Horizon Problem

[asterroc]

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

Comments

[xoder.livejournal.com]

You should see the light of the creation of the Quasar (think of it as a wave-front expanding outward) at 15Gyr. Assuming, of course, you had a telescope in the right direction &c.

[zandperl.livejournal.com]

But the problem with that logic is if we're going to see the start of the quasar at a certain time, we should also see the original host galaxy before that, unless there really IS some sharp cutoff start time when the horizon hits it.

[xoder.livejournal.com]

Hey! You asked about the quasar. No fair changing questions on me ;-)

That said, yes you should see the host galaxy beforehand. But now you are talking about something older, and thus, had more time to reach us.

[zandperl.livejournal.com]

My question wasn't about simple lookback time, it was about the horizon. There are objects further away than we've had time to see yet. What happens when the age of the universe catches up to their distance? An object located at 15Glyr away *cannot* be seen by us, period, because there hasn't been enough time for light to travel that far, whether from the quasar, or its host galaxy before, or the gas before that, or the CMB before that, or the Big Bang before that. :-P

[xoder.livejournal.com]

So, the objects are 15Glyr apart. At 15Gyr after the Big Bang, we shall see whatever happened 15Gyrs ago at that point.

Or at least I'm pretty sure about that.

[zandperl.livejournal.com]

Yes yes, and right now, since the universe is only 13.7 Gyr old, if we look there the Big Bang won't have happened yet, so we won't see anything. So what happens, what do we see, inbetween right now (at t=13.7Gyr) and t=15Gyr?

[xoder.livejournal.com]

The microwave background? A delicious 4 Kelvins hum.

What happens? That I'm not so sure about.

[zandperl.livejournal.com]

I'm starting to doubt myself though. As mentioned above to xoder, if we're saying that we'll be able to see the birth of a quasar, then before that we should be able to see the host galaxy (assuming infinitely good telescopes). There's nothing to my original scenario that distinguishes when the horizon gets to the quasar's location. Therefore, I don't think my original scenario can be right.