Paul Martini, an astronomy professor at Ohio State University, discussed the mystery of cosmic acceleration and his plans to test theories on this phenomenon at an event for the Radcliffe Fellows’ Presentation series Wednesday afternoon.
Cosmic acceleration refers to the increasing rate of expansion of the universe. The phenomenon was first discovered in the 1990s.
Martini compared the unexpected nature of the discovery to a ball flying up and away, thereby ignoring the constraints of gravity.
“This increase is the big mystery: what could be causing the universe to literally defy gravity?” Martini said.
Scientists call the energy that overcomes the force of gravity in cosmic acceleration dark energy. In his talk, Martini outlined three main theories that have been proposed to explain dark energy.
The first theory says that dark energy is a generic and inherent property of spacetime that is consistent with Albert Einstein’s theory of relativity. With the constant presence of dark energy, the more the universe expands, the more space there is — and the more negative pressure.
The second theory postulates that dark energy could be related to an unknown, undiscovered force that scientists call Quintessence. Quintessence would join the four fundamental forces — the strong and weak nuclear forces, the electromagnetic force, and gravity — to describe the universe.
The third theory proposes that Einstein’s theory of general relativity should be modified. Martini called this theory the “most prevalent explanation” and explained that it suggests gravity could change from being an attractive force to a repulsive force on very large scales.
Martini's current work aims to test these theories using the Dark Energy Spectroscopic Instrument (DESI).
“Absence of evidence is not evidence of absence,” he said. “All three of these ideas are exciting, and we need more tools and more data to test them.”
DESI is a four-meter-diameter telescope. Light is reflected through the lens, then focused onto fiber optic cables that transmit the light into 10 thermally controlled spectrographs. Spectrographs are highly sensitive, camera-like apparati that split light into optimized channels and disperse it with holographic grating.
Martini described the features of these fiber cables, including their extraordinary ability to transmit light signals and minimize expensive relay stations.
“All of these technology aspects were critical to design and build an instrument capable of measuring 35 million galaxies and quasars in five years,” he said.
Martini said DESI “dramatically surpasses similar projects” by aiming to measure 10 times more galaxies than previous tools and in a smaller amount of time.
Martini described the importance of designing sensitive equipment to measure photons, or particles of light.
“Every photon from our targets is precious,” he said. “We want to make sure that they do not go astray in the last few tens of meters.”
The DESI project shut down in March due to COVID-19, but it will be on track to restart in about a week, according to Martini. He added that he is looking forward to new experiments and unpredicted discoveries in the future study of cosmic acceleration.
“A lot of the most surprising discoveries [scientists] make are not the ones that they were initially designed to address,” he said.