So much further than we even thought

A fast space fact about fast space.

Galaxy GN-z11, one of the most distant galaxies ever detected, seen as it was 13.4 billion years in the past. Image Credit: NASA, ESA, P. Oesch, G. Brammer, P. van Dokkum, and G. Illingworth

If you read random stuff about astronomy you might know this: one light year is the distance light travels in one year. It’s a really big distance: about 6 trillion miles. I don’t really have a good way to help you conceptualize that, other than to note that light is so fast that it could lap earth 7 times in one second if it so desired. Our fastest spacecrafts take about 10 years to get to Pluto, light gets there in a little over 4 hours. So in one year, light can go a long ways. A whopping 236 million earth laps. Over 2000 trips to Pluto. Yeah.

And yet, on cosmic scales, light is kinda slow. The nearest star to the sun is 4.2 light years away, the nearest major galaxy, Andromeda, is 3 million light years away. Light that left Andromeda before the emergence of Homo sapiens — during an era referred to in geologic time as Pilocene — is just reaching earth now. So we see Andromeda as it was 3 million years ago!

Extrapolating: the universe, we think, is about 13.8 billion years old.¹ So that means that for the most distant visible objects the light has travelled 13.8 billion years to get to earth — travelled through 13.8 billion light years of space. Crazy.

But here’s the little space fact floating around in my mind today. Those distant galaxies are actually 46 billion light years away, not 13.8. That’s nearly 4x further than the “light travel time” distance. Say what?

The reason is because the universe is expanding. Quite quickly. Those galaxies were much closer to earth when the light left them (well, technically, earth didn’t exist yet. So let’s say much closer to the spot where earth would eventually form). But as the universe expands, it moves the galaxies further and further away. So even though the light only has to travel 13.8 billion years, the galaxies, over that time, have packed up and moved away. It’s like if you left home in Michigan for Disney world, and while you were driving, someone picked up your house and moved it the other way. You only drove the the thousand miles to Disney, but your house is now several thousand miles away somewhere in northern Yukon.