On Saturday, January 18, 2020 at 1:00:23 AM UTC-7, Lawrence Crowell wrote: > > On Friday, January 17, 2020 at 7:36:37 PM UTC-6, Alan Grayson wrote: >> >> >> >> On Friday, January 17, 2020 at 6:03:46 PM UTC-7, Lawrence Crowell wrote: >>> >>> On Friday, January 17, 2020 at 5:08:14 PM UTC-6, John Clark wrote: >>>> >>>> On Fri, Jan 17, 2020 at 5:03 PM Alan Grayson <[email protected]> >>>> wrote: >>>> >>>> >> Yes, you can use that to represent a curved path in 4D (one of time >>>>>> 3 of space) Minkowski Space where Special Relativity lives, but as you >>>>>> say >>>>>> that doesn't really get to the fundamental issue because Minkowski Space >>>>>> is >>>>>> flat and Special Relativity says nothing about gravity, for that you >>>>>> need >>>>>> General Relativity and GR doesn't live in Minkowski Space. >>>>>> In General Relativity curved Spacetime is what gravity is, and in GR >>>>>> if there is any curvature in the Spacetime of the universe, and we know >>>>>> there is because we know that gravity exists, then, unless vacuum energy >>>>>> also exists and is fine tuned to one very precise value, the universe >>>>>> can >>>>>> not be stable, it must be either expanding or contracting. There are >>>>>> thermodynamic reasons to think it can't be contracting so it must be >>>>>> expanding. >>>>>> And that is why no physicist would say that Carroll's statement "*the >>>>>> manifestation of spacetime curvature is simply the fact that space is >>>>>> expanding*" was controversial . >>>>>> >>>>> >>>>> > *The question is, what does he mean? Is space expanding BECAUSE of >>>>> curvature? If so it's expanding because of gravity, since you wrote that >>>>> gravity and curvature are equivalent. But since gravity is attractive (as >>>>> far as we know), how could it be responsible for expansion (as >>>>> distinguished from contraction)? AG * >>>>> >>>> >>>> If the universe consisted of a cloud of particles that were not moving >>>> with respect to each other the gravitational attraction between the >>>> particles would indeed cause the universe to contract, but the particles >>>> ARE moving with respect to each other, so what will happen? It depends on >>>> how they are moving, but General Relativity can tell you one thing, unless >>>> you invoke a very fine tuned vacuum energy (aka the Cosmological Constant) >>>> that cloud of particles will NOT remain the same size, it will either >>>> expand or contract. We learn from observation that it's expanding which is >>>> consistent with thermodynamic reasoning. >>>> >>>> John K Clark >>>> >>> >>> >>> Sometimes a picture works best. Below is a diagram that represents how >>> space can be flat in a curved spacetime that expands space. >>> >>> LC >>> >>> [image: vsl.gif] >>> * Can you elaborate further? Not clear what this diagram demonstrates. >>> AG* >>> >> > > This is a sort of light cone diagram. The curves are null rays tangent to > light cones. > > I keep referring to this, but I illustrate here > <https://physics.stackexchange.com/questions/257476/how-did-the-universe-shift-from-dark-matter-dominated-to-dark-energy-dominate/257542#257542> > > how gravitation can generate a repulsive acceleration. This thread is > approaching 100 comments where it then splits and becomes inconvenient. >
*It's not splitting. With a minor digression, we're still discussing whether the universe is really flat. In your diagram I don't see anything relating to an infinite spatial extent, which you earlier agreed is a characteristic of a flat universe. AG * > > LC > > > https://physics.stackexchange.com/questions/257476/how-did-the-universe-shift-from-dark-matter-dominated-to-dark-energy-dominate/257542#257542 > > > > -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/everything-list/a2bc1acc-bb3e-483d-b4ba-f5b2c3a541dd%40googlegroups.com.
