Why is the acceleration of the universe's expansion a mystery?

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SUMMARY

The discussion centers on the mystery of the universe's accelerating expansion, with consensus among scientists that the universe is flat, open, or closed. The acceleration is not attributed to weakening gravitational forces but rather to phenomena such as dark energy and quintessence, which involve a non-zero cosmological constant in general relativity. The possibility of an unknown force influencing this acceleration is also acknowledged, aligning with current observational evidence.

PREREQUISITES
  • Understanding of general relativity and the cosmological constant
  • Familiarity with dark energy and quintessence theories
  • Basic knowledge of cosmic expansion and gravitational forces
  • Awareness of observational cosmology techniques
NEXT STEPS
  • Research the implications of dark energy on cosmic structure formation
  • Explore the concept of quintessence and its potential effects on the universe
  • Study the observational evidence supporting the accelerating expansion of the universe
  • Investigate alternative theories to explain cosmic acceleration beyond current models
USEFUL FOR

Astronomers, physicists, cosmologists, and anyone interested in understanding the dynamics of the universe's expansion and the underlying forces at play.

Jack
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As far as I am aware scientists agree that the universe is flat, open or closed. However, if galaxies get further away from each other the gravitational attraction between them would decrease as the universe expands. Why is the universe speeding up not a real possibility?
 
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Current theory says the expansion is speeding up. The explanation is not based on weakening gravity, but rather some other effect. Yhere are two theories, called (1)dark energy and (2)quintesscence. The firsts (possibly both) seems to be based on a non-zero cosmological constant in general relativity.
 
Originally posted by Jack
As far as I am aware scientists agree that the universe is flat, open or closed. However, if galaxies get further away from each other the gravitational attraction between them would decrease as the universe expands. Why is the universe speeding up not a real possibility?

When gravity decreases, it does not create acceleration of stellar bodies. So it does not speed expansion.

It is possible that an unknown force exists at the large scale of the universe that is causing acceleration of stellar bodies. This is within the range of current observational evidence.
 

Thread 'Estimates — True Size of the Universe?'

I always thought it was odd that we know dark energy expands our universe, and that we know it has been increasing over time, yet no one ever expressed a "true" size of the universe (not "observable" universe, the ENTIRE universe) by just reversing the process of expansion based on our understanding of its rate through history, to the point where everything would've been in an extremely small region. The more I've looked into it recently, I've come to find that it is due to that "inflation"...
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