Thirteen billion years ago our universe was dark. There were neither stars nor galaxies; there was only hydrogen gas left over after the Big Bang. Eventually hydrogen atoms began to clump together to form stars—the very first ones to exist—initiating a major phase in the evolution of the universe, known as the Epoch of Reionization, or EoR.
“The Epoch of Reionization,” Nicole Barry, with ASTRO 3-D, a team that studied a signal that has been traveling across the Universe for 12 billion years, bringing us nearer to understanding the life and death of the very earliest stars told The Daily Galaxy, “will be able to provide insight into some of the last remaining mysteries in the Universe, like the origins of dark matter and dark energy. The EoR signal will also be able to tell us something about how our complex Universe formed from a near-uniform, simple gas. Excitingly, it may also reveal physics that we did not expect. There is a wealth of information, both cosmological and astrophysical, that we could learn from the Epoch of Reionization.”
“Defining the evolution of the EoR is extremely important for our understanding of astrophysics and cosmology,” says Nichole Barry with ASTRO 3-D, a team that studied a signal that has been traveling across the Universe for 12 billion years, bringing us nearer to understanding the life and death of the very earliest stars.
“So far, though, no one has been able to observe it,” said Barry with the University of Melbourne and the ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3-D). “These results take us a lot closer to that goal.”
Shortly after the Big Bang, the Universe was composed of a sea of hot plasma and ions. During the rapid expansion of the early Universe, the protons and electrons cooled and recombined into neutral atoms and molecules, a necessary first step in the star formation process. Eventually, during the EoR, about a billion years after the Big Bang, the first generation of stars produced enough radiation and photons to reionize the surrounding gas in the intergalactic medium.
Before the EoR, there was no light-based signal we could study —”there was no visible light”, said University of Washington professor of physics, Miguel Morales, in a separate study. This dark age was a starless era, a gap of several hundred million years following the Big Bang when elementary particles combined to form hydrogen but no stars or galaxies existed yet to light up the Universe.
In the image of the Epoch of Reionization shown at the top of the page, neutral hydrogen, in red, is gradually ionized by the first stars, shown in white. The image was made by the University of Melbourne’s Dark-ages Reionization And Galaxy Observables from Numerical Simulations (DRAGONS) program. (Paul Geil and Simon Mutch)
Since that point in time, all the energy ever radiated by all the stars that ever existed is still with us, filling the universe with a sort of fog, a sea of photons known as the extragalactic background light (EBL).
Light of the Observable Universe
“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,” observed Dennis Overbye in New York Times, separate from the ASTRO 3-D study. “Most of these stars and galaxies are too far and too faint to be seen with any telescope known to humans.”
The Epoch of Reionization is the billion-year period after hydrogen gas collapsed into the first stars, perhaps 100 million years after the Big Bang, through the ignition of stars and galaxies throughout the universe. These first brilliant objects flooded the universe with ultraviolet light that split or ionized all the hydrogen atoms between galaxies into protons and electrons to create the universe we see today.
The Murchison Widefield Array (MWA), a collection of 4096 dipole antennas set in the remote hinterland of Western Australia, which started operating in 2013, was built specifically to detect electromagnetic radiation emitted by neutral hydrogen—a gas that comprised most of the infant Universe in the period when the soup of disconnected protons and neutrons spawned by the Big Bang started to cool down.
The neutral hydrogen that dominated space and time before and in the early period of the EoR radiated at a wavelength of approximately 21 centimeters. Stretched now to somewhere above two meters because of the expansion of the Universe, the signal persists—and detecting it remains the theoretical best way to probe conditions in the early days of the Cosmos.
The Hidden Signal
However, doing so is fiendishly difficult. “The signal that we’re looking for is more than 12 billion years old,” explains ASTRO 3-D member and co-author Associate Professor Cathryn Trott, from the International Centre for Radio Astronomy Research at Curtin University in Western Australia.
“It is exceptionally weak and there are a lot of other galaxies in between it and us. They get in the way and make it very difficult to extract the information we’re after.” In other words, the signals recorded by the MWA—and other EoR-hunting devices such as the Hydrogen Epoch of Reionization Array in South Africa and the Low Frequency Array in The Netherlands—are extremely messy.
Using 21 hours of raw data, Dr. Barry, co-lead author Mike Wilensky, from the University of Washington in the US, and colleagues explored new techniques to refine analysis and exclude consistent sources of signal contamination, including ultra-faint interference generated by radio broadcasts on Earth.
The result was a level of precision that significantly reduced the range in which the EoR may have begun, pulling in constraints by almost an order of magnitude. “We can’t really say that this paper gets us closer to precisely dating the start or finish of the EoR, but it does rule out some of the more extreme models,” says Professor Trott. “That it happened very rapidly is now ruled out. That the conditions were very cold is now also ruled out.”
Barry also with Australia’s University of Melbourne said the results represented not only a step forward in the global quest to explore the infant Universe, but also established a framework for further research.
“Radio interferometers around the world have made great progress towards detecting the EoR signal, wrote Nichole Barry in an email to The Daily Galaxy. “The two main front-runners in the race to measure the EoR signal have historically been the MWA in Australia and LOFAR in the Netherlands,” she explained. “However, the new Hydrogen Epoch of Reionization Array (HERA) collaboration has recently published their first upper limits. The first EoR upper limits from HERA are astonishing, and we are all excited about where this will lead in the future. These upper limits are from only a subset of what the full array will be able to provide. Hypothetically, this means it will be easy to integrate more and do even better. Not only are these upper limits a huge improvement, but they have also done careful work to simulate all the ways the analysis could bias the measurement.”
HERA is dedicated to observing large scale structure during and prior to the epoch of reionization. HERA is a second generation instrument which combines efforts and lessons learned from the Murchison Widefield Array and the Donald C. Backer Precision Array for Probing the Epoch of Reionization (PAPER).
The Daily Galaxy, Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions and New York Times