The universe’s expansion makes cosmic distance far less intuitive than simply measuring how long light has traveled.
An expanding universe makes distance harder to define because space keeps growing while light is in transit. Objects continue to emit light, but as that light travels toward us, the average separation between galaxies increases (yes, I know that sometimes galaxies can collide, but we’re talking on average, at big scales here).
When light from a distant galaxy finally reaches a telescope after traveling for billions of years, it shows the galaxy as it existed in the past. It does not directly tell us how far away the galaxy is now. To estimate its present distance, astronomers use a cosmological model that accounts for how much the universe expanded while the light was traveling.
The leading model is known as LCDM, which includes both dark matter (different episode) and dark energy (different episode). Its strengths and limitations can be debated (different episode), but alternative models generally do not change the basic picture very much.
Expansion stretches the observable universe
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.
Redshift reveals faster cosmic recession
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.
A second horizon limits future light
We can still observe these galaxies because the light reaching us began its journey billions of years ago, when they were much closer. Some galaxies even farther away may also become visible over time for the same reason. However, there is a boundary known as the cosmological event horizon (which is ever so slightly different from the black hole event horizon). It lies about 17 billion light-years away. Light emitted right now from beyond that distance will never reach us, no matter how long we wait.
The universe’s accelerating expansion, driven by dark energy, makes this limit even more significant. The cosmological event horizon will continue to expand before eventually leveling off at about 60 billion light-years. Even then, we will not be able to see everything. Light from the most distant galaxies will be redshifted to such extreme wavelengths that those galaxies will effectively disappear. In roughly 100 billion years, every galaxy beyond the Local Group will vanish from view forever.
Adapted from an article originally published in The Conversation.
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1 Comment
If or when the universe were to start contracting, how long would it take after the shift for scientists on earth to begin to suspect that has happened