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3Q: Scott Hughes on cosmic distances and the future of gravitational wave astronomy

On Monday, Oct. 16, National Science Foundation Director France Córdova, MIT senior research scientist and LIGO Scientific Collaboration spokesperson David Shoemaker, and other representatives from Caltech and the Virgo detector, announced the detection of GW170817 — the merger of two neutron stars as observed by the two Laser Interferometer Gravitational-wave Observatory (LIGO) detectors, located in Livingston, Louisiana, and Hanford, Washington. Unlike the four binary black hole systems previously detected, the observation of a neutron-neutron star merger opens up a new chapter in the science of gravitational waves: the first correlation between gravitational waves (GWs) and an electromagnetic signal, in this case short-hard gamma ray bursts (SHBs). In research preceding the LIGO detector systems’ first and second observing runs, Scott Hughes, MIT professor in the Department of Physics, working with Daniel Holz of the University of Chicago, developed a theoretical technique by which measuring the gravitational waves and SHBs of this kind of binary system could be used to measure cosmic distances, and to learn about the universe’s expansion. Hughes, who is not a member of the LIGO collaboration, answers questions about this technique and the future of gravitational wave astronomy. Q: What are “standard candles” in astronomy and how does the “standard siren” technique allow us to measure cosmic distances with greater certainty than other methods? A: Measuring distances is one of the hardest problems in astronomy. What kind of yardstick can we use to measure distances so large that light takes millions or billions of years to travel across? Imagine a gigantic lightbulb that puts out 400 trillion trillion watts — that’s the luminosity of our sun. The energy we receive from this lightbulb falls off as the distance squared between us and the bulb. Such a lightbulb 2 light years away would be four times dimmer than if it were 1 light year away. This source is a standard candle: an astronomical object whose luminosity is known so well that we can infer how far away it is from the brightness we measure. Although nature doesn’t provide us with such standard lightbulbs, astronomers have found that certain objects have luminosities that can be calibrated so that they are effectively standardized. Key to standardizing these objects are a series of measurements called the “cosmic distance ladder.” This uses a technique called parallax, which examines how the relative position of nearby stars on the sky changes as the Earth moves in its orbit. Using parallax, astronomers have learned that a class of stars called Cepheid variables — thousands of times more luminous than our sun — are very good standard candles. They can find these candles in distant galaxies, and use them to determine how far away those galaxies are. In 1986, Bernard Schutz of the University of Cardiff in Wales pointed out that binary coalescence — such as the merger of two neutron stars — is a self calibrating standard candle: Measuring its waves makes it possible to directly measure the binary’s distance without the cosmic distance ladder. Schutz’s key observation is that the rate at which the binary’s frequency changes is directly related to the system’s intrinsic gravitational wave “loudness.” (Gravitational waves have a sound-like character — recall the famous “chirp” from the first detection — and it is useful to think of strong events as loud, and weak events as quiet.) Just as the observed brightness of a star depends on both its intrinsic luminosity and how far away it is, the strength of the gravitational waves that we measure depends on both their source’s intrinsic loudness and how far away it is. By observing the waves with detectors like LIGO and Virgo, we learn both the waves’ intrinsic loudness as well as their loudness at the Earth. This allows us to directly determine distance to the source. About 12 years ago, Daniel Holz and I examined how well this idea could be implemented, focusing on how it could be done if the gravitational waves were accompanied by some electromagnetic signature, such as a short-hard gamma-ray burst. Given the sound-like character of gravitational waves, we named such an event a “standard siren.” Our analysis got us excited about how the self-calibrating nature of these events could make them powerful tools for important measurements in cosmology. Q: How does this observation provide a probe of the universe’s expansion and what could other potential observations — such as a black hole-neutron star merger — tell us about the origin of black holes or the expansion of our universe? A: Edwin Hubble first observed that our universe is expanding, finding that distant galaxies move away from us at a rate proportional to their distance. The wavelengths of light from such galaxies are shifted to the red part of the spectrum, a phenomenon in light akin to the Doppler effect in sound. Precise measurements of distance and redshift are needed in order to figure out how fast the expansion is proceeding. The binary inspiral of GW170817 measured its distance; telescope observations of the accompanying gamma-ray burst measured its redshift. Those are exactly the pieces of information needed to measure Hubble’s constant, which tells us how fast the universe is now expanding. Any measurement of binary coalescence that is accompanied by an electromagnetic event, like a gamma-ray burst, can be used to measure the expansion of the universe exactly as was done with GW170817. Indeed, we hope for more events like this: Combining many distance-redshift measurements will make it possible to average out noise and other error effects, and improve our ability to measure Hubble’s constant. Although Hubble’s constant had already been measured by a few different techniques, these techniques appear to be converging to two different values! It is unclear if this discrepancy is because of some currently unknown bit of cosmic physics, or if it is a systematic error in the measurements. Because the standard siren does not require a series of calibrations, it has tremendous promise for resolving this tension in the Hubble constant’s value. In addition to telling us about the distance to the event, each of these measurements such as GW170817 provides a wealth of data about the masses and other properties of the objects involved. As we build up a catalog of data about things like black hole masses and spins and neutron star masses, we will gain more and deeper understanding of how these objects are distributed in the universe, shedding light on the nature of the matter that makes up neutron stars and how some of the heaviest elements were formed. Q: What might space-based gravitational wave and electromagnetic measurements tell us that ground-based measurements from LIGO or Virgo could not? A: This question is near to my heart, since I have spent a lot of my career thinking about measurements using the Laser Interferometer Space Antenna (LISA) — the planned space-based gravitational-wave detector. Indeed, the primary focus of my first standard sirens paper with Holz was on sirens enabled by LISA measurements! LISA will be sensitive to gravitational waves at much lower frequencies than LIGO and Virgo can measure. Low-frequency waves come from much more massive sources, like the coalescence of black holes millions of times more massive than the sun. Such sources may enable LISA to make standard siren measurements from sources that are tremendously far away — perhaps tens of billions of light years, from an epoch when the universe was relatively young. The distant standard sirens that LISA may enable tell us about the expansion of the universe at a very different cosmic time than the relatively nearby sirens (a few hundred million light years away) that LIGO and Virgo measure. Together, these events would make it possible to precisely probe the expansion of the universe over a wide range of cosmic times, enabling a wholly new way of probing the large-scale geometry of our universe. Read the full article

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Apollo 13 commander James Lovell: “Crises don’t bother me anymore”

On April 14, 1970, 56 hours after the Apollo 13 spacecraft launched into space en route to the moon, commander James Lovell began filming inside the spacecraft’s command module as part of a live television program to be beamed down to three major U.S. networks. The broadcast was meant to give people on Earth a glimpse into the mission, which was to be NASA’s third landing on the moon, following its first historic and much celebrated Apollo 11 mission, and later, the equally successful Apollo 12. “But this was the third lunar landing flight,” says Lovell, who spoke at MIT on Wednesday to a rapt audience, packed and overflowing in Room 10-250. “All three networks received the signal — nobody carried it. There was the Dick Cavett show … a rerun of ‘I Love Lucy,’ and a ballgame — even people in the control center were watching the ballgame.” As there seemed to be little interest, NASA’s mission control suggested Lovell cut the program short. “I said goodnight to everybody, turned off the camera, and was coming down the tunnel, when suddenly there was a ‘hiss, bang!’ and the spacecraft rocked back and forth, jets were firing, and there was noise all over,” Lovell recalled. Of course, the cascade of system failures that would follow, and the ingenuity in getting the crew safely back to Earth is a story that has since been retold and aired many times over, on radio, television, and on the silver screen, with the 1995 Hollywood blockbuster “Apollo 13,” with Tom Hanks as Lovell. “That explosion was the best thing to ever happen to NASA,” Lovell told the MIT crowd. “It showed, really, the talent that NASA people had, in mission control and throughout the organization, that turned an almost complete catastrophe into a successful recovery.” Incident-free Lovell spoke as a special guest invited to MIT by the Department of Aeronautics and Astronautics (AeroAstro). He recalled a similar visit to MIT, back in the summer of 1968, when he had recently been selected as the navigator for Apollo 8, which was to be the first crewed mission to orbit the moon. At the time, Institute Professor Charles “Doc” Draper and his team at the Instrument Lab — later renamed the Draper Lab — were leading the development of the guidance system for Apollo 8, which would navigate the spacecraft from Earth to the moon. Working with Draper’s team, Lovell learned to use the system to align the spacecraft with respect to Earth, and navigate to the moon using a map of 37 stars. “This was the first time this system would be used to go to the moon,” Lovell said. “There are probably people here who know it better than I do.” Lovell described Apollo 8 as the high point of his space career, recalling moments when he had the chance to look out the spacecraft’s windows, back toward Earth. “The Earth was a place out there that you could put your thumb on and hide it completely,” Lovell recalled. “Everything you’ve ever known — all the problems that you have — if you put up your thumb, they all disappear. It was really mind-boggling to me. It was one of the most successful, incident-free Apollo flights that we had.” Lead weight Lovell’s next and final spaceflight, Apollo 13, was expected to be similarly smooth. Two weeks before launch, the spacecraft underwent its last test. “As the countdown went on, we could see the whole spacecraft come to life,” Lovell said. “The test was successful — everything looked perfect.” When the ground crew came by to empty the liquid oxygen tanks, which would be refilled before launch, they were unable to do so. It would take a month to replace the tanks, which would have delayed the mission. However, they noticed that one of the tanks was an old design, meant for Apollo 10, that was configured with an oxygen-emptying tube and a heater. “They figured, why not turn on the heater and boil the oxygen out, and therefore save time? Not a bad idea, so that’s what they did,” Lovell said. “The day before liftoff, they filled it up once more with liquid oxygen. It was a bomb waiting to go off” because the fix actually damaged the internal elements of the tank. And go off it did, with a ‘hiss, bang!’ that shook the spacecraft, shortly after Lovell ended the television broadcast on April 14. Checking the instrument gauges, he found that one oxygen tank was completely empty, while another was being rapidly depleted. As he looked out a side window, he witnessed a “gaseous substance, at high speed,” shooting out into space. “That’s when that old lead weight went down in the bottom of my stomach,” Lovell said. “Because we needed oxygen for electricity, the third fuel cell would die, and because we used electricity to control our rocket engine, we’d lose the entire propulsion system. We were in serious trouble.” “A square peg through a round hole” The crew, including NASA astronauts Jack Swigert and Fred Haise, scrambled to find a fix, quickly realizing they would have to abort the lunar mission and attempt to fly home not in the main service module, but in the much smaller lunar module (originally equipped for two men to explore the surface of the moon), before re-docking with the main spacecraft. “We used it as a life raft,” Lovell said. “It’s a fragile device, with skin so thin you could probably punch a hole through it.” The crew worked with mission control to redirect the severely compromised craft back toward Earth, with its limited reserves of power and oxygen. Just as everything appeared to be on course for a successful return, the module’s carbon dioxide light blinked on, indicating the cabin, designed for two people, was saturated with the breathing of three men. “If we weren’t going to do anything, we would be poisoned by our own exhalations,” Lovell said.  The crew would have to transfer canisters of lithium hydroxide from the dead command module to the lunar module, to remove the excess carbon dioxide. The only problem: those canisters were square, whereas the lunar module’s canisters were round. In a moment that the film “Apollo 13” has since made famous, the team would have to “invent a way to put a square peg in a round hole.” They did so using a piece of plastic Lovell recalled that he had stored underwear in, a cardboard cover from a manual, an old sock, and duct tape. Lovell described several more harrowing moments of resetting the craft’s course, before finally splashing down on Earth. “If you come in too shallow, it’ll be like skipping a stone across water, and you’re gone,” Lovell said. “If you come in too steep, the sudden deceleration will put you on fire like a meteorite and that will be it. … I guess I wouldn’t be here if that last maneuver wasn’t successful.” Before closing his talk, Lovell took several questions from the audience, including one from an AeroAstro PhD candidate, who asked his advice for anyone dealing with stress. “You have to have a positive attitude and look ahead,” Lovell responded. “If we got curled up in some sort of attitude, waiting for a miracle to happen, I’d still be up there. And one other thing: Crises don’t bother me anymore. I just look at them and figure out how to get out of them, and that’s it.” Read the full article

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LIGO and Virgo detect neutron star smash-ups

The following press release was issued jointly today by the LIGO Laboratory, LIGO Scientific Collaboration, and Virgo Collaboration. On April 25, 2019, the National Science Foundation's Laser Interferometer Gravitational-Wave Observatory (LIGO) and the European-based Virgo detector registered gravitational waves from what appears likely to be a crash between two neutron stars — the dense remnants of massive stars that previously exploded. One day later, on April 26, the LIGO-Virgo network spotted another candidate source with a potentially interesting twist: It may in fact have resulted from the collision of a neutron star and black hole, an event never before witnessed. "The universe is keeping us on our toes," says Patrick Brady, spokesperson for the LIGO Scientific Collaboration and a professor of physics at the University of Wisconsin-Milwaukee. "We're especially curious about the April 26 candidate. Unfortunately, the signal is rather weak. It's like listening to somebody whisper a word in a busy café; it can be difficult to make out the word or even to be sure that the person whispered at all. It will take some time to reach a conclusion about this candidate." "NSF's LIGO, in collaboration with Virgo, has opened up the universe to future generations of scientists," says NSF Director France Cordova. "Once again, we have witnessed the remarkable phenomenon of a neutron star merger, followed up closely by another possible merger of collapsed stars. With these new discoveries, we see the LIGO-Virgo collaborations realizing their potential of regularly producing discoveries that were once impossible. The data from these discoveries, and others sure to follow, will help the scientific community revolutionize our understanding of the invisible universe." The discoveries come just weeks after LIGO and Virgo turned back on. The twin detectors of LIGO — one in Washington and one in Louisiana — along with Virgo, located at the European Gravitational Observatory (EGO) in Italy, resumed operations April 1, after undergoing a series of upgrades to increase their sensitivities to gravitational waves — ripples in space and time. Each detector now surveys larger volumes of the universe than before, searching for extreme events such as smash-ups between black holes and neutron stars. "Joining human forces and instruments across the LIGO and Virgo collaborations has been once again the recipe of an incomparable scientific month, and the current observing run will comprise 11 more months," says Giovanni Prodi, the Virgo data analysis coordinator, at the University of Trento and the Istituto Nazionale di Fisica Nucleare (INFN) in Italy. "The Virgo detector works with the highest stability, covering the sky 90 percent of the time with useful data. This is helping in pointing to the sources, both when the network is in full operation and at times when only one of the LIGO detectors is operating. We have a lot of groundbreaking research work ahead." In addition to the two new candidates involving neutron stars, the LIGO-Virgo network has, in this latest run, spotted three likely black hole mergers. In total, since making history with the first-ever direct detection of gravitational waves in 2015, the network has spotted evidence for two neutron star mergers; 13 black hole mergers; and one possible black hole-neutron star merger. When two black holes collide, they warp the fabric of space and time, producing gravitational waves. When two neutron stars collide, they not only send out gravitational waves but also light. That means telescopes sensitive to light waves across the electromagnetic spectrum can witness these fiery impacts together with LIGO and Virgo. One such event occurred in August 2017:  LIGO and Virgo initially spotted a neutron star merger in gravitational waves and then, in the days and months that followed, about 70 telescopes on the ground and in space witnessed the explosive aftermath in light waves, including everything from gamma rays to optical light to radio waves. In the case of the two recent neutron star candidates, telescopes around the world once again raced to track the sources and pick up the light expected to arise from these mergers. Hundreds of astronomers eagerly pointed telescopes at patches of sky suspected to house the signal sources. However, at this time, neither of the sources has been pinpointed. "The search for explosive counterparts of the gravitational-wave signal is challenging due to the amount of sky that must be covered and the rapid changes in brightness that are expected," says Brady. "The rate of neutron star merger candidates being found with LIGO and Virgo will give more opportunities to search for the explosions over the next year." The April 25 neutron star smash-up, dubbed S190425z, is estimated to have occurred about 500 million light-years away from Earth. Only one of the twin LIGO facilities picked up its signal along with Virgo (LIGO Livingston witnessed the event but LIGO Hanford was offline). Because only two of the three detectors registered the signal, estimates of the location in the sky from which it originated were not precise, leaving astronomers to survey nearly one-quarter of the sky for the source. The possible April 26 neutron star-black hole collision (referred to as S190426c) is estimated to have taken place roughly 1.2 billion light-years away. It was seen by all three LIGO-Virgo facilities, which helped better narrow its location to regions covering about 1,100 square degrees, or about 3 percent of the total sky. "The latest LIGO-Virgo observing run is proving to be the most exciting one so far," says David H. Reitze of Caltech, executive director of LIGO. "We're already seeing hints of the first observation of a black hole swallowing a neutron star. If it holds up, this would be a trifecta for LIGO and Virgo — in three years, we'll have observed every type of black hole and neutron star collision. But we've learned that claims of detections require a tremendous amount of painstaking work — checking and rechecking — so we'll have to see where the data takes us." LIGO is funded by NSF and operated by Caltech and MIT, which conceived of LIGO and led the Initial and Advanced LIGO projects. Financial support for the Advanced LIGO project was led by the NSF with Germany (Max Planck Society), the U.K. (Science and Technology Facilities Council) and Australia (Australian Research Council-OzGrav) making significant commitments and contributions to the project. Approximately 1,300 scientists from around the world participate in the effort through the LIGO Scientific Collaboration, which includes the GEO Collaboration. A list of additional partners is available at https://my.ligo.org/census.php. The Virgo Collaboration is currently composed of approximately 350 scientists, engineers, and technicians from about 70 institutes from Belgium, France, Germany, Hungary, Italy, the Netherlands, Poland, and Spain. The European Gravitational Observatory (EGO) hosts the Virgo detector near Pisa in Italy, and is funded by Centre National de la Recherche Scientifique (CNRS) in France, the Istituto Nazionale di Fisica Nucleare (INFN) in Italy, and Nikhef in the Netherlands. A list of the Virgo Collaboration members can be found at http://public.virgo-gw.eu/the-virgo-collaboration/. More information is available on the Virgo website at http://www.virgo-gw.eu. Read the full article

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TESS discovers three new planets nearby, including temperate “sub-Neptune”

NASA’s Transiting Exoplanet Survey Satellite, or TESS, has discovered three new worlds that are among the smallest, nearest exoplanets known to date. The planets orbit a star just 73 light-years away and include a small, rocky super-Earth and two sub-Neptunes — planets about half the size of our own icy giant. The sub-Neptune furthest out from the star appears to be within a “temperate” zone, meaning that the very top of the planet’s atmosphere is within a temperature range that could support some forms of life. However, scientists say the planet’s atmosphere is likely a thick, ultradense heat trap that renders the planet’s surface too hot to host water or life. Nevertheless, this new planetary system, which astronomers have dubbed TOI-270, is proving to have other curious qualities. For instance, all three planets appear to be relatively close in size. In contrast, our own solar system is populated with planetary extremes, from the small, rocky worlds of Mercury, Venus, Earth, and Mars, to the much more massive Jupiter and Saturn, and the more remote ice giants of Neptune and Uranus. There’s nothing in our solar system that resembles an intermediate planet, with a size and composition somewhere in the middle of Earth and Neptune. But TOI-270 appears to host two such planets: both sub-Neptunes are smaller than our own Neptune and not much larger than the rocky planet in the system. Astronomers believe TOI-270’s sub-Neptunes may be a “missing link” in planetary formation, as they are of an intermediate size and could help researchers determine whether small, rocky planets like Earth and more massive, icy worlds like Neptune follow the same formation path or evolve separately. TOI-270 is an ideal system for answering such questions, because the star itself is nearby and therefore bright, and also unusually quiet. The star is an M-dwarf, a type of star that is normally extremely active, with frequent flares and solar storms. TOI-270 appears to be an older M-dwarf that has since quieted down, giving off a steady brightness, against which scientists can measure many properties of the orbiting planets, such as their mass and atmospheric composition. “There are a lot of little pieces of the puzzle that we can solve with this system,” says Maximilian Günther, a postdoc in MIT’s Kavli Institute for Astrophysics and Space Research and lead author of a study published today in Nature Astronomy that details the discovery. “You can really do all the things you want to do in exoplanet science, with this system.” Compare and contrast worlds in the TOI 270 system with these illustrations. Temperatures given for TOI 270 planets are equilibrium temperatures, calculated without the warming effects of any possible atmospheres. Credit: NASA’s Goddard Space Flight Center A planetary pattern Günther and his colleagues detected the three new planets after looking through measurements of stellar brightness taken by TESS. The MIT-developed satellite stares at patches of the sky for 27 days at a time, monitoring thousands of stars for possible transits — characteristic dips in brightness that could signal a planet temporarily blocking the star’s light as it passes in front of it. The team isolated several such signals from a nearby  star, located 73 light years away in the southern sky. They named the star TOI-270, for the 270th “TESS Object of Interest” identified to date. The researchers used ground-based instruments to follow up on the star’s activity, and confirmed that the signals are the result of three orbiting exoplanets: planet b, a rocky super-Earth with a roughly three-day orbit; planet c, a sub-Neptune with a five-day orbit; and planet d, another sub-Neptune slightly further out, with an 11-day orbit. Günther notes that the planets seem to line up in what astronomers refer to as a “resonant chain,” meaning that the ratio of their orbits are close to whole integers — in this case, 3:5 for the inner pair, and 2:1 for the outer pair — and that the planets are therefore in “resonance” with each other. Astronomers have discovered other small stars with similarly resonant planetary formations. And in our own solar system, the moons of Jupiter also happen to line up in resonance with each other. “For TOI-270, these planets line up like pearls on a string,” Günther says. “That’s a very interesting thing, because it lets us study their dynamical behavior. And you can almost expect, if there are more planets, the next one would be somewhere further out, at another integer ratio.” “An exceptional laboratory” TOI-270’s discovery initially caused a stir of excitement within the TESS science team, as it seemed, in the first analysis, that planet d might lie in the star’s habitable zone, a region that would be cool enough for the planet’s surface to support water, and possibly life. But the researchers soon realized that the planet’s atmosphere was probably extremely thick, and would therefore generate an intense greenhouse effect, causing the planet’s surface to be too hot to be habitable. But Günther says there is a good possibility that the system hosts other planets, further out from planet d, that might well lie within the habitable zone. Planet d, with an 11-day orbit, is about 10 million kilometers out from the star. Günther says that, given that the star is small and relatively cool — about half as hot as the sun — its habitable zone could potentially begin at around 15 million kilometers. But whether a planet exists within this zone, and whether it is habitable, depends on a host of other parameters, such as its size, mass, and atmospheric conditions. Fortunately, the team writes in their paper that “the host star, TOI-270, is remarkably well-suited for future habitability searches, as it is particularly quiet.” The researchers plan to focus other instruments, including the upcoming James Webb Space Telescope, on TOI-270, to pin down various properties of the three planets, as well as search for additional planets in the star’s habitable zone. “TOI-270 is a true Disneyland for exoplanet science, and one of the prime systems TESS was set out to discover,” Günther says. “It is an exceptional laboratory for not one, but many reasons — it really ticks all the boxes.” This research was funded, in part, by NASA. Read the full article

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Explosions of universe’s first stars spewed powerful jets

Several hundred million years after the Big Bang, the very first stars flared into the universe as massively bright accumulations of hydrogen and helium gas. Within the cores of these first stars, extreme, thermonuclear reactions forged the first heavier elements, including carbon, iron, and zinc. These first stars were likely immense, short-lived fireballs, and scientists have assumed that they exploded as similarly spherical supernovae. But now astronomers at MIT and elsewhere have found that these first stars may have blown apart in a more powerful, asymmetric fashion, spewing forth jets that were violent enough to eject heavy elements into neighboring galaxies. These elements ultimately served as seeds for the second generation of stars, some of which can still be observed today. In a paper published today in the Astrophysical Journal, the researchers report a strong abundance of zinc in HE 1327-2326, an ancient, surviving star that is among the universe’s second generation of stars. They believe the star could only have acquired such a large amount of zinc after an asymmetric explosion of one of the very first stars had enriched its birth gas cloud. “When a star explodes, some proportion of that star gets sucked into a black hole like a vacuum cleaner,” says Anna Frebel, an associate professor of physics at MIT and a member of MIT’s Kavli Institute for Astrophysics and Space Research. “Only when you have some kind of mechanism, like a jet that can yank out material, can you observe that material later in a next-generation star. And we believe that’s exactly what could have happened here.” “This is the first observational evidence that such an asymmetric supernova took place in the early universe,” adds MIT postdoc Rana Ezzeddine, the study’s lead author. “This changes our understanding of how the first stars exploded.” “A sprinkle of elements” HE 1327-2326 was discovered by Frebel in 2005. At the time, the star was the most metal-poor star ever observed, meaning that it had extremely low concentrations of elements heavier than hydrogen and helium — an indication that it formed as part of the second generation of stars, at a time when most of the universe’s heavy element content had yet to be forged. “The first stars were so massive that they had to explode almost immediately,” Frebel says. “The smaller stars that formed as the second generation are still available today, and they preserve the early material left behind by these first stars. Our star has just a sprinkle of elements heavier than hydrogen and helium, so we know it must have formed as part of the second generation of stars.” In May of 2016, the team was able to observe the star which orbits close to Earth, just 5,000 light years away. The researchers won time on NASA’s Hubble Space Telescope over two weeks, and recorded the starlight over multiple orbits. They used an instrument aboard the telescope, the Cosmic Origins Spectrograph, to measure the minute abundances of various elements within the star. The spectrograph is designed with high precision to pick up faint ultraviolet light. Some of those wavelength are absorbed by certain elements, such as zinc. The researchers made a list of heavy elements that they suspected might be within such an ancient star, that they planned to look for in the UV data, including silicon, iron, phosophorous, and zinc. “I remember getting the data, and seeing this zinc line pop out, and we couldn’t believe it, so we redid the analysis again and again,” Ezzeddine recalls. “We found that, no matter how we measured it, we got this really strong abundance of zinc.” A star channel opens Frebel and Ezzeddine then contacted their collaborators in Japan, who specialize in developing simulations of supernovae and the secondary stars that form in their aftermath. The researchers ran over 10,000 simulations of supernovae, each with different explosion energies, configurations, and other parameters. They found that while most of the spherical supernova simulations were able to produce a secondary star with the elemental compositions the researchers observed in HE 1327-2326, none of them reproduced the zinc signal. As it turns out, the only simulation that could explain the star’s makeup, including its high abundance of zinc, was one of an aspherical, jet-ejecting supernova of a first star. Such a supernova would have been extremely explosive, with a power equivalent to about a nonillion times (that’s 10 with 30 zeroes after it) that of a hydrogen bomb. “We found this first supernova was much more energetic than people have thought before, about five to 10 times more,” Ezzeddine says. “In fact, the previous idea of the existence of a dimmer supernova to explain the second-generation stars may soon need to be retired.” The team’s results may shift scientists’ understanding of reionization, a pivotal period during which the gas in the universe morphed from being completely neutral, to ionized — a state that made it possible for galaxies to take shape. “People thought from early observations that the first stars were not so bright or energetic, and so when they exploded, they wouldn’t participate much in reionizing the universe,” Frebel says. “We’re in some sense rectifying this picture and showing, maybe the first stars had enough oomph when they exploded, and maybe now they are strong contenders for contributing to reionization, and for wreaking havoc in their own little dwarf galaxies.” These first supernovae could have also been powerful enough to shoot heavy elements into neighboring “virgin galaxies” that had yet to form any stars of their own. “Once you have some heavy elements in a hydrogen and helium gas, you have a much easier time forming stars, especially little ones,” Frebel says. “The working hypothesis is, maybe second generation stars of this kind formed in these polluted virgin systems, and not in the same system as the supernova explosion itself, which is always what we had assumed, without thinking in any other way. So this is opening up a new channel for early star formation.” This research was funded, in part, by the National Science Foundation. Read the full article

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The music of the spheres

Space has long fascinated poets, physicists, astronomers, and science fiction writers. Musicians, too, have often found beauty and meaning in the skies above. At MIT’s Kresge Auditorium, a group of composers and musicians manifested their fascination with space in a concert titled “Songs from Extrasolar Spaces.” Featuring the Lorelei Ensemble — a Boston, Massachusetts-based women’s choir — the concert included premieres by MIT composers John Harbison and Elena Ruehr, along with compositions by Meredith Monk and Molly Herron. All the music was inspired by discoveries in astronomy. “Songs from Extrasolar Spaces,” part of an MIT conference on TESS — the Transiting Exoplanet Survey Satellite, launched in April 2018. TESS is an MIT-led NASA mission that scans the skies for evidence of exoplanets: bodies ranging from dwarf planets to giant planets that orbit stars other than our sun. During its two-year mission, TESS and its four highly-sensitive cameras survey 85 percent of the sky, monitoring more than 200,000 stars for the temporary dips in brightness that might signal a transit — the passage of a planetary body across that star. “There is a feeling you get when you look at these images from TESS,” says Ruehr, an award-winning MIT lecturer in the Music and Theater Arts Section and former Guggenheim Fellow. “A sense of vastness, of infinity. This is the sensation I tried to capture and transpose into vocal music.”  Supported by the MIT Center for Art, Science and Technology’s Fay Chandler Creativity Grant; MIT Music and Theater Arts; and aerospace and technology giant Northrop Grumman, which also built the TESS satellite, the July 30 concert was conceived by MIT Research Associate Natalia Guerrero. Both the conference and concert marked the 50th anniversary of the Apollo 11 moon landing — another milestone in the quest to chart the universe and Earth’s place in it. A 2014 MIT graduate, Guerrero manages the team finding planet candidates in the TESS images at the MIT Kavli Institute for Astrophysics and Space Research and is also the lead for the MIT branch of the mission’s communications team. “I wanted to include an event that could make the TESS mission accessible to people who aren’t astronomers or physicists,” says Guerrero. “But I also wanted that same event to inspire astronomers and physicists to look at their work in a new way.” Guerrero majored in physics and creative writing at MIT, and after graduating she deejayed a radio show called “Voice Box” on the MIT radio station WMBR. That transmission showcased contemporary vocal music and exposed her to composers including Harbison and Ruehr. Last year, in early summer, Guerrero contacted Ruehr to gauge her interest in composing music for a still-hypothetical concert that might complement the 2019 TESS conference. Ruehr was keen on the idea. She was also a perfect fit for the project. The composer had often drawn inspiration from visual images and other art forms for her music. “Sky Above Clouds,” an orchestral piece she composed in 1989, is inspired by the Georgia O’Keefe paintings she viewed as a child at the Art Institute of Chicago. Ruehr had also created music inspired by David Mitchell’s visionary novel “Cloud Atlas” and Anne Patchett’s “Bel Canto.” “It’s a question of reinterpreting language, capturing its rhythms and volumes and channeling them into music,” says Ruehr. “The source language can be fiction, or painting, or in this case these dazzling images of the universe.” In addition, Ruehr had long been fascinated by space and stars. “My father was a mathematician who studied fast Fourier transform analysis,” says Ruehr, who is currently composing an opera set in space. “As a young girl, I’d listen to him talking about infinity with his colleagues on the telephone. I would imagine my father existing in infinity, on the edge of space.” Drawing inspiration from the images TESS beams back to Earth, Ruehr composed two pieces for “Songs from Extrasolar Spaces.” The first, titled “Not from the Stars,” takes its name and lyrics from a Shakespeare sonnet. For the second, “Exoplanets,” Ruehr used a text that Guerrero extrapolated from the titles of the first group of scientific papers published from TESS data. “I’m used to working from images,” explains Ruehr. “First, I study them. Then, I sit down at the piano and try to create a single sound that captures their essence and resonance. Then, I start playing with that sound.” Ruehr was particularly pleased to compose music about space for the Lorelei Ensemble. “There’s a certain quality in a women’s choir, especially the Lorelei Ensemble, that is perfectly suited for this project,” says Ruehr. “They have an ethereal sound and wonderful harmonic structures that make us feel as if we’re perceiving a small dab of brightness in an envelope of darkness.” At the 2019 MIT TESS conference, experts from across the globe shared results from the first year of observation in the sky above the Southern Hemisphere, and discussed plans for the second-year trek above the Northern Hemisphere. The composers and musicians hope “Songs from Extrasolar Spaces” brought attention to the TESS missions, offers a new perspective on space exploration, and will perhaps spark further collaborations between scientists and artists. George Ricker, TESS principal investigator; Sara Seager, TESS deputy director of science; and Guerrero presented a pre-concert lecture. “Music has the power to generate incredibly powerful emotions,” says Ruehr. “So do these images from TESS. In many ways, they are more beautiful than any stars we might ever imagine.” TESS is a NASA Astrophysics Explorer mission led and operated by MIT in Cambridge, Massachusetts, and managed by Goddard Spaceflight Center. Additional partners include Northrop Grumman, based in Falls Church, Virginia; NASA’s Ames Research Center in California’s Silicon Valley; the Harvard-Smithsonian Center for Astrophysics in Cambridge; MIT Lincoln Laboratory; and the Space Telescope Science Institute in Baltimore, Maryland. More than a dozen universities, research institutes, and observatories worldwide are participants in the mission. Read the full article

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Rocky, Earth-sized exoplanet is missing an atmosphere

Astronomers at MIT, Harvard University, and elsewhere have searched a rocky, Earth-sized exoplanet for signs of an atmosphere — and found none. Atmospheres have previously been detected on planets much larger than our own, including several hot-Jupiters and sub-Neptunes, all of which are primarily made of ice and gas. But this is the first time scientists have been able to nail down whether an Earth-sized, terrestrial planet outside our solar system has an atmosphere. The planet in question, LHS 3844b, was discovered in 2018 by NASA’s Transiting Exoplanet Survey Satellite, TESS, and was measured to be about 1.3 times larger than Earth. The planet zips around its star in just 11 hours, making it one of the fastest orbiting exoplanets known. The star itself is a small, cool M-dwarf that resides just 49 light-years from Earth. In a paper published today in Nature, the team reports that LHS 3844b likely has neither a thick, Venus-like atmosphere nor a thin, Earth-like atmosphere. Instead, the planet is probably more similar to Mercury — a blistering, bare rock. If an atmosphere ever existed, the researchers say the star’s radiation likely blasted it away early in the planet’s formation. “We basically found a hot planet with no gases around it,” says co-author Daniel Koll, a postdoc in MIT’s Department of Earth, Atmospheric, and Planetary Sciences. “This is the first time we’ve known anything in detail about the atmosphere of a planet around these M-dwarfs, which are the most common type of star, making LHS 3844b the most common type of rocky planet in the galaxy.”  Could any form of life manage to take hold in such a barren wasteland? Koll and his colleagues say it’s extremely unlikely, as the lack of an atmosphere would instantly cook off any organisms on the planet’s surface. But that doesn’t mean other terrestrial exoplanets are similarly without cover. “We never thought this particular planet would be hospitable for life,” says lead author Laura Kreidberg, a researcher at the Harvard Center for Astrophysics. “It’s more a question of whether this whole category of planets around smaller stars has atmospheres or not. And our technique is a robust way of assessing whether an atmosphere is present or not.” Kreidberg and Koll’s co-authors from MIT include Jason Dittmann, Ian Crossfield, David Berardo, Xueying “Sherry” Guo, George Ricker, Sara Seager, and Roland Vanderspek, along with colleagues from Harvard, the University of Texas at Austin, the Jet Propulsion Laboratory, Caltech, Stanford University, the University of Maryland, and Vanderbilt University. Read the full article

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For first time, astronomers catch asteroid in the act of changing color

Last December, scientists discovered an “active” asteroid within the asteroid belt, sandwiched between the orbits of Mars and Jupiter. The space rock, designated by astronomers as 6478 Gault, appeared to be leaving two trails of dust in its wake — active behavior that is associated with comets but rarely seen in asteroids. While astronomers are still puzzling over the cause of Gault’s comet-like activity, an MIT-led team now reports that it has caught the asteroid in the act of changing color, in the near-infrared spectrum, from red to blue. It is the first time scientists have observed a color-shifting asteroid, in real-time. “That was a very big surprise,” says Michael Marsset, a postdoc in MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS). “We think we have witnessed the asteroid losing its reddish dust to space, and we are seeing the asteroid’s underlying, fresh blue layers.” Marsset and his colleagues have also confirmed that the asteroid is rocky — proof that the asteroid’s tail, though seemingly comet-like, is caused by an entirely different mechanism, as comets are not rocky but more like loose snowballs of ice and dust. “It’s the first time to my knowledge that we see a rocky body emitting dust, a little bit like a comet,” Marsset says. “It means that probably some mechanism responsible for dust emission is different from comets, and different from most other active main-belt asteroids.” Marsset and his colleagues, including EAPS Research Scientist Francesca DeMeo and Professor Richard Binzel, have published their results today in the journal Astrophysical Journal Letters. A rock with tails Astronomers first discovered 6478 Gault in 1988 and named the asteroid after planetary geologist Donald Gault. Until recently, the space rock was seen as relatively average, measuring about 2.5 miles wide and orbiting along with millions of other bits of rock and dust within the inner region of the asteroid belt, 214 million miles from the sun. In January, images from various observatories, including NASA’s Hubble Space Telescope, captured two narrow, comet-like tails trailing the asteroid. Astronomers estimate that the longer tail stretches half a million miles out, while the shorter tail is about a quarter as long. The tails, they concluded, must consist of tens of millions of kilograms of dust, actively ejected by the asteroid, into space. But how? The question reignited interest in Gault, and studies since then have unearthed past instances of similar activity by the asteroid. “We know of about a million bodies between Mars and Jupiter, and maybe about 20 that are active in the asteroid belt,” Marsset says. “So this is very rare.” He and his colleagues joined the search for answers to Gault’s activity in March, when they secured observation time at NASA’s Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii. Over two nights, they observed the asteroid and used a high-precision spectrograph to divide the asteroid’s incoming light into various frequencies, or colors, the relative intensities of which can give scientists an idea of an object’s composition. From their analysis, the team determined that the asteroid’s surface is composed mainly of silicate, a dry, rocky material, similar to most other asteroids, and, more importantly, not at all like most comets. Comets typically come from the far colder edges of the solar system. When they approach the sun, any surface ice instantly sublimates, or vaporizes into gas, creating the comet’s characteristic tail. Since Marsset’s team has found 6478 Gault is a dry, rocky body, this means it likely is generating dust tails by some other active mechanism. A fresh change As the team observed the asteroid, they discovered, to their surprise, that the rock was changing color in the near-infrared, from red to blue. “We've never seen such a dramatic change like this over such a short period of time,” says co-author DeMeo. The scientists say they are likely seeing the asteroid’s surface dust, turned red over millions of years of exposure to the sun, being ejected into space, revealing a fresh, less irradiated surface beneath, that appears blue at near-infrared wavelengths. “Interestingly, you only need a very thin layer to be removed to see a change in the spectrum,” DeMeo says. “It could be as thin as a single layer of grains just microns deep.” So what could be causing the asteroid to turn color? The team and other groups studying 6478 Gault believe the reason for the color shift, and the asteroid’s comet-like activity, is likely due to the same mechanism: a fast spin. The asteroid may be spinning fast enough to whip off layers of dust from its surface, through sheer centrifugal force. The researchers estimate it would need to have about a two-hour rotation period, spinning around every couple of hours, versus Earth’s 24-hour period. “About 10 percent of asteroids spin very fast, meaning with a two- to three-hour rotation period, and it’s most likely due to the sun spinning them up,” says Marsset. This spinning phenomenon is known as the YORP effect (or, the Yarkovsky-O’Keefe-Radzievskii-Paddack effect, named after the scientists who discovered it), which refers to the effect of solar radiation, or photons, on small, nearby bodies such as asteroids. While asteroids reflect most of this radiation back into space, a fraction of these photons is absorbed, then reemitted as heat, and also momentum. This creates a small force that, over millions of years, can cause the asteroid to spin faster. Astronomers have observed the YORP effect on a handful of asteroids in the past. To confirm a similar effect is acting on 6478 Gault, researchers will have to detect its spin through light curves — measurements of the asteroid’s brightness over time. The challenge will be to see through the asteroid’s considerable dust tail, which can obscure key portions of the asteroid’s light. Marsset’s team, along with other groups, plan to study the asteroid for further clues to activity, when it next becomes visible in the sky. “I think reinforces the fact that the asteroid belt is a really dynamic place,” DeMeo says. “While the asteroid fields you see in the movies, all crashing into each other, is an exaggeration, there is definitely a lot happening out there every moment.” This research was funded, in part, by the NASA Planetary Astronomy Program. Read the full article

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Breakthrough Prize in Fundamental Physics awarded to Event Horizon Telescope Collaboration for black hole observation

The Event Horizon Telescope (EHT) Collaboration, including scientists and engineers from MIT, will receive a 2020 Breakthrough Prize in Fundamental Physics. The team is being honored for making the first direct detection of a black hole. Assistant professor of physics Max Metlitski and several MIT alumni are also receiving awards from the Breakthrough Prize Foundation. The $3 million fundamental physics prize will be shared equally with the 347 EHT researchers from around the world who co-authored the six papers published on April 10, 2019, which reported the detection of the supermassive black hole at the heart of Messier 87, or M87, a galaxy within the Virgo galaxy cluster. The new laureates will be recognized at an awards ceremony in Mountain View, California, on Nov. 3. Earth-sized telescope The EHT is a global network of radio telescopes that work together as one virtual telescope, with a resolution sharp enough to “see” a black hole’s shadow. Researchers at MIT’s Haystack Observatory made several key contributions as members of the global collaboration, such as developing the ultrafast devices that record the vast volumes of data captured by each telescope. After the observing run ended, the data were sent to Haystack and to the Max Planck Institute for Radio Astronomy, where they were processed using a specialized supercomputer called a correlator, also developed by Haystack researchers. Teams at both institutions then undertook the painstaking process of “correlating” the data and ensuring they were rigorously verified before being released to the independent teams that would create the images of M87. The result, according to the Breakthrough Prize citation, was “an image of this galactic monster, silhouetted against hot gas swirling around the black hole, that matched expectations from Einstein's theory of gravity.” MIT-affliated scientists and engineers who will share in the prize include researchers and alumni from Haystack Observatory, the Department of Electrical Engineering and Computer Science, the Department of Physics, the Department of Earth, Atmospheric and Planetary Sciences, and the MIT Kavli Institute for Astrophysics and Space Research. They are: Kazunori Akiyama, Frederick K. Baganoff, John Barrett, Christopher Beaudoin, Lindy Blackburn, Katherine L. Bouman, Roger Cappallo, Geoffrey B. Crew, Joseph Crowley, Mark Derome, Sheperd S. Doeleman, Chris Eckert, Vincent L. Fish, William T. Freeman, Michael H. Hecht, Colin Lonsdale, Sera Markoff, Lynn D. Matthews, Stephen R. McWhirter, James Moran, Kotaro Moriyama, Michael Nowak, Joseph Neilsen, Daniel C. M. Palumbo, Michael Poirier, Alan Rogers, Chet Ruszczyk, Jason SooHoo, Don Sousa, Michael Titus, Alan R. Whitney, and Shuo Zhang. Additional accolades The Breakthrough Prize Foundation has also honored assistant professor of physics Maxim Metlitski, awarding him a New Horizons prize, which recognizes early-career achievements in physics and mathematics. Metlitski will share the prize with three collaborators, two of whom are MIT alumni: Xie Chen PhD ’12 of Caltech, Michael Levin PhD ’06 of the University of Chicago, and Lukasz Fidkowski of the University of Washington. The team is being honored “for incisive contributions to the understanding of topological states of matter and the relationships between them,” according to the Breakthrough Prize citation. “Max is part of a very talented group of experimentalists and theorists working on new materials with very unusual properties,” says Peter Fisher, professor and head of the Department of Physics. “These materials are teaching us how quantum mechanics plays an unexpected role in how electrons and vibrations can travel in materials that could result in new technologies.” Metlitski earned a BS in physics and mathematics and an MS in physics from the University of British Columbia. After obtaining his PhD in physics from Harvard University in 2011, he held a postdoctoral position at the Kavli Institute for Theoretical Physics at the University of California at Santa Barbara. He joined MIT’s Department of Physics as an assistant professor in January 2017, following a faculty appointment at the Perimeter Institute for Theoretical Physics in Waterloo, Canada. “On behalf of the School of Science, I congratulate Max Metlitski for this impressive early-career achievement in condensed matter theory,” says Michael Sipser, dean of the MIT School of Science and the Donner Professor of Mathematics. “In addition, I applaud our researchers in the Event Horizon Telescope Collaboration, who contributed to our first images of a black hole. We celebrate our scientists’ pursuit of fundamental research to advance human knowledge and all recipients of these prestigious awards.” David Jay Julius ’77, a professor at the University of California at San Franciso, has also won a 2020 Breakthrough Prize in Life Sciences, for discovering molecules, cells and mechanisms underlying pain sensation. And last month, Daniel Z. Freedman, professor emeritus in MIT’s departments of Mathematics and Physics, was awarded a Special Breakthrough Prize in Fundamental Physics. Read the full article