Courtesy of Massimo Meneghetti
New research co-authored by a Yale astronomy professor suggests that there may be a large discrepancy between what scientists thought dark matter was and what it actually is.
In a study published in the Sept. 11 issue of “Science,” astrophysicists compared theoretical simulations with observational data on the spatial distribution of dark matter in galaxy clusters. They found that the small-scale gravitational lensing effects of dark matter are 10 times stronger than predicted. This large difference points to two possibilities: there could be a key missing ingredient in the simulations or our current understanding of dark matter is fundamentally misguided.
“Clusters of galaxies are the largest repositories of dark matter in the universe, so they bend light very dramatically,” said Professor of Astronomy and of Physics Priyamvada Natarajan, one of the study’s co-authors. “You can look at other places in the sky, where there is not a lot of dark matter, so you know statistically what the undistorted shapes should be.”
The study aimed to evaluate the accuracy of theoretical simulations for a model known as the cold dark matter paradigm — the currently favored theory explaining the evolution of galaxies — according to the study’s lead author, National Astrophysics Institution of Italy researcher Massimo Meneghetti.
More precisely, the researchers compared a quantity called the Galaxy-Galaxy Strong Lensing Probability for each of these data sets.
“Our entire universe can be thought of as a trampoline, and the presence of matter causes these potholes in spacetime,” Natarajan said. “Light from distant sources, if you have foreground lumps of matter, carries an imprint of these potholes, causing the shapes to get distorted. The way we quantify this is by defining a quantity called Galaxy-Galaxy Strong Lensing Probability.”
The research uses distortions in gravitational lensing curvatures in order to provide insight on the mass distribution in galaxy clusters. Matter condensed into larger scale components, such as galaxies, make up the dark matter halo surrounding the entire cluster. Researchers’ current understanding of dark matter can only be informed by gravitational lensing effects because dark matter itself does not emit, absorb, reflect or interact electromagnetically in any other way with matter or light.
The simulations on which the research is based assume that dark matter is made up of weakly interacting, collisionless, massive particles, although scholars have yet to identify their exact physical nature. Yet, when these characteristics are assumed, “the particles that make up dark matter in the simulation are not able to produce these compact halos,” Meneghetti said.
After having calculated the GGSL for the simulations and observational data, the scientists found that each GGSL calculation differed by an entire order of magnitude, which, according to Natarajan, is “pretty huge.”
Meneghetti said that the discrepancy in the comparisons initially caused surprise in the research team. Unless something, such as a missing ingredient, was wrong with the simulations, this meant that the team could be opening the door to something completely new. They checked for systematic errors in multi-analysis and ensured that they were not missing any key factors in the simulations. They confirmed that none of the possible explanations tested worked to solve the mismatch.
“We know that our theory of gravity is incomplete, that it is not integrated with the physics of the small, which is quantum mechanics,” Natarajan said. “There is no unifying quantum theory of gravity yet. And so you never know, when you see a gap, [whether] it is giving you a clue that there is some other bigger thing that we don’t understand, or if the current theory just needs a little tweak.”
Piero Rosati, one of the study’s co-authors from the University of Ferrara in Italy, spoke about how this research was made possible by the cutting-edge spectrographic data from the European Southern Observatory’s Very Large Telescope (VLT). With this data, the researchers can understand properties of the gravitational lensing.
“It’s critical that you know the distances between you and the lens, and the lens and the source to understand the properties of the lens,” Rosati said.
The team hopes to work with even better data in the future, with the upcoming launch of the James Webb Space Telescope in 2021.
The researchers also have plans to make new simulations to study how galaxies and clusters form and further explore the interaction between ordinary and dark matter. Other scenarios with candidate particles for dark matter, such as axions or non-colliding particles, will also be tested. This analysis will be extended over a much larger sample of galaxy clusters to increase the test’s statistical significance.
According to the cold dark matter paradigm, dark matter makes up about 27 percent of the universe, but the particles that constitute dark matter are yet to be discovered.
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