Emitted just 180 million years after Big Bang, signal indicates universe was much colder than expected.

Jennifer Chu | MIT News Office 

In a study published today in the journal Nature, astronomers from MIT and Arizona State University report that a table-sized radio antenna in a remote region of western Australia has picked up faint signals of hydrogen gas from the primordial universe.

The scientists have traced the signals to just 180 million years after the Big Bang, making the detection the earliest evidence of hydrogen yet observed.

They also determined that the gas was in a state that would have been possible only in the presence of the very first stars. These stars, blinking on for the first time in a universe that was previously devoid of light, emitted ultraviolet radiation that interacted with the surrounding hydrogen gas. As a result, hydrogen atoms across the universe began to absorb background radiation — a pivotal change that the scientists were able to detect in the form of radio waves.

The findings provide evidence that the first stars may have started turning on around 180 million years after the Big Bang.

“This is the first real signal that stars are starting to form, and starting to affect the medium around them,” says study co-author Alan Rogers, a scientist at MIT’s Haystack Observatory. “What’s happening in this period is that some of the radiation from the very first stars is starting to allow hydrogen to be seen. It’s causing hydrogen to start absorbing the background radiation, so you start seeing it in silhouette, at particular radio frequencies.”

Certain characteristics in the detected radio waves also suggest that hydrogen gas, and the universe as a whole, must have been twice as cold as scientists previously estimated, with a temperature of about 3 kelvins, or –454 degrees Fahrenheit. Rogers and his colleagues are unsure precisely why the early universe was so much colder, but some researchers have suggested that interactions with dark matter may have played some role.

“These results require some changes in our current understanding of the early evolution of the universe,” says Colin Lonsdale, director of Haystack Observatory. “It would affect cosmological models and require theorists to put their thinking caps back on to figure out how that would happen.”

Rogers’ co-authors are lead author Judd Bowman of Arizona State University (ASU), along with Thomas Mozdzen, Nivedita Mahesh, and Raul Monsalve, from the University of Colorado.

Turning on, tuning in

The scientists detected the primordial hydrogen gas using EDGES (Experiment to Detect Global EoR Signature), a small ground-based radio antenna located in western Australia, and funded by the National Science Foundation.

The antennas and portions of the receiver were designed and constructed by Rogers and the Haystack Observatory team; Bowman, Monsalve, and the ASU team added an automated antenna reflection measurement system to the receiver, outfitted a control hut with the electronics, constructed the ground plane, and conducted the field work for the project. Australia’s Commonwealth Scientific and Industrial Research Organization provided on-site infrastructure for the EDGES project.

The current version of EDGES is the result of years of design iteration and instrument calibration in order to reach the levels of precision necessary for successfully achieving an extremely difficult measurement.

The instrument was originally designed to pick up radio waves emitted from a time in the universe’s history known as the Epoch of Reionization, or EoR. During this period, it’s thought that the first luminous sources, such as stars, quasars, and galaxies, appeared in the universe, causing the previously neutral intergalactic medium, made mostly of hydrogen gas, to become ionized.

Prior to the appearance of the first stars, the universe was shrouded in darkness, and hydrogen, its most abundant element, was virtually invisible, embodying an energy state that was indistinguishable from the surrounding cosmic background radiation.

MIT News