For many years, astronomers thought the expansion of the universe was slowing down due to the effects of gravity, but in 1998, two teams of researchers confirmed the existence of an unseen “dark energy” that appears to be speeding up the expansion of the universe.
That discovery redirected the astronomy community’s attention from familiar forms of matter to what, at first glance, appears to be nothing.
“Right now, the nature of dark energy is one of the biggest questions in physics,” said Jeffrey Newman, a professor in the University of Pittsburgh Department of Physics and Astronomy, part of the Kenneth P. Dietrich School of Arts and Sciences. “Something is acting like an anti-gravity and we don’t know what. Dark energy is really just a name for our ignorance.”
Dark energy makes up an estimated 68% of the universe, and dark matter, the unseen material that makes up 27% of the universe, is thought to provide the attractive force that keeps galaxies from pulling apart during the expansion.
Newman and his group at Pitt have taken the question of dark energy head on as part of a global team working with the Dark Energy Spectroscopic Instrument (DESI). DESI, a sky scanning instrument mounted on the Mayall Telescope at Kitt Peak National Laboratory in Arizona, is creating three-dimensional maps of the locations of galaxies using light to map their distances from Earth. Those measurements can then be used to calculate how much the universe has expanded since the Big Bang.
By charting galaxies, the map will also document areas holding large amounts of dark matter.
“What we do know is where there’s a lot of dark matter there are also galaxies,” said Newman.
“We’ll take those brightest and best galaxies and, by pointing the instrument at them, can study their full breakdown of light, which allows us to determine with precision their distance from Earth. By having those distances and knowing where they are in the sky, we can build a three-dimensional map of those galaxies, which also gives us a three-dimensional map of where dark matter is in the universe that will span a third of the sky—and more than a quarter of all the volume of the universe that’s visible from Earth.”
Newman developed the techniques to find the right galaxies as a postdoctoral researcher at University of California, Berkeley and brought them to Pitt when he arrived in 2007. The focus of that work was using 2D images of the sky to identify the galaxies that were the best candidates for building a 3D map of where matter in the universe might be found.
“Obtaining those images took 500 nights on three different telescopes,” said Newman. “We get the images, then they go through processing at the National Energy Research Supercomputing Center, which has taken years to do all of the processing. That gives us measurements of the properties of every object—every star and every galaxy—down to pretty faint limits. We use those properties, and, based on what we know about galaxies from other projects, infer which are going to be the most promising targets for DESI.”
DESI captured its first images on Oct. 22 and researchers will continue testing the instrument through early next year before its five-year mission expands to map the entire universe begins.
“After a decade in planning and R&D, installation and assembly, we are delighted that DESI can soon begin its quest to unravel the mystery of dark energy,” said DESI Director Michael Levi of the Department of Energy’s Lawrence Berkeley National Laboratory, the lead institution for DESI’s construction and operations.
In February 2020, Newman and graduate student Biprateep Dey will travel to Arizona to assist with observations and help guide DESI toward more galaxies, a process Newman said will require aligning the instrument’s 5,000 fiber optic sensors with precision.
“If you put a fiber in the wrong place, there’s no galaxy and you don’t get any light,” he said.
The answers DESI seeks could upend thousands of active theories, including Einstein’s theory of gravity, or introduce an entirely new field of study for astrophysicists to understand. Either outcome will yield the field’s most significant finding since the confirmation of the dark forces.
“Just answering the question of is it a problem with our theory of gravity, or is it a new component to the universe, that alone would be extremely exciting. Right now we have a wide open field of possibilities; throwing out half of them would be big progress,” Newman said.
Researcher Helps Find Smallest Known Black Hole
A star survey conducted by another Pitt astronomer has led to the discovery of the smallest known black hole in the universe.
Carles Badenes, an associate professor in the Department of Physics and Astronomy, was part of a team that discovered a star affected by the gravitational pull of the black hole. They discovered the star while looking for binary systems—two or more stars revolving around a common center.
The discovery was published in Science on Nov. 1.
Badenes is leading a statistical study of binary stars at Pitt, observed with the Apache Point Observatory Galaxy Evolution Experiment (APOGEE), which is part of the Sloan Digital Sky Survey at Apache Point Observatory in New Mexico. He collaborated with a team of researchers from The Ohio State University in a search for stars with massive, unseen companions. Together, they found the star—2MASS J05215658+4359220—which showed signs it was attached to an unseen, but massive, source.
The team observed shifts in the spectrum of light coming from the star at different times. They used information from these shifts and Newton’s equations to infer the mass of the unknown object—which was at least three times that of the Earth’s sun.
Their conclusion that the object was a low-mass black hole was “basically a process of elimination,” Badenes explained.
“It can’t be a normal star, because it does not make any light. It can’t be a white dwarf (star) because they only exist in smaller masses. It could be a neutron star, but at three solar masses, it would be the heaviest neutron star known.”
The revelation opens the door for discovering and studying black holes while they’re still hidden in the shadows of the cosmos, before they consume matter and become luminous sources.
“The way we usually study black holes is by watching them as they eat. If you use that method you have a bias. The larger the black hole, the brighter it will be as it accretes material. Because the brighter ones are easier to find, that means that most of the known accreting black holes are quite massive. It also means that we do not have a good idea of what the average mass of a black hole is,” Badenes said.
“With our new method, all we need to do is measure shifts in the spectra of stars. This allows us to measure the mass of black holes directly, not the brightness of material that falls into them. Because it is a more direct measurement, it allows us to have a better idea of what the average mass of a black hole is.”