Using data from the European Space Agency's X-ray space observatory,
XMM-Newton, researchers led by Guido Risaliti of Università di Firenze,
Italy, and Elisabeta Lusso of Durham University, UK, have pegged the
distances to thousands of distant quasars, thus enabling them to be used
as 'standard candles'.
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Originally Posted by ESA
Looking into this previously poorly explored period of cosmic history with the help of quasars, the astronomers have revealed a possible tension in the standard model of cosmology, which might require the addition of extra parameters to reconcile the data with theory.
"One of the possible solutions would be to invoke an evolving dark energy, with a density that increases as time goes by," says Guido.
Incidentally, this particular model would also alleviate another tension that has kept cosmologists busy lately, concerning the Hubble constant – the current rate of cosmic expansion. This discrepancy was found between estimates of the Hubble constant in the local Universe, based on supernova data – and, independently, on galaxy clusters – and those based on Planck's observations of the cosmic microwave background in the early Universe.
"This model is quite interesting because it might solve two puzzles at once, but the jury is definitely not out yet and we'll have to look at many more models in great detail before we can solve this cosmic conundrum," adds Guido.
The team is looking forward to observing even more quasars in the future to further refine their results. Additional clues will also come from ESA's Euclid mission, scheduled for a 2022 launch to explore the past ten billion years of cosmic expansion and investigate the nature of dark energy.
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ESA press release here :-
http://sci.esa.int/xmm-newton/61068-...mic-expansion/
"Cosmological constraints from the Hubble diagram of quasars at high redshifts" by G. Risaliti & E. Lusso in Nature Astronomy (subscription required) :-
https://www.nature.com/articles/s41550-018-0657-z
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Originally Posted by Abstract
The concordance model (Λ cold dark matter (ΛCDM) model, where Λ is the cosmological constant) reproduces the main current cosmological observations assuming the validity of general relativity at all scales and epochs and the presence of CDM and of Λ, equivalent to dark energy with a constant density in space and time.
However, the ΛCDM model is poorly tested in the redshift interval between the farthest observed type Ia supernovae and the cosmic microwave background.
We present measurements of the expansion rate of the Universe based on a Hubble diagram of quasars.
Quasars are the most luminous persistent sources in the Universe, observed up to redshifts of z ≈ 7.5.
We estimate their distances following a method developed by our group, based on the X-ray and ultraviolet emission of the quasars.
The distance modulus/redshift relation of quasars at z < 1.4 is in agreement with that of supernovae and with the concordance model.
However, a deviation from the ΛCDM model emerges at higher redshift, with a statistical significance of ~4σ.
If an evolution of the dark energy equation of state is allowed, the data suggest dark energy density increasing with time.
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