Observations of the gravitational lensing of galaxies have thrown up a new mystery for our best understanding of cosmology; how galaxies are held together with dark matter.
In 1933, Swiss astronomer Fritz Zwicky studied the Coma Cluster – a large galaxy cluster more than 20 million light-years in diameter, containing thousands of galaxies – and found something quite odd. Galaxies within the cluster were moving at such high speeds that they should fly away from each other, given the amount of visible mass within the galaxies and the cluster. He hypothesized that the region must contain a large amount of “Dunkle Materie” (dark matter) in order to keep the cluster stable.
If that sounds unintuitive to you, here is it explained very simply with a rope through a basketball.
Further work, conducted by Vera Rubin and Kent Ford, plotted the velocities of stars within spiral galaxies by looking at the shift in wavelengths of light as they move toward and away from us. They found the same odd phenomenon as Zwicky, and many, many observations since. Stars at the sparsely-populated edges of spiral galaxies were moving just as fast as stars toward their galactic centers.
Physicists largely concluded (though other ideas are available) that there is a mysterious substance in these galaxies and galaxy clusters known as “dark matter” which doesn’t emit, reflect, or absorb light, and only interacts with normal matter through gravity. What’s more, there should be about five times as much of it in the observable universe than regular matter which makes up the stars, planets, dust, and everything else we enjoy.
But nearly a century after it was first proposed, we still don’t know what it is. In a new study, a team of scientists at Case Western Reserve University have found puzzling observations that could throw a new spanner in the works for dark matter models. The team looked at a catalog of 130,000 galaxies and analyzed how much a galaxy in the foreground gravitationally lensed galaxies in the background. This is where objects with large mass bend spacetime, making light bend around them.
If the observed fast rotation of stars at the edge of galaxies is the result of dark matter clumped within them, we should expect the dark matter halo to drop off at a certain distance from the galactic center. However, the team found that the bending of light continued at much further distances than dark matter models would expect.
“The circular velocity curves are consistent with being flat out to hundreds of kiloparsecs,” the team explains in their paper, “perhaps even 1 Mpc, with no sign of having reached the edge of the DM halo.”
Rotation speed remains flat out to 750 kiloparsecs, according to the study.
Image credit: Case Western Reserve University
The observations are challenging to existing models, which would expect a drop-off in velocities of orbiting stars as you move further away from the galactic center. A star placed in these further out regions, according to the team, would show the same flat velocities as stars at the visible galactic edge.
According to the team, it is possible that the observations suggest that dark matter halos extend much further than we thought. Alternatively, if the effect is confirmed or found to extend even further, it could indicate that our understanding of gravity is missing something.
“The implications of this discovery are profound,” Stacy McGaugh, professor and director of astronomy in the College of Arts and Sciences, said in a statement. “It not only could redefine our understanding of dark matter, but also beckons us to explore alternative theories of gravity, challenging the very fabric of modern astrophysics.”
There are alternatives to dark matter cosmology, including Modified Newtonian Dynamics (MOND). In MOND, the odd rotation of galaxies is explained by modifications to gravity experienced by objects with very low acceleration, like those at the edge of galaxies. When gravitational acceleration is tiny enough, different gravitational behavior takes place.
Dark matter remains the explanation favored by the majority of physicists, having the advantage that it allows scientists to make predictions about the universe and objects within it, which MOND has not yet been able to do. Its explanation is also challenged by the existence of the bullet cluster, a collision between two galaxy clusters which shows mass distribution consistent with dark matter models.
It is nevertheless an interesting set of observations and requires further attention and investigation. Perhaps dark matter halos extend further than we thought, or our understanding of gravity is incorrect. Thankfully, we may soon get a more complete picture as the European Space Agency’s Euclid mission maps the large-scale structure of the Universe.
The study is published in the Astrophysical Journal Letters
