On astrophysicists’ charts of star stuff, there’s a substance that still merits the label “here be dragons.” That poorly understood material is found inside neutron stars — the collapsed remnants of once-mighty stars — and is now being mapped out, as scientists better characterize the weird matter.

The detection of two colliding neutron stars, announced in October (SN: 11/11/17, p. 6), has accelerated the pace of discovery. Since the event, which scientists spied with gravitational waves and various wavelengths of light, several studies have placed new limits on the sizes and masses possible for such stellar husks and on how squishy or stiff they are.
“The properties of neutron star matter are not very well known,” says physicist Andreas Bauswein of the Heidelberg Institute for Theoretical Studies in Germany. Part of the problem is that the matter inside a neutron star is so dense that a teaspoonful would weigh a billion tons, so the substance can’t be reproduced in any laboratory on Earth.

In the collision, the two neutron stars merged into a single behemoth. This remnant may have immediately collapsed into a black hole. Or it may have formed a bigger, spinning neutron star that, propped up by its own rapid rotation, existed for a few milliseconds — or potentially much longer — before collapsing. The speed of the object’s demise is helping scientists figure out whether neutron stars are made of material that is relatively soft, compressing when squeezed like a pillow, or whether the neutron star stuff is stiff, standing up to pressure. This property, known as the equation of state, determines the radius of a neutron star of a particular mass.

An immediate collapse seems unlikely, two teams of researchers say. Telescopes spotted a bright glow of light after the collision. That glow could only appear if there were a delay before the merged neutron star collapsed into a black hole, says physicist David Radice of Princeton University because when the remnant collapses, “all the material around falls inside of the black hole immediately.” Instead, the neutron star stuck around for at least several milliseconds, the scientists propose.

Simulations indicate that if neutron stars are soft, they will collapse more quickly because they will be smaller than stiff neutron stars of the same mass. So the inferred delay allows Radice and colleagues to rule out theories that predict neutron stars are extremely squishy, the researchers report in a paper published November 13 at arXiv.org.
Using similar logic, Bauswein and colleagues rule out some of the smallest sizes that neutron stars of a particular mass might be. For example, a neutron star 60 percent more massive than the sun can’t have a radius smaller than 10.7 kilometers, they determine. These results appear in a paper published November 29 in the Astrophysical Journal Letters.

Other researchers set a limit on the maximum mass a neutron star can have. Above a certain heft, neutron stars can no longer support their own weight and collapse into a black hole. If this maximum possible mass were particularly large, theories predict that the newly formed behemoth neutron star would have lasted hours or days before collapsing. But, in a third study, two physicists determined that the collapse came much more quickly than that, on the scale of milliseconds rather than hours. A long-lasting, spinning neutron star would dissipate its rotational energy into the material ejected from the collision, making the stream of glowing matter more energetic than what was seen, physicists Ben Margalit and Brian Metzger of Columbia University report. In a paper published November 21 in the Astrophysical Journal Letters, the pair concludes that the maximum possible mass is smaller than about 2.2 times that of the sun.

“We didn’t have many constraints on that prior to this discovery,” Metzger says. The result also rules out some of the stiffer equations of state because stiffer matter tends to support larger masses without collapsing.

Some theories predict that bizarre forms of matter are created deep inside neutron stars. Neutron stars might contain a sea of free-floating quarks — particles that are normally confined within larger particles like protons or neutrons. Other physicists suggest that neutron stars may contain hyperons, particles made with heavier quarks known as strange quarks, not found in normal matter. Such unusual matter would tend to make neutron stars softer, so pinning down the equation of state with additional neutron star crashes could eventually resolve whether these exotic beasts of physics indeed lurk in this unexplored territory.

Galileo’s most famous experiment has taken a trip to outer space. The result? Einstein was right yet again. The experiment confirms a tenet of Einstein’s theory of gravity with greater precision than ever before.

According to science lore, Galileo dropped two balls from the Leaning Tower of Pisa to show that they fell at the same rate no matter their composition. Although it seems unlikely that Galileo actually carried out this experiment, scientists have performed a similar, but much more sensitive experiment in a satellite orbiting Earth. Two hollow cylinders within the satellite fell at the same rate over 120 orbits, or about eight days’ worth of free-fall time, researchers with the MICROSCOPE experiment report December 4 in Physical Review Letters. The cylinders’ accelerations match within two-trillionths of a percent.

The result confirms a foundation of Einstein’s general theory of relativity known as the equivalence principle. That principle states that an object’s inertial mass, which sets the amount of force needed to accelerate it, is equal to its gravitational mass, which determines how the object responds to a gravitational field. As a result, items fall at the same rate — at least in a vacuum, where air resistance is eliminated — even if they have different masses or are made of different materials.

The result is “fantastic,” says physicist Stephan Schlamminger of OTH Regensburg in Germany who was not involved with the research. “It’s just great to have a more precise measurement of the equivalence principle because it’s one of the most fundamental tenets of gravity.”
In the satellite, which is still collecting additional data, a hollow cylinder, made of platinum alloy, is centered inside a hollow, titanium-alloy cylinder. According to standard physics, gravity should cause the cylinders to fall at the same rate, despite their different masses and materials. A violation of the equivalence principle, however, might make one fall slightly faster than the other.

As the two objects fall in their orbit around Earth, the satellite uses electrical forces to keep the pair aligned. If the equivalence principle didn’t hold, adjustments needed to keep the cylinders in line would vary with a regular frequency, tied to the rate at which the satellite orbits and rotates. “If we see any difference in the acceleration it would be a signature of violation” of the equivalence principle, says MICROSCOPE researcher Manuel Rodrigues of the French aerospace lab ONERA in Palaiseau. But no hint of such a signal was found.

With about 10 times the precision of previous tests, the result is “very impressive,” says physicist Jens Gundlach of the University of Washington in Seattle. But, he notes, “the results are still not as precise as what I think they can get out of a satellite measurement.”

Performing the experiment in space eliminates certain pitfalls of modern-day land-based equivalence principle tests, such as groundwater flow altering the mass of surrounding terrain. But temperature changes in the satellite limited how well the scientists could confirm the equivalence principle, as these variations can cause parts of the apparatus to expand or contract.

MICROSCOPE’s ultimate goal is to beat other measurements by a factor of 100, comparing the cylinders’ accelerations to see whether they match within a tenth of a trillionth of a percent. With additional data yet to be analyzed, the scientists may still reach that mark.

Confirmation of the equivalence principle doesn’t mean that all is hunky-dory in gravitational physics. Scientists still don’t know how to combine general relativity with quantum mechanics, the physics of the very small. “The two theories seems to be very different, and people would like to merge these two theories,” Rodrigues says. But some attempts to do that predict violations of the equivalence principle on a level that’s not yet detectable. That’s why scientists think the equivalence principle is worth testing to ever more precision — even if it means shipping their experiments off to space.

Bigwigs in a more than 600-year-old South American population were easy to spot. Their artificially elongated, teardrop-shaped heads screamed prestige, a new study finds.

During the 300 years before the Incas’ arrival in 1450, intentional head shaping among prominent members of the Collagua ethnic community in Peru increasingly centered on a stretched-out look, says bioarchaeologist Matthew Velasco of Cornell University. Having long, narrow noggins cemented bonds among members of a power elite — a unity that may have helped pave a relatively peaceful incorporation into the Incan Empire, Velasco proposes in the February Current Anthropology.
“Increasingly uniform head shapes may have encouraged a collective identity and political unity among Collagua elites,” Velasco says. These Collagua leaders may have negotiated ways to coexist with the encroaching Inca rather than fight them, he speculates. But the fate of the Collaguas and a neighboring population, the Cavanas, remains hazy. Those populations lived during a conflict-ridden time — after the collapse of two major Andean societies around 1100 (SN: 8/1/09, p. 16) and before the expansion of the Inca Empire starting in the 15th century.

For at least the past several thousand years, human groups in various parts of the world have intentionally modified skull shapes by wrapping infants’ heads with cloth or binding the head between two pieces of wood (SN: 4/29/17, p. 18). Researchers generally assume that this practice signified membership in ethnic or kin groups, or perhaps social rank.
The Callagua people lived in Colca Valley in southeastern Peru and raised alpaca for wool. By tracking Collagua skull shapes over 300 years, Velasco found that elongated skulls became increasingly linked to high social status. By the 1300s, for instance, Collagua women with deliberately distended heads suffered much less skull damage from physical attacks than other females did, he reports. Chemical analyses of bones indicates that long-headed women ate a particularly wide variety of foods.
Until now, knowledge of head-shaping practices in ancient Peru primarily came from Spanish accounts written in the 1500s. Those documents referred to tall, thin heads among Collaguas and wide, long heads among Cavanas, implying that a single shape had always characterized each group.

“Velasco has discovered that the practice of cranial modification was much more dynamic over time and across social [groups],” says bioarchaeologist Deborah Blom of the University of Vermont in Burlington.

Velasco examined 211 skulls of mummified humans interred in either of two Collagua cemeteries. Burial structures built against a cliff face were probably reserved for high-ranking individuals, whereas common burial grounds in several caves and under nearby rocky overhangs belonged to regular folk.
Radiocarbon analyses of 13 bone and sediment samples allowed Velasco to sort Collagua skulls into early and late pre-Inca groups. A total of 97 skulls, including all 76 found in common burial grounds, belonged to the early group, which dated to between 1150 and 1300. Among these skulls, 38 — or about 39 percent — had been intentionally modified. Head shapes included sharply and slightly elongated forms as well as skulls compressed into wide, squat configurations.

Of the 14 skulls with extreme elongation, 13 came from low-ranking individuals, a pattern that might suggest regular folk first adopted elongated head shapes. But with only 21 skulls from elites, the finding may underestimate the early frequency of elongated heads among the high-status crowd. Various local groups may have adopted their own styles of head modification at that time, Velasco suggests.

In contrast, among 114 skulls from elite burial sites in the late pre-Inca period, dating to between 1300 and 1450, 84 — or about 74 percent — displayed altered shapes. A large majority of those modified skulls — about 64 percent — were sharply elongated. Shortly before the Incas’ arrival, prominent Collaguas embraced an elongated style as their preferred head shape, Velasco says. No skeletal evidence has been found to determine whether low-ranking individuals also adopted elongated skulls as a signature look in the late pre-Inca period.

In courtrooms around the United States, computer programs give testimony that helps decide who gets locked up and who walks free.

These algorithms are criminal recidivism predictors, which use personal information about defendants — like family and employment history — to assess that person’s likelihood of committing future crimes. Judges factor those risk ratings into verdicts on everything from bail to sentencing to parole.

Computers get a say in these life-changing decisions because their crime forecasts are supposedly less biased and more accurate than human guesswork.
But investigations into algorithms’ treatment of different demographics have revealed how machines perpetuate human prejudices. Now there’s reason to doubt whether crime-prediction algorithms can even boast superhuman accuracy.

Computer scientist Julia Dressel recently analyzed the prognostic powers of a widely used recidivism predictor called COMPAS. This software determines whether a defendant will commit a crime within the next two years based on six defendant features — although what features COMPAS uses and how it weighs various data points is a trade secret.

Dressel, who conducted the study while at Dartmouth College, recruited 400 online volunteers, who were presumed to have little or no criminal justice expertise. The researchers split their volunteers into groups of 20, and had each group read descriptions of 50 defendants. Using such information as sex, age and criminal history, the volunteers predicted which defendants would reoffend.
A comparison of the volunteers’ answers with COMPAS’ predictions for the same 1,000 defendants found that both were about 65 percent accurate. “We were like, ‘Holy crap, that’s amazing,’” says study coauthor Hany Farid, a computer scientist at Dartmouth. “You have this commercial software that’s been used for years in courts around the country — how is it that we just asked a bunch of people online and [the results] are the same?”

There’s nothing inherently wrong with an algorithm that only performs as well as its human counterparts. But this finding, reported online January 17 in Science Advances, should be a wake-up call to law enforcement personnel who might have “a disproportionate confidence in these algorithms,” Farid says.

“Imagine you’re a judge, and I tell you I have this highly secretive, highly proprietary, expensive software built on big data, and it says the person standing in front of you is high risk” for reoffending, he says. “The judge would be like, ‘Yeah, that sounds quite serious.’ But now imagine if I tell you, ‘Twenty people online said this person is high risk.’ I imagine you’d weigh that information a little bit differently.” Maybe these predictions deserve the same amount of consideration.

Judges could get some better perspective on recidivism predictors’ performance if the Department of Justice or National Institute for Standards and Technology established a vetting process for new software, Farid says. Researchers could test computer programs against a large, diverse dataset of defendants and OK algorithms for courtroom use only if they get a passing grade for prediction.

Farid has his doubts that computers can show much improvement. He and Dressel built several simple and complex algorithms that used two to seven defendant features to predict recidivism. Like COMPAS, all their algorithms maxed out at about D-level accuracy. That makes Farid wonder whether trying to predict crime with anything approaching A+ accuracy is an exercise in futility.

“Maybe there will be huge breakthroughs in data analytics and machine learning over the next decade that [help us] do this with a high accuracy,” he says. But until then, humans may make better crime predictors than machines. After all, if a bunch of average Joe online recruits gave COMPAS a run for its money, criminal justice experts — like social workers, parole officers, judges or detectives — might just outperform the algorithm.

Even if computer programs aren’t used to predict recidivism, that doesn’t mean they can’t aid law enforcement, says Chelsea Barabas, a media researcher at MIT. Instead of creating algorithms that use historic crime data to predict who will reoffend, programmers could build algorithms that examine crime data to find trends that inform criminal justice research, Barabas and colleagues argue in a paper to be presented at the Conference on Fairness, Accountability and Transparency in New York City on February 23.

For instance, if a computer program studies crime statistics and discovers that certain features — like a person’s age or socioeconomic status — are highly related to repeated criminal activity, that could inspire new studies to see whether certain interventions, like therapy, help those at-risk groups. In this way, computer programs would do one better than just predict future crime. They could help prevent it.

Engineers are taking a counterintuitive approach to protecting future spacecraft: shooting at their experiments. The image above and high-speed video below capture a 2.8-millimeter aluminum bullet plowing through a test material for a space shield at 7 kilometers per second. The work is an effort to find structures that could stand up to the impact of space debris.

Earth is surrounded by a cloud of debris, both natural — such as micrometeorites and comet dust, which create meteor showers — and unnatural, including dead satellites and the cast-off detritus of space launches. Those pieces of flotsam can damage other spacecraft if they collide at high speeds, and bits smaller than about a centimeter are hard to track and avoid, says ESA materials engineer Benoit Bonvoisin in a statement.
To defend future spacecraft from taking a hit, Bonvoisin and colleagues are developing armor made from fiber metal laminates, or several thin metal layers bonded together. The laminates are arranged in multiple layers separated by 10 to 30 centimeters, a configuration called a Whipple shield.

In this experiment at the Fraunhofer Institute for High-Speed Dynamics in Germany, the first layer shatters the aluminum bullet into a cloud of smaller pieces, which the second layer is able to deflect. This configuration has been used for decades, but the materials are new. The next step is to test the shield in orbit with a small CubeSat, Bonvoisin says.

Nature may have a few things to teach tennis players about backspin.

The hairyflower wild petunia (Ruellia ciliatiflora) shoots seeds that spin up to 1,660 times per second, which helps them fly farther, researchers report March 7 in Journal of the Royal Society Interface. These seeds have the fastest known rotations of any plant or animal, the authors say. Plants that disperse seeds a greater distance are likely to be more successful in reproducing and spreading.
Glue that holds the flower’s podlike fruit together breaks down on contact with water, allowing the fruit to split explosively, launching millimeter-sized seeds. Little hooks inside the pod help fling these flattened discs at speeds of around 10 meters per second.

Using high-speed cameras that record 20,000 frames per second, the researchers analyzed the seeds’ flight. “Our first thought was: ‘Why doesn’t this throw like a Frisbee?’” says Dwight Whitaker, an applied physicist at Pomona College, in Claremont, Calif. Instead of spinning horizontally, most seeds spin counterclockwise vertically, like a bicycle wheel in reverse.

Whitaker and his colleagues calculated that backspin should help stabilize the seeds as they travel through the air, reducing drag. Experiments backed this up: Stable “spinners” had less drag on average than “floppers,” seeds that tumbled as they fell. Simulations predict that lower drag lets spinners travel 6.7 meters on average — more than twice as far on average as floppers.

Meet STEVE, a newfound type of aurora that drapes the sky with a mauve ribbon and bedazzling green bling.

This feature of the northern lights, recently photographed and named by citizen scientists in Canada, now has a scientific explanation. The streak of color, which appears to the south of the main aurora, may be a visible version of a typically invisible process involving drifting charged particles, or ions, physicist Elizabeth MacDonald and colleagues report March 14 in Science Advances.
Measurements from ground-based cameras and a satellite that passed when STEVE was in full swing show that the luminous band was associated with a strong flow of ions in the upper atmosphere, MacDonald, of NASA’s Goddard Space Flight Center in Greenbelt, Md., and colleagues conclude. But the researchers can’t yet say how a glow arises from this flow.

Part of a project called Aurorasaurus (SN Online: 4/3/15), the citizen scientists initially gave the phenomenon its moniker before its association with ion drift was known. MacDonald and colleagues kept the name, but gave it a backronym: “Strong Thermal Emission Velocity Enhancement.”

We’ll just stick with STEVE.

To craft a new color-switching material, scientists have again taken inspiration from one of nature’s masters of disguise: the chameleon.

Thin films made of heart cells and hydrogel change hues when the films shrink or stretch, much like chameleon skin. This material, described online March 28 in Science Robotics, could be used to test new medications or possibly to build camouflaging robots.

The material is made of a paper-thin hydrogel sheet engraved with nanocrystal patterns, topped with a layer of living heart muscle cells from rats. These cells contract and expand — just as they would inside an actual rat heart to make it beat — causing the underlying hydrogel to shrink and stretch too. That movement changes the way light bounces off the etched crystal, making the material reflect more blue light when it contracts and more red light when it’s relaxed.
This design is modeled after nanocrystals embedded in chameleon skin, which also reflect different colors of light when stretched (SN Online: 3/13/15).

When researchers treated the material with a drug normally used to boost heart rate, the films changed color more quickly — indicating the heart cells were pulsating more rapidly. That finding suggests the material could help drug developers monitor how heart cells react to new medications, says study coauthor Luoran Shang, a physicist at Southeast University in Nanjing, China. Or these kinds of films could also be used to make color-changing skins for soft robots, Shang says.

The center of the Milky Way may be abuzz with black holes. For the first time, a dozen small black holes have been spotted within the inner region of the galaxy in an area spanning just a few light-years — and there could be thousands more.

Astrophysicist Charles Hailey of Columbia University and his colleagues spotted the black holes thanks to the holes’ interactions with stars slowly spiraling inward, the team reports in Nature on April 4. Isolated black holes emit no light, but black holes stealing material from orbiting stars will heat that material until it emits X-rays.
In 12 years of telescope data from NASA’s orbiting Chandra X-ray Observatory, Hailey and colleagues found 12 objects emitting the right X-ray energy to be black holes with stellar companions. Based on theoretical predictions of how many black holes are paired with stars, there should be up to 20,000 invisible solo black holes just in that small part of the galaxy.
The discovery follows decades of astronomers searching for small black holes in the galactic center, where a supermassive black hole lives (SN: 3/4/17, p. 8). Theory predicted that the galaxy should contain millions or even 100 million black holes overall, with a glut of black holes piled up near the center (SN: 9/16/17, p. 7). But none had been found.
“It was always kind of a mystery,” Hailey says. “If there’s so many that are supposed to be jammed into the central parsec [about 3.26 light-years], why haven’t we seen any evidence?” Finding the 12 was “really hard,” he admits.

It’s unclear how the black holes got to the galaxy’s center. Gravity could have tugged them toward the supermassive black hole. Or a new theory from Columbia astronomer Aleksey Generozov suggests black holes could be born in a disk around the supermassive black hole.

The researchers ruled out other objects emitting X-rays, such as neutron stars and white dwarfs, but acknowledged that up to half of the sources they found could be fast-spinning stellar corpses called millisecond pulsars rather than black holes. That could add to the debate over whether a mysterious excess in gamma rays at the galactic center is from pulsars or dark matter (SN: 12/23/17, p. 12).

“The theorists are going to have to slug it out and figure out what’s going on,” Hailey says.

Every few years, a buzz fills the air in the southeastern United States as adolescent cicadas crawl out from the soil to molt and make babies. After a childhood spent sipping tree sap underground, some species emerge every 13 years, others every 17 years, rarely overlapping. Yet somehow in this giant cicada orgy, hybridization happens between species that should be out of sync.

Researchers have sought to explain how the two life cycle lengths developed. A new study published online April 19 in Communications Biology fails to pin the difference on genetics, but finds some interesting things along the way.
Cicadas fall into three species groups that diverged from one another about 3.9 million to 2.5 million years ago. Within each of those groups, species on a 13-year schedule diverged from 17-year-cycle cicadas about 200,000 to 100,000 years ago, the researchers from the United States and Japan report.

But the researchers also found that the 17-year and 13-year broods within each group share genetic code — evidence of hybridization. It’s possible that neighboring broods swapped DNA when their emergence overlapped — something that happens every 221 years — or if stragglers emerged early or late.