Why does it take light from galaxies so long to reach us if they used to be closer?

Could the Universe have expanded at a speed faster than light?

Hubble Ultra Deep Field
Published: January 21, 2022 at 11:44 am
Get your own Space Pen when you subscribe to BBC Sky at Night Magazine today!

In the past, galaxies were closer together, and therefore closer to Earth. When this was the case, their light did take less time to reach us.

Advertisement

Take, for instance, a galaxy we are observing today whose light has taken 100 million years to get here. When it was closer, its light took, say, only 99 million years to get here.

This light, which took a million years less to get here than the light arriving from the galaxy today, arrived on Earth in the past. It is not the light we are observing today.

More from Marcus Chown:

But this doesn’t explain why light from the most distant matter in the Universe – stuff beyond the edge of the observable Universe – has not arrived here yet when, once upon a time, all the matter in the Universe was the barest whisker apart.

How could it get so far apart that light from the most distant objects hasn’t reached us yet when, according to Einstein’s special theory of relativity, no object can travel faster than light?

Illustration of the expansion of the Universe. Understanding more about this phenomenon could reveal clues as to how the Universe will end. Credit: Mark Garlick / Science Photo Library
Illustration showing the expansion of the Universe. Credit: Mark Garlick / Science Photo Library

The answer is that immediately after the Big Bang, the Universe did indeed expand faster than light.

Consequently, light travelling to Earth was like a 100m sprinter trying to reach a finishing tape that is being pulled farther and farther away.

But how can space expand faster than the speed of light? Surely this violates special relativity?

However, the expansion of the Universe involves acceleration, which is not covered by the theory of special relativity.

Advertisement

The theory that must be used instead is Einstein’s general theory of relativity and, according to this, expanding space is like a stretchy fabric which can expand at any rate it likes.

Authors

Marcus Chown is an award-winning writer and broadcaster and a former radio astronomer at the California Institute of Technology in Pasadena. He is the author of Breakthrough: Spectacular stories of scientific discovery from the Higgs particle to black holes (Faber & Faber, 2021).

Sponsored content