Particle horizon

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In physical cosmology, particle horizon is the maximum distance from which particles could have traveled to the observer in the age of the universe. It represents the portion of the universe which we could have conceivably observed at the present day.

Background

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,

$\eta_0 = \int_{t=0}^{t_0} \frac{dt'}{a(t')}$

where $a (t)$ is the scale factor of the Friedmann-Robertson-Walker metric, and we have taken the Big Bang to be at $t = 0$ .

The particle horizon differs from the event horizon in that the particle horizon represents the largest comoving distance from which light could have reached us by now, while the event horizon is the largest comoving distance from which light emitted now can ever reach the observer at any time in the future.