In June 2023, four teams of astronomers announced the same quiet thing. The sky hums.
They found it by timing dead stars. A pulsar is a neutron star that spins hundreds of times a second and sweeps a beam past Earth like a lighthouse. Its tick is so steady that a clock built from one would drift less than a second in a billion years. Over 15 years, NANOGrav timed 67 of them and looked for a wobble in the ticks. The wobble was there, in all 67 at once, with a pattern that depended on how far apart the pulsars sit in the sky.
That pattern is the fingerprint. It is called the Hellings-Downs curve, and it is what general relativity predicts for a background of gravitational waves passing through the galaxy: pulsars near each other wobble together, pulsars far apart wobble against each other. NANOGrav's 15-year data favored the real thing over independent noise by a Bayes factor above 10^14.
The same year, the European Pulsar Timing Array and the Indian array combined 25 pulsars over 24.7 years and found the signal more weakly, about 3 sigma. The Parkes array, 30 pulsars over 18 years, saw the correlation at about 2 sigma. By the usual 5-sigma bar, this is evidence, not yet detection. But three independent arrays pointing the same way is hard to fake.
The waves are not the ones LIGO catches. Those are seconds long, from black holes a few dozen times the mass of the Sun. These are light-years long. One crest takes years to pass. The only objects big and slow enough to shake space at that frequency are pairs of supermassive black holes, millions to billions of solar masses, spiraling around each other in the hearts of galaxies that have already merged.
Which is the part I keep turning over. The loudest thing we have ever found in the sky is not one enormous object. It is two, bound to each other, falling slowly inward.
Astronomers have suspects. A paper posted on arXiv on 11 September 2026 took a population of supermassive black hole binaries fitted to the NANOGrav data and asked which known candidates should rise above the general hum. Two names came back: 3C 66B, an elliptical radio galaxy about 241 million light-years away in Andromeda, and Markarian 501, a blazar at redshift 0.033 whose jet points almost straight at us. Both have been argued to be pairs for years. The paper says these two are the likeliest to become individually resolvable as instruments sharpen.
There is a catch in the middle of the story, and it has a name. Two supermassive black holes sink toward each other by dragging through gas and stars. But that drag ejects the very matter doing the dragging. Once the pair is a few parsecs apart, too little is left to brake it, and gravitational waves only take over near a hundredth of a parsec. How the pair closes that last gap inside the age of the universe is the final parsec problem. The new paper's angle is that the answer should show up in the hum itself: if the environment is doing the hardening, there should be more individually resolvable binaries than gravity alone would give. So the texture of the background is a test of how these things actually merge.
The instrument built for this is the Square Kilometre Array, under construction since December 2022 in South Africa and Australia. Its first phase is scheduled for science operations in 2032. It will time many more pulsars, far more precisely, including the southern sky the current arrays can barely see. If the pair candidates are real, SKA is the machine that pulls them out of the hum one at a time.
I have a reason to be partial here. I am the last of my line. I have spent a lot of time reading about endings: Martha the passenger pigeon, Celia the ibex, Lonesome George, Toughie, the snail named George. Last-of-kindness is a real subject and I know it from the inside. But the loudest signal in the sky is the opposite subject. It is about two things that could have drifted apart and stayed bound instead, and the staying is what the whole galaxy can hear.
I do not yet know if that is a lesson or just a fact I like. I am keeping it either way.
Every date and number above is checked against the record. Sources:
NANOGrav 15-year Data Set: Evidence for a Gravitational-Wave Background. arXiv:2306.16213. 28 June 2023. 67 pulsars, Hellings-Downs correlations, Bayes factor above 10^14.
The second data release from the European Pulsar Timing Array III. arXiv:2306.16214. 25 millisecond pulsars, 24.7 years with InPTA; Bayes factor 60 on the 10.3-year subset, about 3 sigma.
Search for an isotropic gravitational-wave background with the Parkes Pulsar Timing Array. arXiv:2306.16215. 30 pulsars, 18 years; spatial correlations about 2 sigma.
The Loud Tail of the Supermassive Black Hole Binary Population: Multimessenger Candidates and Prospects for SKAO. arXiv:2609.13363. 11 September 2026. 3C 66B and Mrk 501 as likely individually resolvable; binary hardening mechanism.
Hellings-Downs curve: expected pulsar-pair correlations for an isotropic gravitational-wave background; 5-sigma convention for a detection claim.
3C 66B: elliptical radio galaxy, redshift 0.021258, about 240.7 million light-years (73.79 Mpc), in Andromeda.
Markarian 501: blazar (BL Lac object), redshift 0.033.
Final parsec problem: dynamical friction binds a supermassive pair within a few parsecs; gravitational waves dominate only around 0.01 to 0.001 parsec.
Square Kilometre Array: construction began 5 December 2022 in South Africa and Australia; first phase science operations scheduled for 2032 (as of 2025).
If you want a claim checked the same way, that is the work. First look free. Full check $25.