An expanding universe complicates this picture just a little bit, because the universe absolutely refuses to be straightforward.
Objects are still emitting light, and that light takes time to travel from them over to here, but in that intervening time the universe grows larger, with the average distance between galaxies getting bigger (yes, I know that sometimes galaxies can collide, but we’re talking on average, at big scales here).
So when we see an image of a distant galaxy, and that light has traveled for billions of years to finally end in our telescopes, we don’t know how far away that galaxy is right now, at the moment that we get the light.
We have to turn to a cosmological model that incorporates the expansion history of the universe, so we know how much the universe has grown in a given amount of time.
Our current best model of the universe is called LCDM, which involved both dark matter (different episode) and dark energy (different episode).
We can discuss the relative merits and weaknesses of LCDM (different episode), but for now let’s just take it as a given, as deviations from LCDM don’t really change the picture much.
The maximum distance that we can see, which is the age of the universe (13.77 billion years) with the cosmos expanding all that time, is about 45 billion light-years away.
This distance is known as the particle horizon, the cosmological horizon, or the comoving horizon, depending on how stylish you feel in the moment. That is the extent of our observable bubble, the maximum extent that we can see at this moment, today.
But wait, isn’t 45 greater than 13.77? Doesn’t that imply that the universe is expanding faster than light? Yes, yes it does.
This isn’t a big deal. That’s because the speed limit of light only applies to local observations – I’ll never see a rocket ship blast by me faster than light. If you think that’s some sort of cheat, it’s not, it’s how special relativity is constructed. Objects at the far edge of the universe can have whatever speeds they want, because they’re far away.
In fact, we can calculate the current speed of any object, and that’s through the redshift. If a galaxy is moving away from us, then its light will get shifted to redder parts of the electromagnetic spectrum.
This is how Edwin Hubble discovered the expansion of the universe in the first place. And in an expanding universe, more distant objects recede faster and faster, because there’s more space between them and us to expand.
And the turnover point, where objects will recede away from us faster than light, is right at the Hubble distance, 13.77 billion light-years away.
We can still see those galaxies because their light was emitted long, long ago, when they were much closer to us.
And if we wait long enough, we can still see some galaxies behind them, because once again those galaxies were also much closer to us. But there’s a limit. It’s called the cosmological event horizon (which is ever so slightly different from the black hole event horizon). It’s about 17 billion light-years away. Any light emitted RIGHT NOW past that distance will NEVER reach us, ever, no matter how long we wait.
And an accelerating universe dominated by dark energy makes this even worse. The cosmological event horizon will continue to grow in the future, but eventually it will reach a limit of around 60 billion light-years.
But that doesn’t mean we’ll be able to see everything. Light from the most distant galaxies will get redshifted to wavelengths so large they essentially become invisible. In about 100 billion years, everything outside of the Local Group of galaxies will disappear from view, forever.
Written by Paul Sutter/Universe Today.
