Telescopes are, and have always been, time machines. They transport us not only across space, but across the aeons, showing us the light of long-dead stars and the glow of cosmic fires extinguished billions of years in the past. We have seen galaxies so far away that their light has been traveling since before the Earth was here to receive it. And we have seen the light of the Big Bang itself, the cosmic microwave background, a glow that comes to us from every direction at once, carrying the fading light from a time when the entire cosmos was awash in blistering radiation and heat.
Over the past two decades, astronomers have been on a quest to peer into an epoch of cosmic history that has been simultaneously shrouded in darkness and obscured by too much light. The era of Cosmic Dawn, when the first stars were born, their light cutting through the dense fog of ubiquitous neutral hydrogen and stray background photons, should be impossible to observe. Individual stars at that distance would never be visible, and neutral hydrogen absorbs starlight with astonishing efficiency. The only hope is to try to see the neutral hydrogen itself, backlit by the cosmic microwave background and primed by starlight to absorb that radiation at a particular range of radio frequencies.
But those same frequencies are the ones used by FM radio, satellite communications, and digital TV—what’s one more glitch in the spectrum? Even worse, our own galactic home, the Milky Way, creates a radio cacophony so boomingly loud, picking up the tiny neutral hydrogen signal would be like trying to hear a whisper at a rock concert, after someone has set off the fire alarms.
Still, we are scientists, and the mysteries of the universe are right there, just waiting. We persevere.
The announcement today that a team of astronomers has succeeded, after a twelve-year quest, to pick up that faint first blush of starlight in the newborn universe, represents an astonishing achievement, in more ways than one. The EDGES collaboration has been working tirelessly, setting up their radio antenna in an isolated patch of Australian desert to avoid TV and FM signals, testing and retesting and calibrating and recalibrating to try tease out that tiny cosmic whisper. If confirmed, their signal, a jagged little absorption dip in a broad radio spectrum, is the most distant astronomical observation ever made, aside from the cosmic microwave background itself.
It is the earliest indication of any kind of structure in the Universe, and a direct window into the processes that led all that unassuming hydrogen gas to condense, under gravity, into stars, and galaxies, and, eventually, life. It is also the earliest window into the formation of black holes. When a massive star dies, leaving behind a black hole, the doomed gas and dust it captures creates a whirlpool so hot it shines in x-ray radiation. The cosmic dawn’s tiny absorption signal, primed by the light of the stars, is snuffed out again by the x-rays of the first black holes.
The shape of this absorption dip—the downward slope carved out by stars, the upward by black hole x-rays—has been endlessly modeled and predicted, with all possibilities in mind. What if the stars were a little more massive than we thought? What if they were hotter? What if the black holes were a little less efficient? But the signal the EDGES team found doesn’t look like the models. It carries with it a surprise, and another mystery. The dip is too deep. That tiny whisper is more of a shout.
There are only two ways an absorption signal against background radiation could be stronger than expected. One is if the radiation was brighter than we thought. There has been an unexplained radio background seen, but not at these frequencies, and we don’t know what could make so many radio waves at the time of the first stars. The other possibility is that the gas was colder than we thought, thus absorbing more of the background light. As far as we know, there was only one thing in the Universe at that time that was colder than the neutral hydrogen: dark matter.
We’ve never before seen dark matter interact with gas in any way, except through gravity, and gravity alone can’t cool down gas. Every piece of evidence we have for the existence of dark matter thus far comes from the way its gravity moves stars and galaxies, or bends space itself. We have very good evidence that it’s out there, that it helped build our galaxy through its gravitational pull, and that it makes up much more of the universe than regular matter does, but we have no evidence at all that it can touch regular matter, or that it could, through contact, cool it. Every observation we have shows it passing right through itself and everything else, ghostly and invisible. If this signal really is detecting a new kind of dark matter interaction, it’s not only the first confirmation of dark matter making its presence felt, it’s also a magnificent confirmation that dark matter is a real, tangible component of the cosmos. In short, if this signal is what it looks like, it changes everything.
But let’s not break out the champagne just yet. As astronomers, we have learned to be cautious. Spectacular signals of new physics from the early universe are easy to get excited about, but we have been burned before. This is only a single experiment—a lone radio antenna in a dusty desert—and no amount of care and cross-checking can substitute for independent confirmation. More experiments are coming. In just a few short years, half a dozen other groups will confirm or contradict the EDGES result, and the interpretation of its meaning for dark matter will be hotly debated in the weeks and months to come.
Personally, I hope it holds up. I hope the Universe is telling us something new, something amazing, something that will point us theorists in a totally unexpected direction. We will know more soon enough. In the meantime, we astronomers will continue on as we always have: build ourselves another time machine, see what new wonders we might find.