At a given age of the Universe, which we will call *t*, the distance to the event horizon (*D _{eh}*(

*t*)) is given by the following expression

*Where *

t_{max} is the maximum age of the Universe. This is infinity for the **standard model** of the Universe (*known as the lambda cold dark matter model*). However, there are other models of the Universe in which it has a finite maximum age.

*a(t)* is the comic scale factor. It measures the size of the Universe and is discussed in detail in my post https://explainingscience.org/2021/12/01/measuring-distances-in-cosmology/

*c* is the speed of light in a vacuum

The following diagram shows how the event horizon changes over over time in the standard model. In this model: 68% of the mass of the Universe is made up of dark energy, which drives the expansion of the Universe at an increasing rate, 27% dark matter and 5% ordinary matter.