Study on Earliest Evidence of Hydrogen in Universe

In a research published in the Nature journal on March 1, 2018, astronomers from MIT and Arizona State University state that a table-sized radio antenna in a distant region of Western Australia has picked up weak signals of hydrogen gas from the primordial universe.

The research team has traced the signals to just 180 million years after the Big Bang, making the discovery the earliest evidence of hydrogen yet detected.

They also established 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 formerly devoid of light, discharged UV radiation that interacted with the adjacent hydrogen gas. Consequently, hydrogen atoms across the universe started to absorb background radiation — an essential change that the researchers were able to detect in the form of radio waves.

The findings provide proof that the first stars may have begun turning on about 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. 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.

Alan Rogers, Scientist, Haystack Observatory, MIT

Certain features in the detected radio waves also indicate that hydrogen gas, and the universe as a whole, must have been twice as cold as scientists formerly estimated, with a temperature of around 3 K or –454 °F. Rogers and his colleagues are uncertain precisely why the early universe was so much colder, but some scientists have proposed that interactions with dark matter may have had some effect.

“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), together with Thomas Mozdzen, Nivedita Mahesh, and Raul Monsalve, from the University of Colorado.

Turning on, tuning in

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

The antennas and parts of the receiver were designed and built by Rogers and the Haystack Observatory team; Bowman, Monsalve, and the ASU team incorporated an automated antenna reflection measurement system to the receiver, prepared a control hut with the electronics, built 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 present version of EDGES is the outcome of years of design iteration and instrument calibration so as to attain the levels of precision required for effectively achieving a very difficult measurement.

The instrument was initially designed to capture radio waves released from a time in the universe’s history known as the Epoch of Reionization (EoR). During this age, it’s thought that the first luminous sources, such as stars, galaxies, and quasars appeared in the universe, causing the formerly neutral intergalactic medium, made mainly of hydrogen gas, to become ionized.

Before the appearance of the first stars, the universe was cloaked in darkness, and hydrogen, its most copious element, was nearly invisible, representing an energy state that was indistinguishable from the surrounding cosmic background radiation.

Researchers believe that when the first stars came to be, they provided UV radiation that caused variations in the hydrogen atoms’ distribution of energy states. These variations induced hydrogen’s single electron to spin in alignment or opposite to the spin of its proton, causing hydrogen as a whole to “decouple” from the background radiation. Consequently, hydrogen gas started to either discharge or absorb that radiation, at a typical wavelength of 21 cm, equivalent to a frequency of 1,420 megahertz. As the universe grew over time, this radiation became “red-shifted” to lower frequencies. By the time this 21-cm radiation made contact with present-day Earth, it landed somewhere in the range of 100 megahertz.

Rogers and his colleagues have been using EDGES to try to detect hydrogen that was present during the very early evolution of the universe, so as to determine exactly when the first stars turned on.

There is a great technical challenge to making this detection. Sources of noise can be a thousand times brighter than the signal they are looking for. It is like being in the middle of a hurricane and trying to hear the flap of a hummingbird’s wing.

Peter Kurczynski, Program Director, Advanced Technologies and Instrumentation, Division of Astronomical Sciences, National Science Foundation

The instrument, measuring roughly the size of a small table, is positioned in a remote region of Western Australia where there are very little man-made radio signals to obstruct incoming radio waves from the distant universe. The antenna detects radio waves from the whole sky, and the researchers had initially tuned it to listen in at a frequency range of 100 to 200 megahertz.

A switch hit

However, when the team observed within this range, they at first failed to capture much of any signal. They understood that theoretical models had forecast that primordial hydrogen should radiate emissions within this range if the gas was hotter than the neighboring medium. However what if the gas was in fact colder? Models forecast that the hydrogen should then absorb radiation more intensely in the 50 to 100 megahertz frequency range.

“As soon as we switched our system to this lower range, we started seeing things that we felt might be a real signature,” Rogers says.

Specifically, the researchers detected a flattened absorption profile, or a dip in the radio waves, at about 78 megahertz.

We see this dip most strongly at about 78 megahertz, and that frequency corresponds to roughly 180 million years after the Big Bang. In terms of a direct detection of a signal from the hydrogen gas itself, this has got to be the earliest.

Alan Rogers, Scientist, Haystack Observatory, MIT

The dip in radio waves was stronger and deeper than theoretical models projected, signifying that the hydrogen gas at the time was colder than formerly thought. The radio waves’ profile also equals theoretical predictions of what would be formed if hydrogen were indeed impacted by the first stars.

“The signature of this absorption feature is uniquely associated with the first stars,” Lonsdale says. “Those stars are the most plausible source of radiation that would produce this signal.”

“It is unlikely that we’ll be able to see any earlier into the history of stars in our lifetimes,” lead author Bowman of ASU says. “This project shows that a promising new technique can work and has paved the way for decades of new astrophysical discoveries.”

The researchers say this new detection lifts the curtain on a formerly ambiguous phase in the evolution of the universe.

“This is exciting because it is the first look into a particularly important period in the universe, when the first stars and galaxies were beginning to form,” Lonsdale says. “This is the first time anybody’s had any direct observational data from that epoch.”

The National Science Foundation funded this research.