Light Years ... Lots of Light Years

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OK, don't get me wrong. I know it's the distance light travels in a year. One can easily do calculatons and get miles meters parsecs or whatever unit you want. But when we photograph objects close to 13 billion LY away, almost as old as the universe it seems to take on an artificial meaning. The universe was certainly smaller then and the early Milky Way must have been much closer to these objects when their light left to chase us. As the Milky Way raced away, the light stayed in hot persuit for 13 billion years, finally entering the hubble telescope lens. But since the source object has likewise been racing away in the opposite direction and since space has been expanding, it too must be much further away than 13 BLY. So what does that 13 BLY represent? It's not the distance apart we were when the light left the object, nor is it the distance we are today, 13 B years later when the light arrived. It seems to be an artificial distance that says, if that object had stayed fixed (which it hasn't) and if space wasn't expanding (which it is) that object is 13 BLY away. It's tough to wrap my head around this.


Hi there BuzzLY, you are on the right track there, although it is a little more complicated than that. :D

The light that has been travelling for the longest time to reach us is the Cosmic Microwave Background Radation, and it has been travelling for 13.7 billion years. CMBR photons have been hitting the Earth throughout history, but the CMBR photons that are hitting us today were originally emitted only 42 million light years away. Due to the expansion of the universe, the coordinate that those photons were emitted from has receded to around 46 billion light-years by now.

We have seen light from galaxies that has been travelling for nearly 13 billion years. Those galaxies were only 3.5 billion light-years away when that light was emitted, and the region of space that those galaxies were in has now receded to around 29 billion light-years away, due to the expansion of the universe.

See The Distance Scale of the Universe for more information about the different ways astronomers and cosmologists have to measure distance.

To summarise the most commonly used distance measures:

Light-Travel Time The time that light has been travelling.

Angular Diameter Distance The original distance the light was emitted from.

Comoving Radial Distance The distance that emission point will have receded to, due to the expansion of the universe.

The light-travel time is the one most commonly used, but when it is used for distant objects it tells us neither where that object was, or where it is now, it simply tells us how long the light has been travelling for.

The best measurement to use is the redshift, which is usually given as the factor z.

Here is a little list of the different distance measures used in the current mainstream model of the expanding universe.

Light-travel time.

z=0.1___a galaxy whose light is 1.2 billion years old.
z=0.5___a galaxy whose light is 5 billion years old.
z=1____a galaxy whose light is 7.7 billion years old.
z=1.4___a galaxy whose light is 9.1 billion years old.
z=7_____a galaxy whose light is 12.9 billion years old
z=1089___the CMBR, which is 13.7 billion years old.

Angular diameter distance.

z=0.1___a galaxy that was 1.2 billion light-years away
z=0.5___a galaxy that was 4 billion light years away
z=1____a galaxy that was 5.4 billion light years away
z=1.4___a galaxy that was 5.7 billion light-years away
z=7_____a galaxy that was 3.5 billion light-years away
z=1089___a CMBR photon that was emitted 42 million light-years away.

Comoving distance.

z=0.1___a coordinate that has receded to 1.35 billion light-years away.
z=0.5___a coordinate that has receded to 6.1 billion light-years away.
z=1____a coordinate that has receded to 10.8 billion light-years away.
z=1.4___a coordinate that has receded to 13.8 billion light-years away.
z=7_____a coordinate that has receded to 29 billion light-years away.
z=1089___a coordinate that has receded to 46.5 billion light-years away.


I wasn't even gonna touch this answer, since I know not only have you got your head wrapped around it, but have this clear and concise explanation with handy dandy z chart :)


Really, really nice, SpeedFreek! My compliments!


It might be nice to know how long it would take that world weary photon that travelled 13 billion litght years to get home if the poor thing happened to bounce off a mirror and got sent back!


kg":32r7g3di said:
It might be nice to know how long it would take that world weary photon that travelled 13 billion litght years to get home if the poor thing happened to bounce off a mirror and got sent back!

Ha! Interesting question!

Those poor reflected photons will never get home, since the rate of expansion is now accelerating. In a decelerating universe, like ours was doing for 7 billion years or so, light from ever increasing distances can reach us. But once the rate of expansion starts to accelerate there exists a "light horizon" beyond which we can see no more events (and observers at those places can see no more events from this place), and galaxies are forever crossing over that horizon, never to be seen again.

(Actually the "light horizon" also exists in a decelerating universe, but it recedes towards infinity, letting all the galaxies in between into our future light-cone).

The "light horizon" or cosmological event horizon, is currently 16 billion light-years away. This means that if a photon is emitted (or reflected) right now, nobody over 16 billion light years away from that photon will ever see it.

That photon that travelled for 13 billion years was originally emitted only 3.5 billion light-years away, but the source has now receded to around 29 billion light years away, well outside of the cosmological event horizon. If that photon was reflected back, it would not even make it home during the the lifetime of the universe.
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