content_image/308778.jpg content_image/308780.jpg content_image/308779.jpg content_image/308781.jpg content_image/308782.jpg content_image/308783.jpg
We make another definition, the comoving Event Horizon , as 

<img src='content_image/308778.jpg'>

<img src='content_image/308780.jpg'>

<img src='content_image/308779.jpg'>

again with the third line coming from the fact that we take ˙ a ( t ) > 0. This is the maximum distance that light (information) can travel in the (infinite) future. An event that occurs at a spacetime point cannot influence an event at a future spacetime point if the latter is outside of the former’s event horizon. The Event Horizon in physical coordinates is given by 

<img src='content_image/308781.jpg'>

<img src='content_image/308782.jpg'>

In reality, Eqs. (3.5) and (3.10) are definitions that use unrealistic bound- ary conditions, as a = 0 , ∞ is unphysical. Therefore, we usually calculate particle and event horizons between two well-defined limits 

<img src='content_image/308783.jpg'>

