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Old 29-11-2019, 11:13 AM
gary
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Does dark energy exist? Evidence for anisotropy of cosmic acceleration

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 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.

Quote:
Originally Posted by Oxford University Department of Physics 20 Nov 2019
The observed acceleration of the Hubble expansion rate has been attributed to a mysterious ‘dark energy’ which supposedly makes up about 70% of the universe. Professor Subir Sarkar from the Rudolf Peierls Centre for Theoretical Physics, Oxford along with collaborators at the Institut d’Astrophysique, Paris and the Niels Bohr Institute, Copenhagen have used observations of 740 Type Ia supernovae to show that this acceleration is a relatively local effect – it is directed along the direction we seem to be moving with respect to the cosmic microwave background (which exhibits a similar dipole anisotropy). While the physical reason for this acceleration is unknown, it cannot be ascribed to dark energy which would have caused equal acceleration in all directions.
Quote:
Originally Posted by Oxford University Department of Physics 20 Nov 2019
I (Subar Sarkar), along with Jacques Colin and Roya Mohayaee (Institut d’Astrophysique, Paris) and Mohamed Rameez (Niels Bohr Institute, Copenhagen), set out to examine whether dark energy really exists. The primary evidence – rewarded with the 2011 Nobel prize in physics – concerns the “discovery of the accelerated expansion of the universe through observations of distant supernovae” in 1998 by two teams of astronomers. This was based on observations of about 60 Type Ia supernovae but meanwhile the sample has grown and in 2014 the data was made available for 740 objects scattered over the sky (Joint Lightcurve Analysis catalogue). We looked to see if the inferred acceleration of the Hubble expansion rate is uniform over the sky. First, we worked out the supernova redshifts and apparent magnitudes as measured (in the heliocentric system), undoing the corrections that had been made in the JLA catalogue for local ‘peculiar’ (non-Hubble) velocities. This had been done to determine their values in the CMB frame in which the universe should look isotropic – however previous work by our team had shown that such corrections are suspect because peculiar velocities do not fall off with increasing distance, hence there is no convergence to the CMB frame even as far out as a billion light years.

Dark energy

‘When we then employed the standard maximum likelihood estimator statistic to extract parameter values, we made an astonishing finding. The supernova data indicate, with a statistical significance of 3.9σ, a dipole anisotropy in the inferred acceleration (see figure) in the same direction as we are moving locally, which is indicated by a similar, well-known, dipole in the CMB. By contrast any isotropic (monopole) acceleration which can be ascribed to dark energy is 50 times smaller and consistent with being zero at 1.4σ. By the Bayesian information criterion, the best-fit to the data has in fact no isotropic component. We showed that allowing for evolution with redshift of the parameters used to fit the supernova light curves does not change the conclusion – thus refuting previous criticism of our method.

‘Our analysis is data-driven but supports the theoretical proposal due to Christos Tsagas (University of Thessaloniki) that acceleration may be inferred when we are not Copernican observers, as is usually assumed, but are embedded in a local bulk flow shared by nearby galaxies, as is indeed observed. This is unexpected in the standard cosmological model and the reason for such a flow remains unexplained. But independently of that it appears that the acceleration is an artefact of our local flow so dark energy cannot be invoked as its cause.
Full press release :-
https://www2.physics.ox.ac.uk/news/2...c-acceleration

Letter to the Editor, "Evidence for anisotropy of cosmic acceleration"
by Sarkar et. al. :-
https://www.aanda.org/articles/aa/fu...a36373-19.html
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Old 29-11-2019, 11:42 AM
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So, there will be no Big Rip...

Good :-)
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Old 29-11-2019, 05:58 PM
DarkArts
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So, dark energy is a statistical blip. OK, I'm prepared to believe that.

I think the real winner here was science.
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Old 05-01-2020, 11:29 AM
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Shiraz (Ray)
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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 . 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-ener...pernovae-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

Last edited by Shiraz; 05-01-2020 at 06:21 PM.
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