Cross two flashlight beams and they pass right through one another. That’s because particles of light, or photons, are mostly antisocial — they don’t interact with each other. But now scientists have spotted evidence of photons bouncing off other photons at the Large Hadron Collider at CERN, the European particle physics lab in Geneva.

“This is a very basic process. It’s never been observed before, and here it is finally emerging from the data,” says theoretical physicist John Ellis of King’s College London who was not involved with the study. Researchers with the ATLAS experiment at the LHC report the result August 14 in Nature Physics.
Because photons have no electric charge, they shouldn’t notice one another’s presence. But there’s an exception to that rule. According to quantum mechanics, photons can briefly transform into transient pairs of electrically charged particles and antiparticles — such as an electron and a positron — before reverting back to photons. Predictions made more than 80 years ago suggest that this phenomenon allows photons to interact and ricochet away from one another.

This light-by-light scattering is extremely rare, making it difficult to measure. But photons with more energy interact more often, providing additional chances to spot the scattering. To produce such energetic photons, scientists slammed beams of lead nuclei together in the LHC. Photons flit in and out of existence in the lead nuclei’s strong electromagnetic fields. When two nuclei got close enough that their electromagnetic fields overlapped, two photons could interact with one another and be scattered away.

To measure the interaction, ATLAS scientists sifted through their data to find collisions in which only two photons — the two that scattered away from the collision — appeared in the aftermath. “That’s the trickiest part of the whole thing,” says physicist Peter Steinberg of Brookhaven National Laboratory in Upton, N.Y., a member of the ATLAS collaboration. The scientists had to ensure that, in their enormous, highly sensitive particle detector, only two photons appeared, and convince themselves that no other particles had gone unaccounted for. The researchers found 13 such events over 19 days of data collection. Although other processes can mimic light-by-light scattering, the researchers predict that only a few such events were included in the sample.

The number of scattering events the researchers found agrees with the predictions of the standard model, physicists’ theory of particle physics. But a more precise measurement of the interaction might differ from expectations. If it does, that could hint at the existence of new, undiscovered particles.

Artificial intelligence is getting some better perspective. Like a person who can read someone else’s penmanship without studying lots of handwriting samples, next-gen image recognition AI can more easily identify familiar sights in new situations.

Made from a new type of virtual building block called capsules, these programs may cut down the enormous amount of data needed to train current image-identifying AI. And that could boost such technology as machine-made medical diagnoses, where example images may be scarce, or the responsiveness of self-driving cars, where the view is constantly shifting. Researchers with Google will present this new version of an artificial neural network at the Neural Information Processing Systems conference in Long Beach, Calif., on December 5.
Neural networks are webs of individual virtual nerve cells, or neurons, that learn to pick out objects in pictures by studying labeled example images. These networks largely classify pictures based on whether they contain certain features. For instance, a program trained on a series of head shots might conclude that a face has two eyes, a nose and a mouth. Show that program a face in profile with only one eye visible, though, and it may not recognize the photo as a face, explains Roland Memisevic, a computer scientist at the University of Montreal who was not involved in the work.

To overcome that limitation, researchers can train a neural network on millions of photos from myriad angles, and the program memorizes all the different ways a face might look. Compared with the human brain, which doesn’t need anywhere near a million examples to know what a face looks like, this system is wildly inefficient. “It’s a disaster,” Memisevic says. “Capsules try to fix that.”

Instead of webs of individual artificial neurons, these new programs have webs of clusters of neurons, called capsules. These teams of neurons can provide more information than one neuron by itself. Each capsule is designed to track not only whether a certain feature is in an image, but also properties of that feature — say, a nose’s size, orientation and position. This spatial awareness helps the program better recognize objects in previously unseen scenarios.

A capsule-containing network trained on head shots could see a face in profile and deduce — based on the appearance of the visible eye, nose and mouth — that the other eye is simply obscured, and the picture depicts a face. Since capsule networks are better at applying what they know to new situations, these neural networks need less training data to achieve the same performance as their predecessors, says Sara Sabour, a computer scientist with Google Brain in Toronto.
Sabour and her colleagues trained one capsule network on images of handwritten numbers and tested it on pictures where each number was slightly distorted. The capsule network recognized the warped images with 79 percent accuracy; a typical neural network trained on the same amount of data only got 66 percent right.

In another experiment, Sabour and colleagues trained a similar capsule network on tens of thousands of photos of toys, and then asked it to recognize the toys from new viewpoints. In this challenge, reported in a paper submitted to the 2018 International Conference on Learning Representations in Vancouver, the capsule network was wrong only about 1.4 percent of the time. A conventional neural network made almost twice as many errors.

PHILADELPHIA — Flat brains growing on microscope slides may have revealed a new wrinkle in the story of how the brain folds.

Cells inside the brains contract, while cells on the outside grow and push outward, researchers at the Weizmann Institute of Science in Rehovot, Israel, discovered from working with the lab-grown brains, or organoids. This push and pull results in folds in the organoids similar to those found in full-size brains. Orly Reiner reported the results December 5 at the joint meeting of the American Society for Cell Biology and the European Molecular Biology Organization.
Reiner and her colleagues sandwiched human brain stem cells between a glass microscope slide and a porous membrane. The apparatus allowed the cells access to nutrients and oxygen while giving the researchers a peek at how the organoids grew. The cells formed layered sheets that closed up at the edges, making the organoids resemble pita bread, Reiner said. Wrinkles began to form in the outer layers of the organoids about six days after the mini brains started growing.

These brain organoids may help explain why people with lissencephaly — a rare brain malformation in which the ridges and folds are missing — have smooth brains. The researchers used the CRISPR/Cas9 gene-editing system to make a mutation in the LIS1 gene. People with lissencephaly often have mutations in that gene. Cells carrying the mutation didn’t contract or move normally, the team found.

Reiner and her colleagues aren’t the first to propose the push-pull idea for how brains fold. But the researchers were able to show the concept at work in their experimental system, says biophysicist Xavier Trepat of the Institute for Bioengineering of Catalonia in Barcelona, who was not involved in the study. “They really were able to reproduce the shape of what we all imagine the brain should look like,” he says. “It’s not a brain, but they see structures that look like it.”

There’s both inspiring and troubling news for holiday worshippers.

Unlike other historically Christian Western nations, the United States is not losing its religion, say sociologists Landon Schnabel of Indiana University Bloomington and Sean Bock of Harvard University. But America is becoming as polarized religiously as it is politically, the researchers report online November 27 in Sociological Science.

Intense forms of religion, such as Christian evangelicalism, have maintained their popularity for nearly 30 years, Schnabel and Bock find after analyzing almost 30 years of U.S. survey data. At the same time, moderate forms of religion, such as mainline Protestantism, have consistently lost followers.
Religious moderates’ exodus from their churches stems partly from a growing link between religion and conservative politics, exemplified by the rise of the religious right in the late 1980s, the researchers suspect. Political liberals and moderates who already felt lukewarm toward the religion of their parents increasingly report identifying with no organized religion, especially if leaders of their childhood churches have taken conservative stances on social issues. Many Americans still report that they believe in God and pray, so they haven’t turned to atheism, the scientists say.

Population trends also favor intense forms of religion, Schnabel holds. Mainline Protestantism’s decline from 35 percent of the U.S. population in 1972 — about 73.5 million people — to 12 percent in 2016 — nearly 39 million people — reflects low fertility rates among these Protestants and limited numbers of new adherents from immigration and conversion. Opposite trends among U.S. evangelicals helped their form of intense Christianity surge from 18 percent of the population in 1972 to a steady level of about 28 percent from 1989 to 2016.

“More moderate forms of organized religion could become increasingly irrelevant in the United States,” Schnabel says.
The new findings play into an academic debate about the fate of religion in modern societies. Some scholars argue that in wealthy nations marked by scientific advances, religion inevitably withers. National surveys in 13 other Western, historically Christian nations show a general weakening of religious beliefs, even among intense believers, since 1991, the researchers find. But Schnabel and Bock are among those who view the United States as an exception where intense religion holds steady and even many of those leaving churches keep their faith.

The researchers examined data from nationally representative surveys on religion and other topics conducted from 1989 to 2016 by the General Social Survey, or GSS, a project of the National Opinion Research Center at the University of Chicago. GSS surveys include approximately 1,500 people annually.

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The proportion of the U.S. population citing strong ties to any religion held steady at around 36 percent during the study period. But the share of adults identifying themselves as religiously unaffiliated rose from around 9 percent to around 20 percent of the population, the researchers report. In another sign of loosening religious ties, those who never attended religious services rose from around 14 percent to around 25 percent of the population. Occasional attendance dropped from about 80 percent to about 70 percent.

Still, those who rarely or never prayed remained at about 24 percent of the population from 1989 to 2016. People who prayed several times a day rose from around 24 percent to about 30 percent of the total.

A belief in the Bible as God’s literal word held steady at roughly one-third of Americans. A view of the Bible as inspired by a higher power but not literal fell slightly to just under half of the population. Those tagging the Bible as a book of fables rose from around 15 percent to around 22 percent.

The new findings underscore the growing polarization of U.S. religion, say Michael Hout of New York University and Claude Fischer of the University of California, Berkeley. In a 2014 report based on GSS data, the two sociologists found that most political liberals and some political moderates who weakly identified with their parents’ religion have increasingly said that they prefer no particular religion. That trend was most pronounced for those reporting that the church they grew up with had become an advocate of politically conservative positions. Many of those people expressed a qualified belief in God, endorsing neither atheism nor absolute certainty in a higher power’s existence. Political conservatives, including those who seldom attended services or had doubts about church doctrine, had no complaints about religious leaders’ conservative political pronouncements.

Members of the millennial generation born since 1990 report low levels of religious involvement regardless of their politics, Hout adds. Millennials are skeptical of institutions in general although most still believe in God, he says. “Millennials are more comfortable with do-it-yourself religion than none at all.”

Sociologists David Voas of University College London and Mark Chaves of Duke University disagree. Millennials are part of a larger U.S. trend in which each successive generation over nearly the last century has reported slightly less intensity of religious belief than the one before, Voas and Chaves reported in a 2016 analysis of GSS data. For instance, in 2014, only 45 percent of U.S. adults ages 18 to 30 had no doubts that God exists versus 68 percent of those age 65 or over.

“The proportion of intensely religious Americans is being eroded, albeit very slowly,” Voas contends.

NEW ORLEANS — Despite its smooth appearance, the sun’s wispy outer atmosphere is surprisingly full of knots, whorls and blobs.

Newly analyzed observations from NASA’s STEREO spacecraft show that the sun’s outer corona is just as complicated as the highly structured inner corona, solar physicists reported December 12 at the fall meeting of the American Geophysical Union. That previously unseen structure could help solve some of the sun’s biggest puzzles, including how the solar wind is born and why the corona is so much hotter than the solar surface.
The corona is made up of charged plasma, which roils in famous loops and fans that follow magnetic field lines emerging from the surface of the sun (SN Online: 8/17/17). At a certain distance from the sun, though, that plasma escapes the corona and streams through the solar system as the solar wind, a constant flow of charged particles that pummels the planets, including Earth (SN Online: 8/18/17).

But solar physicists don’t know where the plasma gets enough energy to accelerate away from the massive, magnetic sun. And they don’t understand why the corona, which sizzles at several million degrees Celsius, has such higher temperatures than the solar surface, which chills at a mere 5,500° C (SN Online: 8/20/17).

Both problems might be cleared up by better understanding an energetic process called reconnection, which happens when magnetic field lines merge when they get too close to each other. Reconnection releases energy and helps move plasma around, so the process could be important to heating the corona and driving solar wind.

But in the best observations until now, the outer corona appeared smooth and uniform. To explain that smoothness, field lines would have to keep their distance from each other without a lot of reconnection. What’s more, physicists couldn’t tell where the boundary between the corona and the solar wind began, which might help to find that missing energy source.
“That’s changed,” solar physicist Craig DeForest of the Southwest Research Institute in Boulder, Colo., said at the AGU meeting. “Using STEREO, we’ve recently been able to drill in deeply enough to see the transition at the outer edge of the corona, where the dynamics change from what we might call coronal plasma to what we might call the young solar wind plasma.”

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DeForest and colleagues collected data for three days with STEREO in 2014 to gain more detail about small-scale changes in the outer corona than previously obtained. The researchers also processed the resulting images in a new way to bring those changes into focus.

Surprisingly, the team found that the outer corona is full of moving blobs and fine streams of plasma that vary in density by a factor of 10, suggesting that the magnetic field lines there are moving and merging more than scientists thought. “It turns out the apparent smoothness is a reflection of our instruments, not the corona itself,” DeForest says. “There’s almost certainly reconnection in the outer corona.”

The researchers also found that the corona probably fades into the solar wind between 14 million and 56 million kilometers away from the sun — about 10 to 40 times the sun’s diameter. That’s still a big range, but NASA’s Parker Solar Probe spacecraft, scheduled to launch in 2018, will fly right through that boundary. The probe will swoop within 6.4 million kilometers of the sun and take the first direct measurements of the corona — and perhaps figure out more precisely where the corona becomes the solar wind.

For now, the STEREO observations “are just tantalizing hints at an entire new set of phenomena,” DeForest says. Understanding the details of those processes “is going to require both careful analysis from Parker Solar Probe and also new, better imaging instruments.”

Solar physicist Steven Cranmer of the University of Colorado Boulder, who has made simulations of magnetic reconnection in the outer corona, finds the results exciting. Questions about the sun’s hot corona and the acceleration of the solar wind are still unsolved “not because of a lack of ideas, but because there are too many ideas,” he says. “I think we’re getting close to having the data that will let us rule out a good swath of these proposed ideas.”

Newly described little scaly bits could push back the fossil record of the moth-and-butterfly branch on the tree of life by some 70 million years. That raises the question of whether the drinking-straw mouthparts evolved long before the flower nectar many drink today.

The microscopic ridged scales date from roughly 200 million years ago, around the time of one of Earth’s less famous mass extinctions, says fossil-pollen specialist Bas van de Schootbrugge of Utrecht University in the Netherlands. During an unrelated study of ocean oxygen during this dire time, he and his colleagues pulled up cores of sediment in northern Germany near Braunschweig from what had once been a huge lagoon. In the sediment lay mere dots of insect scales.
Comparing the ridges and inner structure of the scales with those from modern insects suggests the fossils came from the evolutionary branch of insects that today gives us moths and butterflies with nectar-sipping mouthparts. No recognizable mouthparts appeared in the sediment. Yet the early existence of distinctive scales might mean this moth-butterfly drinking organ, a proboscis, evolved before the explosion of the classic flowering plants that offer nectar for pollination, van de Schootbrugge and colleagues propose January 10 in Science Advances.
The land already had plants: ferns, mosses and their relatives growing under trees that formed just-about naked seeds, without cushy protective ovaries and other floral coddling. Naked-seeded plants, many of them wind-pollinated such as pines and other conifers, thrive today. But the great evolutionary burst of true flowers—magnolias, roses, legumes, asters and the whole multicolored rainbow — that many moths and butterflies pollinate had yet to arise.These fossils date from a turbulent time when the great land mass called Pangea was cracking into continents. As the Triassic Period ended and the Jurassic dawned, volcanic eruptions on the straining land spewed greenhouse gases and toxins that changed the atmosphere and climate.
The previous record-holder for earliest moth-butterfly fossils came from about 130 million years ago, a bit after a major expansion of flowering plants. But when coauthor Timo van Eldijk, also at Utrecht, compared the newly found insect scales with those from silverfish, beetles and other scaly insects, modern scales of a big branch of the moth-butterfly lineage proved the best match.
In the times of the ancient scales, generally hot and dry conditions might have favored mouthparts specialized for drinking whatever liquids were to be found, the researchers propose.

Other work on how this proboscis evolved proposes that early moths started with chewing mouthparts and ate spores and pollen, says Harald W. Krenn of the University of Vienna. He and colleagues have proposed an intermediate phase of a short, tubelike structure good for slurping up droplets such as “honeydew” copiously excreted by sap-feeding aphids. A big question, though, is when early moths might have evolved such a drinking convenience.

The notion that the moth mouthparts arose before a big floral takeover sounds plausible to paleoecologist Conrad Labandeira of the Smithsonian Institution in Washington, D.C. Drinking-straw mouthparts had evolved in at least three other big insect groups (dipteran flies, lacewings and scorpionflies) somewhat before the full floral evolutionary extravaganza. Even some of the ancient naked-seeded plant groups, such as cycads, secrete nutritious droplets from reproductive structures that modern insects visit.

Interpreting the scales as a sign of an early moth proboscis is “possible,” says taxonomist Erik van Nieukerken of the Naturalis Biodiversity Center in Leiden, the Netherlands, whose specialties include early moths. There are other possibilities, too, for imagining ancient moth mouthparts, he cautions. Saying definitely that the newfound scales reveal the dawn of the proboscis might be “a bit too quick.”

A pair of ancient Egyptian mummies, known for more than a century as the Two Brothers, were actually half brothers, a new study of their DNA finds.

These two, high-ranking men shared a mother, but had different fathers, say archaeogeneticist Konstantina Drosou of the University of Manchester in England and her colleagues. That muted family tie came to light thanks to the successful retrieval of two types of DNA from the mummies’ teeth, the scientists report in the February Journal of Archaeological Science: Reports. The finding highlights the importance ancient Egyptians placed on maternal lines of descent, Drosou’s group contends.
Questions have swirled about the biological backgrounds of the mummified men ever since they were found together in a tomb near the village of Rifeh in 1907. The tomb dates to ancient Egypt’s 12th Dynasty, between 1985 B.C. and 1773 B.C. Coffin inscriptions mention a female, Khnum-Aa, as the mother of both men. And both mummies are described as sons of an unnamed local governor. It has always been unclear if those inscriptions refer to the same man, but discoverers decided the mummies were full brothers, because the two were buried next to each other and had the same mother.

Over time, differences discovered in the men’s skull shapes and other skeletal features raised suspicions that the Two Brothers were not biologically related at all. And some researchers argued that the inscriptions indicating the men had the same mother were misleading.

Adding to those doubts, a 2014 paper reported differences between the two mummies’ mitochondrial DNA, suggesting one or both had no biological link to Khnum-Aa. Mitochondrial DNA typically gets inherited from the mother.

But that study extracted ancient DNA from liver and intestinal samples using a method susceptible to contamination with modern human and bacterial DNA, Drosou’s team argues. In the new work, researchers isolated and assembled short pieces of mitochondrial and Y-chromosome DNA from both mummies’ teeth using the latest methods. The Y chromosome determines male sex and gets passed from father to son. This approach minimizes potential contamination from modern sources (SN Online: 5/31/17).
That new DNA evidence “proves the hieroglyphic text [on the mummies’ coffins] to be accurate,” at least in saying the mummified men had the same mother, says Egyptologist and study coauthor Campbell Price, curator of the Egypt and Sudan collections at the Manchester Museum in England.

Unlike the deference given to Khnum-Aa as a named parent of both interred individuals, he says, the coffin inscriptions must refer to different fathers who were considered peripheral family members and thus left unnamed. “Power may have been transferred down the female line rather than simply by a son inheriting [high rank] from his father,” Price suggests. Khnum-Aa’s background, social standing and genetic makeup, however, remain a mystery.

Genetic evidence that two half brothers were buried in the same tomb and placed in coffins that name only their mother makes sense, says Egyptologist Joann Fletcher at the University of York in England. Many written sources from ancient Egypt show precedence to the maternal line, “from the official lists of Egypt’s early kings whose names are accompanied by those of their mothers to nonroyal individuals, who likewise cite only their mother’s name,” Fletcher explains.

Dates of death on the mummies’ linen wrappings suggest that Khnum-Nakht died first, at around age 40, Price says. A few months later, Nakht-Ankh died at about age 60. The causes of their deaths are unknown.

Unless you are a recent arrival from another universe, you’ve no doubt heard that this one is expanding. It’s getting bigger all the time. What’s more, its growth rate is accelerating. Every day, the universe expands a little bit faster than it did the day before.

Those day-to-day differences are negligible, though, for astronomers trying to measure the universe’s expansion rate. They want to know how fast it is expanding “today,” meaning the current epoch of cosmic history. That rate is important for understanding how the universe works, knowing what its ultimate fate will be and even what it is made of. After all, the prime mission of the Hubble Space Telescope when it was launched in 1990 was to help determine that expansion rate (known, not coincidentally, as the Hubble constant, named for the astronomer Edwin Hubble).
Since then evidence from Hubble (the telescope) and other research projects has established a reasonably precise answer for the Hubble constant: 73, in the units commonly used for this purpose. (It means that two independent astronomical bodies separated by 3.26 million light-years will appear to be moving away from each other at 73 kilometers per second.) Sure, there’s a margin of error, but not much. The latest analysis from one team, led by Nobel laureate Adam Riess, puts the Hubble constant in the range of 72–75, as reported in a paper posted online January 3. Considering that as late as the 1980s astronomers argued about whether the Hubble constant was closer to 40 or 90, that’s quite an improvement in precision.

But there’s a snag in this success. Current knowledge of the universe suggests a way to predict what the Hubble constant ought to be. And that prediction gives a probable range of only 66–68. The two methods don’t match.

“This is very surprising, I think, and very interesting,” Riess, of the Space Telescope Science Institute in Baltimore, said in a talk January 9 at a meeting of the American Astronomical Society.

It’s surprising because astrophysicists and cosmologists thought they had pretty much figured the universe out. It’s made up of a little bit of ordinary matter, a lot of some exotic “dark matter” of unknown identity, and even more of a mysterious energy permeating the vacuum of space, exerting gravitational repulsion. Remember that acceleration of the expansion rate? It implies the existence of such energy. Because nobody knows what it is, people call it “dark energy,” while suspecting that its real name is lambda, the Greek letter that stands for “cosmological constant.” (It’s called a constant because any part of space should possess the same amount of vacuum energy.) Dark energy contributes something like 70 percent of the total mass-energy content of the universe, various lines of evidence indicate.
If all that’s right, then it’s not all that hard to infer how fast the universe should be expanding today. You just take the recipe of matter, dark matter and dark energy and add some ghostly subatomic particles known as neutrinos. Then you carefully measure the temperature of deep space, where the only heat is the faint glow remaining from the Big Bang. That glow, the cosmic microwave background radiation, varies slightly in temperature from point to point. From the size of those variations, you can calculate how far the radiation from the Big Bang has been traveling to reach our telescopes. Combine that with the universe’s mass-energy recipe, and you can calculate how fast the universe is expanding. (You can, in fact, do this calculation at home with the proper mathematical utensils.)

An international team’s project using cosmic microwave background data inferred a Hubble constant of 67, substantially less than the 73 or 74 based on actually measuring the expansion (by analyzing how the light from distant supernova explosions has dimmed over time).

When this discrepancy first showed up a few years ago, many experts believed it was just a mirage that would fade with more precise measurement. But it hasn’t.

“This starts to get pretty serious,” Riess said at the astronomy meeting. “In both cases these are very mature measurements. This is not the first time around for either of these projects.”

One commonly proposed explanation contends that the supernova studies are measuring the local value of the Hubble constant. Perhaps we live in a bubble, with much less matter than average, skewing expansion measurements. In that case, the cosmic microwave background data might provide a better picture of the “global” expansion rate for the whole universe. But supernovas observed by the Hubble telescope extend far enough out to refute that possibility, Riess said.

“Even if you thought we lived in a void…, you still are basically stuck with the same problem.”

Consequently it seems most likely that something is wrong with the matter-energy recipe for the universe (technically, the cosmological standard model) used in making the expansion rate prediction. Maybe the vacuum energy driving cosmic acceleration is not a cosmological constant after all, but some other sort of field filling space. Such a field could vary in strength over time and throw off the calculations based on a constant vacuum energy. But Riess pointed out that the evidence is growing stronger and stronger that the vacuum energy is just the cosmological constant. “I would say there we have less and less wiggle room.”

Another possibility, appealing to many theorists, is the existence of a new particle, perhaps a fourth neutrino or some other relativistic (moving very rapidly) particle zipping around in the early universe.

“Relativistic particles — theorists have no trouble inventing new ones, ones that don’t violate anything else,” Riess said. “Many of them are quite giddy about the prospect of some evidence for that. So that would not be a long reach.”

Other assumptions built into the current cosmological standard model might also need to be revised. Dark matter, for example, is presumed to be very aloof from other forms of matter and energy. But if it interacted with radiation in the early universe, it could have an effect similar to that of relativistic particles, changing how the energy in the early universe is divided up among its components. Such a change in energy balance would alter how much the universe expands at early times, corrupting the calibrations needed to infer the current expansion rate.

It’s not the first time that determining the Hubble constant has provoked controversy. Edwin Hubble himself initially (in the 1930s) vastly overestimated the expansion rate. Using his rate, calculations indicated that the universe was much younger than the Earth, an obvious contradiction. Even by the 1990s, some Hubble constant estimates suggested an age for the universe of under 10 billion years, whereas many stars appeared to be several billion years older than that.

Hubble’s original error could be traced to lack of astronomical knowledge. His early overestimates turned out to be signals of a previously unknown distinction between different generations of stars, some younger and some older, Riess pointed out. That threw off distance estimates to some stars that Hubble used to estimate the expansion rate. Similarly, in the 1990s the expansion rate implied too young a universe because dark energy was not then known to exist and therefore was not taken into account when calculating the universe’s age.

So the current discrepancy, Riess suggested, might also be a signal of some astronomical unknown, whether a new particle, new interactions of matter and radiation, or a phenomenon even more surprising — something that would really astound a visitor from another universe.