Re: Big Crunch

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.
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