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

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The universe's expansion is a mystery primarily due to the acceleration observed, which contradicts expectations based on gravitational attraction. Current theories suggest that this acceleration is not due to weakening gravity but rather the influence of dark energy or quintessence, both linked to a non-zero cosmological constant in general relativity. The discussion emphasizes that as galaxies move apart, gravitational forces diminish, which should not lead to increased acceleration. There is speculation about an unknown force at a cosmic scale that could be responsible for this acceleration. Understanding these phenomena remains a significant challenge in cosmology.
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 'Matter density right after the decoupling'

https://en.wikipedia.org/wiki/Recombination_(cosmology) Was a matter density right after the decoupling low enough to consider the vacuum as the actual vacuum, and not the medium through which the light propagates with the speed lower than ##({\epsilon_0\mu_0})^{-1/2}##? I'm asking this in context of the calculation of the observable universe radius, where the time integral of the inverse of the scale factor is multiplied by the constant speed of light ##c##.
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Thread 'Hubble Constant, Dark Energy and the Expanding Universe'

Why was the Hubble constant assumed to be decreasing and slowing down (decelerating) the expansion rate of the Universe, while at the same time Dark Energy is presumably accelerating the expansion? And to thicken the plot. recent news from NASA indicates that the Hubble constant is now increasing. Can you clarify this enigma? Also., if the Hubble constant eventually decreases, why is there a lower limit to its value?
View full post »

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