From the October 2014 issue

If the light from a galaxy traveled 13.3 billion years to reach Earth, does that mean the light left this galaxy when the universe was 500 million years old? Was the galaxy 13.3 billion light-years away from Earth then?

John Hanson, Huntsville, Alabama
By | Published: October 27, 2014 | Last updated on May 18, 2023

Gemini Sparkle

Key Takeaways:

  • The duration light travels from a distant galaxy does not directly correspond to its distance, either at the time of light emission or presently, due to the universe's expansion.
  • The present-day distance to a galaxy is considerably greater than the light travel time suggests because the intervening space expands while light propagates.
  • Conversely, the distance between galaxies at the moment light was emitted was significantly smaller than the light travel time, aligning with the universe's reduced size at that earlier epoch.
  • These discrepancies between light travel time and physical distance are attributed to the universe's continuous expansion, which stretches the path light traverses and alters intergalactic distances over cosmic timescales.
Key takeaways sponsored by The Space Store
The universe's expansion separates massive objects, like galaxies that lie within space-time, and that means that any light that left one of those galaxies has to cross more of the cosmos to reach our telescopes.
The universe’s expansion separates massive objects, like galaxies that lie within space-time, and that means that any light that left one of those galaxies has to cross more of the cosmos to reach our telescopes.
Astronomy: Roen Kelly
It is true that the universe was only about 500 million years old when the light from this distant galaxy left, given that the light traveled for 13.3 billion years and that the universe is approximately 13.8 billion years old. However, it is not true that the distance from us to this galaxy is 13.3 billion light-years — either now or as measured long ago when the light left.

To understand why, note that the universe expands even while the light travels toward us. The speed of light is constant, but the path it needs to cover — the distance between the other galaxy and our Milky Way — is stretching with time. For example, a mile that the light beam crossed early in the history of the universe some time later becomes 2 miles, still later 3 miles, etc. Clearly, the length that light covers is always larger than the length it would have traveled if the universe were not expanding (which would be 13.3 billion light-years). In our particular case, one can calculate that the light actually has made a trip of about 33 billion light-years. This is the present-day distance between us and the other galaxy.

The distance between the two galaxies was smaller in the past. Light from the distant galaxy in this example was emitted when the universe was only one-thirteenth of its present size. Therefore, when the light left the galaxy, its distance to us was one-thirteenth of 33 billion light-years, or about 2.5 billion light-years. This is not in conflict with the age of the universe of 500 million light-years at the time for the same reason as before: The other galaxy and the Milky Way had the benefit of the expanding universe so as to find themselves farther away from each other than the speed of light traveling in a static universe over 500 million years would indicate.

Dragan Huterer
University of Michigan, Ann Arbor

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