On 2/18/2017 10:18 AM, John Clark wrote:
On Fri, Feb 17, 2017 at 11:19 PM, Brent Meeker <meeke...@verizon.net
<mailto:meeke...@verizon.net>> wrote
>
The cosmological constant appears as an integration constant in
solutions to Einstein's equations.
Yes, so mathematically it could have any value including zero.
>
It would be good to know more about the CC, but we actually "know"
more about it than we do about dark matter.
The Cosmological Constant amount
s
to a repulsive effect that comes from space itself, and
you can set that constant to anything and mathematically the field
equations of General Relativity would still work just fine.
Originally Einstein saw no physical reason for that additional
complication so he set it to zero. But then he noticed that if it
was zero the universe could not be stable,
No, he saw that it could not be in equilibrium. He put in the CC and
gave it a value that balanced the gravitational attraction of the
observed matter so that the system was in equilibrium. However, he
didn't notice that it was an unstable equilibrium - a very elementary
mistake.
it must be expanding or contracting
and at the time everybody including Einstein thought the universe
was stable so he set it to a non zero value and the cosmological
constant was born. However just a few years later Hubble found the
universe was expanding, so Einstein thought the cosmological
constant no longer had a purpose and said that changing it from zero
was the greatest mistake of his life.
Right. Einstein was a genius who was so smart that when the thought he
made a mistake, he was wrong.
Then people working with quantum mechanics found that empty space
should indeed have a repulsive effect, but the numbers were huge,
gigantic astronomical, so large that the universe would blow itself
apart in
much
less than a billionth of a trillionth of a nanosecond. This was
clearly a nonsensical result but most felt that once a quantum theory
of gravity was discovered a way would be found to cancel this out and
the true value of the cosmological constant would be zero.
But then
just a few years ago it was observed that the universe is not just
expanding but accelerating, so now theoreticians must find a way to
cancel out, not the entire cosmological constant, but the vastly more
difficult task of canceling it all out *EXCEPT* for one part in
10^120. There are only about 10^90 atoms in the observable universe.
Nobody has a clue how to do this.
Actually there are several ideas about how to do this - but none that
have been worked out to the point of being testable. One is my friend
Vic Stenger's idea that one should take the Bekenstein bound on entropy
seriously and apply it to the Hubble volume. The 10^120 number comes
from assuming that each quantum field contributes zero-point energy down
to de Broglie wavelengths as small as the Planck length, L. But the
holographic principle can yield a value close the the observed.
http://journals.aps.org.secure.sci-hub.ac/prl/pdf/10.1103/PhysRevLett.89.081301
This is not generally accepted at "the solution" because it seems to
imply the wrong equation of state for inflation; but there may be ways
around this:
https://arxiv.org/pdf/hep-th/0403052.pdf
Sean Carroll has considered this in his review article
https://arxiv.org/abs/astro-ph/0004075v2
/More generally, it is now understood that (at least in some
circumstances) string theory//
//obeys the “holographic principle”, the idea that a theory with gravity
in D dimensions//
//is equivalent to a theory without gravity in D−1 dimensions [148,
149]. In a holographic//
//theory, the number of degrees of freedom in a region grows as the area
of its boundary,//
//rather than as its volume. Therefore, the conventional computation of
the cosmological//
//constant due to vacuum fluctuations conceivably involves a vast
overcounting of degrees//
//of freedom. We might imagine that a more correct counting would yield
a much smaller//
//estimate of the vacuum energy [150, 151, 152, 153], although no
reliable calculation//
//has been done as yet
/Brent/
/
John K Clark
--
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 everything-list+unsubscr...@googlegroups.com
<mailto:everything-list+unsubscr...@googlegroups.com>.
To post to this group, send email to everything-list@googlegroups.com
<mailto:everything-list@googlegroups.com>.
Visit this group at https://groups.google.com/group/everything-list.
For more options, visit https://groups.google.com/d/optout.
--
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 everything-list+unsubscr...@googlegroups.com.
To post to this group, send email to everything-list@googlegroups.com.
Visit this group at https://groups.google.com/group/everything-list.
For more options, visit https://groups.google.com/d/optout.
