On 7/21/2025 5:40 AM, John Clark wrote:
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 beforethe 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 averagedistancebetween 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 universeis NOT smallertoday,except for
the era of inflation which ended about 10^-33 seconds after the start
of the Big Bang, it's largertoday 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. *
I think there may be an ambiguity in the use of "rate of expansion".
There is an observed rate of recession of distant galaxies measured by
redshift and used to calculate a velocity between us an some galaxy.
Then there is the Hubble constant which is a percentage rate of
expansion which is just an inverse time.
/> since the measured value in ancient times is much larger than
it is today, /
*We observe light from a distant galaxy todaybut 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 ener_gy, 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.*
But if the total energy, gravitational potential plus sand kinetic, is
zero then the sphere will expand forever but with a rate asymptotically
approaching zero.
Brent
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 ifthe 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
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