I Is there any evidence of a universe expansion jerk

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Evidence indicates that the expansion of the universe has accelerated over the past few billion years, with the rate of acceleration not being constant due to varying proportions of dark energy and matter density. As the universe expands, dark energy remains relatively constant while the density of matter decreases, leading to an increase in the rate of expansion. Although the change in acceleration, or "jounce," is acknowledged, measuring this change is complicated by noise in the data. Current measurements do not definitively indicate whether dark energy itself has varied over time, contributing to ongoing discussions about the Hubble tension. Understanding these dynamics is crucial for further insights into cosmic expansion.
arusse02
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As stated in the question--is there any evidence of a non zero rate of change of the acceleration of the expansion of the universe. What is the evidence for it and is it negative or positive? Also if there is what would it take to determine if there was a jounce?
 
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arusse02 said:
is there any evidence of a non zero rate of change of the acceleration of the expansion of the universe

Yes. The expansion only started accelerating a few billion years ago (based on the curve of redshift vs. brightness and angular size for distant galaxies), so the rate of acceleration has obviously not been constant. The reason it has not been constant is that the relative proportion of dark energy vs. other types of energy density has not been constant, because the density of dark energy remains the same as the universe expands (as far as we know), while the density of matter (ordinary and dark) decreases.
 
arusse02 said:
As stated in the question--is there any evidence of a non zero rate of change of the acceleration of the expansion of the universe. What is the evidence for it and is it negative or positive? Also if there is what would it take to determine if there was a jounce?
Also, bear in mind that values are easier to measure than how much those values change: looking at the change instead of the value adds noise.

As PeterDonis mentioned, the rate has changed over time for sure because the matter density drops as the universe expands. Earlier, when matter density was higher, the rate at which objects moved apart was decelerating. As dark energy has become more dominant (because it doesn't drop in density much if at all as the universe expands), the distances between objects have started accelerating.

What we can't tell right now is whether or not dark energy has changed over time: that measurement is far too noisy at the moment.
 
One possible explanation for the Hubble tension is that dark energy is not a constant
 

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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