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