During this dark age, there was no light-based signal we can study —”there was no visible light” said University of Washington professor of physics, Miguel Morales, about the dark age, the starless era, a gap of several hundred million years following the Big Bang devoid of data, when elementary particles combined to form hydrogen but no stars or galaxies existed yet to light up the Universe.
“But there is a specific signal we can look for,” Morales points out. “It comes from all that neutral hydrogen. We’ve never measured this signal, but we know it’s out there. And it’s difficult to detect because in the 13 billion years since that signal was emanated, our universe has become a very busy place, filled with other activity from stars, galaxies and even our technology that drown out the signal from the neutral hydrogen.”
The 21-Centimeters Wavelength Signal
The 13 billion-year-old signal that Morales and his team are search for is electromagnetic radio emission that the neutral hydrogen emanated at a wavelength of 21 centimeters. The universe has expanded since that time, stretching the signal out to nearly 2 meters.That signal should harbor information about the dark age and the events that ended it.
That team—led by researchers at the University of Washington, the University of Melbourne, Curtin University and Brown University—reported last year in the Astrophysical Journal that it had achieved an almost 10-fold improvement of radio emission data collected by the Murchison Widefield Array. Team members are currently scouring the data from this radio telescope in remote Western Australia for a telltale signal from this poorly understood “dark age” of our universe.
“We think the properties of the universe during this era had a major effect on the formation of the first stars and set in motion the structural features of the universe today,” said team member Morales. “The way matter was distributed in the universe during that era likely shaped how galaxies and galactic clusters are distributed today.”
“Astronomers estimate that the observable universe — a bubble 14 billion light-years in radius, which represents how far we have been able to see since its beginning — contains at least two trillion galaxies and a trillion trillion stars,” writes Dennis Overbye in New York Times Science. “Most of these stars and galaxies are too far and too faint to be seen with any telescope known to humans.”
Before the Dark Age
Before this dark age, the universe was hot and dense. Electrons and photons regularly snared one another, making the universe opaque. But when the universe was less than a million years old, electron–photon interactions became rare. The expanding universe became increasingly transparent and dark, beginning its dark age.
When the universe was just 1 billion years old, hydrogen atoms began to aggregate and form the first stars, bringing an end to the dark age. The light from those first stars kicked off a new era—the Epoch of Reionization—in which energy from those stars converted much of the neutral hydrogen into an ionized plasma. That plasma dominates interstellar space to this day.
Epoch of Reionization
“The Epoch of Reionization and the dark age preceding it are critical periods for understanding features of our universe, such as why we have some regions filled with galaxies and others relatively empty, the distribution of matter and potentially even dark matter and dark energy,” said Morales.
The Murchison Array is the team’s primary tool. This radio telescope consists of 4,096 dipole antennas, which can pick up low-frequency signals like the electromagnetic signature of neutral hydrogen.
But those sorts of low-frequency signals are difficult to detect due to electromagnetic “noise” from other sources bouncing around the cosmos, including galaxies, stars and human activity. Morales and his colleagues have developed increasingly sophisticated methods to filter out this noise and bring them closer to that signal. In 2019, the researchers announced that they had filtered out electromagnetic interference—including from our own radio broadcasts—from more than 21 hours of Murchison Array data.
Moving forward, the team has about 3,000 hours of additional emission data collected by the radio telescope. The researchers are trying to filter out interference and get even closer to that elusive signal from neutral hydrogen—and the dark age it can illuminate.
Interestingly, all the light in our observable universe provides about as much illumination as a 60-watt bulb seen from 2.5 miles away, says Marco Ajello, an astrophysicist at Clemson University, who led a team that has measured all of the starlight ever produced throughout the history of the observable universe.
Source: W. Li et al. First Season MWA Phase II Epoch of Reionization Power Spectrum Results at Redshift 7, The Astrophysical Journal (2019). DOI: 10.3847/1538-4357/ab55e4
The Daily Galaxy, Max Goldberg, via University of Washington
Image credit at top of page: the universe’s dark age, NASA