The discussion revolves around the behavior of light at the so-called "edge of the universe," exploring theoretical implications, cosmological models, and the nature of space. Participants engage with concepts from cosmology, including the curvature of the universe, the expansion rate, and the implications of the Big Bang.
Participants express multiple competing views regarding the nature of the universe and the behavior of light at its edge. There is no consensus on whether the universe has an edge or how light behaves in relation to it.
The discussion reflects various assumptions about cosmological models, the nature of space, and the implications of the universe's expansion. Some claims depend on definitions of curvature and expansion rates, which remain unresolved.
Chronos said:The center of the universe appears to be 13.7 billion light years distant in every direction.
GodAdoresU said:I'm having a hard time understanding how we are able to see the center of the universe in every direction. Also, how can they tell it's the center?
trainman2001 said:If the "Big Bang" is really "The Great Mist" happening everywhere at once over infinite space, what is all the discussion about "expansion". Sounds to me like it is already expanded and just is morphing from one state of existence to another. Whether it's a single point with infinite density that decides to open up into a universe or a soup of one form of matter and energy over infinite distances that decides to evolve into the matter/energy we now see doesn't help me one bit. Both are inconceivable constructs to me.
I found this forum through my son when I posed a question to him yesterday about what happens to light that's traveling in the opposite direction from us from objects that are at the edge of the observable universe. These objects look very different than the ones near us. They are considerably older and more amorphous than nearby galaxies indicating that they are out near the "edge". I've read the entire forum about "no edge, no center", but as the Webb telescope may see that we do run out of galaxies as we get past the 15 million light year limit. Then what?
Drakkith said:The talk of the expansion is simply because we observe that all objects not bound to us through gravity are receding from us. Moving away.
George Jones said:With just the right amount of dark energy, a photon can circumnavigate the universe an arbitrarily large number of time before the Big Crunch...(but)...Observations indicate that our universe is near the border between open and closed, and that even if our universe is closed, it will expand forever.
DorothyShaw said:Got caught up in this discussion now! I think the answer is no, because of the 1/r squared rule. (I hope this comes out OK!)
russ_watters said:1. That doesn't logically follow. Any arbitrary area or volume can be tracked and observed to expand (or not) regardless of its size, curvature or arbitrary or lack of boundary.
2. False. I have a ruler sitting on my desk to compare the size of the universe to.
Arbitrary means "any sort you want". Of course the volume of the universe falls into that category.Johninch said:1. The universe is not an arbitrary area or volume. We cannot observe the universe to be expanding, because it is everything and we are in it. The observable universe is an area of volume which is not increasing in size.* Just because we observe the most distant galaxies to accelerate out of view, we cannot assume that it gets bigger on the other side.
2. We cannot measure the universe from the inside, therefore not at all. In order to measure something, we have to select limiting parameters, normally the boundaries. If we cannot identify the limiting parameters, we cannot make measurements to determine that the universe is growing.
*Edit: I'm not sure about this, please tell me if I'm wrong on this point, but it doesn't affect the rest of my argument.
Your first point about “observing the expansion of space by measuring some distances” is a strawman argument. Of course you are right. The observed distance between here and a galaxy is finite and measureable. But you can’t do that for the universe.Bandersnatch said:The point was that all you need to do to observe expansion of space is to measure some distances at two different times. You do not need to look at the space from "outside".
In the workhorse analogy of the expanding balloon, the 2d inhabitants on its surface do not need to move into the third dimension to notice expansion. They just measure distances on the surface, see them increase with time, and conclude that there's expansion.
The observable universe is most definitely increasing in size. Before the primordial plasma cooled enough to recombine with electrons, it was merely as big as the average length of photon scattering. Now it's 80-something billion ly across.
jcsd said:An infinite unievres must of always been infinite, at the big bang in an infite unievres, you basically have an inifte, but infinitely dense space, which expands (in other words becomes less dense).
marcus said:But because "singularity" sounds like the word "single"
it suggests to many people that there is a single point
where the singularity happens!
so they imagine a single isolated point
this is a wrong image and leads to much confusion
the BB singularity may have been confined in a small point-like region, but this is not the prevailing view. It could also have occurred at every point of an infinite 3D hypersurface---this is nowadays a very common view
Drakkith said:We know nothing about what the big bang was. We only know that as we look backwards in time it appears that the universe gets denser and denser. We extrapolate this out using known laws and it leads to a point where our math simply breaks down and we get a singularity. It is very likely we simply don't know how the universe works at the densities and temperature of the extremely early universe, leading to this breakdown.
The talk of the expansion is simply because we observe that all objects not bound to us through gravity are receding from us. Moving away.
Johninch said:2. We cannot measure the universe from the inside, therefore not at all. In order to measure something, we have to select limiting parameters, normally the boundaries. If we cannot identify the limiting parameters, we cannot make measurements to determine that the universe is growing.
The suggestion seems to be that when density is too great, time cannot tick. Perhaps someone can help me on this one too.
Drakkith said:We can easily measure the rate of expansion. Just because we can't measure the total size of the universe doesn't mean we can't say that it's getting larger. Instead of saying the universe has increased in size by X cubic meters, we simply say that it has increased in size by X percent.
That would not be correct. An observer ALWAYS measures time passing at a normal rate in their own reference frame.
You are taking the analogy too far - it requires you to think of the 3d universe as a 2d surface of a balloon. The 2d surface of a balloon has no boundaries in 2d space. The third dimension is completely irrelevant, and mathematically unnecessary for expansion.Johninch said:I know it is a workhorse analogy, but it doesn’t help me to compare an expanding balloon or fruitcake to the universe, in the first place because balloons and fruitcakes have clearly defined boundaries. Feynman told us to stop making such potentially misleading analogies.
I think I understand what you're getting at - you're simply challenging the validity of the cosmological principle outside the observable universe. I can sort of get behind that. After all, we do have the idea of eternal inflation, where parts of the universe are still inflating, while others are not.Your first point about “observing the expansion of space by measuring some distances” is a strawman argument. Of course you are right. The observed distance between here and a galaxy is finite and measureable. But you can’t do that for the universe.
Which is what was addressed. In the balloon analogy, whether you compare two close-by distances on the surface, or measure the complete circumference, or even the area(so, "everything"), you get the same result of expanding space, even though the surface is all there is, and there's no more surface to expand into.The distances between galaxies is increasing, but the universe cannot be expanding, because it is everything.
Johninch said:We can easily measure the rate of expansion of what – the observable universe? How do you get from that to the expansion of the universe, which is assumed to be much larger?
If you are of the opinion that the universe is getting larger, I suppose you are also saying that the universe is finite in size?
I think that I did not express my question very well. I was referring to the initial stages of inflation, which some say started with point volume and others say was much larger – everywhere, but with extremely high density. If the latter, we have the phenomenon that space existed in large amounts, before time. So suddenly, time started ticking, although space already existed. That's why I offered the explanation that time cannot or did not exist because of the high density. I know this is all hypothetical, but we still have to know what we mean. I don’t think this has anything to do with an observer.
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Bandersnatch said:In the balloon analogy, whether you compare two close-by distances on the surface, or measure the complete circumference, or even the area(so, "everything"), you get the same result of expanding space, even though the surface is all there is, and there's no more surface to expand into.
I think I understand what you're getting at - you're simply challenging the validity of the cosmological principle outside the observable universe. I can sort of get behind that. After all, we do have the idea of eternal inflation, where parts of the universe are still inflating, while others are not.
Still, since we cannot, in principle, know what's outside the observable universe, it seems sensible to keep the assumption that has worked so splendidly so far in what we do observe.
Johninch said:Irrespective of analogies, you are saying that the total universe, which is everything, can expand. Are you really sure that you don’t just mean that the observed distances between the observed galaxies are observed to be increasing?
How does it look with a finite number of galaxies? What do they collectively expand into?
There is no boundary which can be pushed out, unless there is available space beyond all the galaxies. This would be contrary to the Cosmological Principle which requires an isotropic and homogeneous universe.
Could you just tell me the reasons for why the universe must be expanding in size and not redistributing itself.
Drakkith said:Which would require that the universe expand.
Nothing. Distances grow larger over time, that's it. If the universe is finite and 100 billion light years across, then after a period of time it will be larger than 100 billion light years.
You are correct in that there is no barrier. Nothing is pushed anywhere. Distances grow thanks to the geometry of spacetime. Note that no galaxies actually travel anywhere. Distances between them grow, and nothing gets any closer to anything else.
Redistributing itself according to what known laws? Nothing in science at this time can explain the fact that galaxies appear to be receding from each other better than current theories.
That isn't what "redistribution" means. Redistribution means that the unobserved parts of the universe are behaving differently from our part: ours is epanding while parts that we don't see are contracting. It isn't generally acceptable in science to conclude the opposite of what your data is telling you!Johninch said:I didn’t know that the hypothesis of dark energy is good enough yet to be called a theory. I thought it was more like a stopgap. It is hypothesized that the vacuum strength of dark energy has been increasing in relation to the gravitational strength of dark and baryonic matter. The turning point came about 5 bya when dark energy got the upper hand and started to accelerate the expansion of space. This is an example of what I call redistribution. I assume that the total energy resource of the universe has not increased since the BB.
No, what you are saying is self-contradictory; if it is expanding, then it is getting bigger. Those two words mean the same thing.Wiki (Metric Expansion of Space): “the way we define space in our universe in no way requires additional exterior space into which it can expand since an expansion of an infinite expanse can happen without changing the infinite extent of the expanse. All that is certain is that the manifold of space in which we live simply has the property that the distances between objects are getting larger as time goes on.”
Wiki is saying that for practical purposes we can regard the universe as infinite. I prefer to say that the universe is everything, but I don’t want to quibble. Using Wiki’s formulation, I conclude that we are both right: you are right that the universe appears to be expanding and I am right that it is not getting bigger.
Johninch said:Thanks very much for all the explanations.
I come to the conclusion that the scientific consensus is that the totality of the universe is getting bigger in the spatial dimensions and as a result is moving from a high average density of energy + matter to a lower average density of energy + matter, irrespective of whether the universe already started as being infinitely large or not.
If this is a correct statement of the consensus, I have no further questions.
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Johninch said:Thanks very much for all the explanations.
I come to the conclusion that the scientific consensus is that the totality of the universe is getting bigger in the spatial dimensions and as a result is moving from a high average density of energy + matter to a lower average density of energy + matter, irrespective of whether the universe already started as being infinitely large or not.
If this is a correct statement of the consensus, I have no further questions.
George Jones said:Yes, but if dark energy really is equivalent to a cosmological constant (it may well not be), then the density of dark energy is constant. In this case, even if the scale of the universe grows without bound, the energy/mass density of the universe will approach this (the value for dark energy) finite, non-zero value, even though the density of matter and radiation will both approach zero.