Strange observations of galaxies challenge ideas about dark matter

The new results rely on the fact that massive objects distort the structure of space and time. Galaxies are made of large amounts of stars, visible gas and dust, as well as – according to the theory – a large halo of invisible dark matter. This means that light will bend and distort as it travels across a galaxy, an effect known as gravitational lensing.

Knowing this, astronomer Tobias Mistele of Case Western Reserve University in Cleveland and colleagues decided to look for signs of gravitational lensing around multiple galaxies as a way to probe the contents of galaxies. The team looked at a catalog of about 130,000 galaxies imaged by the VLT Survey telescope at the European Southern Observatory’s Paranal Observatory in Chile, looking for telltale lines indicating the presence of more distant galaxies whose light has been bent and distorted by intervening objects.

The amount of lensing provides a proxy for the masses of foreground galaxies, including their luminous matter and, presumably, the much larger amount of dark matter surrounding each galaxy. The team then calculated the mass at different distances from each galaxy’s center and used that to infer the speed at which a star would rotate at those distances.

According to the prevailing model of cosmology, called Lambda CDM, dark matter accumulates in large globules in the cosmos, and the gravitational pull of these globules pulls the visible matter, which goes on to form a galaxy (SN: 4/4/24). Previous observations have established that these halo-like clusters extend at least 300,000 light-years from the center of a galaxy. Beyond this edge, the stars’ rotation speed should begin to slow.

However, using their gravitational lensing data, Mistele and his colleagues calculated that a star located a million light-years from the center of each galaxy, and potentially up to 3 billion light-years away, would still be spinning very fast. given the visible and the dark. matter believed to be present in the galaxy.

Does this mean there is even more invisible material than previously thought? Maybe not.

A rival theory of Lambda CDM, known as modified Newtonian dynamics, or MOND, removes the concept of dark matter entirely and instead suggests that gravity behaves differently on the scale of galaxies (SN: 28.3.18). Long championed by Mistele’s co-author Stacy McGaugh, also of Case Western, MOND specifically predicts the types of observations found in the study.

But Lambda CDM isn’t down for the count just yet.

“I think it’s a real stretch to say that dark matter can be eliminated, because the lines of evidence [for it] there are so many,” says Bhuvnesh Jain, a cosmologist at the University of Pennsylvania.

For example, the growth of large-scale structures in the universe since the Big Bang is much better explained by Lambda CDM than MOND. Jain suggests that perhaps there are even more exotic ideas in mathematical models of gravity, inspired by the higher-dimensional thinking found in string theory, that could explain the actual large-scale structure of the universe. Some variation of such ideas may be able to explain Mistele and his colleagues’ data while also revising the role of dark matter.

Observations from the European Space Agency’s Euclid satellite, which launched last year, will soon provide researchers with much better data of gravitational lensing, potentially helping to unravel what’s going on with it. strange mystery.