when the universe became transparent (the Cosmic Microwave Background); this corresponds to the particle horizon at a distance of about 46 billion (4.6x10) light years.
In October 1980, Demosthenes Kazanas suggested that exponential expansion could eliminate the particle horizon and perhaps solve the horizon problem, while Sato suggested that an exponential expansion could eliminate domain walls (another kind of exotic relic).
During inflation, the Universe undergoes exponential expansion, and the particle horizon expands much more rapidly than previously assumed, so that regions presently on opposite sides of the observable Universe are well inside each other's particle horizon.
The observed isotropy of the CMB is problematic in this regard: if the Universe had been dominated by radiation or matter at all times up to the epoch of last scattering, the particle horizon at that time would correspond to about 2 degrees on the sky.