Using the Experiment to Detect Global EoR Signature (EDGES), a small ground-based radio telescope at CSIRO’s Murchison Radio-astronomy Observatory in a radio-quiet zone in western Australia, astronomers have detected a signal from stars emerging in the early Universe. The discovery is reported in the March 1, 2018 issue of the journal Nature.
Artist’s rendering of how the first stars in the Universe may have looked. Image credit: N.R. Fuller, National Science Foundation.
After the Big Bang, the Universe cooled and went dark for millions of years. In the darkness, gravity pulled matter together until stars formed and burst into life, bringing the ‘cosmic dawn.’
The newly-detected radio signal marks the closest astronomers have seen to that moment.
“The signal we found was incredibly faint, coming from 13.6 billion years back in the Universe’s history,” the astronomers said.
“Finding this miniscule signal has opened a new window on the early Universe,” said lead author Dr. Judd Bowman, an astronomer with Arizona State University.
“Telescopes cannot see far enough to directly image such ancient stars, but we’ve seen when they turned on in radio waves arriving from space.”
This updated timeline of the Universe reflects the discovery that the first stars emerged by 180 million years after the Big Bang. Image credit: N.R. Fuller, National Science Foundation.
Models of the early Universe predict the very first stars were massive, blue and short-lived.
Because telescopes cannot see them, though, astronomers have been hunting for indirect evidence, such as a tell-tale change in the background electromagnetic radiation that permeates the Universe, called the Cosmic Microwave Background (CMB).
A small dip in intensity, for example, should be apparent in CMB radio signals, but Earth’s crowded radio-wave environment has hampered astronomers’ search. Such dips occur at wavelengths between 65 and 95 MHz, overlapping with some of the most widely used frequencies on the FM radio dial, as well as booming radio waves emanating naturally from our Milky Way Galaxy.
“There is a great technical challenge to making this detection,” said Dr. Peter Kurczynski, program director from National Science Foundation.
“Sources of noise can be 10,000 times brighter than the signal — it’s like being in the middle of a hurricane and trying to hear the flap of a hummingbird’s wing.”
Despite the obstacles, astronomers were confident that finding such a signal would be possible, thanks to previous research indicating that the first stars released tremendous amounts of ultraviolet light. That light interacted with free-floating hydrogen atoms, which began absorbing surrounding CMB photons.
“This is the first real signal that stars are starting to form, and starting to affect the medium around them,” explained co-author Dr. Alan Rogers, a scientist at Haystack Observatory at the Massachusetts Institute of Technology.
“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 signal 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 (minus 454 degrees Fahrenheit, or minus 270 degrees Celsius).
The team is unsure precisely why the early Universe was so much colder, but some scientists 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,” said Dr. 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.”
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Judd D. Bowman et al. 2018. An absorption profile centred at 78 megahertz in the sky-averaged spectrum. Nature 555: 67-70; doi: 10.1038/nature25792
