Tonight's Sky
Sun
Sun
Moon
Moon
Mercury
Mercury
Venus
Venus
Mars
Mars
Jupiter
Jupiter
Saturn
Saturn

Tonight's Sky — Change location

OR

Searching...

Tonight's Sky — Select location

Tonight's Sky — Enter coordinates

° '
° '

How is it possible that scientists estimate that the universe is 13.7 billion years old, but more than 90 billion light-years across?

Allen Mabra, Carlsbad, California
Galaxies-galore
Galaxies galore.
NASA/ESA/S. Beckwith (STScI)/HUDF Team
Distances to objects in the universe refer to their values today. So when we say “an object is 10 billion light-years away from us,” we mean that light emitted by the object traveled the distance, as measured today, of 10 billion light-years on its voyage to us. Because the speed of light is constant and the universe is expanding (and was thus smaller in the past), light would traverse a larger percentage of the distance in the universe’s youth than it does today. For example, a mile that the light crossed early in the history of the universe stretches with time, and hence later becomes 2 miles, 3 miles, etc. The total distance in light-years as measured today is therefore greater than the actual time of travel as measured by a stopwatch multiplied by the speed of light.

Using this logic and some equations, one can show that, given the universe’s age of 13.7 billion years, the most distant object we could possibly see in the universe today is about 47 billion light-years away. This length is called the horizon distance (or simply “horizon”) because that’s how far we could see with even the best telescope. Therefore, the radius of the current observable universe is about 47 billion light-years, and thus it would be more than 90 billion light-years across. While the actual size of the universe may be infinite, we cannot see objects outside the horizon distance because their light has not had enough time to reach us.

— Dragan Huterer, University of Michigan, Ann Arbor


0

JOIN THE DISCUSSION

Read and share your comments on this article
Comment on this article
Want to leave a comment?
Only registered members of Astronomy.com are allowed to comment on this article. Registration is FREE and only takes a couple minutes.

Login or Register now.
0 comments
ADVERTISEMENT

FREE EMAIL NEWSLETTER

Receive news, sky-event information, observing tips, and more from Astronomy's weekly email newsletter. View our Privacy Policy.

ADVERTISEMENT
ADVERTISEMENT
asy_gravitational_eguide

Click here to download a FREE gravitational waves PDF curated by Astronomy magazine.

Find us on Facebook