The whole universe is humming. Actually, the whole universe is Mongolian throat singing. Every star, every planet, every continent, every building, every person is vibrating along to the slow cosmic beat.
That’s the takeaway from yesterday’s remarkable announcement that scientists have detected a “cosmic background” of ripples in the structure of space and time. If the result bears up as more data are gathered, it’s a discovery that promises to open new windows on everything from the evolution of galaxies to the origin of the universe.
Scientists had been awaiting such a discovery for decades. More than 100 years ago, Einstein introduced his radical general theory of relativity. For Einstein, space and time were a single entity, “space-time,” comprising a flexible fabric that could be stretched and compressed, bent and warped. In general relativity, matter makes space-time bend, and space-time, in turn, guides how unconstrained matter will move. Because space-time is flexible, you can make it wave. Just like snapping a bedsheet, if you move enough matter around fast enough, a wave of distorted space-time will ripple outward into the universe.
Scientists predicted the existence of these ripples, called gravitational waves, as far back as the 1910s. But because the distortions they create in space-time are so minute, they weren’t detected until eight years ago. That’s when physicists used mile-long lasers to catch distortions in space-time from two black holes colliding in a distant galaxy. With that first epochal discovery, the doors to gravitational-wave science were thrown open. Since then, scientists have found many other merging black-hole pairs. But an even fainter signal carrying a profound cosmic significance still lay out of their reach.
Along with signals from discrete black-hole mergers, experts believed that a background of gravitational waves should also be washing through the universe, the space-time equivalent of car horns, jackhammers, and shouts all combining into the diffuse cacophony of city life. Plenty of cosmic phenomena could produce such a gravitational-wave background, and astronomers are now busily debating the most likely explanation. Perhaps the culprits are the zillions of supermassive black holes, some billions of times heavier than the sun, that reside at the center of every galaxy. Over cosmic timescales, galaxies collide and merge—and so do their black holes. These are near-apocalyptic events in terms of their effect on space-time, like a wall of speakers at a heavy-metal concert blasting against so many eardrums. Untold numbers of galaxies have merged across the 13.8-billion-year life of the universe, and those blasts should still be echoing in the background of space-time today. And so, perhaps, should the gravitational waves from the birth of the universe itself. The Big Bang was, well, a big bang. Initiating the expansion of everything required so much energy, and did so much violence, that it should have flooded space-time with gravitational waves that continue to ricochet around the universe to this day.
If scientists could find and analyze this background, they’d have a direct look all the way back to the first slivers of time after the moment of creation. First, however, they’d have to prove it exists. And now it seems they might have. A team of astronomers from around the world, working together as the North American Nanohertz Observatory for Gravitational Waves (a.k.a. NANOGrav), made the detection using the rapidly spinning cinders of once-massive stars called pulsars. Pulsars emit bursts of radio waves so perfectly timed that they serve as one of the universe’s most accurate natural clocks. Through 15 long years of sweat and perseverance, the NANOGrav scientists patiently tracked tiny changes in the burst patterns of 67 pulsars scattered across the Milky Way.
They found that a small change in the period of any one pulsar’s signal was linked to changes in the others’. These linked anomalies, they concluded, were reflections of changes in the distance between Earth and the pulsars as passing space-time ripples caused those distances to continually grow and shrink. Putting it all together, the NANOGrav scientists could see that these ripples were not from one discrete source but from a din, a hum, the overlapping echoes of disturbances scattered across the universe.
Over the course of a decade and a half, the NANOGrav team pored over their machines, their numbers, and their mathematical theory to bring us proof that something miraculous—something wonderful—is happening right under our noses. Actually, it’s happening to our noses, and the rest of our bodies as well. Every gravitational wave in that background the NANOGrav team found is humming through the very constitution of the space you inhabit right now. Every proton and neutron in every atom from the tip of your toes to the top of your head is shifting, shuttling, and vibrating in a collective purr within which the entire history of the universe is implicated. And if you put your hand down on a chair or table or anything else nearby, that object, too, is dancing that slow waltz.
The gravitational-wave background is huge news for the cosmos, yes, but it’s also huge news for you. The nature of reality has not changed—you will not suddenly be able to detect vibrations in your morning coffee that you couldn’t see before. And yet, moments like these can and should change how each of us sees our world. All of a sudden, we know that we are humming in tune with the entire universe, that each of us contains the signature of everything that has ever been. It’s all within us, around us, pushing us to and fro as we hurtle through the cosmos.
As an astronomer, I am often asked about UFOs. I’m pretty skeptical about them having anything to do with alien life, but I believe the questions represent something ancient and innate in us all. As children, each of us had a deep and easily triggered sense that the world is full of wonder, that everything is strange and amazing. Stepping out into the backyard, we’d get entranced staring at an anthill or watching leaves pirouette as they fell. As a toddler, my daughter would purposely tip her cup over just to be delighted by how the water spilled across the table.
Today, gifted with a new understanding of the architecture of the universe, each of us has an opportunity to revisit that wonder. After you finish reading this, take a look around you. Ponder how the solid-seeming ground beneath your feet is quietly shaking with the force of billions of years of cosmic collisions. Go outside, if you can, and watch the wind blow through the trees. Perhaps the experience will be different now that you know how the rhythm of giant black holes in distant galaxies also beats out a time in the trees’ gentle swaying.
The universe is an impossibly vast symphony of cause and effect. The endless comings and goings of galaxies, stars, and planets create a melding of songs that you are part of too. The NANOGrav discovery exposes the intricacy and gracefulness of that melding. It’s a reminder that the world always has, and always will be, worthy of wonder. But of course, you already knew that. You always have.
