#quantum gravity and quantum loop theory

[csms-jpg]

My journals ( slight obbsesion with the stars ) and a Book Reality is not what it seems by Carlo Rovelli

[whats-in-a-sentence]

A one-sentence comparison would hold that string theorists start with the small (quantum theory) and move to embrace the large (gravity), while adherents of loop quantum gravity start with the large (gravity) and move to embrace the small (quantum theory).⁹

9. This statement is more one of sociology than of physics. String theory grew out of the tradition of quantum particle physics, while loop quantum gravity grew out of the traditional general relativity. However, it is important to note that, as of today, only string theory can make contact with the successful predictions of general relatively, since only string theory convincingly reduces to general relativity on large distance scales. Loop quantum gravity is understood well in the quantum domain, but bridging the gap to large-scale phenomena has proved difficult.

"The Fabric of the Cosmos" - Brian Greene

[tenth-sentence]

One harmonious possibility is that string enthusiasts and loop quantum gravity aficionados are actually constructing the same theory, but from vastly different starting points.

"The Fabric of the Cosmos" - Brian Greene

[learningbreeze]

[carolkeiter]

A Seahorse Tale ~ A Spin on the Matter of Motion

A Seahorse Tale ~ A Spin on the Matter of Motion is a young adult educational eBook cloaked as a sci-fi. Each of the 170 pages have beautifully illustrated art and science diagrams. Interwoven throughout the story are non-fiction links to various topics and state-of-the-art science, which corroborate the storyline and substantiate the possibility of the occurrence of these fantastical and…

[mysticstronomy]

IS TIME INFINITE IN BLACK HOLES??

Blog#365

Wednesday, January 10th, 2024.

Welcome back,

The singularity at the center of a black hole is the ultimate no man's land: a place where matter is compressed down to an infinitely tiny point, and all conceptions of time and space completely break down. And it doesn't really exist. Something has to replace the singularity, but we're not exactly sure what.

Let's explore some possibilities.

It could be that deep inside a black hole, matter doesn't get squished down to an infinitely tiny point. Instead, there could be a smallest possible configuration of matter, the tiniest possible pocket of volume.

This is called a Planck star, and it's a theoretical possibility envisioned by loop quantum gravity, which is itself a highly hypothetical proposal for creating a quantum version of gravity.

In the world of loop quantum gravity, space and time are quantized — the universe around us is composed of tiny discrete chunks, but at such an incredibly tiny scale that our movements appear smooth and continuous.

This theoretical chunkiness of space-time provides two benefits. One, it takes the dream of quantum mechanics to its ultimate conclusion, explaining gravity in a natural way.

And two, it makes it impossible for singularities to form inside black holes.

As matter squishes down under the immense gravitational weight of a collapsing star, it meets resistance. The discreteness of space-time prevents matter from reaching anything smaller than the Planck length (around 1.68 times 10^-35 meters). All the material that has ever fallen into the black hole gets compressed into a ball not much bigger than this.

Perfectly microscopic, but definitely not infinitely tiny.

This resistance to continued compression eventually forces the material to un-collapse (i.e., explode), making black holes only temporary objects. But because of the extreme time dilation effects around black holes, from our perspective in the outside universe it takes billions, even trillions, of years before they go boom. So we're all set for now.

Another attempt to eradicate the singularity — one that doesn't rely on untested theories of quantum gravity — is known as the gravastar. It's such a theoretical concept that my spell checker didn't even recognize the word.

The difference between a black hole and a gravastar is that, instead of a singularity, the gravastar is filled with dark energy. Dark energy is a substance that permeates space-time, causing it to expand outward. It sounds like sci-fi, but it's real: dark energy is currently in operation in the larger cosmos, causing our entire universe to accelerate in its expansion.

As matter falls onto a gravastar, it isn't able to actually penetrate the event horizon (due to all that dark energy on the inside) and therefore just hangs out on the surface. But outside that surface, gravastars look and act like normal black holes. (A black hole's event horizon is its point of no return — the boundary beyond which nothing, not even light, can escape.)

However, recent observations of merging black holes with gravitational wave detectors have potentially ruled out the existence of gravastars, because merging gravastars will give a different signal than merging black holes, and outfits like LIGO (the Laser Interferometer Gravitational-Wave Observatory) and Virgo are getting more and more examples by the day. While gravastars aren't exactly a no-go in our universe, they are definitely on thin ice.

Originally published https://www.space.com/

COMING UP!!

(Saturday, January 13th, 2024)

"JUPITER HAS A LARGE MAGNETIC FIELD THAN PREVIOUSLY EXPECTED??"

[simpsforscience]

⁉️ What if you're told to forget smooth spacetime and imagine the universe as a cosmic quilt made up of loops? 💭 ➰ That's Loop Quantum Theory! which proposes that space itself is made of tiny, 🤏🏻 interconnected loops, ➿ a fabric of reality finer than any thread. Swipe ➡️ through this post to find how it is a potential quantum upgrade to Einstein's gravity, where the universe isn't just curved, it's knotted!🪢☄️🌠

📸 Image Credits:

Abhay Astekar :- PennState Eberly College of Science

Carlo Rovelli :- Foto LaPresse

Lee Smolin :- footnotes2plato

[frank-olivier]

Not Even Wrong: The Case Against String Theory

Peter Woit is a mathematician and physicist known for his work in quantum field theory and quantum gravity. He is a senior lecturer at Columbia University, where he has been involved in teaching and research for many years. His academic interests lie in the mathematical underpinnings of theoretical physics, particularly in understanding the fundamental forces and particles that make up the universe.

Woit is perhaps best known for his critical stance on string theory, a prominent theoretical framework that attempts to reconcile quantum mechanics with general relativity by proposing that the fundamental constituents of the universe are one-dimensional "strings" rather than point particles. Despite its popularity, string theory has faced criticism for several reasons. One of Woit's primary criticisms is that string theory has not produced any testable predictions that can be verified through experiments. This lack of empirical evidence makes it difficult to validate or falsify the theory. Additionally, Woit argues that string theory has led to a proliferation of possible solutions, known as the "landscape problem," where an enormous number of possible universes exist within the theory's framework. This makes it challenging to identify which, if any, correspond to our observed universe. Furthermore, Woit has expressed concern that the dominance of string theory has stifled alternative approaches to understanding fundamental physics, such as twistor theory and loop quantum gravity. Twistor theory, originally proposed by Roger Penrose, offers a mathematical framework that could potentially unify general relativity and quantum mechanics. Loop quantum gravity is another approach to quantum gravity that attempts to quantize spacetime itself without relying on strings.

The detailed critique of string theory in his book "Not Even Wrong" explores the history and development of theoretical physics. The title refers to a phrase attributed to physicist Wolfgang Pauli, describing theories that are so flawed they cannot even be considered incorrect. Woit argues that string theory fits this description due to its lack of testable predictions. He believes that fostering a diverse range of ideas and approaches is essential for advancing our understanding of fundamental physics and he hopes to inspire future breakthroughs in areas like quantum gravity and particle physics by encouraging open-mindedness and innovation.

Peter Woit: String Theory and the Crisis in Physics (Robinson Erhardt, September 2024)

Saturday, September 21, 2024

[wayti-blog]

"In a continuous pursuit to understand the fundamental laws that govern the universe, researchers have ventured deep into the realms of string theory, loop quantum gravity, and quantum geometry. These advanced theoretical frameworks have revealed an especially compelling concept: the generalized uncertainty principle (GUP).

This principle fundamentally challenges traditional physics by proposing a minimal measurable length, which could profoundly alter our foundational understanding of space and time. It challenges the bedrock of classical mechanics and invites a reevaluation of quantum mechanics and general relativity.

The GUP has catalyzed an impressive range of research efforts, extending from the microscopic domain of atomic physics to the cosmic scales of astrophysics and cosmology. Investigations have explored phenomena such as gravitational bar detectors, condensed matter systems, and the dynamics of quantum optics.

Each study contributes to a broader understanding of the potential implications of the GUP, suggesting it could fundamentally transform our understanding of physics across various scales and systems.

Building upon these insights, our research, published in the International Journal of Modern Physics D, introduces a transformative concept: an "effective" Planck constant. This idea challenges the traditional view of the Planck constant as a static, immutable value, proposing instead that it might vary depending on specific experimental or environmental conditions, particularly the momentum or position of the system under observation."

continue reading

[purplekoop]

I think because of the specific ways I had internet access growing up and what I was dealing with during "traditional fandom baby time", I've felt kind of distant from the standard fandom experience, even for fandoms that I've followed the main official content for while it was ongoing. At most it was a one-way thing, where I could look into the fandom but not really be an active contributor.

I saw 2012 TMNT on the night of the premier and did my best to watch any episodes I could for a good few years after that, but looking up fanart at 2 AM on my phone when my parents couldn't see was the most I really saw people interacting with it further. I saw a Raph x Mona fankid and it blew my mind for... some reason. Look maybe just the image and notion of the tall space lizard lady being affectionate just did something to a young me. I'm not even sure I saw her debut episode before seeing the art so that was. Probably confusing.

There was also Gravity Falls, a show that I was really only able to follow thanks to wiki articles youtubers piecing together the clues that I didn't have the brainpower or ability to pause episodes to figure out myself. Shoot, shoutouts to Valiskibum, a channel I haven't checked on in years but is still a legend for posting about the theory that predicted *Ford.* I don't know if they came up with the theory on their own or if they just made a video sharing it to a wider platform, but in either case they get appreciation from me for making my dumb kid brain aware of it. And even other than that, I watched ANY news about the show LIKE A HAWK while it was still airing. I tried to follow the Cipher hunt but at that point I lost track, and any side media like books and comics that were released after that is news to me whenever it gets brought up now.

But like. Even *then* it feels like there was so much I missed. I've been following Hana Hyperfixates on YouTube lately (their tumblr url is escaping me), and it boggles me that even for a show I stayed caught up with constantly throughout its entire run, fully aware of its deeper mysteries, it still feels like I missed so much of the experience of being a fan of the show because I just. Wasn't in the fandom loop.

And those are just the shows I *did* follow as they were airing, that doesn't even scratch the stuff I wasn't into as it was happening.

I didn't see a full playthrough of *Undertale* until November of *2021* where I played it alongside my partner during thanksgiving break that year. And not making the same mistake twice, I had followed Deltarune soon after its spontaneous reveal, because I didn't have much better to do with my increased internet access. I played Chapter 2 as soon as it came out, so I was lost on everything from the lion waitress to "what's a NEO" until after the fact. Look, I just thought an Undertale was an annoying popular thing my friends with bad taste were into back in middle school. Now here I am, quantum egg on my face, panicking over the fact Papyrus has touched grass.

All this to say that now finally having this here tumblr starting at "only" the age of 20 (which, in the grand scheme of things, isn't as far off from 14 or 16 as people make it out to be, I'm basically just a teenager with more self awareness) is... interesting. In a nice way, I think.

It's this silly little fandom stuff like fics and OCs and RP and AUs that always felt so distant to me, but was basically the later childhood of my partner and many of my current friends. Stuff that they treat so naturally because they basically grew up on it, while I'm basically just now really getting a grasp on it myself.

It's stuff that I'm just now getting the platform for and familiarity with to really let myself indulge in. It's why I'm so happy with the Splatoon OCs, it's me finally getting to have that silly fun with this kind of stuff that it feels like I've been missing out on for a while.

I've had fan OCs before, all the way back in middle school, to the extent they made a full expansive crossover universe combining every series in Smash Bros into a unified cohesive canon (that's where my username comes from in fact, my sona for years was literally just a unique purple-shelled koopa). My friends were in on it and everything. But then after hitting high school, I eventually abandoned that universe, despite the expansive story I made with it. Partly because I felt like it was too convoluted with too many characters at that point, but also because I'd convinced myself it would be stupid to commit to a creative project that I can't make a profit out of. Essentially, I thought it was dumb to put so much energy into something if it wasn't "productive." This was a sentiment it took me way too long to shake.

So! Yeah! This is officially the "living my cringe" era! I'm going to make Splatoon characters with very canon-inaccurate proportions hold hands and give each other a kiss on the cheek! I'm gonna make a Deltarune AU that I know is probably gonna age like milk because the actual story isn't finished yet! I'm going to post my ideas for how I'd turn the TF2 mercs into Overwatch heroes! And NOBODY's gonna stop me!

...

Hey when did 5 AM get here.

[justanotherartblog]

Here are some notes I took during a presentation of a person with a degree in gravitational wave behavior and detection. If you have any points of elaboration to add, I would be pleased to add to this post! Notes are divided into subsections.

QUANTUM MECHANICS:

-There is not a theory of quantum gravity. This theory has been worked on for decades, but the current theories and laws we have for the larger universe are not very easily applicable to the smaller realm of quantum physics. It is believed that the interior of black holes will give hints into the nature of the behavior of the quantum realm. String theory, quantum loop gravity, general relativity can all help as well.

-Does the event horizon radius size determine the internal gravity and size of a black hole? This question is deemed unanswerable by our current understanding of black holes and their central behaviors. This is due to the difficulty of understanding the quantum mechanics of the black holes.

WORM HOLES:

-It is theorized that if someone/something were to travel along the perfect curvature and space time of a black hole, and travel along the event horizon to a certain point, a wormhole could be reached.

-Worm holes are currently theoretical, as a Perfect (symmetrical, perfect orbit, perfect roundness) object in space is near to nonexistent.

It is theorized that as you go into singularity in black hole, its connected to a white hole, which will in turn spew you out. This would be a worm hole. (White holes are the opposite of black holes, black holes are of infinite density and suck in matter, white holes have an infinitely negative density and constantly spew out matter. White holes are also theoretical.

-You can Look for wormholes (and white holes) but its unclear how/why they would form. White holes would keep the law of conservation of mass, by balancing out the presence of black holes, but so far there has been none found in the universe.

GRAVITATIONAL WAVES:

-GRAVITATIONAL WAVE MEMORY: Gravitational wave can PASS THROUGH a distortion in space, and the wave will have an effect on it, it is Altered and cannot be changed back. Gravitational waves are Very hard to alter, and a celestial body or other interference of the wave would have to be of a Very large magnitude or have a very massive effect upon the wave to alter it.

-REDSHIFT IS IMPORTANT TO STUDY: all signals are redshifted to some degree, studying redshift allows to account for it in signals, meaning you can much more accurately pinpoint gravitational signals.

-PULSAR TIMING is another way to detect gravitational waves. However, the data from this form of measurement can become very redshifted, that has to be accounted for.

-The theory of gravitational waves effects diffraction.

-When a star collapses, the mass lost is the mass that was lost into gravitational waves. (Mass is converted into energy, which translates into gravitational waves which ripple the fabric of the universe)

-DETECTING GRAVITATIONAL WAVES:

-Gw170728 is our current largest reach if gravitational wave detection. (9.785 billion light years.) 1/10 size of observable universe.

-More detectors reduce glitches/interference and have better localization. (See where in sky wave is coming from, help triangulate.)

2 detectors = 2 dimensional plane of detection, 3 detectors = 3 dimensional plane, easier to pinpoint.

-LISA is the new and larger version of the LIGO project which will be released into space in ~ 15 years. Its arms will be 150 million kilometers long which will detect longer gravitational waves, which are produced by larger black holes. These supermassive black holes are nearly undetectable by our current observatories.

-Electron Degeneracy Pressure- (RESEARCH)

BLACK HOLES:

-PRIMORDIAL BLACK HOLES do not form by the collapse of stars. (Do they form by means of dark matter? This could explain why there are many black holes we do not see?)

-The size (event horizon) of a black hole is directly related to the mass of the black hole. (Equation: 2gm/c^2)

-NEUTRON STAR center is loose nuclear material (neutrons protons, No atoms; there is too much gravitational pressure for atoms to remain stable) Any larger objects (black holes) would have essentially nothing at their cores due to the immense pressure of gravity.

-CHEMICAL NATURE OF BLACK HOLE: (Too collapsed, not much to measure. Chemical nature is considered unable to be studied or measured.) Center of black hole is a vacuum, nothing, aside from material it spews out. This creates the Black hole information paradox. All black holes of the same size, shape, and density become the same. Where did all the information go that made the bodies identifiable? This question could be understood if the quantum speculation of the black hole could be understood/observed.

-ORBITAL ROTATION OF BLACK HOLES:

Magnetic fields might not play a significant role in the orbital rotation of black holes, as the gravitational forces are so extreme that the magnetic forces are minuscule in comparison, and do not effect the orbital rotation anywhere near to the effect that the gravitational forces have on orbital rotation.

-GEODESICS are a subsection of general relativity. Geodesics are the path a test body will take; in this case it is stages of orbital collapse as an object orbits a black hole.

-Oscillating geodesics: geodesics that are right next to each other.

-Zoom-whirl is what you get when an object orbits closer to a black hole.

[whats-in-a-sentence]

Many string theorists, including me, strongly suspect that something along these lines actually happens, but to go further we need to figure out the more fundamental concepts into which space and time transform.*

* I might note that the proponents of another approach for merging general relativity and quantum mechanics, loop quantum gravity, to be briefly discussed Chapter 16, take a viewpoint that is closer to the former conjecture – that spacetime has a discrete structure on the smallest of scales.

"The Fabric of the Cosmos" - Brian Greene

[tenth-sentence]

By contrast, loop quantum gravity grew out of a tradition tightly grounded in the general theory of relativity; to most practitioners of this approach, gravity has always been the main focus.

"The Fabric of the Cosmos" - Brian Greene

[sciencestyled]

Faster Than a Speeding Photon: Unmasking Quantum Gravity with Yours Truly and Clark Kent

Ah, the dear clutches of quantum gravity, a realm so enigmatic it makes the Bermuda Triangle look like a kiddie maze. We’re your unabashed guides, elegantly treading where angels and physicists alike often tread with a mix of reverence and dread. Today, we’re not just walking through this enigmatic space; we’re waltzing, pirouetting, and perhaps throwing in a saucy salsa dance or two. And guess who’s leading the dance? None other than the bespectacled charmer from The Daily Planet, Clark Kent.

Gravity and quantum mechanics: two cosmic forces, as seemingly harmonious as cats and dogs, oil and water, pineapple on pizza. Clark, with the delicacy of a ballet dancer and the strength of someone who could, hypothetically, bench press a planet (not saying he can, but let’s not rule it out), unravels this dance of celestial elegance.

In the world of leotards and tutus, also known as theoretical physics, quantum gravity plays the elusive prima donna, a blend of gravity’s timeless waltz and quantum mechanics’ unpredictable jive. It's a dance of curving space and time, of particles smaller than a mischievous twinkle in Clark's oh-so-blue eyes.

Yet, beneath those spectacles and winsome smile, Kent explores this dance floor with an agility that’s nothing short of, dare we say, superhuman. He brings us face-to-face with General Relativity and Quantum Mechanics, two titans of physics, explaining their dramatic and somewhat petulant stand-off with the grace of a seasoned diplomat or a superhero mediating between squabbling villains.

We’re not saying Clark Kent is Superman, but have you ever seen them in a room together? Just a thought.

Now, back to gravity, it's as classic as a Shakespearean play, narrating the poetic dance of celestial bodies. Quantum Mechanics, on the other hand, is the edgy, unpredictable newcomer, a dance of particles as spontaneous as jazz hands at a funeral. Combine the two, and you’ve got a performance that’s Tony Award-worthy.

Clark's narration, as smooth as silk and as invigorating as a double shot of espresso, leads us through the convoluted corridors of theories where strings aren’t just for puppets and loops aren’t confined to roller coasters. It’s a realm where the cosmological constant isn’t a magical incantation, although it might as well be, given its enigmatic presence.

In the star-studded universe of Clark Kent’s narration, String Theory and Loop Quantum Gravity aren’t indie bands you’ve never heard of. They’re cutting-edge theories, tantalizing in their complexity, seductive in their potential revelations. In the velvet-toned prose of Metropolis’ darling reporter, each quantum leap and twist of theoretical string is as captivating as a front-page headline.

And oh, the multiverse - a concept as expansive as our collective ego. It’s the cosmic equivalent of having cake and eating it too. In infinite universes, the enigmas of quantum gravity are as varied as the shades of lipstick in a beauty store. Each universe, with its own distinct shade of physical laws, adds a pop of color to the otherwise monochromatic palette of our understanding.

But beware, dear readers, for every action there’s a reaction. For every revelation, there’s a shadow of mystery, as dark and enticing as the space behind Clark’s iconic spectacles. What lurks behind those lenses, you ask? A world where quantum gravity isn’t just a theory but a tangible, touchable entity, as real as the paper The Daily Planet is printed on.

Our romp through the starlit world of quantum gravity, under the gallant guidance of Clark Kent, is akin to a moonwalk in zero gravity – disorienting, exhilarating, and oh-so illuminating. As we take a bow, we leave you with a mystery as enigmatic as the man of steel’s true identity (no spoilers here, though).

Dance on, dear readers, for the stage of the universe is vast, and every step, leap, and pirouette unveils a galaxy of truths, as mesmerizing as the twinkle in the eyes of a certain reporter with an affinity for red capes.

Oh, did we say red capes? We meant headlines, darling, always chasing those exhilarating headlines.

[carolkeiter]

A Seahorse Tale ~ A Spin on the Matter of Motion

A Seahorse Tale ~ A Spin on the Matter of Motion is a young adult educational eBook cloaked as a sci-fi. Each of the 170 pages have beautifully illustrated art and science diagrams. Interwoven throughout the story are non-fiction links to various topics and state-of-the-art science, which corroborate the storyline and substantiate the possibility of the occurrence of these fantastical and…

[xtruss]

An Enormous Gravity ‘Hum’ Moves Through the Universe

Astronomers have found a background din of exceptionally long-wavelength gravitational waves pervading the cosmos. The cause? Probably supermassive black hole collisions, but more exotic options can’t be ruled out.

— By Jonathan O'Callaghan, Contributing Writer | Quantum Magazine | June 28th, 2023

The 100-meter Green Bank Telescope has precisely measured the timing of dozens of pulsars over the course of 15 years.

Astronomers have found an extra-low hum rumbling through the universe. The discovery, announced today, shows that extra-large ripples in space-time are constantly squashing and changing the shape of space. These gravitational waves are cousins to the echoes from black hole collisions first picked up by the Laser Interferometer Gravitational-Wave Observatory (LIGO) experiment in 2015. But whereas LIGO’s waves might vibrate a few hundred times a second, it might take years or decades for a single one of these gravitational waves to pass by at the speed of light.

The finding has opened a wholly new window on the universe, one that promises to reveal previously hidden phenomena such as the cosmic whirling of black holes that have the mass of billions of suns, or possibly even more exotic (and still hypothetical) celestial specters.

“It’s beautiful,” said Chiara Caprini, a theoretical physicist at the University of Geneva and CERN in Switzerland who was not directly involved in the work. “A new era in the observation of the universe has opened up.”

The results come from studies that stretch back more than a decade by four teams based in the U.S., Europe, Australia and China. Today, in a coordinated data release, the teams present evidence for a background “hum” of gravitational waves that were detected by tracking changes in the impossibly regular beats of objects called pulsars.

As long-wavelength gravitational waves pass through our cosmic neighborhood, they distort the space-time around us, which changes the arrival time of a pulsar’s pulses. Researchers had to map the correlations of these arrival times across dozens of different pulsars for decades in order to pick up the signal. “I had butterflies when I first saw this,” said Stephen Taylor, an astrophysicist at Vanderbilt University and chair of the team known as the North American Nanohertz Observatory for Gravitational Waves, or NANOGrav. “I’m so excited we can finally talk about it.”

The NANOGrav team primarily used three large radio observatories in North America (left to right): the Green Bank Telescope in West Virginia, the Very Large Array in New Mexico and the Arecibo Observatory in Puerto Rico. Green Bank Observatory; Susan E. Degginger/Science Source; David Parker/Science Source

Most likely, the gravitational waves come from pairs of supermassive black holes that are spiraling around each other inside merging galaxies. But we might be seeing something else entirely, perhaps something exotic such as ruptures in space-time itself resulting from loops of energy called cosmic strings.

“Finding for the first time the suggestion of background gravitational waves is fascinating,” said Juan García-Bellido, a theoretical cosmologist from the Autonomous University of Madrid who was not involved in the work. “It’s really Nobel Prize-winning research.”

A Galaxy-Size Hack

There’s two ways to start the story of this discovery. The first, as usual, is with Albert Einstein. His general theory of relativity in 1915 suggested that the universe is an ocean of space-time on which objects like black holes and stars sit. Movements of these objects would send ripples across this space-time ocean — gravitational waves.

The other place to start the story is in 1967, with a graduate student from Lurgan, Northern Ireland, named Jocelyn Bell. Using a radio telescope that she helped build near Cambridge, U.K., she spotted an unusual signal in space that repeated every second. She and other astronomers later classified these signals as a new class of celestial object known as pulsars — the rapidly spinning cores of dead stars. Today, some are known to spin exceedingly fast, emitting regular pulses of radio waves hundreds or even thousands of times per second.

The stopwatch-like regularity of pulsars makes them valuable cosmic timekeepers. In 1983, the U.S. astronomers Ron Hellings and George Downs suggested a novel way to put them to use: If gravitational waves were squeezing and stretching space-time, that motion would change the arrival time of the pulsars’ radio flashes.

The key is to look at many pairs of pulsars and compare their time delays. “If they’re close together on the sky, they’re both going to be early or late,” said Sarah Vigeland, an astrophysicist at the University of Wisconsin, Milwaukee and chair of NANOGrav’s Gravitational Wave Detection Working Group. “As you pull them apart, they become out of sync, but in a way you can predict.”

Merrill Sherman/Quanta Magazine; source: nanograv.org

To catch these fluctuations, pulsar timing arrays such as NANOGrav use multiple radio telescopes to observe many pulsars over many years. These projects are cosmic cousins of LIGO and other earthbound observatories that detect gravitational waves by looking for tiny changes in the relative lengths of its two arms.

While LIGO’s arms are each four kilometers long, pulsar timing arrays effectively use the distance from Earth to each pulsar as a much larger arm — one hundreds or thousands of light-years in length. “What we’ve essentially done is hack the entire galaxy to make a giant gravitational wave antenna,” Taylor said.

This longer distance makes pulsar timing arrays sensitive to a different variety of gravitational wave. Whereas LIGO can detect high-frequency gravitational waves, which might occur when star-size black holes orbit each other tens or hundreds of times a second before merging, pulsar timing arrays are sensitive to processes occurring across years or even decades. That’s one reason why pulsar timing arrays need many years of data — if it takes a decade for a single wave to pass by, you can’t detect it in just a few months.

Of the four groups releasing data today, NANOGrav is the most confident in its result. The project was founded in 2007 and has largely used the Green Bank Telescope in West Virginia and the Arecibo radio telescope in Puerto Rico (which collapsed in late 2020, near the end of NANOGrav’s 15 years of data collection). “We’re still mourning the loss of Arecibo,” Taylor said.

Separate pulsar timing array projects were also established in different parts of the globe. The four teams, which together form the International Pulsar Timing Array, coordinated today’s announcements, but they have not yet performed a combined data analysis. “It’s complex,” said Andrew Zic, an astronomer at the Commonwealth Scientific and Industrial Research Organization in Australia and part of that country’s Parkes Pulsar Timing Array team. “We’re ready to move towards being a more unified thing.”

In 2020, NANOGrav released preliminary data from 12.5 years of observations. Those showed a tentative hint of gravitational waves affecting the pulses of some 45 pulsars.

Now they’ve added a few more years of data, along with data from nearly two dozen more sources, and a more consistent pattern has emerged. “It really jumps out to us,” Vigeland said.

“We’re looking at deviations in time that are a couple of hundred nanoseconds,” said Scott Ransom, an astronomer at the National Radio Astronomy Observatory and a founding member of NANOGrav. They’ve detected a particular pattern in the data, called the Hellings-Downs curve, that makes them confident that what they’re seeing is the gravitational-wave background. “That’s the smoking gun of gravitational waves.”

The European team, which observed 25 pulsars over 25 years with six telescopes, sees similar hints of timing delays but is less certain of their results. “The Americans are very confident,” said Michael Keith, an astrophysicist at the Jodrell Bank Center for Astrophysics and part of the European team. The Australian team is reporting observations from 32 pulsars over 18 years, while the Chinese team has observed 57 pulsars for a little more than three years.

Supermassive Dances

So what’s causing these waves? The most likely sources are supermassive black holes — behemoths millions to billions of times the mass of our sun. These are found at the center of massive galaxies such as our own Milky Way. When two galaxies collide, as sometimes happens, the supermassive black holes at their centers may also begin to orbit each other, twirling around at a cosmically ponderous rate, and perturbing space-time as they do.

“If you have a rotating distribution of mass that’s not symmetric” — even something small, like a spinning pen — “gravitational waves are coming out,” Keith said. On big enough scales, with supermassive black holes, the low and steady rumble of these waves becomes detectable as they permeate space.

Sarah Vigeland, an astrophysicist at the University of Wisconsin, Milwaukee, and chair of NANOGrav’s Gravitational Wave Detection Working Group, is one of more than 190 researchers working on NANOGrav. Tonia Klein

NANOGrav can’t yet make out individual gravitational wave sources. Instead, the team has found evidence for the background hum of all low-frequency gravitational waves. It’s like a buoy bouncing up and down in a busy harbor — it can’t distinguish the wake of a single boat, but its motion can reveal that there are some big objects slicing through the water.

Supermassive black holes, however, are not the only possible explanation for the background hum. Another possibility is cosmic strings. First predicted in the 1970s, these would essentially be cracks in space-time caused by the expansion of the universe. The cracks would emit gravitational waves as they spun around in loops.

“The idea of cosmic strings is you have some extension of the Standard Model [of particle physics] in which, in addition to pointlike particles, you can get strings of energy stretching out across the universe,” said John Ellis, a theoretical physicist at King’s College London and CERN who is a proponent of cosmic strings. “Those strings of energy move around and can collide, spawning loops of string that eventually collapse by emitting gravitational waves.”

While the idea is somewhat extravagant, the observations so far from NANOGrav and the other teams are consistent with what we’d expect to see from cosmic strings. “They’d be constantly wriggling, and from time to time they crack like a whip and send out gravitational wave bursts,” said Patrick Brady, an astrophysicist at the University of Wisconsin, Milwaukee. If the pulsar timing arrays don’t see individual sources start to emerge from their upcoming data, that could point toward this exotic physics beyond the Standard Model. “Cosmic strings will give you a much smoother signal,” Ellis said.

But while strings and other exotic phenomena can’t be ruled out, for now supermassive black holes are the favored explanation. “From an Occam’s razor point of view, we know galaxies merge and almost all galaxies have supermassive black holes,” Ransom said. “So we think it’s probably most likely that the signal we’re seeing is from supermassive black holes. But we could be wrong.”

Discovering a population of supermassive black hole pairs would help answer open questions in astrophysics. For example, what happens when two orbiting supermassive black holes get relatively close to each other? There were reasons to think that instead of merging, as smaller black holes do, supermassive black holes just rotate around each other forever. “This is called the last-parsec problem,” Caprini said; a parsec is a unit of distance measuring 3.26 light-years across. “It is an unsolved problem.” If pulsar timing arrays are seeing gravitational waves from these moments, however, it would be “a demonstration that two supermassive black holes do get close enough and merge,” rather than remaining in distant orbits, Caprini said.

Just the existence of such a population has broad implications for our understanding of galactic evolution in the universe. “It would mean that at the center of some galaxies, there are massive black holes that are not just alone,” Caprini said. “We can probe, through the history of the universe, how galaxies collide and the rate of collisions.”

Such work would require the discovery of individual supermassive black hole pairs, and so is not yet feasible. But as researchers combine the data sets from the different teams and take more observations over the next few years, individual sources may start to emerge, perhaps allowing astronomers to pinpoint binary supermassive black holes in space and time.

“Bright individual sources will start poking above this background hum,” said Maura McLaughlin, an astrophysicist at West Virginia University and one of the founding members of NANOGrav. “We’ll be able to say, in that direction, there is a supermassive black hole binary with [a certain] mass. We’ll learn a whole lot about galaxy mergers.”

What is clear is that these projects have given astronomers a completely new tool with which to study the cosmos. The rise of gravitational-wave astronomy “is like when Galileo first turned his telescope on the sky,” Brady said. We now know that a background of ripples in space-time pervades the universe. An ocean of gravitational waves awaits.