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Particle horizon - [2(with comoving distance normally...

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Particle horizon The particle horizon is the maximum distance from which particles could have traveled to the observer in the age of the universe . It represents the boundary between the observable and the unobservable regions of the universe, [56] so its distance at the present epoch defines the size of the observable universe. [57] The existence, properties, and significance of a cosmological horizon depend on the particular cosmological model being discussed. In terms of comoving distance , the particle horizon is equal to the conformal time η 0 that has passed since the Big Bang , times the speed of light c . The quantity η 0 is given by, where a ( t ) is the scale factor of the Friedmann–Lemaître–Robertson–Walker metric , and we have taken the Big Bang to be at t = 0. In other words, the particle horizon recedes constantly as time passes, and the observed fraction of the universe always increases. [56] [58] Since proper distance at a given time is just comoving distance times the scale factor
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Unformatted text preview: [2] (with comoving distance normally defined to be equal to proper distance at the present time, so a ( t ) = 1 at present), the proper distance to the particle horizon at time t is given by [59] The particle horizon differs from the cosmic event horizon in that the particle horizon represents the largest comoving distance from which light could have reached the observer by a specific time, while the event horizon is the largest comoving distance from which light emitted now can ever reach the observer in the future. [60] At present, this cosmic event horizon is thought to be at a comoving distance of about 16 billion light years. [5] In general, the proper distance to the event horizon at time t is given by [59] where t max is the time-coordinate of the end of the universe, which would be infinite in the case of a universe that expands forever....
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