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      Gravitational waves confirm a black hole law predicted by Stephen Hawking

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      Despite their mysterious nature, black holes are thought to follow certain simple rules. Now, one of the most famous black hole laws, predicted by physicist Stephen Hawking, has been confirmed with gravitational waves.

      According to the black hole area theorem, developed by Hawking in the early 1970s, black holes can’t decrease in surface area over time. The area theorem fascinates physicists because it mirrors a well-known physics rule that disorder, or entropy, can’t decrease over time. Instead, entropy consistently increases (SN: 7/10/15).

      That’s “an exciting hint that black hole areas are something fundamental and important,” says astrophysicist Will Farr of Stony Brook University in New York and the Flatiron Institute in New York City.

      The surface area of a lone black hole won’t change — after all, nothing can escape from within. However, if you throw something into a black hole, it will gain more mass, increasing its surface area. But the incoming object could also make the black hole spin, which decreases the surface area. The area law says that the increase in surface area due to additional mass will always outweigh the decrease in surface area due to added spin.

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      To test this area rule, MIT astrophysicist Maximiliano Isi, Farr and others used ripples in spacetime stirred up by two black holes that spiraled inward and merged into one bigger black hole. A black hole’s surface area is defined by its event horizon — the boundary from within which it’s impossible to escape. According to the area theorem, the area of the newly formed black hole’s event horizon should be at least as big as the areas of the event horizons of the two original black holes combined.

      The team analyzed data from the first gravitational waves ever spotted, which were detected by the Advanced Laser Interferometer Gravitational-Wave Observatory, LIGO, in 2015 (SN: 2/11/16). The researchers split the gravitational wave data into two time segments, before and after the merger, and calculated the surface areas of the black holes in each period. The surface area of the newly formed black hole was greater than that of the two initial black holes combined, upholding the area law with a 95 percent confidence level, the team reports in a paper to appear in Physical Review Letters.

      “It’s the first time that we can put a number on this,” Isi says.

      The area theorem is a result of the general theory of relativity, which describes the physics of black holes and gravitational waves. Previous analyses of gravitational waves have agreed with predictions of general relativity, and thus already hinted that the area law can’t be wildly off. But the new study “is a more explicit confirmation,” of the area law, says physicist Cecilia Chirenti of the University of Maryland in College Park, who was not involved with the research.

      So far, general relativity describes black holes well. But scientists don’t fully understand what happens where general relativity — which typically applies to large objects like black holes — meets quantum mechanics, which describes small stuff like atoms and subatomic particles. In that quantum realm, strange things can happen.

      For example, black holes can release a faint mist of particles called Hawking radiation, another idea developed by Hawking in the 1970s. That effect could allow black holes to shrink, violating the area law, but only over extremely long periods of time, so it wouldn’t have affected the relatively quick merger of black holes that LIGO saw.

      Physicists are looking for an improved theory that will combine the two disciplines into one new, improved theory of quantum gravity. Any failure of black holes to abide by the rules of general relativity could point physicists in the right direction to find that new theory.

      So physicists tend to be grumpy about the enduring success of general relativity, Farr says. “We’re like, ‘aw, it was right again.’” More

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      Most planets on tilted orbits pass over the poles of their suns

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      Earth is on an orderly path around the sun, orbiting in nearly the same plane as our star’s equator. In 2008, however, astronomers began finding worlds in other solar systems that sail far above and below their star’s equatorial plane.

      Now a surprising discovery about these wrong-way worlds may eventually reveal their origin: Most of them follow polar orbits (SN: 6/17/16). If Earth had such an orbit, every year we’d pass over the sun’s north pole, dive through its equatorial plane, then pass below the sun’s south pole before coming back up again.

      Astronomers Simon Albrecht and Marcus Marcussen at Aarhus University in Denmark and colleagues analyzed 57 planets in other solar systems for which the researchers could determine the true tilt between a planet’s orbit and its star’s equatorial plane. Two-thirds of the planets have normal orbits, tilted no more than 40 degrees, the team found. The other 19 planets are misaligned.

      But the orbits of those misaligned planets don’t make just any old angle with their star’s equator. Instead, they pile up around 90 degrees. In fact, all but one of the misaligned planets are on polar orbits, having tilts from 80 to 125 degrees, the astronomers report online May 20 at arXiv.org.

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      “It’s very, very strange,” says Amaury Triaud, an astronomer at the University of Birmingham in England who has found a number of misaligned planets but was not involved with the new study. “It’s a beautifully executed idea, and the result is most intriguing,” he says. “It’s so new and so weird.”

      The result may lend insight into the biggest mystery about these planets: how they arose (SN: 10/18/13). Such worlds were a shock to astronomers, because planets form inside pancake-shaped disks of gas and dust orbiting in their stars’ equatorial planes. Thus, planets should lie near the plane of their sun’s equator, too. In our solar system, for example, Earth’s orbit tilts only 7 degrees from the solar equatorial plane, and even Pluto — which many astronomers no longer call a planet — has an orbit tilted only 12 degrees from that plane (and 17 degrees from the Earth’s orbital plane).

      “At the moment, we are not sure what is the underlying mechanism” or mechanisms for creating misaligned planets, Albrecht admits. Whatever it is, though, it should account for the newly discovered plethora of perpendicular planets, he says.

      A possible clue, Albrecht says, comes from the single exception to the rule: the one misaligned planet in the sample that is not on a polar orbit. This planet also happens to be the most massive in the sample, packing the mass of between five and eight Jupiters. Albrecht says that may be just a coincidence — or it may reveal something about how the other planets became misaligned.

      In the future, the astronomers hope to understand how these wayward worlds acquired their odd orbits. All known misaligned planets orbit close to their stars, but are these worlds more likely than normal, close-in planets to have giant planets near them? The scientists don’t yet know, but if they find such a correlation, those companions may have somehow flung these bizarre worlds onto their peculiar planetary paths. More

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      An arc of galaxies 3 billion light-years long may challenge cosmology

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      A giant arc of galaxies appears to stretch across more than 3 billion light-years in the distant universe. If the arc turns out to be real, it would challenge a bedrock assumption of cosmology: that on large scales, matter in the universe is evenly distributed no matter where you look.

      “It would overturn cosmology as we know it,” said cosmologist Alexia Lopez at a June 7 news conference at the virtual American Astronomical Society meeting. “Our standard model, not to put it too heavily, kind of falls through.”

      Lopez, of the University of Central Lancashire in Preston, England, and colleagues discovered the purported structure, which they call simply the Giant Arc, by studying the light of about 40,000 quasars captured by the Sloan Digital Sky Survey. Quasars are the luminous cores of giant galaxies so distant that they appear as points of light. While en route to Earth, some of that light gets absorbed by atoms in and around foreground galaxies, leaving specific signatures in the light that eventually reaches astronomers’ telescopes (SN: 7/12/18).

      The Giant Arc’s signature is in magnesium atoms that have lost one electron, in the halos of galaxies about 9.2 billion light-years away. The quasar light absorbed by those atoms traces out a nearly symmetrical curve of dozens of galaxies spanning about one-fifteenth the radius of the observable universe, Lopez reported. The structure itself is invisible on the sky to human eyes, but if you could see it, the arc would span about 20 times the width of the full moon.

      Astronomers discovered what they say is a giant arc of galaxies (smile-shaped curve in the middle of this image) by using the light from distant quasars (blue dots) to map out where in the sky that light got absorbed by magnesium atoms in the halos (dark spots) that surround foreground galaxies.A. Lopez

      “This is a very fundamental test of the hypothesis that the universe is homogeneous on large scales,” says astrophysicist Subir Sarkar of the University of Oxford, who studies large-scale structures in the universe but was not involved in the new work. If the Giant Arc is real, “this is a very big deal.”

      But Sarkar isn’t convinced it is real yet. “Our eye has a tendency to pick up patterns,” Sarkar says, noting that some people have claimed to see cosmologist Stephen Hawking’s initials written in fluctuations in the cosmic microwave background, the oldest light in the universe.

      Lopez ran three statistical tests to figure out the odds that galaxies would line up in a giant arc by chance. All three suggest that the structure is real, with one test surpassing physicists’ gold standard that the odds of it being a statistical fluke are less than 0.00003 percent.

      That sounds pretty good, but it’s not enough, Sarkar says. “Right now, I would say they still don’t have compelling evidence,” he says. More observations, from Lopez’s group and others, could confirm or refute the Giant Arc.

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      If it is real, the Giant Arc would join a growing group of large-scale structures in the universe that, taken together, would break the standard model of cosmology. This model assumes that when you look at large enough volumes of space — above about 1 billion light-years — matter is distributed evenly. The Giant Arc appears about three times as long as that theoretical threshold. It joins other structures with similarly superlative names, like the Sloan Great Wall, the Giant Gamma-Ray Burst Ring and the Huge Large Quasar Group.

      “We can have one large-scale structure that could just be a statistical fluke,” Lopez said. “That’s not the problem. All of them combined is what makes the problem even bigger.” More

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      NASA will be heading back to Venus for the first time in decades

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      Earth’s evil twin, here we come. NASA’s next two missions, named DAVINCI+ and VERITAS, are heading to Venus, administrator Bill Nelson announced at a news conference June 2.

      “These two sister missions both aim to understand how Venus became an inferno-like world capable of melting lead at the surface,” Nelson said. “We hope these missions will further our understanding of how Earth evolved and why it’s currently habitable, when others in our solar system are not.”

      The missions were selected from four finalists, two headed to Venus, one to Jupiter’s volcanic moon Io, and one to Neptune’s largest moon Triton. The two Venus missions had applied and been rejected in earlier spacecraft selection rounds.

      Venus is almost the same size as Earth, but it seems to have had a different history. Although there’s evidence that it was once covered in oceans and could have been habitable, today it’s a scorched hellscape with clouds of sulfuric acid. No spacecraft has lasted more than two hours on its surface (SN: 2/13/18). And no NASA mission has visited in more than 30 years.

      The DAVINCI+ mission includes a probe (illustrated here) that will drop through the Venusian atmosphere, tasting and measuring as it goes.GSFC/NASA

      One of the newly selected missions, DAVINCI+, will be the first to send a probe into the planet’s thick, hot atmosphere. The spacecraft will be a ball about a meter in diameter that will sink through Venus’ atmosphere over the course of about an hour, taking measurements of how the content of the planet’s atmosphere changes from top to bottom. The probe will also take some of the highest-resolution photos of the Venusian surface yet on its way down.

      Those observations will help scientists figure out how Venus’ water has changed over time, its volcanic activity now and in the past, and the planet’spast potential for habitability (SN: 8/26/16). The data will also help scientistsinterpret observations of Earth-sized exoplanets with atmospheres that could be taken with the upcoming James Webb Space Telescope, giving researchers a way to tell exo-Earths from exo-Venuses (SN: 10/4/19).

      The other mission, VERITAS, will orbit Venus and study the planet’s surface to figure out its history and why it’s so different from Earth. The orbiter will map the surface with radar, chart elevations to make 3-D maps and look for plate tectonics and volcanism still ongoing on Venus. These observations could provide data for afuture mission to land on Venus (SN: 12/23/20).

      The missions are expected to launch sometime between 2028 and 2030, NASA said in a statement.

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      Some fast radio bursts come from the spiral arms of other galaxies

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      Five brief, bright blasts of radio waves from deep space now have precise addresses.

      The fast radio bursts, or FRBs, come from the spiral arms of their host galaxies, researchers report in a study to appear in the Astrophysical Journal. The proximity of the FRBs to sites of star formation bolsters the case for run-of-the-mill young stars as the origin of these elusive, energetic eruptions.

      “This is the first such population study of its kind and provides a unique piece to the puzzle of FRB origins,” says Wen-fai Fong, an astronomer at Northwestern University in Evanston, Ill.

      FRBs typically last a few milliseconds and are never seen again. Because the bursts are so brief, it’s difficult to nail down their precise origins on the sky. Although astronomers have detected about 1,000 FRBs since the first was reported in 2007, only 15 or so have been traced to a specific galaxy.

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      The first burst to be traced to its source came from a small, blobby dwarf galaxy with a lot of active star formation (SN: 1/4/17). That FRB sends off repeated blasts from a single source, which is an unusual feature, and helped astronomers localize its host galaxy.

      “After that, a lot of people thought, well, maybe all FRB hosts are like this,” says astronomer Alexandra Mannings of the University of California, Santa Cruz. But then a second repeating burst was tracked back to a spiral galaxy like the Milky Way (SN: 1/6/20). And a one-off burst was localized to a massive disk-shaped galaxy, also the size of the Milky Way. Others followed.

      Mannings, Fong and colleagues thought they could learn more about the FRBs’ sources by localizing their origins even more precisely. Different parts of spiral galaxies tend to host different types of stars. The bright spiral arms tend to mark sites where new stars are being born, while the older and dimmer stars have had time to drift away from the arms into the rest of the galaxy. So figuring out which galactic neighborhoods FRBs call home can reveal a lot about what kind of objects they come from.

      Using the Hubble Space Telescope, the researchers took high-resolution images of eight galaxies that were already known to host FRBs, then overlaid the FRBs’ positions onto the images. The five FRBs that came from clearly defined spiral galaxies all lay on or close to the galaxies’ spiral arms, which had not been visible in images from ground-based telescopes. The other three host galaxies had inconclusive shapes, Fong says.

      The FRB locales have a fair amount of star formation, but they’re not the brightest and most active parts of their galaxies, Fong says. That suggests FRBs originate with ordinary young stars — not the youngest, most massive stars that occupy the brightest knots in the spiral arms, but not the oldest and dimmest stars that have drifted away from their homes, either.

      That finding is consistent with the idea that FRBs come from highly magnetized stellar corpses called magnetars, Mannings says (SN: 6/4/20). There are a couple of ways to produce magnetars from ordinary stars. There’s the slow way, which involves waiting billions of years for a pair of neutron stars to collide (SN: 12/1/20). Or there’s the fast way, which follows the death of a single massive star. It seems like FRBs might come from an in-between process, like the death of a not-so-massive star, Mannings says.

      “The fact that FRBs are found to be pretty close to, if not on, the spiral arm, near to these star forming regions, that can give us a better idea of what the timeline is like for the progenitor,” whatever created the FRB, Mannings says. “And if it is a magnetar, it lets us know that it’s not through the delayed channel, like a neutron star merger.”

      The finding doesn’t entirely solve the mystery of where FRBs come from, says astrophysicist Emily Petroff of the University of Amsterdam, who was not involved in the new work. But it does help to get a broader picture of their host galaxies.

      “FRBs keep throwing a lot of surprises at us, in terms of what they look like, where they’re found, how they repeat,” Petroff says. “This is maybe providing more evidence that FRBs are more related to just sort of general neutron stars.” The next step, of course, is to find more FRBs. More

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      Laser experiments suggest helium rain falls on Jupiter

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      Sprinkles of helium rain may fall on Jupiter.

      At pressures and temperatures present within the gas giant, the hydrogen and helium that make up the bulk of its atmosphere don’t mix, according to laboratory experiments reported in the May 27 Nature. That suggests that deep within Jupiter’s atmosphere, hydrogen and helium separate, with the helium forming droplets that are denser than the hydrogen, causing them to rain down (SN: 4/19/21).

      Jupiter’s marbled exterior is pretty familiar territory, but it’s still not clear what happens far below the cloud tops. So researchers designed an experiment to compress hydrogen and helium, reaching pressures nearly 2 million times Earth’s atmospheric pressure and temperatures of thousands of degrees Celsius, akin to inner layers of gas giants.

      “We are reproducing the conditions inside the planets,” says physicist Marius Millot of Lawrence Livermore National Laboratory in California.

      Millot and colleagues squeezed a mixture of hydrogen and helium between two diamonds and hit the concoction with a powerful laser to compress it even further. As the pressure and temperature increased, the researchers saw an abrupt increase in how reflective the material was. That suggests that helium was separating from the hydrogen, which becomes a liquid metal under these conditions (SN: 8/10/16). At even higher pressures and temperatures, the reflectivity decreased, suggesting that hydrogen and helium began mixing again.

      The researchers calculated that hydrogen and helium would separate about 11,000 kilometers below the cloud tops of Jupiter, down to a depth of about 22,000 kilometers.

      The results could help scientists explain observations made by spacecraft Galileo (SN: 2/18/02) and Juno (SN: 3/7/18), such as the fact that Jupiter’s outer layers of atmosphere have less helium than expected. More

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      Record-breaking light has more than a quadrillion electron volts of energy

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      The cosmos keeps outdoing itself.

      Extremely energetic light from space is an unexplained wonder of astrophysics, and now scientists have spotted this light, called gamma rays, at higher energies than ever before.

      The Large High Altitude Air Shower Observatory, LHAASO, spotted more than 530 gamma rays with energies above 0.1 quadrillion electron volts, researchers report online May 17 in Nature. The highest-energy gamma ray detected was about 1.4 quadrillion electron volts. For comparison, protons in the largest accelerator on Earth, the Large Hadron Collider, reach mere trillions of electron volts. Previously, the most energetic gamma ray known had just under a quadrillion electron volts (SN: 2/2/21).

      In all, the scientists spotted 12 gamma ray hot spots, hinting that the Milky Way harbors powerful cosmic particle accelerators. In order for gamma rays to reach such energies, electromagnetic fields must first rev up charged particles, namely protons or electrons, to immense speeds. Those particles can then produce energetic gamma rays, for example, when protons interact with other matter in space.

      Scientists aren’t yet sure what environments are powerful enough to produce light with energies reaching more than a quadrillion electron volts. But the new observations point to two possibilities. One hot spot was associated with the Crab Nebula, the turbulent remains of an exploded star (SN: 6/24/19). Another potential source was the Cygnus Cocoon, a region where massive stars are forming, blasting out intense winds in the process.

      LHAASO, located on Haizi Mountain in China’s Sichuan province, is not yet fully operational. When it is completed later this year, it is expected to find even more energetic gamma rays. More

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      Watch this beautiful, high-resolution simulation of how stars are born

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      The most realistic computer simulation of star formation yet offers stunning views of what the inside of a stellar nursery might look like.

      In the Star Formation in Gaseous Environments simulation, or STARFORGE, a giant virtual cloud of gas collapses into a nest of new stars. Unlike other simulations, which could render only a small clump of gas within a larger cloud, STARFORGE simulates an entire star-forming cloud. It’s also the first simulation to account for the whole medley of physical phenomena thought to influence star formation, researchers report online May 17 in Monthly Notices of the Royal Astronomical Society.

      “We sort of know the basic story of star formation … but the devil is in the details,” says Mike Grudić, a theoretical astrophysicist at Northwestern University in Evanston, Ill. (SN: 4/21/20). Astronomers still don’t fully understand, for instance, why stars have different masses. “If you really want to get the full picture, then you really have to just simulate the whole thing.”

      [embedded content]
      In the computer simulation STARFORGE, a massive cloud of cosmic gas — roughly 20 parsecs, or 65 light-years, across — collapses to form new stars. White areas indicate denser regions of gas, including baby stars. Orange highlights places where there’s lots of variation in the gas motion, such as in powerful jets launched by new stars. Gas shown in purple is more tranquil. After 4.3 million years (Myr) have passed, the simulation pauses so the virtual camera can swoop around the cloud, revealing its 3-D structure.

      STARFORGE starts with a blob of gas that can be tens to hundreds of light-years across and up to millions of times the mass of the sun. Turbulence inside the cloud creates dense pockets that collapse to forge new stars. Those stars then launch powerful jets, give off radiation, shed stellar winds and explode in supernovas. Eventually, these phenomena blow the last vestiges of the cloud away and leave behind a hive of young stars. The whole process takes millions of years — or months of computing time, even running on supercomputers.

      Using STARFORGE, Grudić and colleagues have confirmed that jets launched by new stars help regulate how much material a star amasses. In simulations without jets, typical stars were about 10 times the mass of the sun — way bigger than the actual average star. “As soon as you add this jet feedback to your simulation,” Grudić says, “stellar masses start coming out more or less right on the dot for what they’re observed to be.”

      The STARFORGE simulation has helped confirm that jets launched by newborn stars (simulated one shown) determine how much mass stars can accrete.Northwestern University, University of Texas at Austin

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