Eli5: how can astronomers know the age of a fast radio burst from space?
In: Planetary Science
If you can identify the galaxy it’s from, you can use redshift: As the universe expands, all radiation shifts to longer wavelength. Elements like hydrogen absorb radiation at very specific wavelengths, if you see this pattern but it’s shifted then you know how much the universe expanded since the light was emitted. You can convert that to a distance estimate.
If you know what the original frequency of the emission (for example if you know that a certain stellar effect always releases a burst at frequency X) you can compare the frequency you received with what the original would have been, and calculate the distance by the amount of frequency shift.
If it’s a type of emission that doesn’t have a predictable original frequency but you know what galaxy the emission came from, you can check for other predictable emissions from that galaxy – such as certain types of supernovae or stars with predictable variance in their brightness or gasses with known emission spectra. Then you do that same type of frequency drift analysis on that emission source to determine the distance to the galaxy the radio burst came from.
This works because the universe expands at a specific rate, so any electromagnetic emission will be stretched out in predictable ways as it travels through space. If you know how much the frequency has been stretched you know how far it has traveled.
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