Dark matter gravitational lenses bend light surprisingly well

by Rohit Khorana | October 5, 2020

Dark matter in the universe. Image by the NASA Hubble Space Telescope from Wikimedia Commons

As scientists studied galaxies over the last century, they noticed that the stars within them were staying put without floating into space. Those stars all stayed in their galaxies due to gravity, which is affected by two factors: mass and distance. When the scientists added up all of the masses of the stars in each of the galaxies they studied, the number they ended up with was too small for the magnitude of gravity that was exerted. When they checked the distance between stars, they found more evidence that the strength of gravity exerted exceeded their expectations. Scientists then concluded that there was some other invisible substance in galaxies that was adding mass, which in turn, increased the strength of gravity exerted and kept stars in their places. This substance, called dark matter, composes 85% of the matter in the universe and is observable through its effect on gravity.

Initially, scientists tracked down dark matter concentrations around large- and medium-sized galaxies by observing dark matter’s gravitational effect on the stars within. Now, using a new observational technique, scientists at NASA have discovered that the gravitational pull of dark matter bends light more strongly than was previously thought. Through the use of the new observational technique that analyzes images from the Hubble Space Telescope, scientists have been able to discover the existence of small concentrations, or clumps, of dark matter that were not detectable through the old method.

The new technique does not require searching for traces of gravitational influence on stars to detect dark matter concentrations. Rather, it focuses on the gravitational lensing, or warping effect, of dark matter concentrations on the light produced by more-distant quasars. These regions around black holes emit so much light that NASA refers to them as “cosmic streetlights.” Scientists observe the quasars’ light through an intervening foreground galaxy, whose presence warps their emitted light in a way that is predictable through computer simulations. However, these simulations do not account for hidden dark matter. Between the foreground galaxy and the quasar, the presence of previously unseen dark matter clumps is detectable through how the resulting images from the telescopes diverge from the predicted computer model.

The divergence of the telescope images from the predicted computer models is also significant in another respect: the computer simulations claim that clumps of dark matter that are dense enough to warp light should be relatively rare around individual galaxies. The new images show the opposite: the scientists investigated 11 galaxy clusters and found 13 instances of dark matter clumping. Massimo Meneghetti, an astrophysicist at the Astrophysics and Space Science Observatory of Bologna in Italy who reviewed the initial NASA study, notes that these new observations indicate there are more high-density dark matter clumps in real galaxy clusters than in simulated ones. Meneghetti, as well as the authors of the original NASA study and other scientists, believe that the computer simulations are missing some crucial information on the properties of dark matter.

Astrophysicist Priyamvada Natarajan of Yale University says, “The simulations could be missing some physics…or there’s something fundamentally off about our assumptions about the nature of dark matter.” With this problem in mind, many scientists are currently trying to get a better understanding of dark matter and its properties. As shown by the new method for observing this substance, progress is being made, but more research needs to be done.

Categories: Science

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