ALMA (NRAO/ESO/NAOJ); B. Saxton NRAO/AUI/NSF; NASA/ESA Hubble, T. Hunter (NRAO)
A perfect Einstein ring, or gravitational lens. The red glow is from dust in a distant galaxy.
Albert Einstein's general theory of relativity predicts so much about the universe at large, including the existence of gravitational lenses or "Einstein rings."
At the same time, however, Einstein's famous equations struggle to fully explain such objects.
While general relativity says a strong source of gravity - like the sun - will warp the fabric of space, bend light coming from a distant object, and magnify it to an observer, very big objects like galaxies and galaxy clusters make gravitational lenses that are theoretically too strong (like the one above). General relativity also can't fully explain the spinning motions of galaxies and their stars.
That's why most physicists think as much as 80% of the mass in the universe is dark matter: an invisible source of matter, and its resulting gravitational force, that fills the gap. They think dark matter might be made of hard-to-detect particles, or perhaps an unfathomable number of tiny black holes. But we have yet to find smoking-gun evidence of either.
However, a contentious theory by Erik Verlinde at the University of Amsterdam suggests dark matter may not be matter at all. What's more, astronomers say his idea "is remarkable" in its ability to explain the behavior of more than 33,000 galaxies that they studied.
"This does not mean we can completely exclude dark matter, because there are still many observations that Verlinde's cannot yet explain," study leader and physicist Margot Brouwer said in a YouTube video about the research. "However it is a very exciting and promising first step."
A new theory of gravity?
Pablo Carlos Budassi
Called "emergent gravity," or EG, Verlinde's idea was first widely publicized by the New York Times in 2010. However, it took him 6 years to craft into a more testable (though not-yet-peer-reviewed) paper published on arXiv in November 2016.
Emergent gravity borrows from the very tiny (and very weird) world of quantum mechanics to suggest that gravity is really a "dark" gravitational force, though more like a natural side effect of the fabric of space.
You might think of it as the outcome of a spacetime tug-of-war.
On the one hand, matter locally warps the fabric of space. On the other, a powerful and as-yet-unexplained force of nature, called "dark energy," is speeding up the expansion of space and the edge of the universe in all directions. (But don't worry, we may not go through a "big rip" until at least 2.8 billion years from now.)
Verlinde suggests the fabric of space has a kind of "elastic memory" for visible matter against expansion, "which can only relax very slowly" - a friction that, with large pockets of matter, generates a "dark" gravitational force at large distances.
Put another way, gravity may be another way that nature tries to fill a void with chaos, much like air rushing to fill a vacuum, or the heat of your body escaping into the space around you - no exotic, invisible, force-carrying particles required.
"In our view this undercuts the common assumption that the laws of gravity should stay as they are, and hence it removes the rationale of the dark matter hypothesis," Verlinde wrote in his most recent paper. "Indeed, we have argued that the observed dark matter phenomena are a remnant, a memory effect, of the emergence of spacetime together with the ordinary matter in it."
If you're feeling confused by all this, you're not alone: Dennis Overbye wrote for the New York Times in 2010 that "[s]ome of the best physicists in the world say they don't understand Dr. Verlinde's paper, and many are outright skeptical."
However, a team of astronomers recently ran Verlinde's equations through a limited test - and they appeared to check out.
Emergent gravity clears its first hurdle
NASA/ESA/Pontificia Universidad Católica de Chile
A large pocket of gravitational lenses, as seen by the Hubble Space Telescope.
Brouwer and her colleagues tested emergent gravity by studying warped space around 33,613 galaxies.
Specifically, they looked at gravitational lenses caused by the galaxies, and how the background objects behind them were distorted.
"These bent images allow us to reconstruct the gravitational force around foreground galaxies up to a distance that is 100 times larger than the galaxies themselves," Brouwer said of her team's research, which was published December 11 in the British journal Monthly Notices of the Royal Astronomical Society.
"Usually we explain this gravity by assuming that each galaxy has a dark matter cloud of a certain mass," she said. "This time we also compared our data to the new theory of gravity by Erik Verlinde."
Brouwer said Verlinde's equations could explain gravity's distribution "without introducing any free parameters or invisible particles." Translation: No dark matter required.
But the 22 authors of that study (none of whom include Verlinde) are careful to point out that dark matter is far from a dead idea.
"Although [emergent gravity's] performance is remarkable, this study is only a first step," they wrote. "Further advancements on both the theoretical framework and observational tests of EG are needed before it can be considered a fully developed and solidly tested theory."
Timothy Brandt, an astrophysicist at the Institute for Advanced Study who's studied dark matter but wasn't involved in any of the studies, told Business Insider in an email that - new evidence aside - Verlinde's concept leaves him with more questions than answers.
Brandt wondered, for example, if emergent gravity can also explain "the LIGO results, which perfectly match [general relativity]," evidence of the "dark matter content of dwarf galaxies," and the leftover energy of the universe's formation (called the cosmic background radiation).
Given all of the evidence for general relativity, Brandt said he "would bet pretty heavily against" emergent gravity's replacing it.
Even if Verlinde's idea turns out to fail future tests, physics still needs to find a way to solve its biggest problem: how to unite Einstein's general relativity (the physics of the very big) with quantum mechanics (the physics of the very small) into a so-called "theory of everything."
"At large scales, it seems, gravity just doesn't behave the way Einstein's theory predicts," Verlinde told Astronomy Now in November.