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How can space expand when space is not a physical thing? I’ve heard some say that is not expanding but rather it is getting less dense, which to me implies the same thing.
Didn't you ask this question before?PhanthomJay said:How can space expand when space is not a physical thing? I’ve heard some say that is not expanding but rather it is getting less dense, which to me implies the same thing.
I don’t remember asking it. But thanks for responsePeroK said:Didn't you ask this question before?
If space is expanding, then that's your evidence that it can expand, whether it's a physical thing or not.
What do you mean when you say it is not a physical thing? While it is true that what ”space” is is a rather arbitrary separation from spacetime. Cosmology generally uses a very particular coordinate system in which the universe is spatially homogeneous and isotropic. It is in those coordinates we talk about the expansion of space, which is nothing else than noting that the distance between so-called comoving objects (essentially objects at rest wrt the CMB or, equivalently, the cosmic frame) grow with time.PhanthomJay said:How can space expand when space is not a physical thing? I’ve heard some say that is not expanding but rather it is getting less dense, which to me implies the same thing.
No, it's an expansion of space over time - specifically that's how it's described in comoving coordinates.PhanthomJay said:And is time expanding also, being part of spacetime?
Locally (in a small enough spacetime region) it is possible to make a change of coordinates to the locally Minkowski coordinates. In those coordinates, comoving objects are indeed moving apart so it is a matter of coordinates. Hence, this is a matter of nomenclature and interpretation in a particular coordinate system.PhanthomJay said:Thanks, but why would it not be that the galaxies, not space, are moving apart as physical objects that are "spatially extended" (I borrowed that "term" from Einstein)?
Whether you like it or not is irrelevant. The only relevant thing is how accurately it describes the theory. As far as analogies go, it is a surprisingly appropriate one for the description using cosmological coordinates.PhanthomJay said:I don't particularly like the balloon analogy. And is time expanding also, being part of spacetime?
OK, now answer us this: where did you disappear to for 387 days? (from Jul 8 '22 till Jul 30 '23)PhanthomJay said:Once again, I want to thank you all for your time and responses.
I suppose this implies that an expanding flat space corresponds to a curved spacetime. That's right?PeroK said:More generally, the coordinate-free description is that spacetime is curved.
Hah, Dave , nice sense of humor! And good research! I've had some issues this past year, hope to be a regular again.DaveC426913 said:OK, now answer us this: where did you disappear to for 387 days? (from Jul 8 '22 till Jul 30 '23)
In general, yes. There are however some pathological counter examples. The best fit Lambda-CDM model of our universe not counting among those examples.Jaime Rudas said:I suppose this implies that an expanding flat space corresponds to a curved spacetime. That's right?
Here is the curvature of that spacetime, according to Wikipedia:Jaime Rudas said:I suppose this implies that an expanding flat space corresponds to a curved spacetime. That's right?
One clarification is probably appropriate here: what you quoted from Wikipedia only gives the Ricci tensor, which is only a piece of the Riemann tensor. But in this particular case, the other piece of the Riemann tensor, the Weyl tensor, is zero, so the Ricci tensor does capture all of the spacetime curvature. In general that will not be the case.Hill said:Here is the curvature of that spacetime
Thanks a lot. I wondered about the rest of Riemann tensor. Should've asked.PeterDonis said:One clarification is probably appropriate here: what you quoted from Wikipedia only gives the Ricci tensor, which is only a piece of the Riemann tensor. But in this particular case, the other piece of the Riemann tensor, the Weyl tensor, is zero, so the Ricci tensor does capture all of the spacetime curvature. In general that will not be the case.
Space-time expansion refers to the idea that the fabric of space-time itself is expanding, carrying galaxies and other matter along with it. This implies that space is a physical entity that can expand, rather than just a non-physical backdrop for objects to exist in.
One of the key lines of evidence for space-time expansion is the observation that distant galaxies are moving away from us at speeds proportional to their distance. This is known as Hubble's Law, and it suggests that the universe is expanding uniformly in all directions.
The concept of space-time expansion is closely tied to the Big Bang theory, which posits that the universe began as a singularity and has been expanding ever since. The idea of an expanding universe provides a natural explanation for the observed redshift of distant galaxies and the cosmic microwave background radiation.
While space-time expansion suggests that the universe is expanding, it does not necessarily mean that the universe is infinite. The expansion could be occurring within a finite volume of space, or the universe could be infinite in extent but still expanding.
Understanding space-time expansion has profound implications for our understanding of the universe. It suggests that the universe is not static, but dynamic and evolving. It also raises questions about the ultimate fate of the universe and the nature of space and time themselves.