Going off on a complete tangent - I don’t remember the particulars, but I once saw a claim that, had there been no structure (galaxies, clusters, super-clusters…) to the Universe, the night sky would be brighter than the midday sun due to accumulated radiation from all directions (ah, I’m probably butchering this :().
Is it possible that the night sky is, mostly, dark not - or not only - because of clustering, but also because a lot of the radiation just isn’t reaching us yet? In other words, might the night skies become much brighter than they are now - due to pan-univeral radiation, not relatively “local” causes - a few billion years down the road?
That seems to conflict with what was said above, that the light was emitted from the object 2-3 billion years ago. But it has taken 13 billion years for us to see it because of the constant expansion of the universe. That seems to make sense to me. So what am I missing?
An american billion is a thousand million;
almost nobody uses the old British billion as far as I know, but usually it is only astronomy types and economists who talk routinely of billions anyway.
Noone Special; since the universe is thought to have had a definite beginning, and is expanding as well, the amount of light in the sky from distant stars and galaxies will never get very bright; in fact it will fade to practically zero eventually (many many billions of years from now).
I can’t quite see how this would happen. Its because things cluster and heat up due to gravity that we get a lot of the radiation. If there were no clustering, we’d have the CMB, stars and that’s about it. I don’t think there’d be a sort of “background” radiation type thing.
Well, this is essentially the resolution to Olber’s paradox, which put simply, states that the night sky should be brighter than the midday sun, because of the radiation reaching us from all the stars. The resolution to this is that as the universe is expanding, the radiation from the stars and galaxies gets redshifted and fainter, hence we don’t have a bright night sky. The radiation hasn’t reached us yet, and when it does, it’ll be very faint.
OK, from the link:
They look 2 or 3billion light years from us in terms of their size. This is due to a peculiarity of the definition of the apparant angular distance, which makes it non-linear with age. That is, there is some degeneracy between the acutal age and the distance that the apparant angular distance gives you. Which is why this should only be used in the local universe.
The luminosity distance is a better indicator of age, since, it is linear with time, and logically the further away something is, the older it is and the fainter it is.
eburacum45, actually, we tend to talk about gigayears instead.
Exactly, we use Gigayears for 10[sup]9[/sup] years and Megayears for 10[sup]6[/sup] years. None of this messing about with millions and billions which causes confusion.
Its all the other strange units we use that are “fun”, especially when crossing two different areas of astrophysics.
OK - so I actually read the site eburacum45 suggested (thanks! very interesting), and I got some of it, but one thing I do not understand is the D[sub]A[/sub] (Angular Diameter Distance) curve going back down after hitting a high of about 6 GLY - apparently, after some point, the older an object is, the nearer it was to us when it emitted the light we are now observing? Is this somehow due to differing rates of universal expansion over time? Or to the fact that more distant objects are receding at a higher rate than nearer ones (hence also their larger Redshift Value)?
Its to do with the expansion of the universe, and that the expansion of the universe is accelerating - i.e. we’re expanding faster now than the universe was then. So, there was a time when some older objects were closer to us than a linear expansion at constant speed would imply. Essentially, the expansion of our universe is non-linear, so trying to relate an apparant size to an actual distance becomes meaningless outside our own little bit of space.
Lessee if I got this straight. A really old object was created, say 2GLY from us, about a bazzillion years ago. Because it has been carried away so far, the light has, in fact, taken a Bazzillion years to reach us. A younger object was created, say 3 GLY from us, half a bazzillion years ago (when Object #1 was already maybe 5 or 10 GLY away - it had half a Bazzillion years head start on running away from us). So - Object #1 appears at an Ang. Diam. Dist of (surprise!) 2 GLY, while (the younger) object#2 appears at an ADD of 3 GLY, although it is actually physically closer to us now than object#1 is. All this is the result of the light from object #1 not having even reached object #2 when object #2 was created… because of the accelerating expansion.
