On Wed, Apr 9, 2025 at 7:42 PM Alan Grayson <[email protected]> wrote:
*> Einstein initially set the CC to zero, and when it was determined that > his field equations predicted an expanding universe, he reset it to a > positive value, presumably to make gravity stronger so the effect of the > positive CC and gravity would produce a static universe. In this situation, > the CC would augment the attractive force of gravity.* > *In 1915 when Einstein originally published his field equation for general relativity** the left-hand part of the equation is a 4D tensor that described the curvature of spacetime, and the curvature of space time is what we experience as gravity. The right hand side of the equation is a tensor that specifies the distribution of all forms of energy and momentum in a given region of spacetime. Later when Einstein added a cosmological constant he put it on the left-hand side of the equation, the geometry part. He thought that additional term spoiled the mathematical beauty of his original equation but he thought he had no choice because all his astronomer friends insisted that the universe was stable. When astronomers changed their mind about that Einstein immediately dropped the entire cosmological constant idea. * *Much later in the late 1990s astronomers discovered that the universe is accelerating and the cosmological constant idea came back, but this time they put it on the other side of the equation and gave it a minus sign. Doing this is mathematically equivalent but it has a very different physical interpretation, now it's not a modification of spacetime geometry, instead it's a form of energy intrinsic to the vacuum, because of that negative sign its pressure is negative and equal to the energy density. * > *I've never seen any argument affirming that gravity IS related to the > expansion, other than slowing its rate * *I think what you really want to know is why negative pressure produces a repulsive effect. The acceleration of the universe’s expansion is described by:* *-a [(-4πG/3)(ρ+3P)] * *where ρ is the energy density, P is the pressure, G is Newton's gravitational constant, and a is just a scale factor. The 3 is in there because there are three spatial dimensions, so pressure contributes to the push or pull of gravity depending on if the pressure is positive or negative. * *John K Clark See what's on my new list at Extropolis <https://groups.google.com/g/extropolis>* nae -- 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 visit https://groups.google.com/d/msgid/everything-list/CAJPayv0fdquPfqJLFWCw7eot0p7aoKCUuqsZrakNr8AdVD7ktA%40mail.gmail.com.
