By Katherine Wright
A mysterious force is pushing the Universe’s planets, stars, and galaxies away from each other at a continuously increasing speed.
Scientists at Brookhaven National Laboratory hope to unlock the secrets of this enigmatic repulsive force—known as dark energy—with a new radio telescope whose prototype they are currently testing. The proposed telescope is designed to map out the history of the universe from just after the formation of the first stars to the present day, potentially allowing scientists to lift the veil on dark energy and uncover why it makes the universe expand so quickly.
“No one really has any idea what dark energy is,” Chris Sheehy, a cosmologist at Brookhaven National Laboratory, said. “Just getting a hint of what’s going on with dark energy would be amazing.”
What is Dark Energy?
Dark energy manifests in the universe as an “anti-gravity” force, pushing galaxies apart at faster and faster speeds, Jeff Peterson, a cosmologist at Carnegie Mellon University, said.
But, he said, this mysterious force didn’t always have had such a strong repulsive effect on the universe’s matter.
Early in the universe’s life, for the first 8 billion years or so, “ordinary” gravity—the force that keeps chairs and tables firmly on the floor and stops them from floating around in a room—was the stronger force, pulling matter together and creating stars and galaxies.
“For most of the history of the universe ordinary attractive gravity was winning the competition against the repulsive gravity of dark energy,” Peterson said. “But today, on very large scales, repulsive gravity is winning.”
Scientists think this “switch over” happened around 5 billion years ago.
By measuring the expansion rate of the universe at different times in cosmic history, they hope to be pin down when exactly this change happened and in doing so learn the nature of dark energy, uncover the laws that govern this mysterious force, and perhaps even find new particles, Sheehy said.
Radio wave measurements of the universe could help in achieving this goal.
“[Radio telescopes] can probe dark energy by mapping how the geometry of the universe has changed over time,” Paul O’Connor, a scientist at Brookhaven National Laboratory who is involved in building the prototype radio telescope, said. “Using radio waves we can measure the expansion rate of the universe throughout cosmic history.”
So How do Radio Telescopes Peer Back in Time?
Radio telescopes work much like light telescopes, but instead of recording the visible light being emitted by stars and galaxies, they pick up the longer radio waves these astronomical systems send out.
Brookhaven’s prototype radio telescope—a single, four-meter-diameter, quarter-circle “dish,” nestled in a drainage basin behind storage buildings on the eastern side of the lab’s site—is designed to pick up radio waves being emitted from hydrogen, the most abundant atom in the universe.
Hydrogen atoms are made of one proton and one electron. The proton and electron have a property called spin, which can be likened to an arrow attached to the particle that points either up or down. Most of the time the arrows on the proton and electron point the same way, but occasionally one flips, and, when it does, the hydrogen atom releases energy in the form of radio waves at a very particular wavelength: 21 centimeters.
The well-defined wavelength of hydrogen radio wave emission makes it easy to pick out from other radio wave sources, O’Connor said, allowing cosmologists to accurately map the location of hydrogen in the universe.
But how do they distinguish hydrogen radio waves from different times in cosmic history?
While the radio waves are emitted at a very precise wavelength, that wavelength can change as the waves travel through space. As the 21-centimeter radio waves move across the expanding universe they get stretched and their wavelength gets longer. Scientists call this stretching “redshift.”
How much the waves are stretched by depends on when in the Universe’s history the waves were first emitted—the longer ago this happened, the more the universe has stretched in the interim and thus the longer the waves will appear in a radio telescope.
By separating out hydrogen radio signals of different wavelengths, or redshifts, scientists can create snapshots of what the universe looked like at different times in history.
“You can map out the full history of the universe,” Sheehy said.
Capabilities of the Brookhaven’s Prototype Telescope
Mapping the full 13.7 billion year history of the universe will not be possible with the prototype radio telescope currently at Brookhaven National Laboratory.
Initially, with just the single dish collecting incoming radio waves, the team hopes to be able to pick up signals that are up to 5 billion years old, Sheehy said. These signals have a redshift of up to 0.5 in the scientific lingo.
But if the team gets the go ahead to build the envisioned full-scale radio telescope, which would involve placing more than a thousand of these quadrant dishes in an array, then they could potentially look a lot further back in cosmic time.
Such a telescope could capture radio waves that were produced when the universe was just 942 million years old, Peterson said. Today those waves are over 12.7 billion years old and have a redshift of 6.
“No one has measured galaxies and the structure of the universe in this redshift range,” Laura Newburgh, a cosmologist at Yale University, said.
But first the team at Brookhaven needs to show that their detector design works, which is why they have built this single dish.
The purpose of the prototype is to test technologies and gain experience with radio wave detection techniques and equipment, Sheehy said. “You want to build small things to make sure that the larger thing is going to work.”
Over the next few years, the team plans to trial out different components in the telescope to optimize the collection and processing of the signals that it collects, Paul Stankus, a physicist at Oak Ridge National Laboratory and a collaborator on the radio telescope, said.
Brookhaven National Laboratory’s Radio Telescope Won’t be the First
If the technology works, and the team gets funding for their envisioned thousand-strong array of dishes, the 21-centimeter radio telescope they hope to build wouldn’t be the first such telescope of its kind, but it does have an unconventional design.
Rather than focusing incoming radio waves into the center of the dish, like TV radio antenna, Brookhaven’s quadrants focus the beam off to one side, Stankus said.
This so-called off axis design means that the receiver, which records the signal, can be placed so that it doesn’t cover the dish and so doesn’t block any of the incoming waves.
Such a design makes the system more sensitive, allowing the telescope to pick up fainter radio waves, Stankus said.
“None of this is new,” he said. “We are just taking advantage of a better way of doing it.”
The Brookhaven telescope would also potentially be able to pick up much longer radio waves than any other dark-matter-surveying radio telescope, Peterson said.
For example, the recently built Canadian radio telescope, which resembles four giant feeding troughs set side-by-side and will turn on in 2018, will make similar measurements to those the Brookhaven team hopes to make. But CHIME will only be able to look back a modest 11 billion years into the universe’s history, compared to nearly 13 billion years from the Brookhaven telescope, Peterson, who helped design CHIME, said.
The same slice of 11-billion-year slice cosmic history will be observable by HIRAX (the Hydrogen Intensity and Real-time Analysis eXperiment), an array of 6-meter-wide satellite dishes planned for South Africa, Newburgh, who is involved in the development of both CHIME and HIRAX, said.
“The Brookhaven project would be able to look further back in time time than anyone else,” Peterson said. “Brookhaven is paving the way here.”