it was pretty impressive when we built this giant machine that spotted gravitational waves from colliding black holes well we've just taken it to the next level with a galaxy spanning gravitational wave detector that may have detected a foundational element of space itself the gravitational wave background when the laser interferometer gravitational wave observatory ligo turned on in 2015 it quickly spotted exactly what it was built to detect vast ripples in the fabric of space-time produced by colliding black holes a billion light years away since then ligo and its partner virgo have detected 50 similar signals from merging black holes and neutron stars across the universe these sweep past the earth every few days causing space itself to oscillate lengths and widths contract and expand in turn by the tiniest fraction one one thousandth of the width of a single proton over the four kilometer arms of ligo antennas and these are only the most powerful gravitational waves that we can detect produced by only relatively nearby mergers the reality of these strong space-time ripples reveals an awesome truth we know that black holes and neutron stars have been merging for all of cosmic history and supernovae have been exploding and stellar remnants have been spinning and probably at or before the instant of the big bang insanely energetic events took place and all of these produce gravitational waves that means the entire universe must be thrumbing with these vibrations all the time and everywhere we call this underlying buzz the gravitational wave background and it could reveal the lives of the most massive black holes in the universe and provide a glimpse into the time before the big bang and reveal the structure of space time itself oh and we may have just detected it ligo itself may be able to detect a small part of the gravitational wave background or the gwb but it's not there yet but it turns out that we already have another device that may have done the job it's a gravitational wave observatory that spans the entire galaxy and recruits some of the most bizarre stars in the universe as our measuring devices i'm talking about the pulsar timing array and about the first tentative detection of the gravitational wave background revealed in early january by the nanograv collaboration but it's going to take a minute to get to that first let's talk about pulsar timing arrays and why we need them ligo has a fundamental limit it can only see those gravitational waves with frequencies around 1 to ten thousand hertz corresponding to relatively compact waves from tens of kilometers to hundreds of thousands of kilometers in wavelength ligo can't see ripples with either smaller or larger wavelengths ligo is like a rowboat on the ocean you're sitting in the rowboat eyes closed you can feel the rise and fall and the rocking of the boat in the choppy ocean you and your boat are sensitive to those waves very tiny ripples on the ocean surface just lap at the hull but you don't notice them and there are also vast rolling waves that are so big that the rowboat feels steady eyes closed you don't notice those either okay now you're on a giant cargo ship it tips forwards and backwards in the ocean due to those giant waves eyes closed on deck you notice that tipping but are oblivious to the little waves that rocked the rowboat and there are also much larger waves a tsunami perhaps or the planet-sized wave of the tide itself there's no human-built ship that is rocked by those waves it's the same with gravitational waves ligo is rocked by the little ripples produced at the instant of merger of stellar remnants like black holes and neutron stars or potentially in supernova explosions but we know the space-time ocean must have swells that are millions to billions of kilometers long produced by the merger of the most massive black holes in the universe or by the slow in spiral of smaller compact objects in our galaxy for that we're building our giant ocean tanker it's the laser interferometer space antenna or lisa but even the largest ships are oblivious to the ocean tide and ridiculously colossal gravitational waves must also exist waves whose single oscillations span the solar system or even the distance to the nearest stars such gravitational waves are expected to be produced in a few different ways black holes with masses of millions to billions of suns live in the cores of most galaxies these super massive black holes can end up orbiting each other when galaxies collide and that slow fatal in spiral produces enormous gravitational waves and there are theoretical sources also cosmic strings topological seams in the fabric of space would produce waves when their kinks become unkinked in the speculative time before the big bang cosmic inflation was driving exponentially accelerating expansion and huge gravitational waves would have been produced in the phase transition that ended inflation all of these space-time tsunamis would be invisible to our tiny human-made machines to see them we'd need a gravitational wave observatory spanning the galaxy unfortunately nature has provided us with just such a network a system of boys scattered across the milky way each fitted with the most perfect measuring devices known to science obviously i'm talking about pulsars to remind you a pulsar is the collapsed core of a massive star masses of at least one and a half suns packed into balls 20 kilometers across and that extreme density converts all of the matter into neutrons they are on the edge of absolute collapse into black holes supported only by weird quantum forces neutron stars tend to channel jets of high energy particles due to their intense magnetic fields they also rotate rapidly with the rotational axis offset from the magnetic poles causing those jets to sweep through space to process when the jets sweep past the earth we see flashes of electromagnetic radiation and we call those flashing sources pulsars some flash every few seconds but the fastest millisecond pulsars can rotate hundreds of times per second just to give you a feel for that this is what a typical pulsar sounds like that's the sound of a trillion trillion trillion tons rotating 1.4 times per second and this is the crab pulsar with its 30 rotations per second and this is a true millisecond pulsar at 174 rotations per second and the fastest known pulsar with its 642 hertz rotation these pulses are not just fast they're exquisitely regular with the most stable millisecond pulsars the exact instant of one of these ticks can be predicted to within one tenth of a millionth of a second several years prior to that individual pulse so nature has graced us with these perfect clocks scattered across the galaxy but how do we use them to detect gravitational waves well the whole thing about gravitational waves is that they change distances which can change the travel time of the pulses from a pulsar leading to a difference in the arrival time of those pulses compared to what was expected here's a simple example imagine a pair of supermassive black holes that are orbiting each other on the other side of the universe as their gravitational waves pass through the milky way they cause space to stretch and contract in succession at right angle to the direction of the wave's motion in what is known as a quadrupole wave if the wave passes through a ring of points those points would move in and out like this okay so imagine these points are pulsars and the earth is at the center of the ring as the wave passes by the signal from these pulsars would have to travel a little further there'd be a lag in the arrival time during that part of the gravitational wave oscillation but for these pulsars the distances traveled are shorter so the signal arrives early after one oscillation cycle the situation reverses if we had pulsars in all directions we could record the so-called timing residuals the lateness or earliness of the pulses and we'd record this sort of shamrock shape where the petals are large timing residuals and the diagonals show very little timing residual of course space is three-dimensional a pulsar signal coming from the direction of the gravitational wave would actually ride that wave keeping pace with it and so may spend a lot of the journey either in the stretched out region of space or in a compacted region those guys have large timing residuals while a pulsar on the opposite side will skip through many peaks and troughs of the gravitational wave so that its timing residual cancels out the end result is a very predictable pattern to the lag experienced by pulsar signals as a function of their position it's the so-called antenna pattern a gravitational wave coming from here towards the earth here causes the largest atomic offsets for pulsars within this structure now a single pulsar isn't enough to map this structure and it's not enough to be even sure that we're seeing a gravitational wave other things can cause shifts in pulsar signals for example neutron stars experience star quakes which can glitch their rotational rates the magnetized plasma between the stars also slows radio waves from pulsars and the relative motion of the earth the sun and the milky way shifts the frequency of pulsars but if you have an array of pulsars a pulsar timing array in fact then this characteristic pattern leads to unmistakable correlations in the arrival times of many pulsar signals so this is probably where you want me to tell you that our galactic array of quantum star clocks has revealed gigantic gravitational waves from some colossal black hole dance or from the big bang or something well maybe but not quite so simple at least not yet instead we may have detected the sum of all such waves the cosmic gravitational wave background let's get to the actual result nanograv the north american nanohurts observatory for gravitational waves made an announcement at the american astronomical society meeting in early january nanograve is a collaboration of over 100 us and canadian researchers and itself is one of several groups that together make up the international pulsar timing array which combines many of the largest radio telescopes on the planet to monitor around 100 millisecond pulsars nanograv's own pulsar timing array consists of 47 pulsars which it monitors with the green bank observatory in west virginia and with the arecibo observatory in puerto rico but i should say it previously monitored with arecibo you might have heard the iconic arecibo is no longer with us after its hurricane damage cable snapped causing a catastrophic collapse on december 1st we had the great honor to visit arecibo in 2019 and we shot some 180 video there i invite you to climb the catwalk with me one last time it's a tragedy to have lost such an important workhorse for our exploration of the universe but nanograf's findings are now part of arecibo's broad and enduring legacy while nanograv didn't detect a single resolvable gravitational wave its 12.5 years of observations have tentatively detected the gravitational wave background this is the sum of all gravitational waves too weak to be detected individually but which together form this universe wide through of activity the gwb doesn't produce the telltale quadrupole antenna pattern rather it results in more subtle correlations in the timing residuals between pulsars individual fluctuations in this background will result in a correlation across all pulsars in the array and the entire background results in this overlapping set of correlations that can be extracted with sophisticated statistical techniques ultimately the result will be a distribution of wave frequency a spectrum for the background and the shape or the slope of that spectrum will help tell us whether the gravitational wave background is produced by supermassive black holes cosmic strings or events during the inflationary epoch or a combination of all of the above but the current nanograph result isn't quite sensitive enough to measure the shape of the spectrum yet it's just revealed a signal correlation in pulsar timing residuals that can't be explained by other known processes if this is real then there's no question that the gravitational wave background will become better and better resolved as we add pulsars to the timing array and we stare at it for longer and we add more and larger radio telescopes the promise is a deeper understanding of the weirdest things in the universe and if we manage to prove the existence of say cosmic strings or inflation or other weird physics this way then we have opened a window into realms of physics beyond our current imagination all it takes is our galactic fleet of star clocks bobbing in the ripples across the ocean of spacetime.