General relativity reshaped our universe

Einstein presented his general theory of relativity at the end of 1915 in a series of lectures in Berlin. But it wasn’t until a solar eclipse in 1919 that everyone took notice. His theory predicted that a massive object — say, the sun — could distort spacetime nearby enough to bend light from its straight-line course. Distant stars would thus appear not exactly where expected. Photographs taken during the eclipse verified that the position shift matched Einstein’s prediction. “Lights all askew in the heavens; men of science more or less agog,” declared a New York Times headline. Even a decade later, a story in Science News-Letter, the predecessor of Science News, wrote of “Riots to understand Einstein theory.” Apparently extra police had to be called in to control a crowd of 4,500 who “broke down iron gates and mauled each other” at the American Museum of Natural History in New York City to hear an explanation of general relativity.

By 1931, physicist Albert A. Michelson, the first American to win a Nobel Prize in the sciences, called the theory “a revolution in scientific thought unprecedented in the history of science.”

But for all the powers of divination we credit to Einstein today, he was a reluctant soothsayer. We now know that general relativity offered much more than Einstein was willing or able to see. “It was a profoundly different way of looking at the universe,” says astrophysicist David Spergel of the Simons Foundation’s Flatiron Institute in New York City, “and it had some wild implications that Einstein himself didn’t want to accept.” What’s more, says Spergel (a member of the Honorary Board of the Society for Science, publisher of Science News), “the wildest aspects of general relativity have all turned out to be true.”

“[General relativity] had some wild implications that Einstein himself didn’t want to accept.”

astrophysicist David Spergel

What had been masquerading as a quiet, static, finite place is instead a dynamic, ever-expanding arena filled with its own riot of space-bending beasts. Galaxies congregate in superclusters on scales vastly greater than anything experts had considered before the 20th century. Within those galaxies reside not only stars and planets, but also a zoo of exotic objects illustrating general relativity’s propensity for weirdness: neutron stars, which pack a fat star’s worth of mass into the size of a city, and black holes, which pervert spacetime so strongly that no light can escape. And when these behemoths collide, they shake spacetime, blasting out ginormous amounts of energy. Our cosmos is violent, evolving and filled with science fiction–like possibilities that actually come straight out of general relativity.

“General relativity opened up a huge stage of stuff for us to look at and try out and play with,” says astrophysicist Saul Perlmutter of the University of California, Berkeley. From the idea that the universe changes dramatically over its lifetime — “the idea of a lifetime of a universe at all is a bizarre concept,” he says — to the idea that the cosmos is expanding, to the thought that it could collapse and come to an end, and even that there might be other universes. “You get to realize that the world could be much more interesting even than we already ever imagined it could possibly be.”

General relativity has become the foundation for today’s understanding of the cosmos. But the current picture is far from complete. Plenty of questions remain about mysterious matter and forces, about the beginnings and the end of the universe, about how the science of the big meshes with quantum mechanics, the science of the very small. And some astronomers believe a promising route to answering some of those unknowns is another of general relativity’s initially underappreciated features — the power of bent light to magnify features of the cosmos. 

Today’s scientists continue to poke and prod at general relativity to find clues to what they might be missing. General relativity is now being tested to a level of precision previously impossible, says astrophysicist Priyamvada Natarajan of Yale University. “General relativity expanded our cosmic view, then gave us sharper focus on the cosmos, and then turned the tables on it and said, ‘now we can test it much more strongly.’” It’s this testing that turns up cracks that can likely be patched, and perhaps will uncover more dramatic fractures that point the way to a fuller picture.

And so, more than a century after general relativity debuted, there’s plenty left to foretell. The universe may turn out to be even wilder yet.
— Elizabeth Quill

---

“The power of an image is strong,” says Kazunori Akiyama, an astrophysicist at the MIT Haystack Observatory in Westford, Mass., who led one of the teams that created the image. “I somewhat expected that we might see something exotic,” Akiyama says. But after looking at the first image, “Oh my God,” he recalls thinking, “it’s just perfectly matching with our expectation of general relativity.”

“Oh my God,” Kazunori Akiyama recalls thinking after seeing the first-ever image of a black hole. “It’s just perfectly matching with our expectation of general relativity.”

For a long time, black holes were mere mathematical curiosities. Evidence that they actually reside out in space didn’t start coming in until the second half of the 20th century. It’s a common story in the annals of physics. An oddity in some theorist’s equation points to a previously unknown phenomenon, which kicks off a search for evidence. Once the data are attainable, and if physicists get a little lucky, the search gives way to discovery.

In the case of black holes, German physicist Karl Schwarzschild came up with a solution to Einstein’s equations near a single spherical mass, such as a planet or a star, in 1916, shortly after Einstein proposed general relativity. Schwarzschild’s math revealed how the curvature of spacetime would differ around stars of the same mass but increasingly smaller sizes — in other words, stars that were more and more compact. Out of the math came a limit to how small a mass could be squeezed. Then in the 1930s, J. Robert Oppenheimer and Hartland Snyder described what would happen if a massive star collapsing under the weight of its own gravity shrank past that critical size — today known as the “Schwarzschild radius” — reaching a point from which its light could never reach us. Still, Einstein — and most others — doubted that what we now call black holes were plausible in reality.

The term “black hole” first appeared in print in Science News Letter. It was in a 1964 story by Ann Ewing, who was covering a meeting in Cleveland of the American Association for the Advancement of Science. That’s also about the time that hints in favor of the reality of black holes started coming in. Just a few months later, Ewing reported the discovery of quasars — describing them in Science News Letter as “the most distant, brightest, most violent, heaviest and most puzzling sources of light and radio waves.” Though not linked to black holes at the time, quasars hinted at some cosmic powerhouses needed to provide such energy. The use of X-ray astronomy in the 1960s revealed new features of the cosmos, including bright beacons that could come from a black hole scarfing down a companion star. And the motions of stars and gas clouds near the centers of galaxies pointed to something exceedingly dense lurking within.    

---

---

Black holes stand out among other cosmic beasts for how extreme they are. The largest are many billion times the mass of the sun, and when they rip a star apart, they can spit out particles with 200 trillion electron volts of energy. That’s some 30 times the energy of the protons that race around the world’s largest and most powerful particle accelerator, the Large Hadron Collider.

As evidence built into the 1990s and up to today, scientists realized these great beasts not only exist, but also help shape the cosmos. “These objects that general relativity predicted, that were mathematical curiosities, became real, then they were marginal. Now they’ve become central,” says astrophysicist Priyamvada Natarajan of Yale University. We now know supermassive black holes reside at the centers of most if not all galaxies, where they generate outflows of energy that affect how and where stars form. “At the center of the galaxy, they define everything,” she says.

Though visual confirmation is recent, it feels as though black holes have long been familiar. They are a go-to metaphor for any unknowable space, any deep abyss, any endeavor that consumes all our efforts while giving little in return. Real black holes, of course, have given plenty back: Answers to scientists seeking to understand our cosmos plus new questions to ponder. Wonder and entertainment to space fanatics. A lost album from Weezer. Numerous episodes of Doctor Who. The Hollywood blockbuster Interstellar.

For physicist Nicolas Yunes of the University of Illinois at Urbana-Champaign, black holes and other cosmic behemoths continue to amaze. “Just thinking about the dimensions of these objects, how large they are, how heavy they are, how dense they are,” he says, “it’s really breathtaking.”
— Elizabeth Quill

---

Einstein’s math predicted such waves could be created, not only by gigantic collisions but also by explosions and other accelerating bodies. But for a long time, spotting any kind of spacetime ripple was a dream beyond measure. Only the most dramatic cosmic doings would create signals that were large enough for direct detection. Einstein, who called the waves gravitationswellen, was unaware that any such big events existed in the cosmos.

Beginning in the 1950s, when others were still arguing whether gravitational waves existed in reality, physicist Joseph Weber sunk his career into trying to detect them. After a decade-plus effort, he claimed detection in 1969, identifying an apparent signal perhaps from a supernova or from a newly discovered type of rapidly spinning star called a pulsar. In the few years after reporting the initial find, Science News published more than a dozen stories on what it began calling the “Weber problem.” Study after study could not confirm the results. What’s more, no sources of the waves could be found. A 1973 headline read, “The deepening doubt about Weber’s waves.”

Weber stuck by his claim until his death in 2000, but his waves were never verified. Nonetheless, scientists increasingly believed gravitational waves would be found. In 1974, radio astronomers Russell Hulse and Joseph Taylor spotted a neutron star orbiting a dense companion. Over the following years, the neutron star and its companion appeared to be getting closer together by the distance that would be expected if they were losing energy to gravitational waves. Scientists soon spoke not of the Weber problem, but of what equipment could possibly pick up the waves. “Now, although they have not yet seen, physicists believe,” Dietrick E. Thomsen wrote in Science News in 1984. 

---

---

It was a different detection strategy, decades in the making, that would provide the needed sensitivity. The Advanced Laser Interferometry Gravitational-wave Observatory, or LIGO, which reported the first confirmed gravitational waves in 2016, relies on two detectors, one in Hanford, Wash., and one in Livingston, La. Each detector splits the beam of a powerful laser in two, with each beam traveling down one of the detector’s two arms. In the absence of gravitational waves, the two beams recombine and cancel each other out. But if gravitational waves stretch one arm of the detector while squeezing the other, the laser light no longer matches up.

The machines are an incredible feat of engineering. Even spacetime ripples detected from colliding black holes might stretch an arm of the LIGO detector by as little as one ten-thousandth of the width of a proton.

When the first detection, from two colliding black holes, was announced, the discovery was heralded as the beginning of a new era in astronomy. It was Science News’ story of the year in 2016, and such a big hit that the pioneers of the LIGO detector won the Nobel Prize in physics the following year. Within five years of that first report, scientists with LIGO and another gravitational wave detector, Virgo, based in Italy, had logged dozens more detections. Most of the waves have emanated from mergers of black holes, though a few events have featured neutron stars. Smashups so far have revealed the previously unknown birthplaces of some heavy elements, pointed to a bright jet of charged subatomic particles that could offer clues to mysterious flashes of high-energy light known as gamma-ray bursts, and showed that midsize black holes, between 100 and 100,000 times the sun’s mass, do in fact exist — along with reconfirming that Einstein was right, at least so far.

Just five years in, some scientists are already eager for something even more exotic. In a Science News article about detecting black holes orbiting wormholes via gravitational waves, physicist Vítor Cardoso suggested a coming shift to more unusual phenomena: “We need to look for strange but exciting signals,” he said.

Gravitational wave astronomy is truly only at its beginnings. Improved sensitivity at existing Earth-based detectors will turn up the volume on gravitational waves, allowing detections from less energetic and more distant sources. Future detectors, including the space-based LISA, planned for launch in the 2030s, will get around the troublesome noise that interferes when Earth’s surface shakes.

“Perhaps the most exciting thing would be to observe a small black hole falling into a big black hole, an extreme mass ratio inspiraling,” says Nicolas Yunes, an astrophysicist at the University of Illinois at Urbana-Champaign. In such an event, the small black hole would zoom back and forth, back and forth, swirling in different directions as it followed wildly eccentric orbits, perhaps for years. That could offer the ultimate test of Einstein’s equations, revealing whether we truly understand how spacetime is warped in the extreme.
— Elizabeth Quill

---

In the 1920s, Alexander Friedmann, a Russian meteorologist-mathematician, adapted Einstein’s original equations to describe a universe that was growing or shrinking over time. Einstein wasn’t willing to accept an expanding universe — in fact, he had added a term to his equations to keep the cosmos static (the term is now known as the cosmological constant). But astronomer Edwin Hubble eventually made Einstein change his mind. In 1929, with pioneering data from the Lowell Observatory in Arizona, collected by Vesto Slipher, and from the 100-inch telescope at Mount Wilson Observatory in California, then the largest in the world, Hubble had revealed that more distant galaxies, then commonly referred to as “extra-galactic nebulae,” appeared to be zooming away from us faster than nearby galaxies.

In a 1930 article, titled “Nebulae Hold Sky’s ‘Speed Record,’” Science News-Letter reported on a group of nebulae known as the Ursa Major cluster that was flying away from us at 7,200 miles a second. The article summarized Hubble’s findings in the last two paragraphs of the story. “Just what these high speeds mean is not certain,” said the article, attributing the relationship between distance and velocity to the spreading of light waves. Our cosmos, it eventually became clear, was expanding like a baking cake, with the galaxies like raisins spreading outward from one another. What’s more, general relativity had foretold it. Physicist John Archibald Wheeler, a champion of general relativity after Einstein’s death, called the expansion of the universe the “most dramatic prediction that science has ever made.”

From there spilled more questions, answers and questions again about our universe’s past and present. Here’s what scientists currently know, along with some of their ideas:

Big Bang

Within the last century, scientists have determined that the universe as we know it began billions of years ago with a Big Bang. The idea of the bang started with the work of Georges Lemaître, a Belgian cosmologist and priest who in the late 1920s, like Friedmann a few years earlier, had solved Einstein’s equations for an expanding universe. Lemaître went on to reason that an expanding universe must have expanded out of something, a “primeval atom,” he said. More than a decade later, in an effort to explain how chemical elements originated, physicist George Gamow and his Ph.D. student Ralph Alpher described a hot, dense starting point — an “overheated neutral nuclear fluid” — cooking up hydrogen and helium in just 300 seconds, and then all the elements on up. (We now know that the proposed process can’t explain elements beyond hydrogen, helium and some lithium.) Hans Bethe was added as an author of the paper as a joke, so the three last names on the paper (Alpher, Bethe and Gamow) would become a pun on “alpha, beta, gamma,” the first three letters of the Greek alphabet.

Solid evidence of the Big Bang had to wait until 1964, when astronomers Arno Penzias and Robert Wilson serendipitously discovered relic microwave radiation left over from the Big Bang (which had, to no real notice, already been predicted by Alpher and colleague Robert Herman). Increasingly precise studies of this “cosmic microwave background” in the decades since, along with estimates of the ages of globular clusters, have narrowed in on the universe’s birthdate of 13.8 billion years ago.

Accelerated expansion

Not only is the universe expanding, but that expansion is accelerating. When astrophysicist Saul Perlmutter began scanning the sky for supernovas in the late 1980s, he had assumed the universe’s expansion was slowing over time — reined in by the pull of gravity from all the matter in the cosmos. He and other physicists talked of a “deceleration parameter.” Depending on the size of that parameter, expansion might grow ever slower but continue forever, resulting in a cold, dark future. Or the pull of gravity might win out, collapsing the universe in on itself in a “Big Crunch.”

---

---

The supernovas Perlmutter was after, Type Ia supernovas, could offer an answer. The brightness of these explosions should track with their distances; fainter explosions are farther away, and thus further back in time. But Perlmutter’s team found that distant supernovas were fainter than expected, as did another team led by Brian Schmidt. The two teams reported in 1998 that the universe is flying apart faster than ever before. 

In a twist of history, the term Einstein had added to his equations of general relativity to keep the universe static — the “cosmological constant” that Friedmann had ditched — has now been revived. It has been applied to the mysterious force, now called “dark energy,” that appears to be driving the universe’s accelerated expansion.

The accelerated expansion is one of the key finds over the last century, says astrophysicist Priyamvada Natarajan of Yale University. “The universe got unfixed,” she says. “The universe is unmoored.” Whatever this dark energy is, it is a major ingredient in our cosmos.

Matter and energy

“Dark energy” isn’t the only hidden entity we now know of. As early as the 1930s, Swiss astronomer Fritz Zwicky had found galaxies behaving badly in the Coma cluster, more than 300 million light-years from Earth. Galaxies within the cluster were moving faster than could be explained by the pull of visible matter, suggesting the existence of some “dunkle Materie,” as Zwicky described it in German. Astronomer Vera Rubin and colleagues confirmed the existence of dark matter in the 1970s. Those studies of stars revolving around galactic centers found that the farther-out stars traveled just as fast as the inner ones, so fast that the galaxies should be flying apart. Yet they weren’t. Galaxies, the calculations revealed, must have much more dark matter than ordinary stuff.

“There could be a zoology of dark matter particles.”

astrophysicist Tommaso Treu

Efforts to identify what dark matter is have so far come up empty-handed. But the substance might not be as simple as we imagine, says astrophysicist Tommaso Treu of UCLA. “I always think, if I were a dark matter being trying to imagine what the luminous people were like, I would think: one electron, one proton, one photon,” he says. He wouldn’t imagine the vast array of particles that have become part of particle physicists’ standard model. Dark matter could likewise be more complex. “There could be a zoology of dark matter particles,” Treu adds.

Regardless of what exactly dark matter turns out to be, its discovery and the discovery of dark energy dramatically changed our picture of what makes up our cosmos. The ordinary stuff we can detect — the stars, the planets, the people trying to understand it all — is only about 5 percent of the mass and energy content of the universe. Dark energy makes up close to 70 percent, and dark matter is roughly 25 percent.

Inflation

Despite the primordial disorder of the Big Bang, matter appears to be spread out mostly evenly on the largest of scales. Scientists have long puzzled over why this should be so.

In the early 1980s, physicist Alan Guth offered up an idea explaining why the universe is so “smooth,” as well as some other mysteries of the modern cosmos. Guth proposed that in the immediate moments after the Big Bang, during a brief period lasting just fractions of a second, the universe ballooned outward fast enough to spread out the matter uniformly, with only tiny deviations. From those small deviations where matter was slightly denser, galaxies would eventually form. This period of “inflation” was so quick that the visible universe grew from the tiniest of tiny specks to a meter across — still small by what we imagine today, but a ballooning in diameter of more than 50 orders of magnitude.

Inflation has become, astrophysicist David Spergel says, “the most broadly accepted extension” of our standard picture of the cosmos. It explains a lot, and has quite a bit of solid evidence behind it, but it isn’t yet confirmed.

Some critics don’t like some aspects of inflation. “It is easy to get eternal inflation,” for example, says Spergel, who calls himself “one of the more agnostic” on arguments over inflation. In the case of eternal inflation, pockets of the universe — in fact most of the universe — would continue inflating after ours stopped.

That scenario leads to the obvious question: Why are we where we are? Or, in another formulation, what makes our pocket so special? From here, some physicists like to embark on a philosophical journey; Spergel sees it as a sign that some piece is missing.

What’s left?

Despite all we now know, and how far general relativity has brought us, our picture of the universe is far from complete. Many physicists hold out hope that another idea as intuitive as Einstein’s will come along to help clarify some of the big questions that we haven’t answered: What is dark matter? What is dark energy? Why is there more matter than antimatter in the cosmos? How does general relativity fit with quantum mechanics? One question that interests Perlmutter is, what was happening right before the Big Bang?

There’s a good possibility, he notes, that an ultimate answer might not be as satisfying as we would expect — akin to the 42 that the computer Deep Thought reveals as the answer to “the great question of life, the universe and everything” in The Hitchhiker’s Guide to the Galaxy. “It is hard to think of what counts as a satisfying answer, even if you could make anything up at all and imagine it were true,” Perlmutter says. One of the most fundamental parts of doing science, he says, is figuring out what counts as a question and what counts as an answer that you would care about.

His sentiment calls to mind  that of one astronomer who, after taking part in a poll that the publisher of Science News Letter conducted in the late 1950s on the origin of the universe, said that much of the “fun of astronomical research” would be gone if a sure answer were ever found.
— Elizabeth Quill

---

Today “gravitational lensing” is an invaluable tool for probing the cosmos. “Nature gives you a free magnifying lens,” says astrophysicist Tommaso Treu of UCLA. In fact, astronomers have exploited hundreds of lenses since the first example of lensing, when astronomers detected “twin quasars so identical” that they were probably the same object, Science News reported in 1979. By 1990, astronomer Daniel W. Weedman was already impressed by its victories: “Rarely in astronomy can such a simple theory as gravitational lensing be applied to such a wide range of observations,” he told Science News. And the uses have only expanded since.

The principle is simple. In the same way a magnifying glass bends light, creating multiple images or focusing the light to create images larger than the original, massive objects in space can bend and focus the light of more distant objects. The result can be elongated shapes, dramatic arcs, bright rings and multiple images of exceeding beauty. When a foreground and distant galaxy align just right, they can create what’s called an “Einstein ring.” When three galaxies line up, you can get a double Einstein ring.

Unlike some of general relativity’s other wild predictions, “It’s not so much bizarre,” says Liliya Williams, an astrophysicist at the University of Minnesota, “but definitely cool.” What’s more, it’s everywhere. “It affects pretty much every single aspect of astrophysics,” Williams adds.

Among other aims, gravitational lensing has been used to:

1. Study early galaxies

One often-used lens is the galaxy cluster Abell 2744. At about 4 billion light-years away, it has a mass of about 2 quadrillion suns, giving it impressive light-bending power. With the help of Abell 2744, astronomers in 2014 detected one of the faintest and most distant galaxies that had yet been seen. The galaxy, which appeared 10 times larger and brighter than it would otherwise look, dates to just 500 million years after the birth of the universe. Views of such ancient galaxies can help scientists understand how galaxies form and grow over time.

2. Zoom in on stars

In 2018, astronomers reported using lensing to detect a blue supergiant star some 9 billion light-years from Earth, the most distant individual, stable star ever detected. The researchers concluded the star, nicknamed Icarus, was being magnified more than 2,000 times its original brightness.

3. Predict the future

(Future observations at least.)

After seeing four images of the same supernova, lensed by a foreground galaxy cluster, researchers predicted in 2015 that another image would arrive perhaps within a year or so. Sure enough, that image of supernova Refsdal appeared right on schedule. The prediction was possible thanks to an understanding of the distribution of mass in the foreground cluster.

4. Find exoplanets

Astronomers have detected more than a hundred planets orbiting stars other than the sun using a variant of gravitational lensing known as microlensing. In microlensing, the gravitational pull of a star or planet bends the light of a more distant object so slightly that the multiple images overlap, slightly brightening the background object as the lens passes in front. In 2006, astronomers using the technique reported the smallest planet then known to exist beyond the solar system.

5. Probe dark matter

In the 1980s, astronomer Bohdan Paczyński proposed using microlensing to look for dark matter in the form of ordinary stuff — baryonic matter, made of protons and neutrons — that simply wasn’t visible. It was a bold suggestion; these baryonic MACHOs, or massive compact halo objects, would only rarely pass in front of background stars. “If you want any significant chance of observing it, you need to be looking at a million stars,” says Williams. Extensive cosmic surveys by multiple teams have since revealed that there are too few of these baryonic MACHOs to explain dark matter.

What’s more, gravitational lensing is “the best tool to map out the distribution of dark matter on all scales,” Williams adds. Gravity is the only force that dark matter appears to respond to and gravitational lensing is a purely gravitational phenomenon, making the two a perfect match. Mapping dark matter offers clues to what it is made of. Recently, lensing revealed that some globs of dark matter in galaxy clusters might be denser than expected.

6. Map the structure of the universe

Strong lenses — those that create multiple images — are exceedingly rare. But every light source in the universe is bent; the smattering of mass across the cosmos means we’re looking at the stars through a textured shower door. Using this “weak lensing,” researchers analyze these smaller distortions in light across many, many objects to map matter across the sky. A large international collaboration known as the Dark Energy Survey, as just one example, has taken pictures of hundreds of millions of galaxies looking for shape distortions.

7. Find clues to dark energy

The results of such mapping efforts reveal not only how matter is clumped, but can also offer clues to how the universe has expanded over time, and thus provide insights into dark energy’s cosmic push. Using gravitational lensing to probe dark energy is a key goal of upcoming telescopes, including the ground-based Vera C. Rubin Observatory, the European Space Agency’s Euclid spacecraft and the Nancy Grace Roman Space Telescope.

Scientists are likely to find even more uses for gravitational lensing going forward. Astronomer Christopher Kochanek of Ohio State University would like to see microlensing used for surveying black holes. Black holes that are actively feeding, or orbiting with a companion, or colliding into each other and so producing floods of gravitational waves are all detectable. But there’s a presumably much larger population of black holes in the stellar mass range that are floating silently through the cosmos. “We don’t really know how many black holes, stellar-mass black holes, there are in the universe,” Kochanek says. It would be interesting to find out just how many of these beasts are in our midst.

A century on, it’s clearer than ever that general relativity not only gave us such beasts, but also the framework and tools to understand them — along with all the other cosmic bizarreries around us. And as the cosmos opened up to us, the human mind expanded to meet it. So much about general relativity “defies your experience,” says Treu. The theory’s predictions and consequences violate our intuition about the universe. “It’s a wonderful reflection of the ability of the human mind to transcend experience,” he says. “We can think of things that are so counterintuitive, and make sense of them.”
— Elizabeth Quill