One of the biggest mysteries in modern science is the speed at which the universe is expanding. Galaxies that have been moving apart since the Big Bang billions of years ago should be slowing down due to gravity, but instead, they are accelerating. The search for so-called “dark energy” is one part of the potential explanation for this phenomenon, but while hidden forces may be at play, human understanding of gravity itself may be incomplete at these cosmological scales.

To explore this possibility, a team of Penn cosmologists and their colleagues at Cornell are partnering with NASA in search of new wrinkles in the theory of gravity.

The U.S. space agency is providing a three-year, almost $1 million grant to the two institutions to develop models and test theories of how the rules of gravity, as described by Albert Einstein’s General Theory of Relativity, might break down in extreme cases.

The Penn team is led by Mark Trodden, a professor and chair of the Department of Physics & Astronomy in the School of Arts & Sciences, and Justin Khoury, an associate professor of astronomy.

“Deeply theoretical work like this might not seem like something NASA would fund,” Trodden says, “but this grant is explicitly about ways of testing theories using missions NASA is currently flying or will launch in the future.”

“When we think about looking at the laws of gravity on a cosmological scale, you don’t really think of anything in Earth’s neighborhood that would be relevant to that question,” Khoury says. “But in fact, there are extreme places in the solar system that might deviate from Einstein’s relativity where we can put those theories to the test.”

One such opportunity is through NASA’s Cassini orbiter, which was launched in 1997 and has been circling Saturn for the past decade.

“Saturn’s moons can ‘stir’ its rings, producing density waves,” Khoury says. “The speed at which these waves propagate has to do with the gravitational interactions between all of the tiny rocks that make up the rings, so if there is some interaction with the ‘dark’ sector, we could see a higher propagation speed.”

NASA’s space-based telescopes also provide unparalleled views of the universe’s most gravitationally intense objects, such as black holes and neutrons stars, providing windows into other natural experiments and constraining “modified gravity” theories.    

“Everything in the universe is a test of the laws of physics,” says Trodden. “NASA gives access to some of those tests we couldn’t do on Earth, and rather than seeing some new matter or energy that explains cosmic acceleration, we might see that our understanding of the laws aren’t quite right.”