End of the Universe
28 Feb 2006 16:07:29 UTC
Topic 190840
(moderation:
Does anyone have any opinions about the theoretical capabilities of reaching the end of the universe, if there is even the possibility of the existence of such a thing?
Specifically, there is probably a "boundary" of sorts whereby if you attempt to approach, you are either pulled back by the shear gravitational pull of everything emitting gravitons to that point, or it is a massive radiation barrier, still seething with energy and particles forming after the Big Bang...
Any ideas?
End of the Universe
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Some current cosmologies implies expansion of space, not expansion into space, with space being bounded ( there is a maximum separation between points ) but without boundary ( no identifiable discontinuity or edge ). It's well nigh impossible to visualise this in three dimensions, so the usual analogy is in lower dimensions, like the circumference of a circle or the surface of a sphere ( and you ignore the space the object is embedded in ). It is in fact gravity which 'curves space back on itself'.
There are potential points of confusion though.
One is that with ongoing expansion, then the upper bound on separations, while always finite, is increasing.
Another is that due to the finite speed of light, both any travel too and information from any other part of the universe is qualified. Every point has it's 'horizon', which is always increasing, that defines the ( instantaneous ) current sphere centred on that point for which influences which were generated in the past can arrive at lightspeed. So the microwave background radiation, which is the snapshot of radiation from about 300,000 years after the Big Bang, comes from a horizon which represents the 'surface of last scattering' when that radiation stopped interacting generally with matter. This is light from the time when temperatures fell such that energies dropped below ionization levels of atoms, and hence combinations of electrons and nuclei formed which were somewhat less interacting with light. It has a different wavelength than when emitted, but still carries the detail.
Furthermore the so-called 'inflation' period early on in the Big Bang is hypothecated as being superluminal, meaning that space grew faster than the rate that light could travel in it! Heady stuff! It somewhat challenges our intuitive notions of distance, doesn't it? If true this means, in particular, that there are sections of the universe which flew away from our horizon early on. These will now progressively come back into our horizon, because inflation did ease with expansion subsequentally being 'subluminal', and time has since passed for light to travel from there. We may eventually get information from 'everywhere' depending on the subsequent course of events. Current evidence considers this to be the likely case.
Suppose you were to fire off and travel in some direction, and that you could do so superluminally, safely and within our lifetimes. Then you would return from a direction opposite to which you left. Like the early seafarers you have circumnavigated the known universe. I could shake hands with you, turn around, wait, and then greet your return. No doubt by then I would be biologically a lot older than you though... :-)
The above concepts I think really are provisional, and not without their nuances, and as always await falsification/contradiction from observation!
I have made this letter longer than usual because I lack the time to make it shorter ...
... and my other CPU is a Ryzen 5950X :-) Blaise Pascal
RE: RE: Does anyone have
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(edit) So where is/was the 'centre' of the Big Bang? It is/was everywhere! Where you are, where I am, over in the Andromeda Galaxy, at the CMB horizon, out beyond our current horizon......... we were all in the one place at the same time and there was no outside! We are the seething energy and particles formed after the Big Bang! We got cooler but..... :-)
(edit) Bummer, hit the wrong button, double posted! ( Doubled the horizon.... )
I have made this letter longer than usual because I lack the time to make it shorter ...
... and my other CPU is a Ryzen 5950X :-) Blaise Pascal
RE: Does anyone have any
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I'm not sure what you mean by there probably being a boundary, at least not the two sorts you are talking about. As far as we can observe, on large scales matter is pretty uniformly spread around the universe. The technical phrase you'll see in cosmology is "isotropic and homogeneous." When you plug that into the Einstein equations describing gravity you get several possible geometries, depending on how much matter there is, but none of them has an edge.
When scientists talk about the boundary, they are usually referring to something more properly known as the Hubble radius. This is just a statement that if the universe is N years old, you can see at most N light-years away. And there is nothing special about our location - the same could be said of any point in the universe.
Hope this helps,
Ben
RE: When scientists talk
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And here the confusion starts ...
(My English is maybe too bad to explain what I mean, so forgive me)
Yes, you see objects which are N light-years away ... and at the same time N light year back in the past. But since the the universe expands and those N years (with N being a large number) in the past the universe was smaller, what do we actually see?
If some light emitting thing is 11 billions (9 digits) it is currently not 11 billion light years away, it was also not 11 billion light years away when the light starts its travel. It just means that the light had to travel 11 billions of years with the universe expanding during that time.
So: How far away is it now? And what means 'now'? And how far was the object away when the light was emitted?
Is all about measuring a distance on a ballon while it gets blown up.
Wurg! If the universe is
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Wurg!
If the universe is finite then the Hubble horizon could be several times the circumference of the universe.We might be able to look out and see several early copies of the milkyway and rest. In an interesting a side we would see these copies no matter which direction we looked.
MarkF RE: Wurg!If the
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MarkF
Yup! I recall some group has actually examined that, I think concluding that there was a higher than expected correspondence between opposite directions in the sky. Tricky though as you don't really know the true distances and whether light has taken a 'long' or 'short' way around. Any image is dated, and things evolve.....
Wurg!
The way it was when the light was emitted, disregarding adjustments due to redshift, intervening influences like dust etc....
Exactly right.
Key questions indeed. An inevitable consequence of finite light speed and moving targets. It's even worse if you allow for the possibility of an inconstant expansion rate! It's a troublesome area for me, and requires careful definition, so I'll flick you to Cosmic Connections, a cracking site and an excellent source ( more so if you have broadband ) of lot's of stuff. The lectures by Roger Blandford explain what you're after. Browse around the others too!
Cheers, Mike.
I have made this letter longer than usual because I lack the time to make it shorter ...
... and my other CPU is a Ryzen 5950X :-) Blaise Pascal
RE: ... It's a troublesome
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An excellent site that I often see recommended on Usenet physics and astronomy groups is Ned Wright's Cosmology FAQ.
RE: An excellent site that
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It's really quite good, isn't it? Some great diagrams. I was initially a little put off by this one-liner though "The Big Bang is a singularity extending through all space at a single instant, while a black hole is a singularity extending through all time at a single point." - but I'm picky. :-)
I have made this letter longer than usual because I lack the time to make it shorter ...
... and my other CPU is a Ryzen 5950X :-) Blaise Pascal
Wurgl, You've raise a good
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Wurgl,
You've raise a good point but I'm not sure how to address it without pictures. Yes, it's something like a balloon.
To the others who answered:
There have been papers in recent years asking if the universe had (in 4 dimensions) a shape more like a donut than a ball, would we see any consequences? If the thin dimension of the donut is less than a Hubble radius, we would see "circles on the sky" as they call it. Neat idea, but there hasn't been any conclusive evidence of it. As far as the instruments can tell (mainly WMAP), the universe is pretty flat.
Hope this helps,
Ben
> So: How far away is it now?
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> So: How far away is it now? And what means 'now'? And how far was the object away when the light was emitted?
we somehow acustomed to think the universe exists as flat 3D space and that's why we assume such things as "There - Now"
Actually "There - Now" is not real but rather our attempt to extend the definition of our imagined 3D flat space as we acustomed to see it here nearby.
even in special relativity "There-Now" definition is dependant on the speed of our movement. At given direction and speed of our movement we 'calculate/estimate' certain distant events to be happening at the same time and as we are now we think of them as 3D subspace of "There-Now" dots of 4D space-time (our own reality projection). If u move in different direction and/or with different speed then the dots of There-Now become different dots of 4D space-time (other 'new reality' projection).
To make it more apparent we assume Sun to be There-Now 8 light minutes away. But this does not mean it actually is there now, we only calculate/asume it to be there now and there is no garantee that some UFOs did not explode or maybe moved the Sun to other position. So "There-Now" is what we estimate the world to be depending on what we persieve "Here-Now". "Here-Now" is real but "There-Now" is not real but rather estimation based on our experience of the info we recieved from the distant past :)
I sometimes think it would be better to abandon these intuitive analogies when explaining einstein relativity because often these artificial but intuitive definitions lead reader to confusions. For example when in relativity one says moving objects shrink for outer observer it does not mean he actually sees them to shrink it simply means that his calculated length of that object in his estimated "There-Now" 3D flat world (his estimated reality) is shrinked. where in reality he simply has no means to know what is actually happening with that object until its light comes to him some time later.
one more example is how for traveling Twin his brother on earth can suddenly age many years in a matter of seconds (while making turning-back manouvre involving acceleration to change the speed direction to opposite).
It is not what traveling (turning back) Twin actually sees about his brother but it is rather what he estimates about his brother's "There-Now" (traveling Twin while turning back changes his speed to opposite und thus changes drastically his estimation about what the intuitive contemporaly with him 3D flat world around him is)
what in reality travelling Twin sees is that when he moves away from earth his brother is ageing SLOWER than he himself so he may even assume that when he returns to earth he will be older than his brother.
At point of turning back nothing special happens (if earth brother is 20 years old he still stays 20 years old) but what's funny - his earth brother suddenly starts to age much FASTER. Having that in mind returning Twin reestimates his brother age upon arrival and suddenly realizes his earth brother will be much older upon arrival. So traveling twin must recalculate what his brothers "There-Now" is and since he is traveling back with high speed it does not take much his own years so his earth brother's There-Now is not that far in the past (probably he must be there already at 60 years old instead of 21-23)