Ask Ethan: How Many Galaxies Have Already Disappeared From Our Perspective?
“So how many earth observable galaxies have dropped out of sight? That is, how many galaxies (with the highest redshift) have disappeared from our point of view?”
When we look out at the distant reaches of space, there are some 2 trillion galaxies observable within our Universe. But our Universe is expanding, the expansion is accelerating, and light can only travel at the speed of light. Does that mean that galaxies are dropping out of sight?
There are two ways to look at this: from the point of view of not being able to see galaxies that we can presently see, and from the point of view of whether we can see the light those galaxies are emitting today, 13.8 billion years after the Big Bang? If we take the first definition, not only is the answer “zero,” but there will be trillions more galaxies revealed to us over time. But if we take the second, we find that most of the galaxies we can see today are already gone.