An expanding universe makes measuring distance more complicated because space does not remain fixed while light travels through it. Galaxies continue to emit light, but that light may spend billions of years crossing the cosmos before reaching us. During that journey, the universe keeps expanding, increasing the average separation between galaxies (yes, I know that sometimes galaxies can collide, but we’re talking on average, at big scales here).
This means that when a telescope captures the light from a distant galaxy, the image does not reveal where that galaxy is today. It shows the galaxy as it appeared when the light began its journey. To estimate its present distance, astronomers must use a cosmological model that accounts for how the universe has expanded over time.
The leading model used today is called LCDM. It includes dark matter (different episode) and dark energy (different episode). The strengths and limitations of LCDM are worth discussing separately (different episode), but alternative models do not significantly change the overall picture described here.
The Edge of the Observable Universe
The universe is about 13.77 billion years old, but the most distant regions we can observe are now roughly 45 billion light-years away. Space expanded during the entire time that their light was traveling toward us.
This boundary is known as the particle horizon, the cosmological horizon, or the comoving horizon, depending on how stylish you feel in the moment. It defines the outer edge of our observable bubble and marks the greatest distance we can see today.
At first, the numbers seem contradictory. How can the observable universe extend 45 billion light years when the universe is only 13.77 billion years old? The answer is that the universe can expand faster than light.
Why Faster Than Light Expansion Is Allowed
This does not violate the laws of physics. The speed of light limits how quickly objects can move through space in a local region. An observer will never see a nearby rocket ship pass by faster than light.
Cosmic expansion is different. Faraway galaxies are not necessarily speeding through space in the usual sense. Instead, the space between us and those galaxies is growing. Special relativity does not place the same restriction on how quickly large distances can increase across the universe.
Astronomers can estimate how quickly a galaxy is receding by measuring its redshift. As a galaxy moves away, its light is stretched toward redder wavelengths in the electromagnetic spectrum. Edwin Hubble used this effect to uncover evidence that the universe is expanding.
In an expanding universe, more distant galaxies generally recede more quickly because a greater amount of space lies between them and us. More space means more distance that can expand.
The point at which galaxies begin receding faster than light is called the Hubble distance. It lies about 13.77 billion light-years away.
Why We Can Still See Faster Moving Galaxies
We can observe galaxies beyond the Hubble distance because the light reaching us today was emitted long ago, when those galaxies were much closer. We may also eventually receive light from some galaxies located even farther away, provided that the light began traveling toward us when the galaxies were nearer.
However, there is an ultimate limit called the cosmological event horizon (which is ever so slightly different from the black hole event horizon). It is currently about 17 billion light years away.
Any light emitted RIGHT NOW from beyond that boundary will NEVER reach us, ever, no matter how long we wait. The expansion of space will prevent it from crossing the growing distance.
Dark Energy and the Vanishing Universe
The accelerating expansion driven by dark energy makes this separation even more extreme. The cosmological event horizon will continue to expand in the future, but it will eventually approach a maximum distance of about 60 billion light years.
Even then, observers will not be able to see everything within that distance. Light from the most remote galaxies will become stretched to such enormous wavelengths that it will effectively disappear from view.
In about 100 billion years, every galaxy beyond the Local Group of galaxies will fade from sight, forever. Future observers will live in a universe that appears far smaller and emptier than the one we can see today.