Discussion in 'Astronomy, Exobiology, & Cosmology' started by Hayden, Jul 24, 2018.
How is the universe isotropic for extreme horizon objects?
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Define extreme horizon?
If you mean the earliest celestial object we can image, that would be the cosmic microwave background.
Outer most point of any instant after the big bang.
An instant of course is time related not tropic or related to real estate. Instant is a good word because we spend 0 time in the now, as we move through time. so,
all matter has travelled the same distance through time, but not at the same rate through space or direction.
The furthest matter should by now have travelled is 13.8 billion years in the opposite direction from us and be 43 billion light years away the way the crow flies,. beyond our personal horizon of ~13 billion light years.
Our horizon, (distance from which we receive signals), is not the furthest reach of the universe.
Matter is isotemporal, but not isotropic.
After the big-bang the space is being added, so naturally it will have outer horizon. For an object on the outer horizon, the universe won't be isotropic.
For such an observer, half the sky would be dark. I do not think such a place exists, nor a favoured real estate, the now empty spatial site of the former big bang. In a perverse way we are both at the fringe, what you call "horizon", and the centre of the visual universe. (everything looks equidistant from our viewpoint).
My best contribution to this quandary is, to see us in a model where all has travelled the same time distance , we living in the expanding sphere, one spatial dimension exchanged for the time radius. So in that membrane universe we are flatlanders, Not being able to see the dark outside of the universe its future destination, time; only seeinwhat comes aloh the membrane.
Here is another rough sketch from "Alma" (for milk, basic food). might not be relevant, but shows dark sky # F for future. ( you would need serious insulation for contact and radiation heat loss in the exposed Horizon observer position, , at the edge of a 3D isotropic universe.
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Even within the Big Bang model, it's possible that the universe is infinite in size. In such a case, what exactly do you mean by "outer horizon"?
There is no outer most point.
There is no outer horizon.
You appear to be operating under the misconception that the universe is expanding into something - it isn't, therefore there is no 'edge' or horizon, just as there is no center.
But that shouldn't stop you from continuing to, well, do whatever it is you are doing.
Of course, if the universe is infinite in any model, then by definition it cannot have horizon.
But, I think anything which has an origin, cannot be infinite. Our universe has an origin around 14 billion light years ago, so can it be infinite?
I don't think it is, but it could be. It could have always been infinite in space and all that happened at the Big Bang is that any two points within that (infinite) space are now further apart.
A horizon is just a line of sight concept anyway. Whether space is finite or infinite, the horizon would just be as far as you can see (or theoretically see).
Thus we have the concept of the Observable Universe and the entire Universe. The Observable Universe could be described as a "horizon". It's just the radius that light has had time to travel since the Big Bang (including expansion).
OK, so you mean something like "edge" or "boundary" when you say "horizon", and you're not talking about some observational horizon due to the limited speed of light?
Why should something that has an origin in time be limited in the spatial dimensions as well? I don't see how that necessarily follows?
While it is true that Hayden/The God/Rajesh is an idiot troll, this idea of a universe of finite age but infinite size confuses me.
If there is a starting point for the universe then even if the universe was expanding at a rate c x 10^1000000000000000 for 13 billion years it would not be infinite in size. In fact any rate that you pick and any time frame you pick will not result in a universe of infinite size. It seems that the only way for an infinite size is an infinite age of the universe or an infinite expansion rate so if there is a starting point then there would have to be an infinite expansion rate.
I do not doubt I am missing something here, so any help on this would be appreciated.
You are assuming the (space of the) universe was created as a point, or at least finitely sized. Since we don't really understand what happened before/during the creation of spacetime, why would it be impossible for an infinitely sized space to be created? As far as I'm aware, that's still an option. (Note that an infinitely sized universe can still expand and contract just fine.)
I am, because as I understand it that it the generally accepted concept of the big bang theory. I have also read that inflation resulted in the universe growing from a tiny point to the size of an apple or football, which again points to a finite size at some point in the past.
That would certainly solve the problem in my mind (at least one problem..)
So I think you are saying that if at the creation of the universe it started as a tiny finite size the universe is not infinite in size and if the universe started of infinite then it still is.
I can live with that.
I'm not sure that's correct. I agree it's often displayed as such, and it makes the whole thing mentally easier to handle, but I'm quite sure it's not demanded by the theory.
What's meant there is that some region of that size grows to the visible universe, not that the entire universe (necessarily) was that size at that point in time.
Yes, you've understood me correctly, and I don't see a third option. Note that I'm currently unaware of evidence proving the universe is either finite or infinite in (spatial) size, so I'm not advocating for either.
My understanding is that if the Universe is now infinite then it was always infinite. Regarding the Big Bang, that started as (supposedly) a infinitely dense point that expanded.
Picture an infinite sized Universe that contracted down to an infinitely dense point. If that now starts to expand, that means that any two points are further apart as time progresses.
I don't personally think the Universe (or anything) is infinite but would easily change my mind with confirming data.
If that point (in space) was indefinitely dense, it was so only because it was infinitely small, unlikely. some physicists do not like infinites. Even the energy content of the universe is thought to be finite.
and evidence is that the expansion is again accelerating (it was expanding faster in the past), so it has not stopped, An infinite universe would have stopped expanding a long long time ago.
By whom? It seems weird to me to have a finite amount of energy if the universe is infinite in size?
Why would it?
below a partial quote from New Scientist July 12 2017 " 5 things physicists hate "
Mathematicians are pretty convinced that infinity exists. In fact, there’s not just one infinity, but an infinite number of infinities of varying sizes. And there’s nothing mathematicians like better than to invent new levels of infinity to plug logical holes in their subject. For physicists, though, infinity is a pain in the proverbial. Infinities are unruly quantities that tend to blow up any theory they appear in.--"
The Hilbert paradox notwithstanding, once an entity has reached infinite size, how could it get any bigger? The universe is still moving through time right, so it has not reached an infinite age yet. thank you.
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