When you sweep across the FM radio band, you don’t always hear music—mostly, you hear static. Lots of this ambient noise is actually garbled signals from throughout the Milky Way. If you had perhaps the most sensitive FM receiver on Earth, you might pick up the tiniest dip in volume: a signal that comes not from our galaxy, but from the earliest stars in the Universe.
A team of scientists at the University of Arizona and MIT are reporting today the first observation of a long-predicted radio signal coming from stars formed just a hundred million years after the Big Bang. It’s only initial evidence, and further hunting might change our interpretation of what it means. But it’s a crucial step that could have important implications for telling the story of the early universe.
“After all of the steps over two years, we’re left now with no instrumental explanation—this is truly from the sky,” Judd Bowman, the study’s first author from Arizona State University, told Gizmodo. “It’s a very difficult measurement with a faint signal, and this is the first time that anyone is claiming to see it.”
The observation is basically just a teeny dip in the amplitude of ambient radio waves coming from the lowest part of the FM band. That dip carries lots of meaning.
A radio wave signature from hydrogen, the most common element and building block of stars, permeates the universe. Hydrogen consists of two particles, an electron and a proton, each with a property called “spin” whose values can be “up” or “down.” If the spins are the same, the atom has slightly more energy than if they’re opposite—so if the electron spin flips from parallel to anti-parallel, the atom releases a blip of light with a frequency of around 1.5 GHz. Those light waves are stretched as they travel to us through space, due to the expansion of the universe. The further we look back in time, the more stretched the light is—so older hydrogen emits a longer wavelength than newer hydrogen.
Observing changes and fluctuations to this ever-present signal could reveal new insights about what was going on in the earliest years of the universe, especially the first several hundred million years after the Big Bang, which is mostly invisible to modern telescopes. In these regions, the hydrogen signature has been stretched to frequencies of 50 MHz and 100 MHz—a radio band that mostly encompasses the FM band’s 88 MHz to 101 MHz. Centred around 78MHz, the researchers spotted a slight dip in the amplitude of these waves using a radio antennae in Australia called the Experiment to Detect the Global Epoch of Reionisation Signature, or EDGES.
The EDGES experiment at the CSIRO Murchison Radio Observatory in Western Australia. (Photo: CSIRO Australia)
This dip would have been perfectly explained by the earliest star formation. During this epoch, there’d be lots of ambient hydrogen gas beginning to clump into stars. These stars would have emitted ultraviolet radiation, energy that excites the hydrogen. This excited hydrogen gas, in turn, would absorb the ambient light stemming from 380,000 years after the Big Bang. This dip is a depiction of that absorption from all directions, stemming from around 100 million to 200 million years after the Big Bang.
Bowman’s team wanted to make sure that they understood the universe’s radio emission well enough to interpret their signal. “We went through many tasks ruling out the alternative possibilities we came up with,” he said. Essentially, they assumed the signal was noise until they ruled everything else out, according to the paper published today in Nature.
And the discovery is a really big deal. The dip in amplitude is twice as large as they expected, a hint of dark matter—read more about that here. “Basically, it’s worth two Nobel Prizes if the detection is correct,” Harvard astrophysicist Avi Loeb, who was not involved in this research, told Gizmodo. “One for the first detection of hydrogen from when the universe was 100 million years old, and the second for detecting new physics.”
Loeb gave me a warning, though—this is a single detection from a single experiment. Huge scientific discoveries, like the Higgs Boson or gravitational waves, had two different experiments looking independently at the same data to confirm the discovery. Only one experiment has spotted this dip so far, though others around the world are looking to confirm it.
But should it hold, this discovery may have potentially opened up a whole new field of astronomy, in which scientists could analyse and even map this ancient hydrogen.
“We’re hoping it’s an important milestone and the first step of a productive path ahead,” said Bowman. That path will be long, he said, but one “that’s been eagerly awaited by lots of us.” [Nature]