The Evolving Universe

Smithsonian Institution

How do we define distance in space?

image showing changes in distance between galaxies over time.

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We can't use a giant tape measure, but astronomers use a variety of methods to determine how far something is from Earth. But the question is stickier than which method to use.

Consider this. Billions of years ago, a hypothetical galaxy was 5 billion light-years from Earth. You might think its light would take only 5 billion years to reach us.

But, the universe is expanding.

That means that the space between the two galaxies is getting bigger. The light has to travel even farther to catch up with our galaxy. By the time it does, the universe has expanded so much the that two galaxies could now be 10 billion light years apart.

Things can get even trickier, as Einstein's theory of relativity implies different possible ways of defining distance in the first place.

In this exhibit, we use the distance that light has to travel (the ‘lookback time’) to describe the distance from Earth.