Cosmological Horizon Help

[DamNative]

Hi,

I'm new to astronomy and I'm having trouble understanding the concept of the cosmological horizon.

I don't understand how there is anything beyond the horizon. If the age of the universe is 13.7 billion years old, and the light emitted at the beginning has covered that distance, doesn't that set the edge of the universe? Also is the universe aging every day? I'm having a hard time explaining my misunderstanding, hopefully it makes sense?

[brianb]

I don't understand how there is anything beyond the horizon. If the age of the universe is 13.7 billion years old, and the light emitted at the beginning has covered that distance, doesn't that set the edge of the universe?

No, that sets the "cosmological horizon".

Perhaps it helps to think in terms of a lower dimensional analogue. Consider a universe which consists of what we see as the circumference of a circle, whose radius is expanding by one inch per second (the speed of light in this universe). An ant - who cannot see anything except round the circumference - can only see N inches in each of two directions (in front of and behind it) or 2N inches in total when the universe is N seconds old. Yet the whole universe has a perimeter of 2πN inches ... so the ant can see not quite one third of its universe.

Also is the universe aging every day?

Yes. Different bits of it at different rates, depending on the curvature of space/time at different places.

[DamNative]

No, that sets the "cosmological horizon".

Perhaps it helps to think in terms of a lower dimensional analogue. Consider a universe which consists of what we see as the circumference of a circle, whose radius is expanding by one inch per second (the speed of light in this universe). An ant - who cannot see anything except round the circumference - can only see N inches in each of two directions (in front of and behind it) or 2N inches in total when the universe is N seconds old. Yet the whole universe has a perimeter of 2πN inches ... so the ant can see not quite one third of its universe.

Yes. Different bits of it at different rates, depending on the curvature of space/time at different places.

Thanks for the quick reply, but how can we know there are galaxies BEYOND the horizon?

[brianb]

how can we know there are galaxies BEYOND the horizon?

Empirically - we can't. Cosmology is a model of a process we can't see the whole of, or experiment on in any practical way. But the standard model (and most of the non standard ones) work on the Copernican principle i.e. there is nothing special about out position in space/time - therefore we assume that wherever you are in the universe, things look pretty much the same. Even if you happen to be looking from a part which is beyond our horizon.

[DamNative]

Thank-you for the insight, I think I have a better understanding now.