The End of Everything (Astrophysically Speaking)

by Katie Mack

Everything you see is in the past, as far as you’re concerned. If you look up at the Moon, you’re seeing a little over a second ago. The Sun is more than eight minutes in the past. And the stars you see in the night sky are deep in the past, from just a few years to millennia.

It also means that cosmology doesn’t really have a well-defined concept of “now.” Or rather, the “now” you experience is highly specific to you, to where you are and to what you are doing.II What does it mean to say “that supernova is going off now” if we see the light of it now, and we can watch the star explode now, but that light has been traveling for millions of years? The thing we’re watching is essentially fully in the past, but the “now” for that exploded star is unobservable to us, and we won’t receive any knowledge of it for millions of years, which makes it, to us, not “now,” but the future.

the fact that pretty much all the hydrogen in the universe was produced in the first few minutes means that a pretty large fraction of what you and I are made of has been hanging around the universe in one form or another for almost as long as the universe has been here. You may have heard that “we are made of stardust”