Dark matter could interact in totally unexpected ways with the universe

According to Sir Isaac Newton’s theory of universal gravitation, gravity is an action at a distance, where one object feels the influence of another regardless of distance. This became a central feature of classical Newtonian physics which remained the accepted canon for over two hundred years. In the 20th century, Einstein began reconceptualizing gravity with his theory of General Relativity, in which gravity alters the curvature of local spacetime. From this we get the principle of locality, which states that an object is directly influenced by its surroundings and distant objects cannot communicate instantaneously.

However, the birth of quantum mechanics prompted another conceptualization, as physicists discovered that non-local phenomena not only exist, but are fundamental to reality as we know it. This includes quantum entanglement, where the properties of one particle can be transferred to another instantaneously and regardless of distance. In a new study by the International School for Advanced Studies (SISSA) in Trieste, a team of researchers suggests that dark matter could interact with gravity in a non-local way.

The team was led by Francesco Benetti and Giovanni Gandolfi, two Ph.D. students of the SISSA Astrophysics and Cosmology Group. He was joined by researchers from the Institute of Fundamental Physics of the Universe (IFPU), the National Institute of Nuclear Physics (INFN) and the Institute of Radio Astronomy of the National Institute of Astrophysics (IRA-INAF). Their article, “Dark Matter in Fractional Gravity. I. Astrophysical tests on galactic scales” (the first of a series dealing with DM interactions), which appeared in The astronomical journal.

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Image of KK 246 (ESO 461-036), an irregular dwarf galaxy that resides within the Local Void, taken by Hubbles Wide Field Camera 3 (WFC3) and Advanced Camera for Surveys (ACS). Credit: NASA/ESA/Hubble/E. Shaia et al.

According to the most widespread cosmological theory, Dark Matter is the mysterious mass that makes up about 85% of the matter in the Universe. This matter does not interact with baryonic matter (also known as normal or “visible” matter) through electromagnetism or nuclear forces, only gravity (the weakest fundamental force). It was present shortly after the Big Bang, where it formed halos that caused all the neutral hydrogen to clump together, giving birth to the first stars in the Universe that were bound together to create the first galaxies.

In theory, DM is a fundamental building block of nature, responsible for the formation of cosmic structures ranging from galaxies to galaxy clusters. It is also responsible for the rotation curves of galaxies, causing stars to orbit a galaxy’s disk around a common center. Its existence is also necessary for General Relativity, which has been infinitely verified through observation and experimentation, to work on the largest scales. However, the nature of the DM remains a mystery in terms of its composition (WIMPs or Axions?) and how it interacts with smaller galaxies.

According to the authors, their study proposes a new model of the non-local interaction between a galaxy’s DM and gravity, which could provide a new perspective on the still mysterious nature of this invisible mass. As Einstein said, describing General Relativity in a nutshell, “matter tells spacetime how to curve, and curved spacetime tells matter how to move.” As Benetti described his team’s theory to Universe Today via email, “In the innermost part of small galaxies, DarkMatter behaves like a nonlocal object, interacting with all other masses in the Universe.”

This runs counter to the prevailing view that dark matter is “cold,” meaning that it is composed of weakly interacting massive particles (WIMPS). These particles move slowly relative to the speed of light and interact with normal matter weakly and locally. As Benetti indicated:

“Although the most widely used model of Dark Matter (the so-called Cold Dark Matter, CDM) provides predictions well confirmed by experimental data on a cosmological scale, it suffers from problems inside galaxies, especially in the nuclei of smaller ones. Our model is able to overcome these problems by proposing a non-local interaction between dark matter within galaxies.”

The Hubble eXtreme Deep Field (XDF) combines Hubble observations taken over the past decade of a small patch of sky in the constellation Fornax. Credit: NASA/ESA/UCSC/Leiden University/HUDF09 Team

To model the non-locality theory of DM, the team used fractional calculus, a branch of mathematical analysis first developed in the 17th century. In recent years, fractional calculus has been found to have applications in various areas of physics, but it has never been tested before in astrophysics. When used to describe DM in a confined system (small galaxies), this non-locality emerges as a collective behavior of the particles. Benetti and his colleagues applied their theory to the rotation curves of thousands of different types of galaxies, from small DM-dominated dwarfs to large spirals.

Their results showed that their theory could predict the rotational speed of galaxies (especially smaller ones) better than the CDM in Newtonian gravity, which they confirmed using Bayesian statistical analysis. Benetti said:

“In particular, the theory correctly predicts many scaling laws observed in galactic environments (the radial acceleration relationship, nucleus surface density versus nucleus radius, nucleus radius versus disk scale length) and that the Dark matter density is expected to be suppressed at the center of dwarf galaxies than predicted by a model of Cold Dark Matter in Newtonian gravity. This is confirmed by observations and represents one of the major problems in Cold Dark Matter models in Newtonian gravity.”

In particular, the theory proposed by Benetti and his colleagues could provide clues to what is known as the “Cusp-Core Problem” (or “Cuspy Halo Problem”). This refers to the discrepancy between the inferred DM density profiles of low-mass galaxies and the density profiles predicted by cosmological simulations. Several possible solutions have been proposed for this problem, including possible feedback mechanisms for alternative theories of the DM (including the possibility that it may be “hot”).

The non-locality theory proposed by Benetti and his team offers a potentially revolutionary solution within the CDM framework and could have significant implications for cosmology. “Moreover, if the mechanism by which Dark Matter develops non-local behaviors in these systems were the result of the quantum nature, it would represent an example of a quantum system on macroscopic, galactic scales, a very interesting phenomenon in itself,” Benetti added. “In particular, other quantum Dark Matter models are already known to the community, but none of them introduces a non-local interaction via a fractional derivative.”

This research is part of a growing effort to constrain the nature of dark matter and dark energy, two of the biggest mysteries facing astronomers and cosmologists today. These efforts will benefit significantly from next-generation telescopes such as ESA’s Euclid mission (which launched today!) and the Nancy Grace Roman Space Telescope (RST), due to launch by 2027.

Further reading: SISSA

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