The LIGO (Laser Interferometer Gravitational-wave Observatory) began its fourth run yesterday afterover two years of maintenance work and upgrades.In its latest operational run, the experiment will work in tandem with the Virgo Interferometer in Italy and the KAGRA observatory in Japan.
LIGO made history in 2015 when it detected gravitational waves for the first time, and its fourth run, which began yesterday, will be its most sensitive yet, according to Caltech. The run will last about 20 months, including two monsters of commissioning breaks when work will be done to improve instrument performance further.
LIGO has already begun its fourth run, while Virgo is set to join later in the year. KAGRA has joined for one month starting yesterday and should rejoin later in the fourth run after some upgrades
“Our LIGO teams have worked through hardship during the past two-plus years to be ready for this moment, and we are indeed ready: our engineering run leading up to tomorrow’s official start of 04 has already revealed a number of candidate events, which we have shared with the astronomical community,” Albert Lazzarini, the deputy director of the LIGO Laboratory, in a press statement released before the beginning of the third run.
According to Lazzarini, the detectors will begin the run with a 30 per cent increased sensitivity. This means that they will be able to observe a larger fraction of the universe than before and will pick up gravitational-wave signals at a higher rate.
Apart from being able to detect a larger fraction of the universe, LIGO will be able to extract more physical information from the data thanks to its sensitivity. This will let scientists test Albert Einstein’s general theory of relativity and infer the real population of dead stars in the local universe.
These observatories have instruments so sensitive that they can produce false readings due to the influence of earthquakes, landslides, other natural events and even the movement of trucks. This is where multiple observatories can be helpful in revalidating the signal.
Seven years ago, researchers with the Laser Interferometer Gravitational-Wave Observatory (LIGO) reported the first detection of gravitational waves. Now, the list of gravitational wave candidates numbers nearly 100.
And astronomers are confident they can find more of these ripples in the fabric of space-time, which are caused by the acceleration of massive objects — for example, two black holes spiraling toward each other for a cataclysmic merger.
May 24 marked the start of Observation Run 4 (O4), the newest effort of the LIGO-Virgo-KAGRA (LVK) Collaboration. With newly upgraded gravitational wave detectors, astronomers hope O4 can make seeing gravitational waves — and the objects that produce them — an everyday occurrence.
"We expect to go from what we had in our previous run — one neutron star every couple of months, one [binary] black hole every week or 10 days," said Salvatore Vitale, an astrophysicist at the Massachusetts Institute of Technology (MIT), "to getting a binary black hole every day or two, getting a neutron star every week."
Gravitational waves are a byproduct of general relativity, as envisioned by Albert Einstein a century ago. General relativity holds that space and time are like a fabric. Every object leaves a dent in that fabric, which we perceive as gravity. In that world, disturbances — such as two black holes colliding — can send ripples across the fabric. Astronomers can use laser-based detectors spot those ripples.
As LVK's name suggests, the collaboration is a multi-pronged effort, combining four detectors on three continents: LIGO's two detectors, one in Livingston, Louisiana and another in Hanford, Washington; Virgo in Europe, stretching across the Tuscan plains southeast of Pisa, Italy; and KAGRA, under the mountains of central Japan.
Alas, as O4 begins, only LIGO's pair is fully operational. Virgo must undergo repairs to a damaged mirror and will remain deactivated for an uncertain amount of time. KAGRA, meanwhile, will observe for just a month before going offline again; it hasn't reached its target sensitivity, and its operators hope to restart it again in late 2024.
Astronomers want more detectors because a single gravitational wave detector doesn't provide details about the direction in which the waves are traveling. So, they need multiple detectors to actually triangulate the source of gravitational waves. With all four, astronomers could trace a source to just a few square degrees of sky. With just two detectors, they're stuck with a far larger wedge of the sky.
"It's going to be harder for us to tell our friends with telescopes where to point their telescope," said Vitale.
But even two detectors might reap a bounty of science. With upgraded sensitivity, the detectors can pick out weaker or more distant gravitational waves. That means scientists can pick up more events.
And with more events, they hope to begin answering a looming question: Where did the black holes they're seeing tend to form?
Perhaps black holes formed inside galaxies; perhaps they formed outside, in globular clusters or in dwarf galaxies. Or, perhaps, they're primordial, having formed in raw space at the beginning of the universe.
"To answer this question, you need to have a large dataset," said Vitale.
LVK's current schedule calls for O4 to run for 18 months, into 2025. Afterward, the gravitational wave detectors will shut down for upgrades and engineering work — and start back up again around 2027 for a fifth, longer observing run.
All gravitational waves detected so far have been from binary sources.
So far all the gravitational wave sources detected have been from binary systems, from a merger between two black holes, two neutron stars, or a black hole and a neutron star. While astronomers in theory know that gravitational waves are emitted by non-binary sources as well, such a signal has eluded detectors so far. In new research, scientists have demonstrated that the chaotic cocoons surrounding newly dead stars or newly born black holes, can be a source of gravitational waves that can be registered by gravitational wave detectors.
When a star runs out of nuclear fuel at the end of its life cycle, it balloons up and violently sheds its outer layers in an explosion known as a supernova. A dense core remains behind, which can be a white dwarf, a neutron star, or a black hole. If a dying star collapses into a black hole, it can form powerful polar jets, containing relativistic particles. The jets can collide with the collapsing layers of the star, potentially producing gravitational waves that can be spotted by gravitational wave detectors.
A paper describing the research has been presented at the 242nd meeting of the American Astronomical Society. Led author of the research, Ore Gottlieb, says, “Our study is a call to action to the community to look at cocoons as a source of gravitational waves. We also know cocoons to emit electromagnetic radiation, so they could be multi-messenger events. By studying them, we could learn more about what happens in the innermost part of stars, the properties of jets and their prevalence in stellar explosions.”
The LIGO-VIRGO-KAGRA collaboration has initiated the fourth run for the detection of gravitational waves, with the BlackGEM observatory looking for optical counterparts. Other ground and space based astronomical instruments are prepared to interrupt their planned operations to turn their gaze towards any source that is flagged. A new gravitational wave source is expected to be spotted at least once every three days.
Although astrophysicists have never sensed supermassive black hole binary systems, a galaxy-sized detector composed of dead stars is hot on their trail.
In a new Northwestern University-led study, astrophysicists crunched 12.5 years of data from 45 dead stars (called pulsars) to set the best limits yet on the gravitational wave signatures emitted from pairs of monster black holes. Knowing these limits will help astrophysicists constrain the number of binaries existing in the nearby universe, confirm or deny existing binary candidates and, someday, detect gravitational waves from these complex pairs.
In another breakthrough, the study also found that when searching for pairs of supermassive black holes, researchers need to account for the steady hum of background noise made by the symphony of gravitational waves from all the supermassive black hole binaries in the universe.
“We genuinely think that detection of a supermassive black hole binary through gravitational waves is right around the corner,” said Northwestern’s Caitlin Witt, who led the study. “That would be an important discovery for many scientific fields. It would enable us to perform further experiments like testing gravity to explore whether supermassive black hole binaries evolve the way we think they do, and it will teach us how to look for them in future surveys. We also will be able to look back through cosmic time and trace the history of the universe in which we live.”
Located in the center of most galaxies, supermassive black holes can be several billion times the mass of our sun. Compared to typical stellar-mass black holes, which are 10 to 100 times more massive than our sun, supermassive black holes are unfathomably gigantic.
When two galaxies — each with a central supermassive black hole — merge together, it can create a binary system of these monstrous black holes.
“Someday, our galaxy will collide with the Andromeda galaxy,” Witt said. “Millions of years after that, the black holes eventually find each other to form a little buddy system. Detecting gravitational waves from systems like these will help us understand how galaxies interact and how the universe evolves.”
In 2016, an international team co-led by Northwestern professor Vicky Kalogera used the Laser Interferometer Gravitational-Wave Observatory (LIGO) to first detect gravitational waves from the merger of two stellar-mass black holes, which resulted in obvious, short-lived ripples in space-time. But supermassive black hole binaries are too big and much too far apart for Earth-based equipment like LIGO to detect. These monster pairs create waves so long that it could take years or even decades for their gravitational waves to fully wash over Earth. Even when NASA and the European Space Agency launch LISA (a space-based gravitational-wave detector for which Northwestern professor Shane Larson is a co-principal investigator) in the early 2030s, it still will not be able to detect such enormous waves.
“LIGO can only detect wavelengths that fit within its arms,” Witt said. “We have to look for much lower wave frequencies. We are sensitive to supermassive black hole pairs that can take a month or even up to 15 years to orbit each other. So, we’re looking for a steady signal that could blend into the background.”
Pulsars tick like a clock
To overcome this obstacle, an international collaboration of researchers established the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), which hunts for gravitational waves using pulsars, a type of rapidly rotating neutron star born in the supernova explosion of a massive star at the end of its life. Just like a lighthouse, a pulsar emits a light beam that flashes by as it rotates.
“Because pulsars spin so stably, we see little flashes of light that tick like a clock,” Witt said. “We watch that light with ground-based radio telescopes. If the clock ticks arrive either a little bit early or a little bit late, this is a sign that it could have been affected by a gravitational wave.”
NANOGrav tracks 75 pulsars — 45 of which were used in this study — located all across the night sky. Their beams of light take mere milliseconds to flicker past Earth. So, in this case, “a little bit early or a little bit late” could mean a fraction of a nanosecond. Therefore, NANOGrav’s techniques must be incredibly sensitive to capture these nearly imperceptible changes.
By looking across the entire sky, Witt and the NANOGrav team search for specific patterns from all pulsars together. According to theory, supermassive black hole binaries should emit gravitational waves that literally stretch and squeeze (or strain) space-time on their way to Earth. Warped space-time will affect pulsars’ light beams in such a way that indicates an elusive pair of monster black holes.
‘Red noise can trick us’
But, of course, pulsars also generate their own noise, which can muddy the signals.
“Pulsars do have some intrinsic noise called ‘red noise,’” Witt said. “Their insides might slowly wobble a little bit, which you wouldn’t be able to see unless you were looking as closely as we are. That red noise looks similar to the broad gravitational wave noise that we’re looking for. We have to tease that apart.”
Last year, the NANOGrav team published a study finding a red noise process in all pulsars that shares the same common characteristics. Without more evidence, however, NANOGrav cannot attribute this to gravitational waves. In the new study, Witt and her team found that this red noise still must be carefully considered in order to definitively detect gravitational waves from individual supermassive black hole binaries.
“When a gravitational wave becomes detectable, it looks very similar to red noise at first glance,” Witt said. “The red noise can trick us. Our new study tells us that we must look closely to avoid getting confused. That will be important to watch for when we do finally detect gravitational waves.”
Although NANOGrav has yet to detect supermassive black hole binaries with gravitational waves, Witt’s new paper brings the field closer than ever. By leveraging the 12.5-year dataset, the researchers created new models to accurately account for uncertainties in the pulsar data and implement new techniques to account for the red noise.
Confirming candidates
These new models provide the tightest limits yet on the strength of gravitational waves emitted from supermassive black hole pairs. Previously, other researchers discovered potential supermassive black hole binaries with light-based telescopes. NANOGrav could eventually confirm that these potential candidates are, indeed, supermassive black hole binaries.
“With our new methods, we might be able to confirm this sooner,” Witt said. “Or, if we continue gathering and analyzing data, then we might be able to rule it out as a candidate. It might just be something else weird going on in the galaxy.”
Its of the stars are all around us, and in us, too. About half of the abundance of elements heavier than iron originates in some of the mostviolent explosions in the cosmos. As the universe churns and new stars and planets form out of old gas and dust, these elements eventually make their way to Earth and other worlds. After 3.7 billion years of evolution on our planet, humans and many other species have come to rely on them in our bodies and our lives. Iodine, for instance, is a component of hormones we need to control our brain development and regulate our metabolism.
Ocean microplankton called Acantharea use the element strontium to create intricate mineral skeletons. Gallium is critical for the chips in our smartphones and our laptop screens. And the mirrors of the JWST are gilded with gold, an element useful for its unreactive nature and ability to reflect infrared light (not to mention its popularity in jewelry).
Scientists have long had a basic idea of how these elements come to be, but for many years the details were hazy and fiercely debated. That changed recently when astronomers observed, for the first time, heavy-element synthesis in action. The process, the evidence suggests, went something like this.
Eons ago a star more than 10 times as massive as our sun died in a spectacular explosion, giving birth to one of the strangest objects in the universe: a neutron star. This newborn star was a remnant of the stellar core compressed to extreme densities where matter can take forms we do not understand. The neutron star might have cooled forever in the depths of space, and that would have been the end of its story. But most massive stars live in binary systems with a twin, and the same fate that befell our first star eventually came for its partner, leaving two neutron stars circling each other. In a dance that went on for millennia, the stars spiraled in, slowly at first and then rapidly.
As they drew closer together, tidal forces began to rip them apart, flinging neutron-rich matter into space at velocities approaching one-third the speed of light. At last the stars merged, sending ripples through spacetime and setting off cosmic fireworks across the entire electromagnetic spectrum.
At the time of the crash, our own pale blue planet, in a quiet part of the Milky Way about 130 million light-years away, was home to the dinosaurs. The ripples in spacetime, called gravitational waves, began making their way across the cosmos, and in the time it took them to cover the vast distance to Earth, life on the planet changed beyond recognition.
New species evolved and went extinct, civilizations rose and fell, and curious humans began looking up at the sky, developing instruments that could do incredible things such as measure minute distortions in spacetime. Eventually the gravitational waves (traveling at light speed) and the light from the merger reached Earth together. Astrophysicists recognized a distinctive glow that showed the presence of new elements. Humanity had just witnessed heavy-element production.
As an expert in cosmic cataclysms, I’m enthralled by both the science and the romance of this story—the creation of something new and enduring, even precious, from an ancient remnant of a once luminous star. And I’m thrilled that we finally get to see it happening.
The discovery has answered several long-standing questions in astrophysics while also raising entirely new questions. But I and many scientists are energized. Our newfound ability to detect gravitational waves, as well as light from the same cosmic source, promises to help us understand astrophysical explosions and the synthesis of elements in a way that was previously impossible.
WE ARE STARDUST
The quest to understand heavy-element formation is part of a larger scientific effort to answer a fundamental question: Where did everything come from? The cosmic history of the elements of the periodic table extends from a few minutes after the big bang to the present. The synthesis of the first elements—hydrogen, helium and lithium—occurred roughly three minutes after the birth of the universe. From these ingredients, the first stars formed, shining bright and fusing new elements in their cores during both their lives and their explosive deaths.
The next generation of stars was born from the debris of these blasts, enriched with the elements formed by the first stars. This process continues today and accounts for all the elements from helium on the light end, with two protons per atom, all the way up to iron, which has 26 protons in its atomic nucleus. The heaviest elements, such as tennessine with 117 protons, aren’t created by nature at all. But physicists can force them into being inside particle accelerators, where they typically last for mere thousandths of a second before decaying.
Several decades ago scientists theorized that about half of the elements heavier than iron are produced through a process called rapid neutron capture, or the r-process. The rest are thought to originate through slow neutron capture, or the s-process—a relatively well-understood sequence of reactions that occurs in long-lived, low-mass stars.
Both the r-process and the s-process involve adding one or more neutrons to an atomic nucleus. Adding neutrons, however, does not produce a new element, because elements are defined by the number of protons in their nucleus. What we do get is a heavier isotope of the same element—a nucleus containing the same number of protons but a different number of neutrons.
This heavy isotope is often unstable and radioactive. Through what’s called beta-minus decay, a neutron will transform into a proton, spitting out an electron and another subatomic particle called a neutrino in the process. In this way, the number of protons in an atom’s nucleus increases, and a new element is born.
