25 March 2006

Horizon Problem

I've been talking lately with Jethereal about a cosmology conundrum. The universe is roughly 13.7 billion years old. Due to the speed of light, this means that we cannot see objects further away than 13.7 billion light years. The universe is actually larger than that 13.7 Glyr due to inflation, but we just can't see those parts because there hasn't been enough time for the light to get to us yet. Let's assume there's a quasar located, oh, 15 Glyr away from us. It's moving away from us true, but at a slower speed than that of light. We'll assume that it's actually 15 Glyr away when the Universe hits 15 Gyr old, and our horizon hits the quasar.

The question is as follows: What do we see when the horizon gets to the quasar? Jethereal proposed that it springs into our view fully formed. I countered that the quasar had to form at some point in time, so where did the light from its formation go? I proposed that the light from formation gets to us when the horizon hits the quasar.

Neither of us are actually cosmologists, so we're not sure of the answer. Anyone else want to weigh in?

x-posted

2 comments:

Thomas Siefert said...

I'm no cosmologist either, but here goes:
It is true that our horizon is constantly expanding like a sphere, but there is objects outside that sphere and those object are sending photons towards that horizon and when they reach it will be the formation of the quasar that we.

So by that logic; you would be right.

(I'm no Spock either, I just like the word "logic" and I find the subject fascinating)

zandperl said...

I have problems with both potential things we could see. Scenario 1 is the quasar appears fully formed, scenario 2 is we watch its birth.

1) Quasar is located 15 Glyr away. When the universe is 15 Gyr old, the horizon hits the quasar, we see it appear fully formed. But then, what happened to the light from the formation of the quasar? That light was also sent towards us, where'd it go? Did it hit the "edge" of the horizon somehow?

2) Quasar's light is always going out, light from the quasar, light from its birth, light from the host galaxy before its birth, light from the Big Bang before the host galaxy... But then, what's so special about the horizon? What happens when the horizon actually "hits" the quasar? What do we see from Earth prior to a universe age of 15 Gyr?

We should just do away with inflation, that would fix this problem. Alan Greenspan, where are you? :)