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We measure distances in universe by the units of light year/s or parsec. Which means distance traveled by light in one year equals one light year. Thus the lights we receive from the distant stars or galaxies are coming from many light years away.

So how do we know the age of the light so that we determine the distance it has traveled to reach earth?

Qmechanic
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user990909
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2 Answers2

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We actually measure the distance, and infer the age of the light from the distance. There are many answers on the site discussing how cosmological distances are measured.

Zo the Relativist
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    This is not entirely true. We can measure the age of light from standard candles directly by measuring the redshift of the light, which tells us how long it has been travelling. That is also where some distance measurements come from. – Jim Jul 18 '14 at 15:31
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    And not just standard candles but also Hydrogen emission spectra, and similar sources where we know the expected spectral pattern – Jim Jul 18 '14 at 15:32
  • Addendum: the redshift distance from standard candles is also backed up with luminosity distance, which is not inferred from age of light. I don't want people thinking I forgot about luminosity distance – Jim Jul 18 '14 at 15:33
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    Red shift is due to relative source-observer speed, not age or distance! – Michael Jul 18 '14 at 15:33
  • @Michael from the expansion of the universe, the longer a ray of light travels through the universe, the more the neighbouring peaks expand apart, thus redshifting – Jim Jul 18 '14 at 15:34
  • ok, maybe that spreading is another phenomenon. But shifting is a Doppler like effect. – Michael Jul 18 '14 at 15:36
  • Most of the red-shifting in the universe happens because of the expansion of the universe and not because of source/observer relative speeds – Jim Jul 18 '14 at 15:37
  • @Jim: sure, if you know the hubble law. But we generally use cosmological distance and redshift data to fit to the hubble constant, not the other way around (though it can be used to find ranges to newly discovered galaxies). Irrespectively, it's still a distance measurement, at least in my head. That is certainly the case with luminosity distance -- you measure the distance, and then infer the age. Not vice versa. In any case, the fundamental thing is the distance from which the light was emitted, not the travel time. – Zo the Relativist Jul 18 '14 at 15:40
  • Just saying we "can" directly measure the age of the light. There's no real value in knowing the age of the light if you already know the hubble law, and knowing how long light from a new galaxy has been en route to us is not nearly as useful as knowing how far away the new galaxy is. So we never really care to measure the age, but we can do it was my point – Jim Jul 18 '14 at 15:44
  • @Jim: but we do it not by direct measurement, but by measuring the distance first and dividing by c. That was my point. – Zo the Relativist Jul 18 '14 at 16:06
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You can misure the redshift $z$, from which $z=\frac{1}{a_e}$ you obtain the Scale Factor $a_e$ that the Universe had when that light was emitted. But the scale factor is directly related to the age of the universe, so you know the age that the universe had when the light was emitted.

Andrea Di Pinto
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