In the early 1980’s Israeli physicist Mordehai Milgrom from Weizmann Institute proposed a new theory, “the smoking gun” –a new theory of the universe’s structure that offers the most plausible alternative yet to dark matter– that defied the laws of gravity suggesting that galaxies are governed by modified dynamics rather than obeying the laws of Newton and of general relativity. “This theory became known as MOND for modified Newtonian Dynamics, Milgromian Dynamics, or “modified gravity”–a viable explanation for a cosmological dilemma: that galaxies appear to defy the long-accepted rules of gravity traced to Sir Isaac Newton in the late 1600’s. In short, the mathematics of gravity may hint at weirder phenomena than many thought.
Rules of Gravity Slightly Altered
MOND theory, says that rather than attributing the excess gravitational pull to an unseen, undetectable dark matter, this gravitational pull exists because the rules of gravity are slightly altered– that gravity at low accelerations is stronger than would be predicted by a pure Newtonian understanding.
In June of 2020, theorists Tom Złosnik and Constantinos Skordis, from the Central European Institute for Cosmology and Fundamental Physics, say they’ve finally squeezed an alternative theory of gravity that agrees with the data of the precise maps of ancient light, known as the CMB, or the cosmic microwave background generally thought to have been emitted about 400,000 years after the Big Bang. “Their work,” reports The Atlantic, “which was posted online in late June as a preprint and has not yet passed peer review—uses a tweaked version of Einstein’s theory of gravity to reproduce an iconic map of the early universe, a feat that even some rebels feared to be impossible.”
Fast forward to December 2020, an international group of scientists, solved the anomaly that for decades, we’ve measured more gravitational pull in space than we think we should have–that there’s not enough visible or known matter to account for it all. The new research contends that the rival MOND hypothesis more accurately predicts a galactic phenomenon that appears to defy the classic rules of gravity.
“For 15 years we’ve just been dead in the water,” says Stacy McGaugh, an astronomer at Case Western Reserve University and a longtime advocate for modified-gravity theories who wasn’t involved in the research. “It’s a huge leap forward.”
In his Triton Station blog –About the Science and Sociology of Cosmology and Dark Matter–McGaugh, says “MOND passed test after test after test, successfully predicting observations both basic and detailed that dark matter theory got wrong or did not even address. It was only after this experience that I realized that what I thought was evidence for dark matter was really just evidence that something was wrong: the data cannot be explained with ordinary gravity without invisible mass. The data – and here I mean ALL the data – were mostly ambiguous: they did not clearly distinguish whether the problem was with mass we couldn’t see or with the underlying equations from which we inferred the need for dark matter.”
“Most cosmologists still prefer dark matter as the simpler of the two paradigms,” reports The Atlantic, but they agree that the new theory could be intriguing, observes Dan Hooper, an astrophysicist at the University of Chicago—if it can truly match additional cosmological observations. “That would be a big barrier,” says Hooper.
Dark matter proponents theorize that most of the known universe is actually made of material that doesn’t interact with light, making it invisible and undetectable– but that this material accounts for much of the gravitational pull among galaxies. It has been the prevailing theory for nearly 50 years.
New Research Detects the External Field Effect
Case Western Reserve University astronomy chair Stacy McGaugh, and his collaborators, led by Kyu-Hyun Chae, from Sejong University in South Korea, say they detected what is called “the external field effect” (EFE). in more than 150 galaxies studied. MOND predicted that the internal motions of an object in the cosmos should not only depend on the mass of the object itself, but also the gravitational pull from all other masses in the universe–or EFE.
“The external field effect is a unique signature of MOND that does not occur in Newton-Einstein gravity,” McGaugh said. “This has no analogy in conventional theory with dark matter. Detection of this effect is a real head-scratcher.”
“I have been working under the hypothesis that dark matter exists, so this result really surprised me,” said lead author Chae. “Initially, I was reluctant to interpret our own results in favor of MOND. But now I cannot deny the fact that the results as they stand clearly support MOND rather than the dark matter hypothesis.”
Galaxies Defy the Dark Matter Hypothesis
The group analyzed 153 rotation curves of disk galaxies as part of their study. The galaxies were selected from the Spitzer Photometry and Accurate Rotation Curves (SPARC) database, created by another collaborator, Federico Lelli, during his postdoctoral studies at Case Western Reserve, McGaugh and co-author James Schombert, of the University of Oregon. In addition to the authors of the research were Pengfei Li from Case Western Reserve and Harry Desmond from the University of Oxford.
The scientists said they deduced the EFE by observing that galaxies in strong external fields slowed (or exhibited declining rotation curves) more frequently than galaxies in weaker external fields–as predicted by MOND alone.
Lelli said he was skeptical by the results at first “because the external field effect on rotation curves is expected to be very tiny. We spent months checking various systematics. In the end, it became clear we had a real, solid detection.”
MOND, says McGaugh in his Triton Blog, “has successfully predicted many galaxy rotation curves, highlighting some remarkable correlations with their visible mass. This is unexpected if they mostly consist of invisible dark matter with quite different properties to visible mass. The Local Group satellite galaxy planes also strongly favor MOND.”
“I came from the same place as those in dark matter community,” McGaugh said that of the skepticism is part of the scientific process and understands the reluctance of many scientists to consider MOND as a possibility. “It hurts to think that we could be so wrong. But Milgrom predicted this over 30 years ago with MOND. No other theory anticipated the observed behavior.”
The Daily Galaxy Edit Staff, Max Goldberg, via Stacy McGaugh Triton Station Blog, Case Western Reserve University and A Breakthrough Some Cosmologists Thought Would Never Come, The Atlantic
Image credit: Shutterstock License
Read about The Daily Galaxy editorial team here