On Sun, Jul 20, 2025 at 4:59 PM Alan Grayson <[email protected]> wrote:
* > I forget, but there's another important prediction of inflation, which > is verified in the CMB. Do you recall what it is? AG * *Inflation correctly predicted that the intensity of the CMB should be almost constant from one place to another, but not perfectly constant, it should change in about one part in 100,000. It's not perfect because of Heisenberg's Uncertainty Principle, tiny changes in the very very early universe were enormously magnified by inflation from the sub microscopic level to cosmic scale. Inflation also explains why at the largest scale space is flat, inflation ironed out any wrinkles that were produced before the era of inflation. It even explains why we have never seen a Magnetic Monopole even though many particle physics theories predicted they should be stable and have been produced in huge numbers back when the entire universe was super hot, inflation diluted their concentration so much that now there may only be one Magnetic Monopole in the entire visible universe, and perhaps none at all. * *>>back then the average distance between galaxies was less than it is now, > and the rate at which that distance was increasing was also smaller. * > > > *> I really don't see how the rate of expansion can be smaller today,* > *The RATE of expansion of the universe is NOT smaller today, except for the era of inflation which ended about 10^-33 seconds after the start of the Big Bang, it's larger today than it has ever been. The average distance between galaxies was less 5 billion years ago than it is today, and the RATE of increase of those galactic distances was also less 5 billion years ago than it is today. I don't understand why you think those two statements are contradictory. * *> since the measured value in ancient times is much larger than it is > today, * > *We observe light from a distant galaxy today but it took that light 5 billion years to reach our telescope, and during that long journey the expansion of space has distorted that light. If we want to know what things were like back then we have to understand that distortion and take it into account. * > *>> if I'm being pedantic it's a little more complicated than that. In the > late 1990s it was discovered that during the first 9 billion years of the > universe's existence the rate of expansion of the universe was indeed > slowing down just as you'd expect because gravity is attractive but then, > about 5 billion years ago, there was a cosmic "jerk" (the rate of increase > of acceleration) and the rate at which the universe was expanding started > to increase. * > *This can be explained if a property of empty space is that it has a > negative pressure which according to Einstein causes space to expand, * > > > *> Can you say more about negative pressure and its cause? AG* > *The gravitational potential energy of a sphere of particles of matter like sand is always negative, this is true in Newtonian Physics and remains true in General Relativity, so the gravitational potential energy of a sphere of particles of mass-energy M and radius R is PE= (-G*M^2)/R where G is the gravitational constant. It’s important to note that this is negative energy, so the larger R gets the closer the potential energy gets to zero, and if it was at infinity it would be precisely zero. * *If the sphere expands and is made of sand, which is normal matter, then M stays the same but R increases, so the gravitational potential energy becomes less negative and more positive, and that means it's uphill; It would take an external expenditure of work to do that, so if you let the sphere go to rest it would fall inward as you'd expect.* However if the sphere is primarily made of empty space and *IF* empty space contains energy, which Einstein's General Theory Of Relativity allows for but does not insist on, then things would be different because unlike an expanding sphere made of sand, the density of mass/energy inside an expanding sphere of empty space would NOT decrease with expansion. So if the sphere expands then, although R increases, M^2 increases even more, so the overall gravitational potential energy becomes larger, and thus more negative.* Thus if the vacuum contains negative energy then as the sphere increases in size it becomes more negative and that means expansion is downhill, and so no work is used but instead work is produced. So in any universe in which vacuum energy dominates that universe will expand, it will fall outward and accelerate. * *Most physicists (but not all, see the next paragraph) think vacuum energy is what makes our universe accelerate. You might ask if the sphere gets larger what makes it get larger, where did that mass/energy come from? The answer is: It comes from the gravitational energy released as the sphere of vacuum energy falls outward. So at any point in this process if you add up all the positive kinetic energy and energy locked up in matter (remember E=MC^2) of the universe and all the negative potential gravitational energy of the universe you always get precisely zero.* *As I said a minority of physicists reject the idea that Dark Energy is an intrinsic part of space itself and think it's really caused by a scalar (something that has a magnitude but no direction) quantum field that fills the entire universe, and they call that field "Quintessence". If Dark Energy is a field then that opens up the possibility of it changing with time. * *In just the last few months astronomers have found hints that Dark Energy might be getting weaker. The evidence is good enough to be interesting but not good enough to claim a discovery; IF it turns out to be true then Dark Energy cannot be an intrinsic part of space and some form of Quintessence must be true. But that's a big "IF". * *John K Clark See what's on my new list at Extropolis <https://groups.google.com/g/extropolis>* tbi -- 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/CAJPayv1VPXf6OWv3skJvwVKL9SJP9cVvmYJfNatf14tQSe7uiw%40mail.gmail.com.
