Dark energy may not be a constant

ack

VIP/Donor & WBF Founding Member
May 6, 2010
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#1
There are recent indications that dark energy may not be constant, potentially dealing a blow to Einstein’s cosmological constant, and potentially implying that dark energy is getting stronger, perhaps to a point where the universe may eventually experience the Big Rip, whereby all regular matter is destroyed.

Two fascinating articles here, with Forbes going deeper


http://www.forbes.com/sites/startsw...-which-would-lead-to-a-revolution-in-physics/

Recently, however, a team of scientists has begun using X-ray emitting quasars, which are much brighter and, hence, visible at even earlier times: when the Universe was only one billion years old. In an interesting new paper, scientists Guido Risaliti and Elisabeta Lusso use quasars as a standard candle to go farther back than we ever have in measuring the nature of dark energy. What they found is still tentative, but astounding nonetheless.

Using data from around 1,600 quasars, and a new method for determining the distances to them, they found a strong agreement with the supernova results for quasars from the past 10 billion years: dark energy is real, about two thirds of the energy in the Universe, and appears to be a cosmological constant in nature.

But they also found more distant quasars, which showed something unexpected: at the greatest distances, there’s a deviation from this “constant” behavior. Risaliti has written a blog post here, detailing the implications of his work, including this gem:
Our final Hubble Diagram gave us completely unexpected results: while our measurement of the expansion of the Universe was in agreement with supernovae in the common distance range (from an age of 4.3 billion years up to the present day), the inclusion of more distant quasars shows a strong deviation from the expectations of the standard cosmological model! If we explain this deviation through a dark energy component, we find that its density must increase with time.​

https://www.sciencealert.com/the-un...ught-it-might-take-new-physics-can-explain-it
 

astrotoy

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May 25, 2010
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#2
Thanks, ack. The Forbes article is very well done and the graphs make some pretty difficult stuff much clearer. One major way in which astronomers have tried to tackle the big problems in cosmology is illustrated in the 3rd illustration from the top of the article. In this graph three completely different measurements are laid on the graph and where they intersect shows a result that is consistent with all three measurements, and consistent with a "flat" w=-1 universe, that is one that is neither positively nor negatively curved and the current standard model of the universe. However, the third illustration from the bottom is another graph that shows different measurements that do not all overlap, showing particularly the yellow data not consistent with w=-1 (the current standard model of the universe)

What the last, most controversial, observations show is data from objects that are even farther away (and therefore closer in time to the Big Bang, since farther away means further back in time) and appear to indicate that the universe may be accelerating its expansion, rather than expanding at a constant rate or slowing its expansion. However, these observations are the most difficult to make and have the greatest uncertainty. So at this point they are suggestive, but by no means conclusive. The author states that if the observations are correct, then the universe would have a negative curvature and the universe would end in a "Big Rip" where everything would be torn apart by the dark energy (as opposed to a "Big Crunch" where the universe has a positive curvature and would eventually collapse upon itself.)

One major problem which has always plagued astronomers is that for the most distant objects, one often does not have independent ways to measure the distance to these objects and must assume that they are the same intrinsic brightness (or at least cluster around the same intrinsic brightness) even though some are farther away and therefore younger than others.

Very interesting, but the final conclusions are still very speculative.

Larry (your astronomer friend)
 
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ack

VIP/Donor & WBF Founding Member
May 6, 2010
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Boston, MA
#3
Very good summary of what's reported!
 

ack

VIP/Donor & WBF Founding Member
May 6, 2010
5,545
195
63
Boston, MA
#4
 

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