I Galaxy GNZ11 33 billion light-years away

[Andy DS]

Galaxy GNZ11, we are told, is 33 billion light-years away, so why can we still see it if it is beyond the cosmic event horizon?

 

[Ibix]

Galaxy GNZ11, we are told, is 33 billion light-years away, so why can we still see it if it is beyond the cosmic event horizon?

It wasn't when the light we are now seeing was emitted.

 

[Ibix]

Davis and Lineweaver is a useful introductory reference on the various kinds of distance measures in cosmology. It's about as non-technical as you can get and still be accurate.

 

[Andy DS]

Why is it then we can't see all the other galaxies that have receded beyond the cosmic event horizon?

 

[Ibix]

Some galaxies were always behind it. Those we can never see. (Note: it's possible to play around with the dark energy/matter mix to get a universe where you can eventually see some of the history of all of space, but we don't appear to live in one of those.)

Light from some distant galaxies hasn't reached us yet but will do so eventually. Those galaxies are already beyond the horizon, but weren't when the light we will see was emitted.

Some galaxies we could see in principle but they are just too faint. This problem will get worse and worse as expansion continues and increasing redshift fades out more and more galaxies.

 

[Andy DS]

This seems a reasonable explanation for why we don't see those galaxies but how can a galaxy be measured at 33 billion light years away if what we can see is this side of the cosmic event horizon.

 

[Ibix]

Essentially you measure how far away it looks and multiply by the amount the universe has expanded since the light you are seeing was emitted. There are quite a few challenges both theoretical and practical in measuring those two numbers, and the result depends on our cosmological models being correct, but the multiplication really is that simple.

 

[PeroK]

This seems a reasonable explanation for why we don't see those galaxies but how can a galaxy be measured at 33 billion light years away if what we can see is this side of the cosmic event horizon.

In general, if something is moving away from you, there are three possible measures of the "distance" of that object. Note, of course, that the distance of the object from you varies over time.

The first is the distance of the object was when the light was emitted.

The second is the distance of the object when the light is received.

The third is some measure of the time the light has been travelling.

For example, imagine that the Voyager spacecraft sent a signal back to Earth from where it is now. The signal would take about 20 hours to reach us. By the time the light reaches us, Voyager is slightly further away, but only about another 4 light seconds. That's almost negligible, so the three measures of distance almost coincide, and there is negligible ambiguity is how far away it is. It's about 20 light hours by all three measures. Note that the expansion of the universal is negligible or even irrelevant over such short distances.

A distance galaxy, however, might have been 5 billion light years away when the light was emitted, the light might have take 10 billion years to reach Earth and that galaxy might now be 25 billion light years away. I've made those numbers up, by the way. You could research for yourself what would be valid numbers, but you get the idea. In any case, the three possible measues of distance are all significantly different.

Finally, the concept of a cosmological event horizon arises because some objects are so far away when the light is emitted that the light will never reach Earth - owing to the continued expansion of the universe. For further reading on this, you could try this excellent Insight:

https://www.physicsforums.com/insights/inflationary-misconceptions-basics-cosmological-horizons/