An international team of researchers led by University of Portsmouth Professor Gong-Bo Zhao has found that the nature of dark energy may not be the cosmological constant introduced by Albert Einstein a century ago.
This Hubble image shows the inner region of Abell 1689, an immense cluster of galaxies located 2.2 billion light-years away. The cluster’s gravitational field is warping light from background galaxies, causing them to appear as arcs. Dark matter in Abell 1689, which represents about 80% of its mass, is mapped by plotting these arcs. Image credit: NASA / ESA / JPL-Caltech / Yale / CNRS.
Revealing the nature of dark energy is one of key goals of modern science.
The physical property of dark energy is represented by its Equation of State (EoS), which is the ratio of pressure and energy density of dark energy.
In the traditional Lambda-Cold Dark Matter model, dark energy is essentially the cosmological constant, i.e., the vacuum energy, with a constant EoS of minus 1. In this model, dark energy has no dynamical features.
In 2016, the SDSS-III (BOSS) Collaboration performed a successful measurement of the Baryonic Acoustic Oscillations at multiple cosmic epochs with a high precision.
Based on this measurement, Professor Zhao and colleagues found an evidence of dynamical dark energy at a significance level of 3.5 sigma.
This suggests that the nature of dark energy may not be the vacuum energy, but some kind of dynamical field, especially for the quintom model of dark energy whose EoS varies with time and crosses the minus 1 boundary during evolution.
The cosmological constant (illustrated by the straight yellow line) is introduced to explain the accelerated expansion of the Universe (shown as the expanding blue cone) due to the presence of dark energy. The new study instead suggests that the contribution of dark energy to this expansion is time-dependent (grey curve). The uncertainty of this time dependency is also shown (blue shaded area). Image credit: Gong-Bo Zhao et al.
“We are excited to see that current observations are able to probe the dynamics of dark energy at this level, and we hope that future observations will confirm what we see today,” Professor Zhao said.
“Since its discovery at the end of last century, dark energy has been a riddle wrapped in an enigma,” added co-author Professor Bob Nichol, Director of the Institute of Cosmology and Gravitation at the University of Portsmouth.
“We are all desperate to gain some greater insight into its characteristics and origin. Such work helps us make progress in solving this 21st century mystery.”
“The dynamics of dark energy needs to be confirmed by next-generation astronomical surveys,” the scientists said.
The research is published in the journal Nature Astronomy.
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Gong-Bo Zhao et al. 2017. Dynamical dark energy in light of the latest observations. Nature Astronomy 1: 627-632; doi: 10.1038/s41550-017-0216-z
