An international research team led by Dr. Massimo Viola from the Leiden Observatory and Dr. Hendrik Hildebrandt from the Argelander-Institut für Astronomie has gained fresh insight into the nature of dark matter.
This map of dark matter was obtained from data from the KiDS survey, using ESO’s VLT Survey Telescope. It reveals an expansive web of dense (light) and empty (dark) regions. This image is one out of five patches of the sky observed by KiDS. The invisible dark matter is seen rendered in pink, covering an area of sky around 420 times the size of the full Moon. This image reconstruction was made by analyzing the light collected from over 3 million galaxies more than 6 billion light-years away. The observed galaxy images were warped by the gravitational pull of dark matter as the light traveled through the Universe. Some small dark regions, with sharp boundaries, appear in this image. They are the locations of bright stars and other nearby objects that get in the way of the observations of more distant galaxies and are hence masked out in the map as no weak-lensing signal can be measured in these areas. Image credit: Kilo-Degree Survey Collaboration / H. Hildebrandt & B. Giblin / ESO.
Dr. Viola, Dr. Hildebrandt and their colleagues used images from the Kilo Degree Survey (KiDS), made with ESO’s VLT Survey Telescope in Chile, to study how the light from nearly 15 million galaxies was affected by the gravitational influence of matter on the largest scales in the Universe.
The new results, published online Nov. 2 in the Monthly Notices of the Royal Astronomical Society (arXiv.org preprint), suggest that dark matter is less dense and more smoothly distributed throughout space than previously thought.
The findings contradict previous predictions from a survey of the far-off Universe, representing a point in time soon after the Big Bang, imaged by ESA’s Planck satellite.
“Unraveling what has happened since the Big Bang is a complex challenge, but by continuing to study the distant skies, we can build a picture of how our modern Universe has evolved,” said team member Prof. Catherine Heymans, from the University of Edinburgh, UK.
By exploiting the exquisite image quality available to the VLT Survey Telescope, and using innovative computer software, the scientists were able to carry out one of the most precise measurements ever made of an effect known as cosmic shear.
This is a subtle variant of weak gravitational lensing, in which the light emitted from distant galaxies is slightly warped by the gravitational effect of large amounts of matter, such as galaxy clusters.
In cosmic shear, it is not galaxy clusters but large-scale structures in the Universe that warp the light, which produces an even smaller effect.
Very wide and deep surveys, such as KiDS, are needed to ensure that the very weak cosmic shear signal is strong enough to be measured and can be used by astronomers to map the distribution of gravitating matter.
Intriguingly, the team’s results appear to be inconsistent with deductions from the results of the Planck team.
In particular, the KiDS team’s measurement of how clumpy matter is throughout the Universe — a key cosmological parameter — is significantly lower than the value derived from the Planck data.
“This latest result indicates that dark matter in the cosmic web, which accounts for about one-quarter of the content of the Universe, is less clumpy than we previously believed,” Dr. Viola said.
The results of this study also have implications for our wider understanding of the Universe, and how it has evolved during its almost 14-billion-year history.
Such an apparent disagreement with previously established results from Planck means that astronomers may now have to reformulate their understanding of some fundamental aspects of the development of the Universe.
“Our findings will help to refine our theoretical models of how the Universe has grown from its inception up to the present day,” Dr. Hildebrandt said.
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H. Hildebrandt et al. 2017. KiDS-450: cosmological parameter constraints from tomographic weak gravitational lensing. MNRAS 465 (2): 1454-1498; doi: 10.1093/mnras/stw2805
