If the universe is 13 billion years old how do we have stars 45 billion light years from earth? Wouldn’t that light take 45 billion years to reach us?
In: Physics
This is due to the expansion of space. If a very distant object emitted light a long time ago, that light would have to travel across expanding space to reach us. By the time the light hits our telescope, the distance between us and the object has increased because of the expansion of space. In the circumstance you mentioned, the light has not actually travelled for 45 billion years because that clearly violates causality. It is just that the distance between us and the object that emitted the light is now 45 billion light years due to the expansion of space. But how can we determine this distance just by measuring light? This increase in distance can be detected by analysing the redshift in the light. Because of general relativity, we know that light gets redshifted as it moves through expanding space. We also know the spectra that elements emit when hot. If we analyse the pattern of redshifted wavelengths we are seeing, we can tell which elements initially emitted this light and compare the observed wavelengths to ones that we know these elements emit. This information is plugged into the equations of general relativity to determine the actual distance between us and the object that emitted the light.
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