Rotational Curves of Galaxies Stay Parallel Indefinitely

In his classic book On the Structure of the Scientific Revolution, philosopher Thomas Kuhn suggested that, for a new scientific system to take root, there must be evidence that does not sit well within the existing system. For more than a century now, Einstein’s theory of relativity and gravitation has been the prevailing system. However, cracks are beginning to appear, and a new paper from researchers at Case Western Reserve University recently added another when they failed to find a decrease in rotational energy in galaxies even millions of light years from the center of the galaxy.

Galaxies are known to rotate – even our own solar system travels in a circle around the center of the Milky Way galaxy at around 200 kilometers per second, although we cannot detect any motion on human time scales. According to Newtonian dynamics, this rotational speed should slow down the further away the star is from the center of the galaxy. However, the observations did not support this, showing that the speed remained high regardless of how far away the star was.

That led scientists to create another force that affects the speed of rotation of the most distant stars. Today, we usually call it dark matter. However, scientists have also spent decades trying to puzzle out what dark matter is made of and have yet to come up with a solid theory.

But in some cases, even the existence of dark matter as we know it does not match the observational data. Dr. Tobias Mistele, a post writer in Observations, found that the speed of rotation of galaxies does not decrease, no matter how far away and no matter how long they have been doing so. This data flies in the face of the traditional understanding of dark matter, where its gravitational influence is felt by a “halo” surrounding dark matter itself. Even these dark halos have an effective area. Dr. Mistele and his co-authors found evidence of a sustained rotation speed that should be outside the sphere of influence of any dark halo present in these galaxies.

To collect this data, the authors used a favorite tool of cosmologists – gravitational lensing. They collected data on the most distant galaxies and their light was enhanced by a group of galaxies or a similar large object that was closer. While collecting the data, Dr. Mistele analyzed the rotation speed of the stars in the galaxy and plotted it according to the distance of the stars from the center of the galaxy. This is known as the “Tully-Fisher” relation in cosmology.

The result was a perfectly straight line – the speed of rotation of the stars in the galaxy did not seem to decrease with distance from the center of the galaxy, as traditional Newtonian dynamics and relativity through dark matter predicted. So, what are the alternative explanations?

Paper co-author Stacy McGaugh points out in a press release that one theory in physics accurately predicted the data her team had collected—the modified theory of Newtonian Dynamics (or MOND). Designed expressly to deal with things like the rotation of galaxies, MOND was created in 1983 and remains controversial to this day. It contends with factors such as the gravitational lensing through which the paper’s data were collected.

That disconnect points to the need for a deeper understanding of gravity — what Kuhn called “crisis,” which many psychologists already believe affects the discipline. Although there is no current consensus on what might resolve the conflict, evidence is mounting for the need for resolution. If we are to truly understand our place in the world, we will eventually need to find a solution – it may take time.

Learn more:
CWRU – A new, ground-breaking study shows that galactic rotation curves stay flat indefinitely, confirming predictions of modified gravity as an alternative to dark matter.
Mistele et al. – Uncertain Velocity of the Flat Curve and Baryonic Tully-Fisher Relation from Weak Lensing
UT – Will Wide Binaries Be the End of MOND?
UT – New Measurements of the Rotation of the Galaxy Lena towards Altered Gravity as an Explanation for Dark Matter
UT – The Earliest Clusters Rotated Slowly, Expanding Over Billions of Years

Leadership Image:
A diagram of the rotation curve of the galaxy used in the study.
Credits – Mistele et al.