Re: Big Crunch

On Thu, 27 Mar 2025 10:15:19 -0400, Kestrel Clayton
<richZIG.e.clayZIGton@gmail.com> wrote:

On 27-Mar-25 03:34, jillery wrote:

On Wed, 26 Mar 2025 13:26:51 -0500, RonO <rokimoto557@gmail.com>
wrote:
 

On 3/26/2025 9:41 AM, erik simpson wrote:

Sorry, a senior moment in my previous reply. I wrote about something I
was thinking about while "replying" to something else.  The new
information about the possible evolution of dark energy is a work in
progress.  This website is probably the best source of information at
present.
>
https://www.desi.lbl.gov/2025/03/19/desi-dr2-results-march-19-guide/
>
My degree is in astrophysics, but concentrating on stellar evolution,
not cosmology, and decades ago.  I am reasonably competent with general
relativity, no expert. The results of further research in dark energy
will probably be published in the Astrophysical Journal, Monthly Reviews
of the Royal Astronomical Society or Physical Reviews.  They will be
pretty hard sledding for the lay reader.
>

So far I have found estimates that our observable universe (only a
fraction of the whole universe) is 93 billion light years in diameter.
The estimate is that when the initial inflation ended, 380,000 years
after the Big Bang, the current visible part of the universe was only
around 85 million light years in diameter after the initial inflation.
The rate of expansion slowed down, and the acceleration of the rate of
expansion due to dark energy didn't start until 7.7 billion years ago.
Before this time the rate of expansion was slowing down.  I can't find
an estimate for the size of the universe when the dark energy
acceleration started.  In over 13 billion years the visible portion of
the universe has expanded from a diameter of only 85 million light years
to a diameter of 93 billion light years.  Over a thousand fold increase
in diameter after the initial inflation and the volume is increasing at r^3.
>
Why was it ever assumed that dark energy would maintain a constant
density in an expanding universe?  Dark energy would need to increase at
the same rate as the expansion of space, or they would have had to
believe that dark energy effects were independent of the volume of space
with a specific amount of dark energy. Originally the density of dark
energy in space would have been something like 1:1.  The effect of dark
energy on increasing the volume of space would have to be the same when
the ratio increased to 1:2.

 
 
My understanding is the phenomena collectively referred to as dark
energy appears to be intrinsic to spacetime itself.  If so, as the
volume of spacetime increases, so too does the amount of dark energy
within that volume by the same amount aka a constant proportional to
volume.
 
 

Is the milky way galaxy expanding at the same rate as the space between
galaxies?  We can see galaxies from 8 billion years ago with the Webb
telescope, but the images would be of galaxies 8 billion years in the
past.  Today how large would they be?  They believe that our galaxy is
13.6 billion years old.  How large was it 13 billion years ago?
>
Ron Okimoto

 
 
Dark energy density is very low.  Objects which are currently
gravitationally bound don't have enough spacetime among them to
overcome their mutual gravity.  This includes not just galaxies, but
even clusters of galaxies.

>
Quite true! Although if the scale factor of the universe continues to
accelerate, eventually galaxy clusters, galaxies, and even solar systems
are spread so far apart they are no longer gravitationally bound.
Eventually the stretching of space will overwhelm even the strong
nuclear force, and rip atoms apart.

I confess to not understanding Big Rip cosmology.  If the amount of
dark energy between objects is relative to the distances between them,
then even if the rest of the cosmos were to expand to infinity, the
spacetime between gravitationally bound objects would still remain the
same and so insufficient to overcome their mutual gravity.

The good news is about a dozen OTHER things will have happened by then
to make the universe utterly inhospitable to life as we know it, so
worrying about the Big Rip (as they call it) is somewhat like trying to
figure out what you'll wear for your ten thousandth birthday party, when
you are presently 22 and suffering a heart attack, while while falling
from an airplane without a parachute, into a volcano.

--
To know less than we don't know is the nature of most knowledge