- #1
Robert100
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First, a quick note to explain why this question is being posted in the "Beyond the Standard Model" forum, as opposed to the relativity forum. Many studies of relativity discuss the expansion of our universe, but I haven't seen many which explicitly target the question of what our universe is expanding into. In contrast, when people like Alex Vilenkin and Alan Guth speculate about these topics, they do so in the context of GUTs, superstring theory, and linkages between gravity and QM.
Question: What is our universe expanding into? Many people describe the Big Bang as an expansion, but simultaneously write that the universe is not expanding into anything. They say that it is mathematically possible to describe space as an expanding manifold, without that manifold expanding into a higher dimensional space, and also (perhaps a simpler explanation) without expanding into a prior 3D space. But they don't offer any physical interpretation for their claim, which is very unphysics-like.
Here is one such example, a FAQ about Cosmology
http://www.astro.ucla.edu/~wright/cosmology_faq.html#BBevidence
This answer is common, but enormously unhelpful. Are we really to believe that the volume of our universe is constant? If so, then how could the space between distance galaxies be expanding? The universe cannot be expanding, while simultaneously remaining constant in volume, having rulers with a constant size.
The quote also seems to be misleading: Many physicists openly state that our universe did appear in a pre-existing 3D space of some sort. (Or perhaps a higher dimensional space.) Many cosmologists posit that other Big Bangs likely have occurred in this same huge 3D (or more D) space. These other Big Bangs would then create many other universes, and we probably have no way of observing these other universes.
So here is my observation: When physicists talk about multiple universes, they never make the argument that space expands without the need for a larger volume for it to expand into. They openly state that we are expanding into some larger domain. I've seen articles by Alan Guth (known for inflation) and Max Tegmark (known for positing many types of multiverses) which seem to openly state that the Big Bang occurred (better: is still occurring) in some 3 or more dimensional space.
Only when physicists discuss our universe alone do they fall back on the "expansion without expansion" argument.
Sometimes I get the idea that many physicists were brainwashed by their teachers into thinking that such questions were simply not allowed. This has happened before: From the 1920s to the 1970s most physicists were brainwashed into believing that Quantum Mechanics requires no interpretation. The Copenhagen interpretation, which required magic observors that mystically collapsed the waveform, was the only "interpretation" allowed, and it basically said nothing about the ontological reality of the universe. Hundreds of physicists were brow-beated into the "shut up and calculate" approach.
As we now know, that approach is now rejected. Today we admit that QM requires some kind of interpretation, and this has become an active field .
So is it still ok to say that that our universe is DEFINITELY NOT expanding into any 3D space, or into any higher dimensional space? If so, then what physical interpretation allows the expansion of our universe, yet without a change in volume?
Or, as I suspect, is the old answer based on outdated dogma? Are physicists now comfortable in saying that our universe is expanding into some space (even though we cannot currently detect it?)
Consider the recent paper by Guth and Vilenkin, "Eternal inflation, bubble collisions, and the persistence of memory"
http://arxiv.org/abs/hep-th/0612242
A ``bubble universe'' nucleating in an eternally inflating false vacuum will experience, in the course of its expansion, collisions with an infinite number of other bubbles. In an idealized model, we calculate the rate of collisions around an observer inside a given reference bubble. We show that the collision rate violates both the homogeneity and the isotropy of the bubble universe. Each bubble has a center which can be related to ``the beginning of inflation'' in the parent false vacuum, and any observer not at the center will see an anisotropic bubble collision rate that peaks in the outward direction. Surprisingly, this memory of the onset of inflation persists no matter how much time elapses before the nucleation of the reference bubble.
Any thoughts would be greatly appreciated.
Robert
Question: What is our universe expanding into? Many people describe the Big Bang as an expansion, but simultaneously write that the universe is not expanding into anything. They say that it is mathematically possible to describe space as an expanding manifold, without that manifold expanding into a higher dimensional space, and also (perhaps a simpler explanation) without expanding into a prior 3D space. But they don't offer any physical interpretation for their claim, which is very unphysics-like.
Here is one such example, a FAQ about Cosmology
http://www.astro.ucla.edu/~wright/cosmology_faq.html#BBevidence
"What is the Universe expanding into?" This question is based on the ever popular misconception that the Universe is some curved object embedded in a higher dimensional space, and that the Universe is expanding into this space. This misconception is probably fostered by the balloon analogy which shows a 2-D spherical model of the Universe expanding in a 3-D space. While it is possible to think of the Universe this way, it is not necessary, and there is nothing whatsoever that we have measured or can measure that will show us anything about the larger space. Everything that we measure is within the Universe, and we see no edge or boundary or center of expansion. Thus the Universe is not expanding into anything that we can see, and this is not a profitable thing to think about. Just as Dali's Corpus Hypercubicus is just a 2-D picture of a 3-D object that represents the surface of a 4-D cube, remember that the balloon analogy is just a 2-D picture of a 3-D situation that is supposed to help you think about a curved 3-D space, but it does not mean that there is really a 4-D space that the Universe is expanding into."
This answer is common, but enormously unhelpful. Are we really to believe that the volume of our universe is constant? If so, then how could the space between distance galaxies be expanding? The universe cannot be expanding, while simultaneously remaining constant in volume, having rulers with a constant size.
The quote also seems to be misleading: Many physicists openly state that our universe did appear in a pre-existing 3D space of some sort. (Or perhaps a higher dimensional space.) Many cosmologists posit that other Big Bangs likely have occurred in this same huge 3D (or more D) space. These other Big Bangs would then create many other universes, and we probably have no way of observing these other universes.
So here is my observation: When physicists talk about multiple universes, they never make the argument that space expands without the need for a larger volume for it to expand into. They openly state that we are expanding into some larger domain. I've seen articles by Alan Guth (known for inflation) and Max Tegmark (known for positing many types of multiverses) which seem to openly state that the Big Bang occurred (better: is still occurring) in some 3 or more dimensional space.
Only when physicists discuss our universe alone do they fall back on the "expansion without expansion" argument.
Sometimes I get the idea that many physicists were brainwashed by their teachers into thinking that such questions were simply not allowed. This has happened before: From the 1920s to the 1970s most physicists were brainwashed into believing that Quantum Mechanics requires no interpretation. The Copenhagen interpretation, which required magic observors that mystically collapsed the waveform, was the only "interpretation" allowed, and it basically said nothing about the ontological reality of the universe. Hundreds of physicists were brow-beated into the "shut up and calculate" approach.
As we now know, that approach is now rejected. Today we admit that QM requires some kind of interpretation, and this has become an active field .
So is it still ok to say that that our universe is DEFINITELY NOT expanding into any 3D space, or into any higher dimensional space? If so, then what physical interpretation allows the expansion of our universe, yet without a change in volume?
Or, as I suspect, is the old answer based on outdated dogma? Are physicists now comfortable in saying that our universe is expanding into some space (even though we cannot currently detect it?)
Consider the recent paper by Guth and Vilenkin, "Eternal inflation, bubble collisions, and the persistence of memory"
http://arxiv.org/abs/hep-th/0612242
A ``bubble universe'' nucleating in an eternally inflating false vacuum will experience, in the course of its expansion, collisions with an infinite number of other bubbles. In an idealized model, we calculate the rate of collisions around an observer inside a given reference bubble. We show that the collision rate violates both the homogeneity and the isotropy of the bubble universe. Each bubble has a center which can be related to ``the beginning of inflation'' in the parent false vacuum, and any observer not at the center will see an anisotropic bubble collision rate that peaks in the outward direction. Surprisingly, this memory of the onset of inflation persists no matter how much time elapses before the nucleation of the reference bubble.
Any thoughts would be greatly appreciated.
Robert