Galaxy Ages

How does look-back time affect observations of galaxies?

Galaxies are really, really far away from Earth compared to planets or stars. So look-back times to galaxies can be a substantial fraction of the total age of the universe. Every light-year of distance creates a look-back time of one year. If a galaxy is five billion light-years away, then we are seeing the galaxy as it existed five billion years ago, which is before our planet was even formed.


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