Astronomers face a unique problem. While scientists from most fields can conduct experiments—particle physicists build massive particle colliders to test their theories of subatomic material, and microbiologists probe the properties of microbes on petri dishes—astronomers cannot conduct experiments with the stars and planets. Even the most advanced telescopes can provide only snapshots of the cosmos, and very little changes during our lifetimes. 

Yet many questions remain, such as how the Milky Way formed, what dark matter is and the role of supermassive black holes at the center of galaxies. In an attempt to edge closer to answering these unsolved mysteries, some scientists have embarked on ambitious projects: creating virtual universes.

Evolving the cosmos 

The earliest observational evidence of the universe come from the cosmic microwave background, the afterglow created by the Big Bang. Computational cosmologists use this data to model the conditions at this time, when the universe was around a few hundred thousand years old. 

Then they add the basic ingredients: baryonic (or ordinary) matter, from which the stars and planets form; dark matter, which enables galactic structures to grow; and dark energy, the mysterious force behind cosmic acceleration. These are coded into a simulation along with equations that describe various physical processes such as supernova explosions and black holes. Cosmologists then wait as the simulation evolves: The virtual universe expands, gas condenses into small structures and eventually form into stars and galaxies. 

“The exciting thing is that if you do this, the universe that develops in a computer looks remarkably like the real universe,” says Joop Schaye of Leiden University and the principal investigator of the EAGLE (Evolution and Assembly of GaLaxies and their Environments) Project. “You get galaxies of all kinds of sizes and morphologies that look a lot like the real galaxies.”

A number of groups around the world are working on these simulations. In 2014, both the EAGLE Project and the Illustris Project, led by theoretical astrophysicist Mark Vogelsberger from MIT, made major steps forward with their groundbreaking, realistic universes. Both simulations are massive, covering a cubic space of around 300 million light years on each side. They also require a hefty amount of computing power—just one complete run requires large supercomputers to run for months at a time. 

“What we ended up doing is running the big simulation once, but we want to understand why the universe behaved as it did,” says Richard Bower, a cosmologist at Durham University and member of the EAGLE Project. “So we’ve been running lots of other simulations where we change things a little bit.” 

These simulations have already revealed some interesting properties of evolving galaxies. Bower and his colleagues, for instance, discovered that the number and size of galaxies is dependent on a fine balance between supernovae and black holes. 

Using their simulation, they found that without supernovae, the universe created far too many galaxies. This is because without supernovae exploding, many small galaxies were not being blown apart. 

On the other hand, they found that including only supernovae made galaxies grow too massive—10 times the mass of the Milky Way. To manage the size of those galaxies, they needed to also include black holes. 

“The supernovae and the black holes are both kind of competing to use up the material that's supplied to the galaxy,” explains Bower. “Once the supernovae begin to wane, the black hole takes over, and it's the end of forming stars and the beginning of forming bigger and bigger black holes.”

Zooming in  

There are two type of simulations in this field of study—representative volume simulations, which model huge volumes of the observable universe, and zoom simulations, which focus on individual galaxies or galaxy clusters. 

As astronomers collect more and more detailed snapshots of the universe, cosmologists such as Andrew Pontzen at the University College London are using zoom simulations to try to investigate the properties of individual galaxies at the same level of specificity. “We’re trying to push forward on understanding the individual galaxies in enough detail that we can make meaningful comparisons to this really cutting-edge data,” Pontzen says. 

To do so, Pontzen and his colleagues have developed a technique called genetic modification, which involves creating many different versions of galaxies. “It almost becomes like an experiment,” says Pontzen. “You have your control over how a particular object forms, and then you can say if it forms in this particular way, then the galaxy that comes out at the end looks like this.” For example, they can change the way that mass arrives in galaxies over time and see how it affects the galaxy that emerges. 

In a similar way, cosmologists working on larger-scale simulations can “turn the knobs” by changing certain variables—the laws of gravity or the properties of dark matter, for example—and see what the universe that emerges looks like. “I think what's very interesting is to try to constrain the properties of dark matter and dark energy through these simulations,” says Vogelsberger. “We don't know what they are, but by tweaking minor parameters of these models we can try to constrain the properties of dark matter or dark energy in more detail.”

These scientists also work closely with observers to compare how the simulations stack up against what is actually out there in the universe. “That’s the critical part,” says Pontzen. “We want to be able to link all of these things together.”

The LHC is colliding protons at a faster rate than ever before, approximately 1 billion times per second. Those collisions are adding up: This year alone the LHC has produced roughly the same number of collisions as it did during all of the previous years of operation together.

This faster collision rate enables scientists to learn more about rare processes and particles such as Higgs bosons, which the LHC produces about once every billion collisions.

“Every time the protons collide, it’s like the spin of a roulette wheel with several billion possible outcomes,” says Jim Olsen, a professor of physics at Princeton University working on the CMS experiment. “From all these possible outcomes, only a few will teach us something new about the subatomic world. A high rate of collisions per second gives us a much better chance of seeing something rare or unexpected.”

Since April, the LHC has produced roughly 2.4 quadrillion particle collisions in both the ATLAS and CMS experiments. The unprecedented performance this year is the result of both the incremental increases in collision rate and the sheer amount of time the LHC is up and running.

“This year the LHC is stable and reliable,” says Jorg Wenninger, the head of LHC operations. “It is working like clockwork. We don’t have much downtime.”

Scientists predicted that the LHC would produce collisions around 30 percent of the time during its operation period. They expected to use the rest of the time for maintenance, rebooting, refilling and ramping the proton beams up to their collision energy. However, these numbers have flipped; the LHC is actually colliding protons 70 percent of the time.

“The LHC is like a juggernaut,” says Paul Laycock, a physicist from the University of Liverpool working on the ATLAS experiment. “We took around a factor of 10 more data compared to last year, and in total we already have more data in Run 2 than we took in the whole of Run 1. Of course the biggest difference between Run 1 and Run 2 is that the data is at twice the energy now, and that’s really important for our physics program.”

This unexpected performance comes after a slow start-up in 2015, when scientists and engineers still needed to learn how to operate the machine at that higher energy.

“With more energy, the machine is much more sensitive,” says Wenninger. “We decided not to push it too much in 2015 so that we could learn about the machine and how to operate at 13 [trillion electronvolts]. Last year we had good performance and no real show-stoppers, so now we are focusing on pushing up the luminosity.”

The increase in collision rate doesn’t come without its difficulties for the experiments.

“The number of hard drives that we buy and store the data on is determined years before we take the data, and it’s based on the projected LHC uptime and luminosity,” Olsen says. “Because the LHC is outperforming all estimates and even the best rosy scenarios, we started to run out of disk space. We had to quickly consolidate the old simulations and data to make room for the new collisions.”

The increased collision rate also increased the importance of vigilant detector monitoring and adjustments of experimental parameters in real time. All the LHC experiments are planning to update and upgrade their experimental infrastructure in winter 2017.

“Even though we were kept very busy by the deluge of data, we still managed to improve on the quality of that data,” says Laycock. “I think the challenges that arose thanks to the fantastic performance of the LHC really brought the best out of ATLAS, and we’re already looking forward to next year.”

Astonishingly, 2.4 quadrillion collisions represent just 1 percent of the total amount planned during the lifetime of the LHC research program. The LHC is scheduled to run through 2037 and will undergo several rounds of upgrades to further increase the collision rate.

“Do we know what we will find? Absolutely not,” Olsen says. “What we do know is that we have a scientific instrument that is unprecedented in human history, and if new particles are produced at the LHC, we will find them.”

Physics can often seem inconceivable. It’s a field of strange concepts and special terms. Language often fails to capture what’s really going on within the math and theories. And to make things even more complicated, physics has repurposed a number of familiar English words. 

Much like Americans in England, folks from beyond the realm of physics may enter to find themselves in a dream within a dream, surrounded by a sea of words that sound familiar but are still somehow completely foreign. 

Not to worry! Symmetry is here to help guide you with this list of words that acquire a new meaning when spoken by physicists.

Quench

The physics version of quench has nothing to do with Gatorade products or slaking thirst. Instead, a quench is what happens when superconducting materials lose their ability to superconduct (or carry electricity with no resistance). During a quench, the electric current heats up the superconducting wire and the liquid coolant meant to keep the wire at its cool, superconducting temperature warms and turns into a gas that escapes through vents. Quenches are fairly common and an important part of training magnets that will focus and guide beams through particle accelerators. They also take place in superconducting accelerating cavities.

Cannibalism, strangulation and suffocation

These gruesome words take on a new, slightly kinder meaning in astrophysics lingo. They are different ways that a galaxy's shape or star formation rate can be changed when it is in a crowded environment such as a galaxy cluster. Galactic cannibalism, for example, is what happens when a large galaxy merges with a companion galaxy through gravity, resulting in a larger galaxy.

Chicane

Depending on how much you know about racecars and driving terms, you may or may not have heard of a chicane. In the driving world, a chicane is an extra turn or two in the road, designed to force vehicles to slow down. This isn’t so different from chicanes in accelerator physics, where collections of four dipole magnets compress a particle beam to cluster the particles together. It squeezes the bunch of particles together so that those in the head (the high-momentum particles at the front of the group) are closer to the tail (the particles in the rear).

Cooler

A beam cooler won’t be of much use at your next picnic. Beam cooling makes particle accelerators more efficient by keeping the particles in a beam all headed the same direction. Most beams have a tendency to spread out as they travel (something related to the random motion, or “heat,” of the particles), so beam cooling helps kick rogue particles back onto the right path—staying on the ideal trajectory as they race through the accelerator.

House

In particle physics, a house is a place for magnets to reside in a particle accelerator. House is also used as a collective noun for a group of magnets. Fermilab’s Tevatron particle accelerator, for example, had six sectors, each of which had four houses of magnets.

Barn

A barn is a unit of measurement used in nuclear and particle physics that indicates the target area (“cross section”) a particle represents. The meaning of the science term was originally classified, owing to the secretive nature of efforts to better understand the atomic nucleus in the 1940s. Now you can know: One barn is equal to 10^-24 cm². In the subatomic world, a particle with that size is quite large—and hitting it with another particle is practically like hitting the broad side of a barn.

Cavity

Most people dread cavities, but not in particle physics. A cavity is the name for a common accelerator part. These metal chambers shape the accelerator’s electric field and propel particles, pushing them closer to the speed of light. The electromagnetic field within a radio-frequency cavity changes back and forth rapidly, kicking the particles along. The cavities also keep the particles bunched together in tight groups, increasing the beam’s intensity.

Doping

Most people associate doping with drug use and sports. But doping can be so much more! It’s a process to introduce additional materials (often considered impurities) into a metal to change its conducting properties. Doped superconductors can be far more efficient than their pure counterparts. Some accelerator cavities made of niobium are doped with atoms of argon or nitrogen. This is being investigated for use in designing superconducting magnets as well.

Injection

In particle physics, injections don’t deliver a vaccine through a needle into your arm. Instead, injections are a way to transfer particle beams from one accelerator into another. Particle beams can be injected from a linear accelerator into a circular accelerator, or from a smaller circular accelerator (a booster) into a larger one.

Decay

Most people associate decay with things that are rotting. But a particle decay is the process through which one particle changes into other particles. Most particles in the Standard Model are unstable, which means that they decay almost immediately after coming into being. When a particle decays, its energy is divided into less massive particles, which may then decay as well.

Hello Kitty is known throughout Japan as the poster girl (poster cat?) of kawaii, a segment of pop culture built around all things cute.

But recently she took on a new job: representing the proposed International Linear Collider.

At the August International Conference on High Energy Physics in Chicago, ILC boosters passed out folders featuring the white kitty wearing a pair of glasses, a shirt with pens in the pocket and a bow with an L for “Lagrangian,” the name of the long equation in the background. Some picture the iconic cat sitting on an ILC cryomodule.

Hello Kitty has previously tried activities such as cooking, photography and even scuba diving. This may be her first foray into international research.

Japan is considering hosting the ILC, a proposed accelerator that could mass-produce Higgs bosons, along with other fundamental particles. Japan’s Advanced Accelerator Association partnered with the company Sanrio to create the special kawaii gear in the hopes of drawing attention to the large-scale project.

The ILC: Science you’ll want to snuggle.

The universe is unbalanced.

Gravity is tremendously weak. But the weak force, which allows particles to interact and transform, is enormously strong. The mass of the Higgs boson is suspiciously petite. And the catalog of the makeup of the cosmos? Ninety-six percent incomplete.

Almost every observation of the subatomic universe can be explained by the Standard Model of particle physics—a robust theoretical framework bursting with verifiable predictions. But because of these unsolved puzzles, the math is awkward, incomplete and filled with restrictions.

A few more particles would solve almost all of these frustrations. Supersymmetry (nicknamed SUSY for short) is a colossal model that introduces new particles into the Standard Model’s equations. It rounds out the math and ties up loose ends. The only problem is that after decades of searching, physicists have found none of these new friends.

But maybe the reason physicists haven’t found SUSY (or other physics beyond the Standard Model) is because they’ve been looking through the wrong lens.

“Beautiful sets of models keep getting ruled out,” says Jessie Shelton, a theorist at the University of Illinois, “so we’ve had to take a step back and consider a whole new dimension in our searches, which is the lifetime of these particles.”

In the past, physicists assumed that new particles produced in particle collisions would decay immediately, almost precisely at their points of origin. Scientists can catch particles that behave this way—for example, Higgs bosons—in particle detectors built around particle collision points. But what if new particles had long lifetimes and traveled centimeters—even kilometers—before transforming into something physicists could detect?

This is not unprecedented. Bottom quarks, for instance, can travel a few tenths of a millimeter before decaying into more stable particles. And muons can travel several kilometers (with the help of special relativity) before transforming into electrons and neutrinos. Many theorists are now predicting that there may be clandestine species of particles that behave in a similar fashion. The only catch is that these long-lived particles must rarely interact with ordinary matter, thus explaining why they’ve escaped detection for so long. One possible explanation for this aloof behavior is that long live particles dwell in a hidden sector of physics.

“Hidden-sector particles are separated from ordinary matter by a quantum mechanical energy barrier—like two villages separated by a mountain range,” says Henry Lubatti from the University of Washington. “They can be right next to each other, but without a huge boost in energy to get over the peak, they’ll never be able to interact with each other.”

High-energy collisions generated by the Large Hadron Collider could kick these hidden-sector particles over this energy barrier into our own regime. And if the LHC can produce them, scientists should be able to see the fingerprints of long-lived particles imprinted in their data.

Long-lived particles jolted into our world by the LHC would most likely fly at close to the speed of light for between a few micrometers and a few hundred thousand kilometers before transforming into ordinary and measurable matter. This incredibly generous range makes it difficult for scientists to pin down where and how to look for them.

But the lifetime of a subatomic particle is much like that of any living creature. Each type of particle has an average lifespan, but the exact lifetime of an individual particle varies. If these long-lived particles can travel thousands of kilometers before decaying, scientists are hoping that they’ll still be able to catch a few of the unlucky early-transformers before they leave the detector. Lubatti and his collaborators have also proposed a new LHC surface detector, which would extend their search range by many orders of magnitude.

Because these long-lived particles themselves don’t interact with the detector, their signal would look like a stream of ordinary matter spontaneously appearing out of nowhere.

“For instance, if a long lived particle decayed into quarks while inside the muon detector, it would mimic the appearance of several muons closely clustered together,” Lubatti says. “We are triggering on events like this in the ATLAS experiment.” After recording the events, scientists use custom algorithms to reconstruct the origins of these clustered particles to see if they could be the offspring of an invisible long-lived parent.

If discovered, this new breed of matter could help answer several lingering questions in physics.

“Long-lived particles are not a prediction of a single new theory, but rather a phenomenon that could fit into almost all of our frameworks for beyond-the-Standard-Model physics,” Shelton says.

In addition to rounding out the Standard Model’s mathematics, inert long-lived particles could be cousins of dark matter—an invisible form of matter that only interacts with the visible cosmos through gravity. They could also help explain the origin of matter after the Big Bang.

“So many of us have spent a lifetime studying such a tiny fraction of the universe,” Lubatti says. “We’ve understood a lot, but there’s still a lot we don’t understand—an enormous amount we don’t understand. This gives me and my colleagues pause.”

If you chop a magnet in half, you end up with two smaller magnets. Both the original and the new magnets have “north” and “south” poles. 

But what if single north and south poles exist, just like positive and negative electric charges? These hypothetical beasts, known as “magnetic monopoles,” are an important prediction in several theories. 

Like an electron, a magnetic monopole would be a fundamental particle. Nobody has seen one yet, but many—maybe even most—physicists would say monopoles probably exist.

“The electric and magnetic forces are exactly the same force,” says Wendy Taylor of Canada’s York University. “Everything would be totally symmetric if there existed a magnetic monopole. There is a strong motivation by the beauty of the symmetry to expect that this particle exists.”

Dirac to the future

Combining the work of many others, nineteenth-century physicist James Clerk Maxwell showed that electricity and magnetism were two aspects of a single thing: the electromagnetic interaction. 

But in Maxwell’s equations, the electric and magnetic forces weren’t quite the same. The electrical force had individual positive and negative charges. The magnetic force didn’t. Without single poles—monopoles—Maxwell’s theory looked asymmetrical, which bugged him. Maxwell thought and wrote a lot about the problem of the missing magnetic charge, but he left it out of the final version of his equations.

Quantum pioneer Paul Dirac picked up the monopole mantle in the early 20th century. By Dirac’s time, physicists had discovered electrons and determined they were indivisible particles, carrying a fundamental unit of electric charge. 

Dirac calculated the behavior of an electron in the magnetic field of a monopole. He used the rules of quantum physics, which say an electron or any particle also behaves like a wave. For an electron sitting near another particle—including a monopole—those rules say the electron’s wave must go through one or more full cycles wrapping around the other particle. In other words, the wave must have at least one crest and one trough: no half crests or quarter-troughs.

For an electron in the presence of a proton, this quantum wave rule explains the colors of light emitted and absorbed by a hydrogen atom, which is made of one electron and one proton. But Dirac found the electron could only have the right wave behavior if the product of the monopole magnetic charge and the fundamental electric charge carried by an electron were a whole number. That means monopoles, like electrons, carry a fundamental, indivisible charge. Any other particle carrying the fundamental electric charge—protons, positrons, muons, and so forth—will follow the same rule.

Interestingly, the logic runs the other way too. Dirac’s result says if a single type of monopole exists, even if that type is very rare, it explains a very important property of matter: why electrically charged particles carry multiples of the fundamental electric charge. (Quarks carry a fraction—one-third or two-thirds—of the fundamental charge, but they always combine to make whole-number multiples of the same charge.) And if more than one type of monopole exists, it must carry a whole-number multiple of the fundamental magnetic charge.

The magnetic unicorn

Dirac’s discovery was really a plausibility argument: If monopoles existed, they would explain a lot, but nothing would crumble if they didn’t. 

Since Dirac’s day, many theories have made predictions about the properties of magnetic monopoles. Grand unified theories predict monopoles that would be over 10 quadrillion times more massive than protons. 

Producing such particles would require more energy than Earthly accelerators can reach, “but it’s the energy that was certainly available at the beginning of the universe,” says Laura Patrizii of the Italian National Institute for Nuclear Physics. 

Cosmic ray detectors around the world are looking for signs of these monopoles, which would still be around today, interacting with molecules in the air. The MACRO experiment at Gran Sasso in Italy also looked for primordial monopoles, and provided the best constraints we have at present. 

Luckily for scientists like Patrizii and Taylor, grand unified theories aren’t the only ones to predict monopoles. Other theories predict magnetic monopoles of lower masses that could feasibly be created in the Large Hadron Collider, and of course Dirac’s original model didn’t place any mass constraints on monopoles at all. That means physicists have to be open to discovering particles that aren’t part of any existing theory. 

Both of them look for monopoles created at the Large Hadron Collider, Patrizii using the MoEDAL detector and Taylor using ATLAS.

“I think personally there's lots of reasons to believe that monopoles are out there, and we just have to keep looking,” Taylor says. 

“Magnetic monopoles are probably my favorite particle. If we discovered the magnetic monopole, [the discovery would be] on the same scale as the Higgs particle.”

The historic detection of gravitational waves announced earlier this year breathed new life into a theory that’s been around for decades: that black holes created in the first second of the universe might make up dark matter. It also inspired a new idea: that those so-called primordial black holes could be contributing to a diffuse background light.

The connection between these perhaps seemingly disparate areas of astronomy were tied together neatly in a theory from Alexander Kashlinsky, an astrophysicist at NASA’s Goddard Spaceflight Center. And while it’s an unusual idea, as he says, it could be proven true in only a few years.

Mapping the glow

Kashlinsky’s focus has been on a residual infrared glow in the universe, the accumulated light of the earliest stars. Unfortunately, all the stars, galaxies and other bright objects in the sky—the known sources of light—oversaturate this diffuse glow. That means that Kashlinsky and his colleagues have to subtract them out of infrared images to find the light that’s left behind.

They’ve been doing precisely that since 2005, using data from the Spitzer space telescope to arrive at the residual infrared glow: the cosmic infrared background (CIB).

Other astronomers followed a similar process using Chandra X-ray Observatory data to map the cosmic X-ray background (CXB), the diffuse glow of hotter cosmic material and more energetic sources.

In 2013, Kashlinsky and colleagues compared the CIB and CXB and found correlations between the patchy patterns in the two datasets, indicating that something is contributing to both types of background light. So what might be the culprit for both types of light?

“The only sources that could be coherent across this wide range of wavelengths are black holes,” he says.

To explain the correlation they found, roughly 1 in 5 of the sources had to be black holes that lived in the first few hundred million years of our universe. But that ratio is oddly large.

“For comparison,” Kashlinsky says, “in the present populations, we have 1 in 1000 of the emitting sources that are black holes. At the peak of star formation, it’s 1 in 100.”

He wasn’t sure how the universe could have ever had enough black holes to produce the patterns his team saw in the CIB and CXB. Then the Laser Interferometric Gravitational-wave Observatory (LIGO) discovered a pair of strange beasts: two roughly-30-solar-mass black holes merging and emitting gravitational waves.

A few months later, Kashlinsky saw a study led by Simeon Bird analyzing the possibility that the black holes LIGO had detected were primordial—formed in the universe’s first second. “And it just all came together,” Kashlinsky says.

Gravitational secrets

The crucial ripples in space-time picked up by the LIGO detector on September 14, 2015, came from the last dance of two black holes orbiting each other and colliding. One black hole was 36 times the sun’s mass, the other 29 times. Those black-hole weights aren’t easy to make.

The majority of the universe’s black holes are less than about 15 solar masses and form as massive stars collapse at the end of their lives. A black hole weighing 30 solar masses would have to start from a star closer to 100 times our sun’s mass—and nature seems to have a hard time making stars that enormous. To compound the strangeness of the situation, the LIGO detection is from a pair of those black holes. Scientists weren’t expecting such a system, but the universe has a tendency to surprise us.

Bird and his colleagues from Johns Hopkins University next looked at the possibility that those black holes formed not from massive stars but instead during the universe’s first fractions of a second. Astronomers haven’t yet seen what the cosmos looked like at that time, so they have to rely on theoretical models.

In all of these models, the early universe exists with density variations. If there were regions of very high-contrasting density, those could have collapsed into black holes in the universe’s first second. If those black holes were at least as heavy as mountains when they formed, they’d stick around until today, dark and seemingly invisible and acting through the gravitational force. And because these primordial black holes formed from density perturbations, they wouldn’t be comprised of protons and neutrons, the particles that make up you, me, stars and, thus, the material that leads to normal black holes.

All of those characteristics make primordial black holes a tempting candidate for the universe’s mysterious dark matter, which we believe makes up some 25 percent of the universe and reveals itself only through the gravitational force. This possible connection has been around since the 1970s, and astronomers have looked for hints of primordial black holes since. Even though they’ve slowly narrowed down the possibilities, there are a few remaining hiding spots—including the region where the black holes that LIGO detected fall, between about 20 and 1000 solar masses.

Astronomers have been looking for explanations of what dark matter is for decades. The leading theory is that it’s a new type of particle, but searches keep coming up empty. On the other hand, we know black holes exist; they stem naturally from the theory of gravity.

“They’re an aesthetically pleasing candidate because they don’t need any new physics,” Bird says.

A glowing contribution

Kashlinsky’s newest analysis took the idea of primordial black holes the size that LIGO detected and looked at what that population would do to the diffuse infrared light of the universe. He evolved a model of the early universe, looking at how the first black holes would congregate and grow into clumps. These black holes matched the residual glow of the CIB and, he found, “would be just right to explain the patchiness of infrared background by sources that we measured in the first couple hundred million years of the universe.”

This theory fits nicely together, but it’s just one analysis of one possible model that came out of an observation of one astrophysical system. Researchers need several more pieces of evidence to say whether primordial black holes are in fact the dark matter. The good news is LIGO will soon begin another observing run that will be able to see black hole collisions even farther away from Earth and thus further back in time. The European gravitational wave observatory VIRGO will also come online in January, providing more data and working in tandem with LIGO.

More cases of gravitational waves from black holes around this 30-solar-masses range could add evidence that there is a population of primordial black holes. Bird and his colleague Ilias Cholis suggest looking for a more unique signal, though, in future gravitational-wave data. For two primordial black holes to become locked in a binary system and merge, they would likely be gravitationally captured during a glancing interaction, which could result in a signal with multiple frequencies or tones at any one moment.

“This is a rare event, but it would be very characteristic of our scenario,” Cholis says. “In the next 5 to 10 years, we might see one.”

This smoking-gun signature, as they call it, would be a strong piece of evidence that primordial black holes exist. And if such objects are floating around our universe, it might not be such a stretch to connect them to dark matter.

When Galileo first introduced the telescope in the 1600s, astronomers gained the ability to view parts of the universe that were invisible to the naked eye. This led to centuries of discovery—as telescopes advanced, they exposed new planets, galaxies and even a glimpse of the very early universe. 

Last September, scientists gained yet another invaluable tool: the ability to hear the cosmos through gravitational waves.

Ripples in space-time

Newton described gravity as a force. Thinking about gravity this way can explain most of the phenomena that happens here on Earth. For example, the force of gravity acting on an apple makes it fall from a tree onto an unsuspecting person sitting below it. However, to understand gravity on a cosmic scale, we need to turn to Einstein, who described gravity as the bending of space-time itself. 

Some physicists describe this process using a bowling ball and a blanket. Imagine space-time as a blanket. A bowling ball placed at the center of the blanket bends the fabric around it. The heavier an object is, the further it sinks. As you move the ball along the fabric, it produces ripples, much like a boat travelling through water.

“The curvature is what makes the Earth orbit the sun—the sun is a bowling ball in a fabric and it's that bending in the fabric that makes the Earth go around,” explains Gabriela González, the spokesperson for the Laser Interferometer Gravitational-Wave Observatory (LIGO) collaboration. 

Everything with mass—planets, stars and people—pulls on the fabric of space-time and produces gravitational waves as they move through space. These are passing through us all time, but they are much too weak to detect. 

To find these elusive signals, physicists built LIGO, twin observatories in Louisiana and Washington. At each L-shaped detector, a laser beam is split and sent down two four-kilometer arms. The beams reflect off the mirrors at each end and travel back to reunite. A passing gravitational wave slightly alters the relative lengths of the arms, shifting the path of the laser beam, creating a change that physicists can detect.  

Unlike telescopes, which are pointed toward very specific parts of the sky, detectors like LIGO scan a much larger area of the universe and hear sources from all directions. “Gravitational waves detectors are like microphones,” says Laura Nuttall, a postdoctoral researcher at Syracuse University. 

First detections

On the morning of September 14, 2015, a gravitational wave from two black holes that collided 1.3 billion years ago passed through the two LIGO detectors, and an automatic alert system pinged LIGO scientists around the world. “It took us a good part of the day to convince ourselves that this was not a drill,” González says. 

Because LIGO was still preparing for an observing run—researchers were still running tests and diagnostics during the day—they needed to conduct a large number of checks and analyses to make sure the signal was real. 

Months later, once researchers had meticulously checked the data for errors or noise (such as lightning or earthquakes) the LIGO collaboration announced to the world that they had finally reached a long-anticipated goal: Almost 100 years after Einstein first predicted their existence, scientists had detected gravitational waves. 

A few months after the first signal arrived, LIGO detected yet another black hole collision. “Finding a second one proves that there's a population of sources that will produce detectible gravitational waves,” Nuttall says. “We are actually an observatory now.”

Cosmic microphones

Many have dubbed the detection of gravitational waves as the dawn of the age of gravitational wave astronomy. Scientists expect to see hundreds, maybe even thousands, of these binary black holes in the years to come. Gravitational-wave detectors will also allow astronomers to look much more closely at other astronomical phenomena, such as neutron stars, supernovae and even the Big Bang.

One important next step is to detect the optical counterparts—such as light from the surrounding matter or gamma ray bursts—of the sources of gravitational waves. To do this, astronomers need to point their telescopes to the area of the sky where the gravitational waves came from to find any detectable light. 

Currently, this feat is like finding a needle in a haystack. Because the field of view of gravitational wave detectors is much, much larger than telescopes, it is extremely difficult to connect the two. “Connecting gravitational waves with light for the first time will be such an important discovery that it's definitely worth the effort,” says Edo Berger, an astronomy professor at Harvard University.

LIGO is also one of several gravitational wave observatories. Other ground-based observatories, such as Virgo in Italy, KAGRA in Japan and the future LIGO India have similar sensitivities to LIGO. There are also other approaches that scientists are using—and plan to use in the future—to detect gravitational waves at completely different frequencies. 

The evolved Laser Interferometer Space Antenna (eLISA), for example, is a gravitational wave detector that physicists plan to build in space. Once complete, eLISA will be composed of three spacecraft that are over a million kilometers apart, making it sensitive to much lower gravitational wave frequencies, where scientists expect to detect supermassive black holes.

Pulsar array timing is a completely different method of detection. Pulsars are natural timekeepers, regularly emitting beams of electromagnetic radiation. Astronomers carefully measure the arrival time of the pulses to find discrepancies, because when a gravitational wave passes by, space-time warps, changing the distance between us and the pulsar, causing the pulses to arrive slightly earlier or later. This method is sensitive to even lower frequencies than eLISA. 

These and many other observatories will reveal a new view of the universe, helping scientists to study phenomena such as merging black holes, to test theories of gravity and possibly even to discover something completely unexpected, says Daniel Holz, a professor of physics and astronomy at the University of Chicago. “Usually in science you're just pushing the boundaries a little bit, but in this case, we're opening up a whole new frontier.”