Hubble Observations Hint That Something Is Missing From Our Dark Matter Models

This image from Hubble shows the galaxy cluster MACS J0416.1–2403, one of six being studied by the Hubble Frontier Fields program. ESA/Hubble, NASA, HST Frontier Fields Acknowledgement: Mathilde Jauzac (Durham University, UK and Astrophysics & Cosmology Research Unit, South Africa) and Jean-Paul Kneib (École Polytechnique Fédérale de Lausanne, Switzerland)

New Hubble observations suggest that our current model of dark matter might be missing some important components. While models have been pretty good at describing the universe so far, these latest observations don’t match with them.

The existence of dark matter is a cornerstone of leading cosmological theories. Dark matter is a mysterious substance that outweighs regular matter (that makes us, planets, and stars) five to one. It only interacts with gravity and it is very diffuse, so the best place to study it is in galaxy clusters.

Dart matter fills the space between galaxies and surrounds a cluster, and its combined gravitational pull warps space-time. The cluster itself acts as a gravitational lens, distorting and magnifying the light of background galaxies.

As reported in Science, the new observations showed that not only galaxy clusters produce intense lensing, as was expected, but also within the six clusters studied there are some small-scale concentrations of dark matter that create lensing effects 10 times stronger than expected.

This Hubble Space Telescope image shows the massive galaxy cluster MACSJ 1206, and the many big and small features created by gravitational lensing. NASA, ESA, G. Caminha (University of Groningen), M. Meneghetti  (Observatory of Astrophysics and Space Science of Bologna), P. Natarajan (Yale University), and the CLASH team.

“Galaxy clusters are ideal laboratories in which to study whether the numerical simulations of the universe that are currently available reproduce well what we can infer from gravitational lensing,” lead author Massimo Meneghetti of the INAF-Observatory of Astrophysics and Space Science of Bologna in Italy said in a statement.

“We have done a lot of testing of the data in this study, and we are sure that this mismatch indicates that some physical ingredient is missing either from the simulations or from our understanding of the nature of dark matter.”

The observations showed that the most massive galaxies in each cluster studied also produced arcs and elongated images of much more distant background galaxies. When the team compared the observations with simulations, they did not see the same amount of dark matter concentration on a smaller scale.

"There's a feature of the real universe that we are simply not capturing in our current theoretical models," added senior author Priyamvada Natarajan of Yale University. "This could signal a gap in our current understanding of the nature of dark matter and its properties, as these exquisite data have permitted us to probe the detailed distribution of dark matter on the smallest scales."

"The results of these analyses further demonstrate how observations and numerical simulations go hand in hand", said team member Elena Rasia of the INAF-Astronomical Observatory of Trieste, Italy.

The work was possible thanks to detailed observations from the Hubble Space Telescope, matched with measurements collected by the European Southern Observatory’s Very Large Telescope.

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