s-c-i-guy

[s-c-i-guy]

Mood

#me #stranger things #david harbour

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M87 Black Hole Size Comparison

#science #science comics #xkcd #science news #event horizon telescope

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What the Sight of a Black Hole Means to a Black Hole Physicist

The astrophysicist Janna Levin reflects on the newly unveiled, first-ever photograph of a black hole.

“At this historic moment, the world has paused to take in the sight of humanity’s first image of the strangest phenomenon in the known universe, a remarkable legacy of the general theory of relativity: a black hole. I am moved not just by the image; overwhelmingly I am moved by the significance of sharing this experience with strangers around the globe. I am moved by the image of a species looking at an image of a curious empty hole looming in space.

I am at the National Press Club, in Washington, D.C., a hive of excitement. Scientists with the Event Horizon Telescope aspired for years to take the first-ever picture of a supermassive black hole, so when they gathered journalists and scientists together today for a press conference, there wasn’t much doubt as to what we were here to see.

But still, there are surprises.

At the podium is Sheperd Doeleman, the director of the Event Horizon Telescope. He welcomes us, ‘black hole enthusiasts.’ I have the strongest memory of standing at the chalkboard in an otherwise empty classroom at the Massachusetts Institute of Technology with Shep, my funny friend with his funny, unmistakable, burnt-mahogany hair. Covered in chalk dust, we acquired the hard-earned mathematics of Albert Einstein’s theory of relativity.

We knew the words already, the standard lore: All forms of matter and energy bend space and time, and light and matter follow those curves. The words have to be taken on trust. But the mathematics we could acquire. It would belong to us. When Einstein conceived of relativity, he gave us a gift that has been passed from person to person around the world. Relativity, defying its name, is true for all of us.

Maybe my memory of that particular board is so crisp precisely because that moment defines the cusp between before and after acquiring relativity. Now I cannot imagine my own mind without it. Relativity permeates my thoughts so that I think in relativity the way writers think in their natural language. Since that time at MIT, Shep and I have both found our way via relativity to the most remarkable of its predictions, black holes.

Black holes were conceived of as a thought experiment, a fantastical imagining. Imagine matter crushed to a point. Don’t ask how. Just imagine that. While enlisted in the German army during World War I, Karl Schwarzschild discovered this possible solution to Einstein’s newly published theory of relativity, apocryphally between calculating ballistic trajectories from the trenches on the Russian front. Schwarzschild inferred that space-time effectively spills toward the crushed center. Racing at its absolute speed, even light gets dragged down the hole, casting a shadow on the sky. That shadow is the event horizon, the stark demarcation between the outside and anything with the misfortune to have fallen inside.

Einstein thought nature would protect us from the formation of black holes. To the contrary, nature makes them in abundance. When a dying star is heavy enough, gravity overcomes matter’s intrinsic resistance and the star collapses catastrophically. The event horizon is left behind as an archaeological record while the stellar material continues to fall inward to an unknown fate. In our own Milky Way galaxy there could be billions of black holes.

Supermassive black holes, millions or even billions of times the mass of the sun, anchor the centers of nearly all galaxies, though nobody yet knows how they formed or got so heavy. Maybe they formed from dead stars that merged and escalated in size, or maybe they directly collapsed out of more primordial material in a younger universe. However they formed, there are as many supermassive black holes as there are galaxies — hundreds of billions in the observable universe.

We had never seen a black hole before today. No telescope had ever taken a picture of one. We have indirectly inferred the presence of black holes when they’ve cannibalized companion stars, powered energetic jets in twisted magnetic fields, and captured stars in their orbit. We have even heard black holes collide and merge, ringing space-time like mallets on a drum.

We had never taken a direct picture of a black hole before because black holes are tiny, despite their dramatic reputation as weapons of mayhem and destruction (yes, the Nova film I hosted was called ‘Black Hole Apocalypse’). A black hole the mass of the sun would have an event horizon a mere 6 kilometers across. Compare that to the 1.4-million-kilometer breadth of the sun itself. The supermassive black hole at the center of the Milky Way, dubbed Sagittarius A*, is 4 million times the mass of the sun but only about 17 times wider.

Consider the challenge of capturing a portrait of an entirely dark object only 17 times the width of an ordinary star at a distance of 26,000 light-years. Resolving an image of Sagittarius A* is comparable to resolving the image of a piece of fruit on the moon.

To resolve such a minuscule image requires a telescope the size of the entire Earth. Since those days in that chalk-dusted classroom at MIT, my funny, utterly unconventional friend has been determined to capture the image of a supermassive black hole all the same.

During our years in graduate school, Shep’s hair was an allegory for his mind — wild and spirited. I admired the freedom I sensed in the way he thought, always forging unexpected connections, sometimes at the expense of the required lesson. His shocked eyes would warn me that a crazy idea had struck him just at that precise moment, as though he was as surprised as I was by the thought.

The Event Horizon Telescope is a testament to bold ideas, as well as scientific ingenuity and collaboration. Exploiting large radio telescopes around the globe — relying on the newest, most sophisticated observatories and reviving some that were nearly defunct — EHT became a composite telescope the size of the Earth. As the planet spins and orbits, the target black holes rise into the field of view of component telescopes around the planet. To render a precise image, the telescopes need to operate as one, which involves sensitive time corrections so that one global eye looks toward the black hole.

Combining telescopes for better resolution was the basis of Shep’s doctoral thesis in the ’90s. By 2008, he led a small team that imaged structures comparable in size to nearby supermassive black holes. That proof of concept drove the EHT project, whose team was now confident that the required resolution was in reach. In the decade since, EHT had to address challenges the data posed and advance technologically, and Shep is quick to credit the international team for their stamina and for the cleverness of their collective contributions.

Our supermassive black hole, Sagittarius A*, became the obvious target to pursue. Despite the abundance of supermassive black holes in galaxies, all others are too far away to resolve even with a telescope the size of the Earth. There is one exception. Messier 87, or M87, is an enormous elliptical galaxy 55 million light-years away that is known to harbor a staggering supermassive black hole somewhere between 3.5 billion and 7.2 billion times the mass of the sun. At the small end of that range, M87 would be an impossible target for EHT. At the high end, it is possibly suitable. So M87 became a secondary target in the heated pursuit of Sagittarius A*.

A black hole against the dark backdrop of empty space would be truly invisible. Sagittarius A* and M87 are helpfully illuminated by debris caught in hot disks orbiting very near their event horizons. The path of the light from the luminous orbiting material is bent along the curved space so that even light behind a black hole gets redirected our way. The disk appears to surround the black hole, allowing for a bright contrast against which its shadow is visible.

EHT actually sees an area slightly outside the event horizon itself — a region defined by the location closest to the black hole where a beam of light could orbit on a circle, known as the ‘last photon orbit.’ (Were you to float there, you could see light reflected off the back of your head after completing a round trip. Or, if you turned around quickly enough, you might see your own face.) Closer than that, all the light falls in.

We are gathered here, black hole theorists and observers, journalists and friends, in this room together to share an image we could already pretty well imagine and were excited to celebrate. But this was the surprise on hearing the announcement: It’s not Sagittarius A* they saw. It’s not our black hole. It’s M87!

The image is unmistakable — a dark shadow the size of our solar system, enveloped by a bright, beautiful blob.

While the scientific implications will take time to unpack, some of the anthropological impact feels immediate. The light EHT collected from M87 headed our way 55 million years ago. Over those eons, we emerged on Earth along with our myths, differentiated cultures, ideologies, languages and varied beliefs. Looking at M87, I am reminded that scientific discoveries transcend those differences. We are all under the same sky, all of us bound to this pale blue dot, floating in the sparse local territory of our solar system’s celestial bodies, under the warmth of our yellow sun, in a sparse sea of stars, in orbit around a supermassive black hole at the center of our luminous galaxy.

When asked his thoughts at the moment he first saw the image of the black hole in M87, Shep replied, ‘We saw something so true.’ And it’s true for all of us.”

#science #science news #event horizon telescope #general relativity #black holes

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Astronomers capture first image of a black hole

The Event Horizon Telescope (EHT) -- a planet-scale array of eight ground-based radio telescopes forged through international collaboration -- was designed to capture images of a black hole. Today, in coordinated press conferences across the globe, EHT researchers reveal that they have succeeded, unveiling the first direct visual evidence of a supermassive black hole and its shadow. This breakthrough was announced in a series of six papers published in a special issue of The Astrophysical Journal Letters. The image reveals the black hole at the center of Messier 87, a massive galaxy in the nearby Virgo galaxy cluster. This black hole resides 55 million light-years from Earth and has a mass 6.5-billion times that of the Sun.

#science #science news #black hole #event horizon telescope #general relativity

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How Our Universe Could Emerge as a Hologram

Physicists have devised a holographic model of “de Sitter space,” the term for a universe like ours, that could give us new clues about the origin of space and time.

The fabric of space and time is widely believed by physicists to be emergent, stitched out of quantum threads according to an unknown pattern. And for 22 years, they’ve had a toy model of how emergent space-time can work: a theoretical “universe in a bottle,” as its discoverer, Juan Maldacena, has described it.

The space-time filling the region inside the bottle — a continuum that bends and undulates, producing the force called gravity — exactly maps to a network of quantum particles living on the bottle’s rigid, gravity-free surface. The interior “universe” projects from the lower-dimensional boundary system like a hologram. Maldacena’s discovery of this hologram has given physicists a working example of a quantum theory of gravity.

But that doesn’t necessarily mean the toy universe shows how space-time and gravity emerge in our universe. The bottle’s interior is a dynamic, Escheresque place called anti–de Sitter (AdS) space that is negatively curved like a saddle. Different directions on the saddle curve in opposite ways, with one direction curving up and the other curving down. The curves tend toward vertical as you move away from the center, ultimately giving AdS space its outer boundary — a surface where quantum particles can interact to create the holographic universe inside. However, in reality, we inhabit a positively curved “de Sitter (dS) space,” which resembles the surface of a sphere that’s expanding without bounds.

Ever since 1997, when Maldacena discovered the AdS/CFT correspondence — a duality between AdS space and a “conformal field theory” describing quantum interactions on that space’s boundary — physicists have sought an analogous description of space-time regions like ours that aren’t bottled up. The only “boundary” of our universe is the infinite future. But the conceptual difficulty of projecting a hologram from quantum particles living in the infinite future has long stymied efforts to describe real space-time holographically.

In the last year, though, three physicists have made progress toward a hologram of de Sitter space. Like the AdS/CFT correspondence, theirs is also a toy model, but some of the principles of its construction may extend to more realistic space-time holograms. There is “tantalizing evidence,” said Xi Dong of the University of California, Santa Barbara, who led the research, that the new model is a piece of “a unified framework for quantum gravity in de Sitter [space].”

Dong and co-authors Eva Silverstein of Stanford University and Gonzalo Torroba of the Bariloche Atomic Center in Argentina constructed a hologram of dS space by taking two AdS universes, cutting them, warping them and gluing their boundaries together.

The cutting is needed to deal with a problematic infinity: the fact that the boundary of AdS space is infinitely far away from its center. (Picture a ray of light traveling an infinite distance up the saddle’s curve to reach the edge.) Dong and co-authors rendered AdS space finite by chopping off the space-time region at a large radius. This created what’s known as a “Randall-Sundrum throat,” after the physicists Lisa Randall and Raman Sundrum, who devised the trick. This space is still approximated by a CFT that lives on its boundary, but the boundary is now a finite distance away.

Next, Dong and co-authors added ingredients from string theory to two of these theoretical Randall-Sundrum throats to energize them and give them positive curvature. This procedure, called “uplifting,” turned the two saddle-shaped AdS spaces into bowl-shaped dS spaces. The physicists could then do the obvious thing: “glue” the two bowls together along their rims. The CFTs describing both hemispheres become coupled with each other, forming a single quantum system that is holographically dual to the entire spherical de Sitter space.

“The resulting space-time has no boundary, but by construction it is dual to two CFTs,” Dong said. Because the equator of the de Sitter space, where the two CFTs live, is itself a de Sitter space, the construction is called the “dS/dS correspondence.”

Silverstein proposed this basic idea with three co-authors back in 2004, but new theoretical tools have enabled her, Dong and Torroba to study the dS/dS hologram in greater detail and show that it passes important consistency checks. In a paper published last summer, they calculated that the entanglement entropy — a measure of how much information is stored in the coupled CFTs living on the equator — matches the known entropy formula for the corresponding spherical region of de Sitter space.

They and other researchers are further exploring the de Sitter hologram using tools from computer science. As I described in a recent Quanta article, physicists have discovered in the last few years that the AdS/CFT correspondence works exactly like a “quantum error-correcting code” — a scheme for securely encoding information in a jittery quantum system, be it a quantum computer or a CFT. Quantum error correction may be how the emergent fabric of space-time achieves its robustness, despite being woven out of fragile quantum particles.

Dong, who was part of the team that discovered the connection between AdS/CFT and quantum error correction, said, “I believe that de Sitter holography also works as a quantum error-correcting code, and I would very much like to understand how.” There’s little hope of experimental evidence verifying that this new perspective on de Sitter space-time is correct, but according to Dong, “you instinctively know you are on the right track if the pieces start to fit together.”

Patrick Hayden, a theoretical physicist and computer scientist at Stanford who studies the AdS/CFT correspondence and its relationship to quantum error correction, said he and other experts are mulling over Dong, Silverstein and Torroba’s dS/dS model. He said it’s too soon to tell whether insights about how space-time is woven and how quantum gravity works in AdS space will carry over to a de Sitter model. “But there’s a path — something to be done,” Hayden said. “You can formulate concrete mathematical questions. I think a lot is going to happen in the next few years.”

#science #physics #quantum physics #quantum gravity

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29 years ago today, the photograph Pale Blue Dot was taken by the Voyager I spacecraft as it exited our solar system, four billion miles away.

#pale blue dot #perspective #voyager #nasa #history #science history #space history #humanism #science #space #physics #astronomy #astrophotography

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This is the face I wake up to every morning

#dogs #border collie #Scout

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NASA has released new images of Jupiter, taken by the Juno Spacecraft.

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#wow

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#electronegative little bitch

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Why Black Hole Interiors Grow (Almost) Forever

The renowned physicist Leonard Susskind has identified a possible quantum origin for the ever-growing volume of black holes.

Leonard Susskind, a pioneer of string theory, the holographic principle and other big physics ideas spanning the past half-century, has proposed a solution to an important puzzle about black holes. The problem is that even though these mysterious, invisible spheres appear to stay a constant size as viewed from the outside, their interiors keep growing in volume essentially forever. How is this possible?

In a series of recent papers and talks, the 78-year-old Stanford University professor and his collaborators conjecture that black holes grow in volume because they are steadily increasing in complexity — an idea that, while unproven, is fueling new thinking about the quantum nature of gravity inside black holes.

Black holes are spherical regions of such extreme gravity that not even light can escape. First discovered a century ago as shocking solutions to the equations of Albert Einstein’s general theory of relativity, they’ve since been detected throughout the universe. (They typically form from the inward gravitational collapse of dead stars.) Einstein’s theory equates the force of gravity with curves in space-time, the four-dimensional fabric of the universe, but gravity becomes so strong in black holes that the space-time fabric bends toward its breaking point — the infinitely dense “singularity” at the black hole’s center.

According to general relativity, the inward gravitational collapse never stops. Even though, from the outside, the black hole appears to stay a constant size, expanding slightly only when new things fall into it, its interior volume grows bigger and bigger all the time as space stretches toward the center point. For a simplified picture of this eternal growth, imagine a black hole as a funnel extending downward from a two-dimensional sheet representing the fabric of space-time. The funnel gets deeper and deeper, so that infalling things never quite reach the mysterious singularity at the bottom. In reality, a black hole is a funnel that stretches inward from all three spatial directions. A spherical boundary surrounds it called the “event horizon,” marking the point of no return.

Since at least the 1970s, physicists have recognized that black holes must really be quantum systems of some kind — just like everything else in the universe. What Einstein’s theory describes as warped space-time in the interior is presumably really a collective state of vast numbers of gravity particles called “gravitons,” described by the true quantum theory of gravity. In that case, all the known properties of a black hole should trace to properties of this quantum system.

Indeed, in 1972, the Israeli physicist Jacob Bekenstein figured out that the area of the spherical event horizon of a black hole corresponds to its “entropy.” This is the number of different possible microscopic arrangements of all the particles inside the black hole, or, as modern theorists would describe it, the black hole’s storage capacity for information.

Bekenstein’s insight led Stephen Hawking to realize two years later that black holes have temperatures, and that they therefore radiate heat. This radiation causes black holes to slowly evaporate away, giving rise to the much-discussed “black hole information paradox,” which asks what happens to information that falls into black holes. Quantum mechanics says the universe preserves all information about the past. But how does information about infalling stuff, which seems to slide forever toward the central singularity, also evaporate out?

The relationship between a black hole’s surface area and its information content has kept quantum gravity researchers busy for decades. But one might also ask: What does the growing volume of its interior correspond to, in quantum terms? “For whatever reason, nobody, including myself for a number of years, really thought very much about what that means,” said Susskind. “What is the thing which is growing? That should have been one of the leading puzzles of black hole physics.”

In recent years, with the rise of quantum computing, physicists have been gaining new insights about physical systems like black holes by studying their information-processing abilities — as if they were quantum computers. This angle led Susskind and his collaborators to identify a candidate for the evolving quantum property of black holes that underlies their growing volume. What’s changing, the theorists say, is the “complexity” of the black hole — roughly a measure of the number of computations that would be needed to recover the black hole’s initial quantum state, at the moment it formed. After its formation, as particles inside the black hole interact with one another, the information about their initial state becomes ever more scrambled. Consequently, their complexity continuously grows.

Using toy models that represent black holes as holograms, Susskind and his collaborators have shown that the complexity and volume of black holes both grow at the same rate, supporting the idea that the one might underlie the other. And, whereas Bekenstein calculated that black holes store the maximum possible amount of information given their surface area, Susskind’s findings suggest that they also grow in complexity at the fastest possible rate allowed by physical laws.

John Preskill, a theoretical physicist at the California Institute of Technology who also studies black holes using quantum information theory, finds Susskind’s idea very interesting. “That’s really cool that this notion of computational complexity, which is very much something that a computer scientist might think of and is not part of the usual physicist’s bag of tricks,” Preskill said, “could correspond to something which is very natural for someone who knows general relativity to think about,” namely the growth of black hole interiors.

Researchers are still puzzling over the implications of Susskind’s thesis. Aron Wall, a theorist at Stanford (soon moving to the University of Cambridge), said, “The proposal, while exciting, is still rather speculative and may not be correct.” One challenge is defining complexity in the context of black holes, Wall said, in order to clarify how the complexity of quantum interactions might give rise to spatial volume.

A potential lesson, according to Douglas Stanford, a black hole specialist at the Institute for  Advanced Study in Princeton, New Jersey, “is that black holes have a type of internal clock that keeps time for a very long time. For an ordinary quantum system,” he said, “this is the complexity of the state. For a black hole, it is the size of the region behind the horizon.”

If complexity does underlie spatial volume in black holes, Susskind envisions consequences for our understanding of cosmology in general. “It’s not only black hole interiors that grow with time. The space of cosmology grows with time,” he said. “I think it’s a very, very interesting question whether the cosmological growth of space is connected to the growth of some kind of complexity. And whether the cosmic clock, the evolution of the universe, is connected with the evolution of complexity. There, I don’t know the answer.”

#science #physics #string theory #black holes

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How Holography Could Help Solve Quantum Gravity

In the latest campaign to reconcile Einstein’s theory of gravity with quantum mechanics, many physicists are studying how a higher dimensional space that includes gravity arises like a hologram from a lower dimensional particle theory.

How does gravity work at the particle level? The question has stumped physicists since the two bedrock theories of general relativity (Albert Einstein’s equations envisioning gravity as curves in the geometry of space-time) and quantum mechanics (equations that describe particle interactions) revolutionized the discipline about a century ago.

One challenge to solving the problem lies in the relative weakness of gravity compared with the strong, weak and electromagnetic forces that govern the subatomic realm. Though gravity exerts an unmistakable influence on macroscopic objects like orbiting planets, leaping sharks and everything else we physically experience, it produces a negligible effect at the particle level, so physicists can’t test or study how it works at that scale.

Confounding matters, the two sets of equations don’t play well together. General relativity paints a continuous picture of space-time while in quantum mechanics everything is quantized in discrete chunks. Their incompatibility leads physicists to suspect that a more fundamental theory is needed to unify all four forces of nature and describe them at all scales.

One relatively recent approach to understanding quantum gravitymakes use of a “holographic duality” from string theory called the AdS-CFT correspondence. Our latest In Theory video explains how this correspondence connects a lower dimensional particle theory to a higher dimensional space that includes gravity.

This holographic duality has become a powerful theoretical tool in the quest to understand quantum gravity and the inner workings of black holes and the Big Bang, where extreme gravity operates at tiny scales.

We hope you enjoyed this second episode from season two of Quanta’s In Theory video series. Season two opened in August with an animated explainer about a mysterious mathematical pattern that has been discovered in disparate settings — in the energy spectra of heavy atomic nuclei, a function related to the distribution of prime numbers, an independent bus system in Mexico, spectral measurements of the internet, Arctic ponds, human bones and the color-sensitive cone cells in chicken eyes.

#science #quantum gravity #physics #general relativity #quantum physics

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Significant other? Sure. Let’s see those p-values

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Writing in a diary is a really strange experience for someone like me. Not only because I’ve never written anything before, but also because it seems to me that later on neither I nor anyone else will be interested in the musings of a thirteen-year old school girl. Oh well, it doesn’t matter. I feel like writing.

Anne Frank

#quotes #anne frank #writing

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Dark Energy May Be Incompatible With String Theory

A controversial new paper argues that universes with dark energy profiles like ours do not exist in the “landscape” of universes allowed by string theory.

On June 25, Timm Wrase awoke in Vienna and groggily scrolled through an online repository of newly posted physics papers. One title startled him into full consciousness.

The paper, by the prominent string theorist Cumrun Vafa of Harvard University and collaborators, conjectured a simple formula dictating which kinds of universes are allowed to exist and which are forbidden, according to string theory. The leading candidate for a “theory of everything” weaving the force of gravity together with quantum physics, string theory defines all matter and forces as vibrations of tiny strands of energy. The theory permits some 10500 different solutions: a vast, varied “landscape” of possible universes. String theorists like Wrase and Vafa have strived for years to place our particular universe somewhere in this landscape of possibilities.

But now, Vafa and his colleagues were conjecturing that in the string landscape, universes like ours — or what ours is thought to be like — don’t exist. If the conjecture is correct, Wrase and other string theorists immediately realized, the cosmos must either be profoundly different than previously supposed or string theory must be wrong.

After dropping his kindergartner off that morning, Wrase went to work at the Vienna University of Technology, where his colleagues were also buzzing about the paper. That same day, in Okinawa, Japan, Vafa presented the conjecture at the Strings 2018 conference, which was streamed by physicists worldwide. Debate broke out on- and off-site. “There were people who immediately said, ‘This has to be wrong,’ other people who said, ‘Oh, I’ve been saying this for years,’ and everything in the middle,” Wrase said. There was confusion, he added, but “also, of course, huge excitement. Because if this conjecture was right, then it has a lot of tremendous implications for cosmology.”

Researchers have set to work trying to test the conjecture and explore its implications. Wrase has already written two papers, including one that may lead to a refinement of the conjecture, and both mostly while on vacation with his family. He recalled thinking, “This is so exciting. I have to work and study that further.”

The conjectured formula — posed in the June 25 paper by Vafa, Georges Obied, Hirosi Ooguri and Lev Spodyneiko and further explored in a second paper released two days later by Vafa, Obied, Prateek Agrawal and Paul Steinhardt — says, simply, that as the universe expands, the density of energy in the vacuum of empty space must decrease faster than a certain rate. The rule appears to be true in all simple string theory-based models of universes. But it violates two widespread beliefs about the actual universe: It deems impossible both the accepted picture of the universe’s present-day expansion and the leading model of its explosive birth.

Dark Energy in Question

Since 1998, telescope observations have indicated that the cosmos is expanding ever-so-slightly faster all the time, implying that the vacuum of empty space must be infused with a dose of gravitationally repulsive “dark energy.”

In addition, it looks like the amount of dark energy infused in empty space stays constant over time (as best anyone can tell).

But the new conjecture asserts that the vacuum energy of the universe must be decreasing.

Vafa and colleagues contend that universes with stable, constant, positive amounts of vacuum energy, known as “de Sitter universes,” aren’t possible. String theorists have struggled mightily since dark energy’s 1998 discovery to construct convincing stringy models of stable de Sitter universes. But if Vafa is right, such efforts are bound to sink in logical inconsistency; de Sitter universes lie not in the landscape, but in the “swampland.” “The things that look consistent but ultimately are not consistent, I call them swampland,” he explained recently. “They almost look like landscape; you can be fooled by them. You think you should be able to construct them, but you cannot.”

According to this “de Sitter swampland conjecture,” in all possible, logical universes, the vacuum energy must either be dropping, its value like a ball rolling down a hill, or it must have obtained a stable negative value. (So-called “anti-de Sitter” universes, with stable, negative doses of vacuum energy, are easily constructed in string theory.)

The conjecture, if true, would mean the density of dark energy in our universe cannot be constant, but must instead take a form called “quintessence” — an energy source that will gradually diminish over tens of billions of years. Several telescope experiments are underway now to more precisely probe whether the universe is expanding with a constant rate of acceleration, which would mean that as new space is created, a proportionate amount of new dark energy arises with it, or whether the cosmic acceleration is gradually changing, as in quintessence models. A discovery of quintessence would revolutionize fundamental physics and cosmology, including rewriting the cosmos’s history and future. Instead of tearing apart in a Big Rip, a quintessent universe would gradually decelerate, and in most models, would eventually stop expanding and contract in either a Big Crunch or Big Bounce.

Paul Steinhardt, a cosmologist at Princeton University and one of Vafa’s co-authors, said that over the next few years, “all eyes should be on” measurements by the Dark Energy Survey, WFIRST and Euclid telescopes of whether the density of dark energy is changing. “If you find it’s not consistent with quintessence,” Steinhardt said, “it means either the swampland idea is wrong, or string theory is wrong, or both are wrong or — something’s wrong.”

Inflation Under Siege

No less dramatically, the new swampland conjecture also casts doubt on the widely believed story of the universe’s birth: the Big Bang theory known as cosmic inflation. According to this theory, a minuscule, energy-infused speck of space-time rapidly inflated to form the macroscopic universe we inhabit. The theory was devised to explain, in part, how the universe got so huge, smooth and flat.

But the hypothetical “inflaton field” of energy that supposedly drove cosmic inflation doesn’t sit well with Vafa’s formula. To abide by the formula, the inflaton field’s energy would probably have needed to diminish too quickly to form a smooth- and flat-enough universe, he and other researchers explained. Thus, the conjecture disfavors many popular models of cosmic inflation. In the coming years, telescopes such as the Simons Observatory will look for definitive signatures of cosmic inflation, testing it against rival ideas.

In the meantime, string theorists, who normally form a united front, will disagree about the conjecture. Eva Silverstein, a physics professor at Stanford University and a leader in the effort to construct string-theoretic models of inflation, thinks it is very likely to be false. So does her husband, the Stanford professor Shamit Kachru; he is the first “K” in KKLT, a famous 2003 paper (known by its authors’ initials) that suggested a set of stringy ingredients that might be used to construct de Sitter universes. Vafa’s formula says both Silverstein’s and Kachru’s constructions won’t work. “We’re besieged by these conjectures in our family,” Silverstein joked.  But in her view, accelerating-expansion models are no more disfavored now, in light of the new papers, than before. “They essentially just speculate that those things don’t exist, citing very limited and in some cases highly dubious analyses,” she said.

Matthew Kleban, a string theorist and cosmologist at New York University, also works on stringy models of inflation. He stresses that the new swampland conjecture is highly speculative and an example of “lamppost reasoning,” since much of the string landscape has yet to be explored. And yet he acknowledges that, based on existing evidence, the conjecture could well be true. “It could be true about string theory, and then maybe string theory doesn’t describe the world,” Kleban said. “[Maybe] dark energy has falsified it. That obviously would be very interesting.”

Mapping the Swampland

Whether the de Sitter swampland conjecture and future experiments really have the power to falsify string theory remains to be seen. The discovery in the early 2000s that string theory has something like 10500 solutions killed the dream that it might uniquely and inevitably predict the properties of our one universe. The theory seemed like it could support almost any observations and became very difficult to experimentally test or disprove.

In 2005, Vafa and a network of collaborators began to think about how to pare the possibilities down by mapping out fundamental features of nature that absolutely have to be true. For example, their “weak gravity conjecture” asserts that gravity must always be the weakest force in any logical universe. Imagined universes that don’t satisfy such requirements get tossed from the landscape into the swampland. Many of these swampland conjectures have held up famously against attack, and some are now “on a very solid theoretical footing,” said Hirosi Ooguri, a theoretical physicist at the California Institute of Technology and one of Vafa’s first swampland collaborators. The weak gravity conjecture, for instance, has accumulated so much evidencethat it’s now suspected to hold generally, independent of whether string theory is the correct theory of quantum gravity.

The intuition about where landscape ends and swampland begins derives from decades of effort to construct stringy models of universes. The chief challenge of that project has been that string theory predicts the existence of 10 space-time dimensions — far more than are apparent in our 4-D universe. String theorists posit that the six extra spatial dimensions must be small — curled up tightly at every point. The landscape springs from all the different ways of configuring these extra dimensions. But although the possibilities are enormous, researchers like Vafa have found that general principles emerge. For instance, the curled-up dimensions typically want to gravitationally contract inward, whereas fields like electromagnetic fields tend to push everything apart. And in simple, stable configurations, these effects balance out by having negative vacuum energy, producing anti-de Sitter universes. Turning the vacuum energy positive is hard. “Usually in physics, we have simple examples of general phenomena,” Vafa said. “De Sitter is not such a thing.”

The KKLT paper, by Kachru, Renata Kallosh, Andrei Linde and Sandip Trivedi, suggested stringy trappings like “fluxes,” “instantons” and “anti-D-branes” that could potentially serve as tools for configuring a positive, constant vacuum energy. However, these constructions are complicated, and over the years possible instabilities have been identified. Though Kachru said he does not have “any serious doubts,” many researchers have come to suspect the KKLT scenario does not produce stable de Sitter universes after all.

Vafa thinks a concerted search for definitely stable de Sitter universe models is long overdue. His conjecture is, above all, intended to press the issue. In his view, string theorists have not felt sufficiently motivated to figure out whether string theory really is capable of describing our world, instead taking the attitude that because the string landscape is huge, there must be a place in it for us, even if no one knows where. “The bulk of the community in string theory still sides on the side of de Sitter constructions [existing],” he said, “because the belief is, ‘Look, we live in a de Sitter universe with positive energy; therefore we better have examples of that type.’”

His conjecture has roused the community to action, with researchers like Wrase looking for stable de Sitter counterexamples, while others toy with little-explored stringy models of quintessent universes. “I would be equally interested to know if the conjecture is true or false,” Vafa said. “Raising the question is what we should be doing. And finding evidence for or against it — that’s how we make progress.”

#science #mathematical physics #dark energy #string theory

[s-c-i-guy]

NASA, ULA Launch Parker Solar Probe on Historic Journey to Touch Sun

Hours before the rise of the very star it will study, NASA’s Parker Solar Probe launched from Florida Sunday to begin its journey to the Sun, where it will undertake a landmark mission. The spacecraft will transmit its first science observations in December, beginning a revolution in our understanding of the star that makes life on Earth possible.

Roughly the size of a small car, the spacecraft lifted off at 3:31 a.m. EDT on a United Launch Alliance Delta IV Heavy rocket from Space Launch Complex-37 at Cape Canaveral Air Force Station. At 5:33 a.m., the mission operations manager reported that the spacecraft was healthy and operating normally.

The mission’s findings will help researchers improve their forecasts of space weather events, which have the potential to damage satellites and harm astronauts on orbit, disrupt radio communications and, at their most severe, overwhelm power grids.

“This mission truly marks humanity’s first visit to a star that will have implications not just here on Earth, but how we better understand our universe,” said Thomas Zurbuchen, associate administrator of NASA’s Science Mission Directorate. “We’ve accomplished something that decades ago, lived solely in the realm of science fiction.”

During the first week of its journey, the spacecraft will deploy its high-gain antenna and magnetometer boom. It also will perform the first of a two-part deployment of its electric field antennas. Instrument testing will begin in early September and last approximately four weeks, after which Parker Solar Probe can begin science operations.

“Today’s launch was the culmination of six decades of scientific study and millions of hours of effort,” said project manager Andy Driesman, of the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland. “Now, Parker Solar Probe is operating normally and on its way to begin a seven-year mission of extreme science.”

Over the next two months, Parker Solar Probe will fly towards Venus, performing its first Venus gravity assist in early October – a maneuver a bit like a handbrake turn – that whips the spacecraft around the planet, using Venus’s gravity to trim the spacecraft’s orbit tighter around the Sun. This first flyby will place Parker Solar Probe in position in early November to fly as close as 15 million miles from the Sun – within the blazing solar atmosphere, known as the corona – closer than anything made by humanity has ever gone before.

Throughout its seven-year mission, Parker Solar Probe will make six more Venus flybys and 24 total passes by the Sun, journeying steadily closer to the Sun until it makes its closest approach at 3.8 million miles. At this point, the probe will be moving at roughly 430,000 miles per hour, setting the record for the fastest-moving object made by humanity.

Parker Solar Probe will set its sights on the corona to solve long-standing, foundational mysteries of our Sun. What is the secret of the scorching corona, which is more than 300 times hotter than the Sun’s surface, thousands of miles below? What drives the supersonic solar wind – the constant stream of solar material that blows through the entire solar system? And finally, what accelerates solar energetic particles, which can reach speeds up to more than half the speed of light as they rocket away from the Sun?

Scientists have sought these answers for more than 60 years, but the investigation requires sending a probe right through the unrelenting heat of the corona. Today, this is finally possible with cutting-edge thermal engineering advances that can protect the mission on its daring journey.

“Exploring the Sun’s corona with a spacecraft has been one of the hardest challenges for space exploration,” said Nicola Fox, project scientist at APL. “We’re finally going to be able to answer questions about the corona and solar wind raised by Gene Parker in 1958 – using a spacecraft that bears his name – and I can’t wait to find out what discoveries we make. The science will be remarkable.”

Parker Solar Probe carries four instrument suites designed to study magnetic fields, plasma and energetic particles, and capture images of the solar wind. The University of California, Berkeley, U.S. Naval Research Laboratory in Washington, University of Michigan in Ann Arbor, and Princeton University in New Jersey lead these investigations.

Parker Solar Probe is part of NASA’s Living with a Star program to explore aspects of the Sun-Earth system that directly affect life and society. The Living with a Star program is managed by the agency’s Goddard Space Flight Center in Greenbelt, Maryland, for NASA’s Science Mission Directorate in Washington. APL designed and built, and operates the spacecraft.

The mission is named for Eugene Parker, the physicist who first theorized the existence of the solar wind in 1958. It’s the first NASA mission to be named for a living researcher.

A plaque dedicating the mission to Parker was attached to the spacecraft in May. It includes a quote from the renowned physicist – “Let’s see what lies ahead.” It also holds a memory card containing more than 1.1 million names submitted by the public to travel with the spacecraft to the Sun.

#science #news #science news #nasa

[s-c-i-guy]

Star-Swallowing Black Holes Reveal Secrets in Exotic Light Shows

Black holes occasionally reveal themselves when passing stars get ripped apart by their gravity. These tidal disruption events have created a new way for astronomers to map the hidden cosmos.

Black holes, befitting their name and general vibe, are hard to find and harder to study. You can eavesdrop on small ones from the gravitational waves that echo through space when they collide — but that technique is new, and still rare. You can produce laborious maps of stars flitting around the black hole at the center of the Milky Way or nearby galaxies. Or you can watch them gulp down gas clouds, which emit radiation as they fall.

Now researchers have a new option. They’ve begun corralling ultrabright flashes called tidal disruption events (TDEs), which occur when a large black hole seizes a passing star, shreds it in two and devours much of it with the appetite of a bear snagging a salmon. “To me, it’s sort of like science fiction,” said Enrico Ramirez-Ruiz, an astrophysicist at University of California, Santa Cruz, and the Niels Bohr Institute.

During the past few years, though, the study of TDEs has transformed from science fiction to a sleepy cottage industry, and now into something more like a bustling tech startup.

Automated wide-field telescopes that can pan across thousands of galaxies each night have uncovered about two dozen TDEs. Included in these discoveries are some bizarre and long-sought members of the TDE zoo. In June, a study in the journal Nature Astronomy described an outburst of X-ray light in a cluster of faraway stars that astronomers interpreted as a midsized black hole swallowing a star. That same month, another group announced in Science that they had discovered what may be brightest ever TDE, one that illuminated faint gas at the heart of a pair of merging galaxies.

These discoveries have taken place as our understanding of what’s really happening during a TDE comes into sharper focus. At the end of May, a group of astrophysicists proposed a new theoretical model for how TDEs work. The model can explain why different TDEs can appear to behave differently, even though the underlying physics is presumably the same.

Astronomers hope that decoding these exotic light shows will let them conduct a black hole census. Tidal disruptions expose the masses, spins and sheer numbers of black holes in the universe, the vast majority of which would be otherwise invisible. Theorists are hungry, for example, to see if TDEs might unveil any intermediate-mass black holes with weights between the two known black hole classes: star-size black holes that weigh a few times more than the sun, and the million- and billion-solar-mass behemoths that haunt the cores of galaxies. The Nature Astronomy paper claims they may already have.

Researchers have also started to use TDEs to probe the fundamental physics of black holes. They can be used to test whether black holes always have event horizons — curtains beyond which nothing can return — as Einstein’s theory of general relativity predicts.

Meanwhile, many more observations are on the way. The rate of new TDEs, now about one or two per year, could jump up by an order of magnitude even by the end of this year because of the Zwicky Transient Facility, which started scanning the sky over California’s Palomar Observatory in March. And with the addition of planned observatories, it may increase perhaps another order of magnitude in the years to come.

“The field has really blossomed,” said Suvi Gezari at the University of Maryland, one of the few stubborn pioneers who staked their careers on TDEs during leaner years. She now leads the Zwicky Transient Facility’s TDE-hunting team, which has already snagged unpublished candidates in its opening months, she said. “Now people are really digging in.”

Searching for Star-Taffy

In 1975, the British physicist Jack Hills first dreamed up a black-hole-eats-star scenario as a way to explain what powers quasars — superbright points of light from the distant universe. (Quasars are now known to be supermassive black holes feeding on surrounding gas, not stars.) But in 1988, the British cosmologist Martin Rees realized that black holes snacking on a star would exhibit a sharp flare, not a steady glow. Looking for such flares could let astronomers find and study the black holes themselves, he argued.

Nothing that fit the bill turned up until the late 1990s. That’s when Stefanie Komossa, at the time a graduate student at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, found massive X-ray flares from the centers of distant galaxies that brightened and dimmed according to the Rees predictions.

The astronomical community responded to these discoveries — based on just a few data points — with caution. Then in the mid-2000s, Gezari, then beginning a postdoc at the California Institute of Technology, searched for and discovered her own handful of TDE candidates. She looked for flashes of ultraviolet light, not X-rays as Komossa had. “In the old days,” Gezari said, “I was just trying to convince people that any of our discoveries were actually due to a tidal disruption.”

Soon, though, she had something to sway even the doubters. In 2010, Gezari discovered an especially clear flare, rising and falling as modelers predicted. She published it in Nature in 2012, catching other astronomers’ attention. In the years since, large surveys in optical light, sifting through the sky for changes in brightness, have taken over the hunt. And like Komossa’s and Gezari’s TDEs, which had both been fished out of missions designed to look for other things, the newest batch showed up as bycatch. “It was, oh, why didn’t we think about looking for these?” said Christopher Kochanek, an astrophysicist at Ohio State University who works on a project designed to search for supernovas.

Now, with a growing number of TDEs in hand, astrophysicists are within arm’s reach of Rees’s original goal: pinpointing and studying gargantuan black holes. But they still need to learn to interpret these events, divining their basic physics. Unexpectedly, the known TDEs fall into separate classes. Some seem to emit mostly ultraviolet and optical light, as if from gas heated to tens of thousands of degrees. Others glow fiercely with X-rays, suggesting temperatures an order of magnitude higher. Yet presumably they all have the same basic physical root.

To be disrupted, an unlucky star must venture close enough to a black hole that gravitational tides exceed the internal gravity that binds the star together. In other words, the difference in the black hole’s gravitational pull on the near and far sides of the star, along with the inertial pull as the star swings around the black hole, stretches the star out into a stream. “Basically it spaghettifies,” said James Guillochon, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics.

The outer half of the star escapes away into space. But the inner half — that dense stream of star-taffy — swirls into the black hole, heating up and releasing huge sums of energy that radiate across the universe.

With this general mechanism understood, researchers had trouble understanding why individual TDEs can look so distinct. One longstanding idea appeals to different phases of the star-eating process. As the star flesh gets initially torn away and stretched into a stream, it might ricochet around the black hole and slam into its own tail. This process might heat the tail up to ultraviolet-producing temperatures— but not hotter. Then later — after a few months or a year — the star would settle into an accretion disk, a fat bagel of spinning gas that theories predict should be hot enough to emit X-rays.

But there’s another option, argued a team led by Jane Lixin Dai at the Niels Bohr Institute and including Ramirez-Ruiz in May. According to their simulations, which include the effects of general relativity, the two kinds of TDEs might just be the same thing seen at different angles. If astronomers are viewing a bagel-like accretion disk from the top, they can see X-rays from the hot inner material swirling right down the drain. When the accretion disk is edge-on, though, colder gas stands in the way. This gas catches X-rays and reemits them as ultraviolet light.

Ultimately, theorists hope to read each event as a variation on the same core theme — and then do deeper science. “Maybe we’ll learn something fundamental about accretion,” Kochanek said. Or maybe “every one will be sufficiently idiosyncratic that it will be like worrying about the shape of a cloud.”

Testing Einstein

The newly discovered TDEs are also helping astronomers to understand supermassive, galaxy-ruling black holes. Only about 10 percent of these giants emit radiation as they feed on surrounding gas, leaving the other 90 percent of them invisible.

TDEs change that. Komossa, an astronomer at the Max Planck Institute for Radio Astronomy in Bonn, hopes to find more binary supermassive black holes: black holes forced to cohabitate after their own galaxies collided, which future space-based gravitational wave experiments will also search for. As a star drains into one black hole, the presence of another supermassive maw nearby would tug at the stream of matter falling in. Instead of a smooth dimming, the TDE would exhibit dips and rises.

Other teams want to test a fundamental, eerie correlation. Somehow the masses of central black holes and their host galaxies seem to increase in tandem. “The mass of the black hole knows about the mass of the galaxy, which is kind of mesmerizing,” Ramirez-Ruiz said. TDEs, plumbing black hole masses in an independent sample of galaxies, could either strengthen or weaken this relationship.

TDEs can also reveal an oxymoronic population: the shrimpiest massive black holes around. While the very biggest known black holes can weigh 10 billion times the sun, and galaxies like the Milky Way host specimens that tip the scales at millions of solar masses, it isn’t clear whether smaller dwarf galaxies are ruled by proportionally pipsqueak versions, in the hundreds of thousands of solar masses or below.

Spotting TDEs from these intermediate mass black holes would settle the question, helping astronomers understand how giant black holes form in the first place. The June paper in Nature Astronomy claims to have found such an intermediate object, one weighing a few tens of thousands of solar masses. That event appeared in 2003, peaked in 2006 and then declined for the following decade. Instead of happening at the center of a galaxy, the X-ray flare occurred in a star cluster, a place where intermediate-mass black holes could coalesce from the mergers of stars. But a single event does not a population make. “We need to find more similar events, to confirm our result,” said Dacheng Lin, an astrophysicist at the University of New Hampshire who led the study.

Then come still deeper goals. TDEs are also starting to test general relativity’s picture of black holes, probing for places where the theory might break.

For example, as a black hole increases in mass, its predicted event horizon creeps steadily outward. But the radius at which the black hole’s tides can crack open a star increases more slowly. At a theoretical limit called the Hills mass, about 100 million times the mass of the sun, a black hole’s star-tearing radius exactly matches its own border. That should put a mass cap on TDEs. “Below that, you can tear something apart. Above that, stars get swallowed hole,” said Nicholas Stone, a theoretical astrophysicist at Columbia University.

So far, the data matches this idea. The rise and fall of known TDEs — already as reliable at weighing supermassive black holes as other techniques — show they all happened around black holes that weigh less than the Hills mass, suggesting that heavier objects likely do have the event horizons that relativity predicts.

But Stone and colleagues are eager to exploit an additional wrinkle. A spinning black hole that weighs 10 times above the Hills mass can still swallow stars. Eventually, after discovering more TDEs, astronomers can watch how the rate of events fades off at high mass, which should help them understand the fastest black hole spins, Stone said.

That might put relativity’s idea of event horizons right back in the crosshairs. A rotating black hole has a theoretical maximum speed, and any black holes seen spinning faster would violate the idea that a black hole has a firm outer boundary.

Thankfully, the observational grist needed to test these various ideas is already on its way. In a dramatic reversal of the field’s beginnings, the new Zwicky Transient Facility is now turning up too many candidates for comfort, Gezari said. She’s starting to strain her resources, trying to get enough telescope time for follow-up observations on each worthy target.

The next leaps come soon. A long-delayed joint German-Russian mission called eRosita, if it goes up in 2019 as planned, should spot hundreds or thousands of TDEs as X-ray flashes. So should the Einstein Probe, a Chinese mission that Komossa collaborates on, scheduled to launch in 2022. And then there’s the Large Synoptic Survey Telescope, currently being built in Chile and scheduled to start scanning the sky in 2022, which should catch its own hundreds or thousands of TDEs among whatever else goes bump in the night.

For Ramirez-Ruiz, this growth since the field’s humble beginnings is a natural consequence of modern “celestial cinematography” — telescopes that shoot night-by-night time-lapse video across the entire sky. TDEs only happen about once every 10,000 years in a given galaxy, when an unlucky star wanders close enough to a black hole. But now that we monitor enough galaxies at once, he said, “the field actually has exploded.”

#science #astrophysics #general relativity #long reads #gif