Does dark energy exist? Evidence for anisotropy of cosmic acceleration

[gary]

The Cosmic Microwave Background (CMB) has an extremely uniform
temperature of 2.725 Kelvin.

However, there is a small apparent gradient from 0.0035 Kelvin below
average in the direction of the constellation Aquarius, to 0.0035 Kelvin
above average in the direction of the constellation Leo across the sky.

Soon after the CMB was discovered, it was realized that this apparent
dipole was simply the result of our Galaxy and in fact the Local Group
of Galaxies, moving at 600 km/sec with respect to the CMB radiation
towards the Great Attractor.

Once the cosmic microwave background dipole is removed, the variation in
the temperature of the CMB is astonishingly uniform with variations of only
one part in ten thousand.

Now an analysis by three theoretical physicists (https://www2.physics.ox.ac.uk/news/2019/11/27/evidence-for-anisotropy-of-cosmic-acceleration) of Type Ia supernovae
data looked to see if the inferred acceleration of the Hubble expansion
rate is uniform over the sky.

If dark energy exists, one would expect the force to be isotropic - that is
the same value when measured in all directions.

However, the supernova data indicates a dipole anisotropy in the inferred
acceleration in the same direction as we are moving locally. That is
the same direction as we see are moving with respect the CMB.





Full press release :-
https://www2.physics.ox.ac.uk/news/2019/11/27/evidence-for-anisotropy-of-cosmic-acceleration

Letter to the Editor, "Evidence for anisotropy of cosmic acceleration"
by Sarkar et. al. :-
https://www.aanda.org/articles/aa/full_html/2019/11/aa36373-19/aa36373-19.html

[bojan]

So, there will be no Big Rip...

Good :-)

[DarkArts]

So, dark energy is a statistical blip. OK, I'm prepared to believe that.

I think the real winner here was science. :einstein:

[Shiraz]

thanks Gary. that was a very interesting read. Follows on from a related paper by the same group in ~2016.

Of course it is not over yet - not even close :lol:. I understand that Wiggle Z for example found acceleration based on a different technique and the assumptions and conclusions in the Oxford paper have been somewhat taken apart in
https://arxiv.org/pdf/1912.02191.pdf ... maybe it is not yet time to give up on dark energy (whatever it is).

and from https://physicsworld.com/a/dark-energy-debate-reignited-by-controversial-analysis-of-supernovae-data/

"By re-converting the red-shift data back to their raw “heliocentric” form as best they could, and plugging the data into a model, Sarkar and colleagues found that the monopole component – the universal acceleration – yielded just a 1.4σ signal, while the dipole – presumably a local motion – was present at 3.9σ. What’s more, they found that this dipole lines up with the one in the CMB.

“If you look at supernovae in only a small part of the sky, it would look like you had cosmic acceleration,” says Sarkar. “But we are saying that it is just a local effect, that we are non-Copernican observers. It has nothing to do with the overall dynamics of the universe and therefore nothing to do with dark energy.”

According to Riess, however, the supernovae data used by Sarkar’s group are out of date. He says that he and some colleagues, including D’Arcy Kenworthy of Johns Hopkins University, plugged data from a sample of about 1300 supernovae with lower systematic uncertainties into the model used in the latest work. The results, he says, were unambiguous, with the existence of a dipole rejected at more than 4σ and cosmic acceleration confirmed at over 6σ.

More importantly, says Riess, the objections against Sarkar and colleagues’ original statistical analysis still stand, as do the criticisms of neglecting other data. “The evidence for cosmic acceleration and dark energy are much broader than only the supernovae Ia sample, and any scientific case against cosmic acceleration needs to take those into account,” he says.

Even here, however, Sarkar insists the evidence is lacking. He claims that the data on baryon acoustic oscillations are too sparse to chose between models with and without cosmic acceleration, while dark energy would have been too weak to leave a significant imprint in the early universe. “The CMB does not directly measure dark energy,” he says. “That is a widely propagated myth.”


cheers and thanks, Ray