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. 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/314d8ced-6c95-4754-aa1e-f4ab61cf328e%40googlegroups.com.
