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.
“Defining the evolution of the EoR is extremely important for our understanding of astrophysics and cosmology,” says Nichole Barry with ASTRO 3-D, about 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.”
In the image of the Epoch of Reionization below, 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 pint in time, 12 billion years ago, 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.
Astrophysicists believe that our universe, which is about 13.7 billion years old, began forming the first stars when it was a few hundred million years old. Since then, the universe has become a star-making machine.
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.”
On December 6, 2018, The Galaxy reported that all the light in the observable universe provides about as much illumination as a 60-watt bulb seen from 2.5 miles away.
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.
In a paper on the preprint site arXiv and soon to be published in the Astrophysical Journal, a team led by Barry, reports a 10-fold improvement on data gathered by the Murchison Widefield Array (MWA) – a collection of 4096 dipole antennas set in the remote hinterland of Western Australia.
The MWA, 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.”
Dr. 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.
“We have about 3000 hours of data from MWA,” she explains, “and for our purposes some of it is more useful than others. This approach will let us identify which bits are most promising, and analyze it better than we ever could before.”
The Daily Galaxy, Max Goldberg, via ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions and New York Times
Image top of page: shows the new version of Hubble’s Ultra-Deep Field. In dark grey you can see the new light that has been found around the galaxies in this field. That light corresponds to the brightness of more than one hundred billion suns. It took researchers at the Instituto de Astrofísica de Canarias almost three years to produce this deepest image of the Universe ever taken from space, by recovering a large quantity of ‘lost’ light around the largest galaxies in the iconic Hubble Ultra-Deep Field.