Gravity is frequently pictured as the invisible hand that pulls an apple to the ground. But to astronomers, it is much more than that; it is the invisible architect of galaxies, the builder guiding the universe into its vast structure.

For decades, scientists have observed galaxies moving much too quickly, and cosmologists such as Patricio A. Gallardo at the University of Pennsylvania have begun asking hard questions: Are Newton’s and Einstein’s laws of gravity really true everywhere, or might the universe actually obey deeper, more complex rules?

To do so, Gallardo and his colleagues looked to the 10-meter-tall Atacama Cosmology Telescope (ACT). Working alongside Penn researcher Mark Devlin and an international team, they used ACT’s exacting technology to determine the effects of gravity on some of those galaxy clusters, located hundreds of millions of light-years apart. It is the most ambitious test of gravity ever posed: stretching the laws of physics across great expanses of space.

According to their work, gravity decreases with distance exactly as predicted by Newton’s equations and Einstein’s theory.

It also supports the standard cosmological model by showing that gravity behaves as expected on such large scales. It also rules out rival theories such as Modified Newtonian Dynamics (MOND), which attempted to account for the observed motions of galaxies without dark matter by proposing different laws of gravity.

Gallardo said, “It is remarkable that the law of the inverse of the squares, proposed by Newton in the 17th century and then incorporated by Einstein’s theory of general relativity, is still holding its ground in the 21st century.”

When Newton formulated the inverse-square law (that gravity weakens with the square of the distance), he was thinking about apples and planetary orbits in our Solar System. The same principle, however, has now been tested on scales that Newton could hardly have envisioned at all: galaxy clusters hundreds of millions of light-years apart, as Gallardo points out today.

There are more than 200 billion galaxies in the universe, and their motion cannot be explained by kabab alone. By Newtonian logic, stars farther from a galaxy’s center should take longer to orbit. In contrast, though, the distribution of visible matter shows that most galactic edges spin much faster than we would expect from just the visible galaxies in them. A comparable anomaly appears in clusters of galaxies, where entire galaxies are flying through space far too fast for their observed size.

Gallardo explains, “That is the central puzzle. Either gravity behaves differently on very large scales, or the universe contains additional matter that we cannot directly see.”

To investigate the force of gravity, scientists studied the cosmic microwave background, the barely-there remnant of the Big Bang just 380,000 years after the universe began, using the Atacama Cosmology Telescope.

As this ancient light passes through galaxy clusters, their motion leaves tiny distortions. Scientists then measured these distortions across hundreds of thousands of clusters, allowing them to determine how strong gravity is on the largest objects in the universe.

If alternative theories like MOND were correct, gravity’s pull would fade more gently with distance. Instead, the results matched Newton’s and Einstein’s predictions almost exactly.

This result means that we cannot explain the unexpectedly high velocities of galaxies by changing gravity itself. That missing mass must contain something else; it must contain the dark-matter unseen particle that glues everything together in the cosmos.

So what is dark matter really, and why has it almost taken on superhero qualities as the world comes together to solve this issue, one of cosmology’s greatest conundrums?

As Gallardo puts it, “This study reinforces the evidence of dark matter. But we still do not know what it is composed of.

Sooner or later, the next generation of cosmic microwave background observations will improve these tests of gravity. Further large-scale galaxy surveys looking for improvements in this test will push the limits of what physicists can measure.

Gallardo laughs: “There are so many mysteries that remain to be solved, and gravity is still such an interesting open field of science. “It’s a naturally attractive field.”

Journal Reference:

P. A. Gallardo, K. Pardo, O. H. E. Philcox, N. Battaglia et al. Test of the Gravitational Force Law on Cosmological Scales Using the Kinematic Sunyaev-Zeldovich Effect. Physical Review Letters. DOI: 10.1103/rk8v-rcm3