Shoreless Seas, Stars Uncounted

Shoreless Seas, Stars Uncounted[1]

A fantastical float down the Milky Way.

The realm of fairy-story is wide and deep and high and filled with many things: all manner of beasts and birds are found there; shoreless seas and stars uncounted; beauty that is an enchantment, and an ever-present peril; both joy and sorrow as sharp as swords. In that realm a man may, perhaps, count himself fortunate to have wandered, but its very richness and strangeness tie the tongue of a traveler who would report them.

— J. R. R. Tolkien, “On fairy-Stories”

We journey down the Heavenly River, the Milky Way. As in the quote from Tolkien above, the “richness and strangeness” of this realm in the sky are so great they defy our attempts to describe them.

The fantastic Milky Way river. One of the major points of Tolkien’s essay is that the wonder at the heart of fantasy can sometimes be found even in its simplest stories, those usually considered to be aimed at children. But who would have thought that this wonder, or a large part of it, could also be found in the section of the Milky Way from Scutum to Sagittarius?

Do we have in the summer Milky Way “all manner of beasts and birds” as we do in Faerie (the realm at the heart of fairy-story)? Certainly. We have constellation beasts and birds galore—Scorpius (the scorpion), Sagittarius (half-man/half-horse), Serpens (the serpent), Delphinus (the dolphin), Vulpecula (the fox), Cygnus (the swan), Aquila (a soaring eagle), and Lyra (not just a lyre, but at one time, a stooping eagle or vulture). We also have a bevy of strange “beasts” from the “astrophysical zoo”—things like the variable stars Chi (χ) Cygni and Beta (β) Lyrae, and the visible remnants of the most bizarre stellar corpses, like the Dumbbell and Ring Nebulae, derived from solar-mass stars dying to become white dwarfs, and the more elusive Veil Nebula, derived from a massive star dying in a supernova to become a neutron star or black hole.

The summer Milky Way also has “stars uncounted” and, if not “shoreless seas,” at least the shoreless bank of a celestial river, for the edges of the Milky Way band fade off imperceptibly. Are there in the Milky Way joys and sorrows as sharp as swords? Well, with larger aperture telescopes there are innumerable blade-edge sharp images of the stars that incite joy in the observer. And you know in winter there’s a sword—Orion’s—as sharp as joys or sorrows.

Scutum to Sagittarius: a Milky Way river torrent of grandeur. Do we go over a kind of waterfall when we reach the Scutum Star Cloud with its foreground telescopic “avalanche of stars” (or spate of stars) called the star cluster Messier 11? Do we then pour down past the equilateral triangle formed by the Gamma Scuti Star Cloud, M16 (the Eagle Nebula), and M17 (the Omega Nebula)? And onward, through the intense bright “foam” of M24 (the Small Sagittarius Star Cloud), flanked by the open clusters M25 and M23, until we arrive at a vision of the broadly spread central bulge (or river delta?) of the Milky Way with M20 (the Trifid Nebula), M8 (the Lagoon Nebula), and the gorgeous Large Sagittarius Star Cloud? And don’t forget Sagittarius’s supreme globular cluster, M22, which hangs just upper left of the star at the top of the Sagittarius Teapot, and the pair of big naked-eye clusters levitating above Scorpius’s stinger, MG and M7.

River Anduin or path of souls? Tolkien’s fantasy river Anduin has a waterfall, Rauros. Downstream stand the grand cities Osgiliath and Minas Tirith. But maybe the Milky Way more closely resembles the path that dead souls take to their final destination near Antares as envisioned by some Native Americans. If the latter is true, then what do we make of Mars and Saturn passing through that high holy ground this year? (2016)

[1] Fred Schaaf, “Shoreless Seas, Stars Uncounted,” Sky and Telescope (132, 3, September, 2016, p. 45).


Neutron Stars Collide

Neutron Stars Collide and Are Seen[1]

The image below is linked to a video explaining the kilonova merging of two neutron stars.

For the first time, astronomers have seen and heard a pair of neutron stars collide in a crucible of cosmic alchemy.

Watch in Times Video »

Astronomers announced on Monday, October 16, 2017 that they had seen and heard a pair of dead stars collide, giving them their first glimpse of the violent process by which most of the gold and silver in the universe was created.

The collision, known as a kilonova, rattled the galaxy in which it happened 130 million light-years from here in the southern constellation of Hydra, and sent fireworks across the universe. On Aug. 17, 2017 the event set off sensors in space and on Earth, as well as producing a loud chirp in antennas designed to study ripples in the cosmic fabric. It sent astronomers stampeding to their telescopes, in hopes of answering one of the long-sought mysteries of the universe.

Such explosions, astronomers have long suspected, produced many of the heavier elements in the universe, including precious metals like gold, silver and uranium. All the atoms in your wedding band, in the pharaoh’s treasures and the bombs that destroyed Hiroshima and still threaten us all, so the story goes, have been formed in cosmic gong shows that reverberated across the heavens.

This gong show happened when a pair of neutron stars, the shrunken dense cores of stars that have exploded and died, collided at nearly the speed of light. These stars are masses as great as the sun packed into a region the size of Manhattan brimming with magnetic and gravitational fields.

An artist’s rendering of the merger of two neutron stars from Aug. 17. Credit Robin Dienel/The Carnegie Institution for Science

Studying the fireball from this explosion, astronomers have concluded that it had created a cloud of gold dust many times more massive than the Earth, confirming kilonovas as agents of ancient cosmic alchemy.

“For the first time ever, we have proof,” said Vicky Kalogera, an astronomer at Northwestern University.

She was one of thousands of astronomers that reported their results Monday (October 16, 2017) in a globe-girdling set of news conferences and academic conferences.

A blizzard of papers is being published, including one in Astrophysical Journal Letters that has some 4,000 authors . “That paper almost killed the paperwriting team,” said Dr. Kalogera, one of 10 people who did the actual writing.

An artist’s rendering of a neutron star compared with the skyline of Chicago. Neutron stars are about 12 miles in diameter and are extremely dense. Credit Daniel Schwen/Northwestern, via LIGO-Virgo

More papers are appearing in Nature, Physical Review Letters and in Science, on topics including nuclear physics and cosmology.

“It’s the greatest fireworks show in the universe,” said David Reitze of the California Institute of Technology and the executive director of the Laser Interferometer Gravitational-Wave Observatory, or LIGO.

Daniel Holz, an astrophysicist at the University of Chicago and a member of the LIGO Scientific Collaboration, a larger group that studies gravitational waves, said, “I can’t think of a similar situation in the field of science in my lifetime, where a single event provides so many staggering insights about our universe.”

The key to the discovery was the detection of gravitational waves, emanating like ripples in a pond vibrating the cosmic fabric, from the distant galaxy. It was a century ago that Albert Einstein predicted that space and time could shake like a bowl of jelly when massive things like black holes moved around. But such waves were finally confirmed only in 2016, when LIGO recorded the sound of two giant black holes colliding, causing a sensation that eventually led this month to a Nobel Prize.

For the researchers, this is in some ways an even bigger bonanza than the original discovery. This is the first time they have discovered anything that regular astronomers could see and study. All of LIGO’s previous discoveries have involved colliding black holes, which are composed of empty tortured space-time—there is nothing for the eye or the telescope to see.

But neutron stars are full of stuff, matter packed at the density of Mount Everest in a teaspoon. When neutron stars slam together, all kinds of things burst out: gamma rays, xrays, radio waves. Something for everyone who has a window on the sky.

“Joy for all,” said David Shoemaker, a physicist at the Massachusetts Institute of Technology who is the spokesman for the LIGO Scientific Collaboration.

This is the story of a gold rush in the sky.

It began on the morning of Aug. 17, Eastern time. Dr. Shoemaker was on a Skype call when alarms went off. One of the LIGO antennas, in Hanford, Wash., had recorded an auspicious signal and sent out an automatic alert. Twin antennas, in Washington and Livingston, Louisiana, monitor the distance between a pair of mirrors to detect the submicroscopic stretching and squeezing of space caused by a passing gravitational wave. Transformed into sound, the Hanford signal was a long 100-second chirp, that ended in a sudden whoop to 1000 cycles per second, two octaves above middle C. Such a high frequency indicated that whatever was zooming around was lighter than a black hole.

Checking the data from Livingston to find out why it had not also phoned in an alert, Dr. Shoemaker and his colleagues found a big glitch partly obscuring the same chirp.

Meanwhile, the Fermi Gamma-Ray Space Telescope, which orbits Earth looking at the highest-energy radiation in the universe, recorded a brief flash of gamma rays just two seconds after the LIGO chirp. Fermi sent out its own alert. The gamma-ray burst lasted about two seconds, which put it in a category of short gamma ray bursts, which astronomers suspect are neutron stars colliding.

“When we saw that,” Dr. Shoemaker said, “the adrenaline hit.”

Dr. Kalogera, who was in Utah hiking and getting ready for August’s total solar eclipse when she got the alarm, recalled thinking: “Oh my God, this is it. This 50-year-old mystery, the holy grail, is solved.”

Together the two signals told a tale of a pair of neutron stars spiraling around each other like the blades

But where?

Luckily the European Virgo antenna had joined the gravitational wave network only two weeks before, and it also showed a faint chirp at the same time. The fact that it was so weak allowed the group to localize the signal to a small region of the sky in the Hydra constellation that was in Virgo’s blind spot.

The hunt was on. By then Hydra was setting in the southern sky. It would be 11 hours before astronomers in Chile could take up the chase.

One of them was Ryan Foley, who was working with a team on the Swope telescope run by the Carnegie Institution on Cerro Las Campanas in Chile. His team made a list of the biggest galaxies in that region and set off to photograph them all systematically.

The fireball showed up in the ninth galaxy photographed, as a new bluish pinprick of light in the outer regions of NGC 4993, a swirl of stars about 130 million light-years from here. “These are the first optical photons from a kilonova humankind has ever collected,” Dr. Foley said.

Within 10 minutes, another group of astronomers, led by Marcelle Soares-Santos of Brandeis University and using the Dark Energy Camera, which could photograph large parts of the sky with a telescope at the nearby Cerro Tololo Interamerican Observatory, had also spotted the same speck of light.

Emails went flying about in the Chilean night.

When it was first identified, the fireball of 8,000-degree gas was about the size of Neptune’s orbit and radiating about 200 million times as much energy as the sun.

Nine days later, the orbiting Chandra X-ray Observatory detected x rays coming from the location of the burst, and a week after that, the Very Large Array in New Mexico recorded radio emissions. By then the fireball faded from blue to red.

From all this, scientists have begun patching together a tentative story of what happened in the NGC 4993 galaxy.

“It’s actually surprising how well we were able to anticipate what we’re seeing,” said Brian David Metzger, a theorist at Columbia University who coined the term kilonova back in 2010.

As they tell it, the merging objects were probably survivors of stars that had been orbiting each other and had each puffed up and then died in the supernova explosions in which massive stars end their luminous lives some 11 billion years ago, according to an analysis by Dr. Kalogera. Making reasonable assumptions about their spins, these neutron stars were about 1.1 and 1.6 times as massive as the Sun, smack in the known range of neutron stars.

As they approached each other swirling a thousand times a second, tidal forces bulged their surfaces outward. Quite a bit of what Dr. Metzger called “neutron star guts” were ejected and formed a fat doughnut around the merging stars.

At the moment they touched, a shock wave squeezed more material out of their polar regions, but the doughnut and extreme magnetic fields confined the material into an ultra-high-speed jet emitting a blitzkrieg of radiation, the gamma rays.

As the jet slowed down, encountering interstellar gas in the galaxy, it began to glow in x rays and then radio waves.

The subatomic nuggets known as neutrons meanwhile were working their cosmic alchemy. The atoms in normal matter are mostly empty space: a teeny tiny nucleus of positively charged protons and electrically neutral neutrons enveloped in a fluffy cloud of negatively charged electrons. Under the enormous pressures of a supernova explosion, however, the electrons get squeezed back into the protons turning them into neutrons packed into a ball denser than an atomic nucleus.

The big splat liberates these neutrons into space where they inundate the surrounding atoms, transmuting them into heavy elements. The radioactivity of these newly created elements keeps the fireball hot and glowing.

Dr. Metzger estimated that an amount of gold equal to 40 to 100 times the mass of the Earth could have been produced over a few days and blown into space. In the coming eons, it could be incorporated into new stars and planets and in some far, far day become the material for an alien generation’s jewels.

The discovery filled a long-known chink in the accepted explanation of how the chemistry of the universe evolved from pure hydrogen and helium into the diverse place it is today. Stars and supernovas could manufacture the elements up to iron or so, according to classic papers in the 1950s but heavier elements required a different thermonuclear chemistry called r-process and lots of free neutrons floating around. Where would they have come from?

One idea was neutron star collisions, or kilonovas, which now seem destined to take their place on the laundry list of cosmic catastrophes along with the supernova explosions and black hole collisions that have shaped the history of the universe.

Until now there was only indirect evidence of kilonovas. Astronomers found a fireball from a gamma-ray burst in 2013, but there was no proof that neutron stars were involved. Now astronomers know they are, completing the picture of the origin of bling.

One burning question is what happened to the remnant of this collision. According to the LIGO measurements, it was about as massive as 2.6 suns. Scientists say that for now they are unable to tell whether it collapsed straight into a black hole, formed a fat neutron star that hung around in this universe for a few seconds before vanishing, or remained as a neutron star. They may never know, Dr. Kalogera said.

Neutron stars are the densest form of stable matter known. Adding any more mass over a certain limit will cause one to collapse into a black hole, but nobody knows what that limit is.

Future observations of more kilonovas could help physicists understand where the line of no return actually is.

Dr. Holz, the University of Chicago astrophysicist, said, “I still can’t believe how lucky we all are,” reciting a list of fortuitous circumstances. They had three detectors running for only a few weeks, and it was the closest gamma-ray burst ever recorded and the loudest gravitational wave yet recorded. “It’s all just too good to be true. But as far as we can tell it’s really true. We’re living the dream.”


[1] See Dennis Overbye, “LIGO Detects Fierce Collision of Neutron Stars for the First Time,” New York Times (October 16, 2017). Accessed at  The associated video may be linked at as well as from the image at the top, or from the link in the body of the article.

Why Study Astronomy

That’s Astronomy, Too[1]

A student needing science credits to graduate flips through the catalog. He considers biology for a moment, but that sounds squishy. Chemistry sounds smelly. Geology sounds. . . well, how much fun could rocks be? He doesn’t even glance at physics.

That leaves astronomy. “Stars? I can do stars. Sign me up?”

The irony is that astronomy is physics, chemistry, geology, biology, and most other kinds of science you can think of. Throw in some history, politics, math, computer science, engineering, and philosophy for good measure. Astronomy is an any-port-in-a storm science. Astronomers don’t get to put a star in a laboratory where we can poke it and prod it under controlled conditions. We have to work with what nature gives us. Astronomy requires its devotees to be clever and to draw on absolutely everything that we know.

Humankind’s historical conception of the universe was built on two pillars. The first was that Earth is the center of all things. The second was the belief that the heavens are other. From Hindus and Buddhists in the East to the Greeks in the West, our ancestors spoke of the four classical elements: earth, air, fire, and water. But there was a heavenly fifth element as well, described by Aristotle as unchanging and incorruptible. To this day we call the perfect example “quintessential,’ literally “made of the fifth element.”

It is perhaps ironic that grasping the reality of the universe meant standing those traditional beliefs on their heads. We aren’t the center. We are residents of an ordinary planet, orbiting an ordinary star in the disk of an ordinary spiral galaxy. And rather than other, the heavens are the same. The “principle” that terrestrial physics applies throughout the universe is actually a testable scientific theory. It is corroborated every time we observe familiar features in the spectrum of a distant galaxy or use computer models to build a virtual star with properties that match the real thing. The universal applicability of physical law is so ingrained today that we forget what a radical and world-changing idea it was.

That brings us back to that first day of class when, wearing a puckish smile, I would disavow students of the notion that by taking astronomy they had avoided all the hard stuff.

Physics is everywhere in astronomy. From the interaction of electromagnetic radiation with matter to the theories of space time that describe the fabric of the universe, they don’t call it astrophysics for nothing! Likewise, interplanetary dust, molecular clouds, and the oxidation that gives Mars its red color are chemistry. Thoughts about extraterrestrial life are guided by what we know of terrestrial biology and evolution.

Comparative planetology is the cornerstone of modern planetary science. Starting with the geology, atmospheric physics, and chemistry of Earth, we look at other worlds and study how they are similar and how they are different. Comparative planetology is a two-way street. What we have learned from our sister worlds, along with the tools developed to explore them, has revolutionized the way we think about our own planet.

You can’t talk meaningfully about astronomy without grappling with historical, social, and philosophical currents like those present at the birth of the Renaissance. We revere Copernicus, Galileo, Newton, and others because their discoveries about the heavens changed the course of civilization.

Astronomy benefits from technology, but it also has driven innovation from the dawn of time. Imagine the new technologies needed to build Stonehenge! Physics was invented in large part to explain planetary motions. More recently you might know that Riccardo Giacconi won the 2002 Nobel Prize in Physics “for pioneering contributions to astrophysics, which have led to the discovery of cosmic x-ray sources.” You might not know that x-ray astronomers are responsible for the technologies that form the heart of x-ray machines at airport security checkpoints and the CT scans that remade medicine.

Astronomy is mind-bendingly cool, but it’s not comfort able. It demands that we change how we think about everything. To claim to know things about the distant universe, we have to carefully consider what knowledge is in the first place. We have to be willing to put even our most cherished notions on the chopping block. And we have to broaden our perspective. When Apollo 8 astronauts took the famous photo of Earth rising above the lunar horizon, it marked the first time human eyes saw our seemingly limitless and inexhaustible world as it truly is: a small, beautiful, fragile oasis adrift in space.

Astronomy is the study of the cosmos. If you run across something that is not part of the cosmos, be sure and let me know!

From time to time someone will ask me why an astronomer would spend so much time thinking about philosophy, history, evolution, climate science, cognition, and on down the list. I always give the same reply.

“Because that’s astronomy too.”

[1] Jeff Hester, “That’s Astronomy, Too,” Astronomy (44, 3, March 2016, p 14). Jeff Hester is a keynote speaker, coach, and astrophysicist. Follow his thoughts at .

Why We Need Dark Mattter

Why We Need Dark Matter[1]

All of the matter we see around us—in our own bodies, in everyday objects, in planets, stars, nebulae, and galaxies—amounts to just 4.9 percent of the mass-energy content of the universe. Astronomers know this from recent studies of the cosmic microwave background, a remnant of the very first light in the cosmos, from the European Space Agency’s Planck spacecraft.

Gravitational lensing caused by galaxies distorts the shape of galaxies farther away. From the distortion, the total mass
inside the galaxies causing the lensing can be inferred. Image credits: NASA

By contrast, a substance of unknown nature—dark matter—makes up 26.8 percent of today’s universe, outnumbering normal matter by better than 5 to 1. The remainder of the universe’s mass-energy comes in the form of dark energy, a negative pressure that has been accelerating the expansion of space over the past 5 billion years and will determine the ultimate fate of the cosmos (see “How The Universe Will End,” Astronomy, September 2014). Our story here will focus only on material matters.

Don’t look—it would, after all, be pointless—but there’s dark matter near you right now. It accounts for more than 80 percent of the matter in the universe, but it neither emits nor absorbs light and primarily interacts with the rest of the universe through gravity, the weakest force in nature. Astronomers see its effects throughout the cosmos—in the rotation of galaxies, in the distortion of light passing through galaxy clusters, in the web-like structure of the large-scale universe, and in the cosmic microwave background. They even see it in the abundances of light elements, like hydrogen and helium, produced during the first minutes of the expanding universe. None of these observations add up without some type of matter we can’t currently detect that was also moving comparatively slowly—that is, it’s cold—when structures began to form in the early universe.

In 2015, astronomers discovered a large, dim, nearly featureless galaxy named Dragonfly 44 in the Coma Cluster of galaxies about 330 million light-years away. Dragonfly 44 is one of a growing number of large, low-surface-brightness, featureless objects called ultra-diffuse galaxies now being discovered by the robotic Dragonfly Telephoto Array, a project led by Pieter van Dokkum at Yale University and Roberto Abraham at the University of Toronto.

What is unusual about this galaxy is that it contains so few stars that it should simply shear apart in the hurly-burly environment of a galaxy cluster. Using the Keck II and Gemini North telescopes on Mauna Kea, Hawaii, Dokkum and his colleagues studied the galaxy’s stellar motions to determine its mass[2]. The galaxy has a mass comparable to the Milky Way’s, but 99.99 percent of it is in the form of dark matter. The team suggests Dragonfly 44 may be a failed version of the Milky Way, perhaps one stripped of the gas needed to build stars in its youth through processes we don’t yet understand.

The most detailed computer simulations of the evolution of cosmic structure require daark matter in order to form galaxies at all. Small dark matter halos condense first, and their gravitational pull draws in normal matter, creating small galaxies that collide and merge to form larger ones. This “bottom up” assembly process implies astronomers should be seeing far more left over debris—such as dwarf galaxies and streams of stars stripped from them—than we see around large galaxies like the Milky Way today. Ultra-diffuse galaxies like Dragonfly 44 offer a new possibility for reconciling theory and observation.

Crashing clusters

Galaxies can congregate into groups as small as a few dozen members up to rich objects like the Coma Cluster, containing some 10,000 galaxies. With typical sizes spanning tens of millions of light-years and masses reaching 100 trillion Suns or more, galaxy dusters are the largest objects held together by gravity in the universe. Gas with temperatures exceeding 20 million degrees Fahrenheit—hot enough to glow in x rays—fills these objects and typically accounts for more than twice the mass of the galaxies. Even with this, these cosmic behemoths couldn’t stay together without a heaping helping of dark matter, typically about 85 percent of the duster’s mass. Indeed, galaxy motions in the Coma and Virgo clusters provided the first hints that dark matter must be present within them.

Colliding clusters offer an opportunity for astronomers to explore how these various components interact. The Bullet Cluster, about 3.8 billion light-years away toward the constellation Carina, is a small cluster that has passed through a larger duster in the past 200 million years, moving at about 10 million mph. In 2000, x-ray observations by NASA’s Chandra X-ray Observatory revealed a conical cloud of x-ray gas—a bow shock lagging behind the interloper. While the galaxies of both clusters simply passed by each other without interacting, their vast inventory of hot gas experienced a drag force akin to air resistance (called ram-pressure stripping) and was wrenched away from the galaxies and, presumably, their dark matter halos.

The dark matter in clusters is so massive that it distorts space-time in a noticeable way, warping the light from more distant objects. Using a technique called weak lensing, astronomers measured the distorted shapes of background galaxies to map out the distribution of dark matter in both clusters. Another technique, called strong lensing, looks for repeated warped images of background galaxies to accomplish the same goal. Astronomers have employed both methods on the Bullet

Cluster with the same results: The dark matter halos line up with the galaxies of each cluster and show no indication of interaction. Similar studies of other interacting clusters demonstrate the presence and dominance of dark matter in these objects.

Beyond The Standard Model

Particle physicists had their own reasons for positing the existence of new particles, many of which seem ready-made to play the role of dark matter. The so-called standard model of particle physics, developed in the early 1970s, describes a wide range of phenomena by the interactions of fundamental particles (for example, electrons and quarks) through three of the four known fundamental forces (electromagnetism, and the strong and weak forces operating in atomic nuclei). The standard model’s most recent success is the 2012 discovery of the Higgs particle in proton-smashing runs at the Large Hadron Collider (LHC) in Europe, the most powerful particle accelerator on the planet.

But the standard model doesn’t include gravity, the weakest force, and there remain other aspects of the universe it cannot explain, such as why matter is so much more common than antimatter. So physicists have devised a number of extensions to try to address these issues. The most popular solution, called super-symmetry, predicts a massive partner for each particle in the standard model. The lightest supersymmetric particle predicted is the neutralino, which appealed to astronomers because its properties matched their favorite dark matter model, the weakly interacting massive particle (WIMP). WIMPs interact with normal matter rarely and only through the weak nuclear force. In some versions, WIMPs serve as their own antiparticle, annihilating when they collide and giving off gamma rays that can be detected by space-based observatories, such as NASA’s Fermi Gamma-ray Space Telescope.

Physicists expected supersymmetric particles to start appearing in high-energy collisions at the LHC, but to the frustration of many, nothing new has turned up. “From the point-of-view of supersymmetric dark matter, I think the situation is becoming increasingly tight,” says Troy Porter, an astrophysicist at Stanford University. “There is no evidence from the LHC and, so far, no detection of gamma-ray emission from places where there could be a chance for a relatively ‘clean’ astrophysical signal,” such as dwarf galaxies orbiting the Milky Way, which have been monitored by Fermi. Detectors like the Large Underground Xenon experiment, located nearly a mile below Lead, South Dakota, similarly have nothing to show.

“There are two very different ways to read the current situation,” says Dan Hooper, an astrophysicist at Fermilab in Batavia, Illinois. “On the one hand, no underground detectors or the LHC have seen anything that looks like a WIMP, constraining the range of models that are consistent with the data.” This has had the effect of moving the dark matter community toward “hidden sector” models, where dark matter particles interact differently from normal matter with standard model forces, and other scenarios that are more difficult to detect. “If these experiments continue to come up empty-handed, the move away from conventional WIMPs will become only more and more evident,” he says.

On the other hand, Fermi sees residual gamma-ray emission at energies greater than 2 billion electron volts (GeV) from the center of our galaxy, which is both a nearby source and expected to hold a large store of dark matter. “This represents the most promising possible signal of dark matter particles that we have had to date,” says Hooper. “If future observations strengthen the case for this interpretation, then the community will refocus back to WIMPs, and in particular toward those WIMP models that can generate a signal like that seen from the galactic center.”

Gamma-Ray Eyes

While the great amount of dark matter expected at the galactic center should produce a strong signal, it has to compete with many other gamma-ray sources, such as pulsars and cosmic ray interactions with interstellar gas. That’s why the gamma-ray “excess” Fermi sees at the galactic center remains far from a slam dunk.

Astronomers recently used Fermi data to investigate the possibility that dark matter might consist of hypothetical particles called axions or other particles with similar properties. These particles don’t require supersymmetry and rank highly among dark matter candidates. An intriguing aspect of axion-like particles is their ability to convert into gamma rays and back again when they interact with strong magnetic fields. These conversions would leave behind characteristic traces, like gaps or steps, in the spectrum of a bright gamma-ray source.

In 2015, Manuel Meyer at Stockholm University led a search for these effects in gamma rays from NGC 1275 (Perseus A), the central galaxy of the Perseus galaxy duster, located about 240 million light-years away. Magnetic fields threading the duster could enable gamma rays emitted by the galaxy to switch into axion-like particles as they make their way to us. Meyer’s team searched for predicted distortions in the Fermi data and was ultimately able to exclude a small range of axion-like particles that could have made up about 4 percent of dark matter.

Regina Caputo at the University of California, Santa Cruz, sought these signals from the Small Magellanic Cloud (SMC), which is about 200,000 light-years away and is the second largest of the small satellite galaxies near the Milky Way. Part of the SMC’s appeal for a dark matter search is that it lies comparatively close to us, and its gamma-ray emission from conventional sources, like star formation and pulsars, is well understood. Most importantly, astronomers have high-precision measurements of the SMC’s rotation curve, which shows how its rotational speed changes with distance from its center and indicates how much dark matter is present. Caputo and her colleagues showed that the SMC possessed enough dark matter to produce detectable signals for two WIMP types. While Fermi definitely sees gamma rays from the galaxy, Caputo’s team can explain them all by conventional sources.

In another study, researchers led by Marco Aiello at Clemson University in South Carolina and Mattia Di Mauro at SLAC National Accelerator Laboratory in California took the search in a different direction. Instead of looking at specific astronomical targets, the team used more than 6.5 years of data to analyze the background glow of gamma rays seen all over the sky.

The nature of this light, called the extragalactic gamma-ray background (EGB), has been debated since NASA’s Small Astronomy Satellite 2 first measured it in the early 1970s. Fermi has shown that much of this light arises from unresolved gamma-ray sources, particularly galaxies called blazars, which are powered by material falling toward gigantic black holes in their centers. Blazars constitute more than half of the total gamma-ray sources seen by Fermi. EGB gamma rays could arise from distant interactions of dark matter particles, such as the annihilation or decay of WIMPs, but Ajello and his colleagues found that blazars and other discrete sources can account for nearly all of the emission.

“Fermi has done great in cutting into the parameter space”—that is, shrinking the theoretical box—“of dark matter models,” Ajello says. That’s because its Large Area Telescope surveys the whole sky every three hours, deepening its exposure with every orbit. WIMPs can produce gamma rays through a variety of mechanisms, such as converting into pairs of quarks, gluons, muons, and other particles, which then decay to emit gamma rays and stable particles. This provides scientists with many avenues to explore in the hunt for dark matter using Fermi. “Direct detections and collider searches test different aspects of dark matter and are complementary to indirect searches like Fermi’s,” he says. “These three approaches probe different regions of dark matter spaces, cutting even more into realistic models.”

Dark matter candidates may fall out of fashion, but everything we know about the universe seems to require the substance itself.

“I’d say that in the next few years 2 WIMPs could be more or Iess ruled out, but it’s quite plausible that dark matter is made up of whole families of particles,” says Caputo. “Think how diverse the standard model is—and it’s only 5 percent of the matter budget of the universe.”

[1] Francis Reddy, “Why We Need Dark Matter,” Astronomy (45, 11, November 2017, pp. 30-35)

[2] See “Dark Ages of Dark Matter,” from Astronomy (45, 11, November 2017, p. 35)

Dark Ages of Dark Matter

Dark Ages of Dark Matter[1]

In 1932, Dutch astronomer Jan Oort [pictured at left] analyzed the vertical motions of stars near the plane of our galaxy-and concluded the density of known stars was too low, suggesting unseen matter near the galactic pia ne was needed to explain them.

The following year, Swiss astronomer Totz Zwieky showed that the motions of galaxies in the Coma Cluster implied the presence of much more mass than could be accounted for by Its glowing galaxies alone. Zwicky suggested this extra mass was in the form of “dunkle (kalte) materie”—dark cold matter. In 1936, American astronomer Sinclair Smith found a similar discrepancy in the Virgo Cluster

Just a few years later came the first evidence of a “missing mass” problem in individual galaxies. Measurements published by the American astronomer Horace Babcock in 1939 showed that the outer parts of the Andromeda Galaxy (M31) were-rotating at a constant angular velocity. Astronomers assumed most of the mass of a spiral galaxy was concentrated at its center, which contained the highest concentration of stars.

In such a case, the rotational motion of stars in the galaxy’s outer reaches should decrease steadily when measured at greater distances from the center. Instead they were constant, which implied the mass enclosed at those distances was still increasing.

Although Babcock did not attribute the issue to missing mass, his study stands as one of the earliest indications of dark matter and foreshadows later work.

In 1970, Vera Rubin and Kent Ford, Jr. at the Carnegie Institution of Washington tracked M31’s rotation out to 78000 light-years from its center using optical and radio data, revealing near constant rotation in the outer regions, a finding confirmed in 1978. Over the next decade, Rubin, Ford, and other astronomers studied the rotation of hundreds of galaxies. Nearly all show similar flat rotational structure requiring that they reside within massive halos of dark matter.


[1] Francis Reddy, “Dark Ages of Dark Matter,” Astronomy (45, 11, November 2017, p. 35)

Creationism vs. Science a la Dan Brown’s Book

Creationism vs. Science a la Dan Brown’s Book[1]

Dan Brown has thrown off the doldrums of Inferno with a brisk new book that pits creationism against science, and is liable to stir up as much controversy as The Da Vinci Code did. In Origin, the brash futurist Edmond Kirsch comes up with a theory so bold, so daring that, as he modestly thinks to himself in Brown’s beloved italics, “It will not shake your foundations. It will shatter them.” Kirsch is of course addressing The World, because that’s the scale on which Brown writes.

And Kirsch is right. Millions of people learn of his shocking, religion-flouting ideas. Entire belief systems are thrown into jeopardy. Action is triggered—the kind that sends Brown’s hunky, beloved Harvard symbology professor, Robert Langdon, chasing all over Spain on Kirsch’s trail, accompanied by the inevitable beautiful and brilliant woman. As one admirer says to Langdon, the full flap this generates “reminds me of the Vatican denouncing your book Christianity and the Sacred Feminine, which, in the aftermath, promptly became a best seller.”

No need to be so modest. The book the Vatican fought in real life was The Da Vinci Code. It sold tens of millions of copies in dozens of languages, and has led to Brown’s selling hundreds of millions of books (not to mention movie tickets) around the world. The voice speaking to Langdon about his popularity is that of Winston, Kirsch’s A.I. avatar. Winston’s sensibilities are so highly developed that he sounds wiser than most people—which is a good thing, since he has to lead Langdon to many of the hoops through which Origin makes him leap.

Origin grows out of questions raised by scientists who adopt atheism in a world where strict creationism has less and less relevance. The novel doesn’t paint Kirsch as an enemy of religion, though its prologue does show him arriving threateningly at a scenic abbey in Montserrat to challenge three religious leaders just after a meeting of the Parliament of the World’s Religions.

We soon cut to the Guggenheim Museum, Bilbao, in Spain, where Langdon sports the white tie and tails he wore nearly 30 years ago as a member of the Ivy Club at Princeton. Being fit and studly (and who would want it otherwise?), he swims enough daily laps for the tails to fit. A large crowd has been summoned by Kirsch to hear his earthshaking announcement, and of course the 40-year-old genius wants his favorite professor to be present. So Langdon kills time staring aghast at the modern art, which Brown describes in detail. This book does some spectacularly funny stalling in order to postpone the moment of truth. It also plants a bitterly troubled assassin in the crowd. An assassin who means to do God’s will and deludes himself by thinking (in italics, naturally): “I have returned from the abyss.”

Does anyone think Kirsch will get through 100 pages of introductory fanfare and make his big announcement unscathed?

Now Langdon, still in tails, dashes out of the museum alongside its director, the beautiful Ambra Vidal. She happens to be the fiancée of Spain’s Prince Julien, who will soon be king. (Brown has made up his own Spanish royal family.) Her clout, Kirsch’s money and Winston’s disembodied smarts empower the two runaways to go anywhere in their search for … what? Touristy sites to keep the book interesting, for starters. Barcelona is big on the agenda because its Gaudi architecture eerily embodies Kirsch’s theories about the intersection of science and nature; because it poses fabulous challenges to Brown’s fascination with logistics; and because everyone seems to have forgotten how hard it is for a man wearing tails to clamber all over the place without getting tied up in knots.

The Sagrada Família, the towering, incomplete Gaudi cathedral, is one of several inspired physical embodiments of the serious ideas this book means to contemplate. (Brown and serious ideas: they do fit together, never more than they have in Origin.) Here in this unfinished building, God and science mysteriously coexist in bizarrely engineered spires and the flora and fauna sculpted to climb the foundations.

But in the world of quantum computing, where Kirsch’s earlier pioneering work had broken boundaries, the divine was harder to apprehend. The book’s final destination reveals the essence of what Kirsch saw and created, and it inspires awe. Getting there is worth the roundabout journey.

Part of the fun in reading Brown comes from not taking him too seriously as a stylist. He brings to mind Joseph Heller’s Yossarian in Catch-22, who has the job of censoring letters and turns it into an arbitrary game. There are Brown sentences that could happily lose their modifiers: “The grisly memory was mercifully shattered by the chime of the jangling bar door.” There are phrases that beg you to ask friends to fill in the blanks: “Clear and penetrating, ____ _ ____.” (Like a bell.) There’s an air of overstatement that’s more gleeful than egregious, but it can’t be mistaken for good. And the hyperbole is sometimes the stuff of giggles: “I am not exaggerating when I tell you that my discovery will have repercussions on the scale of the Copernican revolution.” If Brown didn’t mean that, his books wouldn’t be so well worth waiting for.

Then there are the tricks. All that symbology he and Langdon bring to the game is never without its gee-whiz excitement. Brown has told The Times that he loved the Hardy Boys books, and it shows. The hunt here for a 47-character password yields the niftiest feat of gamesmanship in the book, as does Langdon’s self-important analysis of what looks like an exotic symbol on a car’s window. It appears to be something that not even an expert of his caliber has ever seen before. It’s not.

Brown loves winking at Langdon, the literally dashing version of himself, and inviting readers to share the joke. And for all their high-minded philosophizing, these books’ geeky humor remains a big part of their appeal. Not for nothing does Kirsch’s Tesla have a license plate frame reading: “THE GEEKS SHALL INHERIT THE EARTH.” Brown continues to do everything in his playful power to ensure that will happen.

[1] Janet Maslin, “In Dan Brown’s Origin, Robert Langdon Returns, With an A.I. Friend in Tow,” New York Times (October 3, 2017). Accessed at A version of this review appears in print on October 5, 2017, on Page C1 of the New York edition with the headline: Atlas, Buckling a Little.

Bird Photobombs the Sun

Bird Photobombs the Sun[1]

A bird (flying up the center of the image) crosses the face of the Sun at the same time as the International Space Station (diagonal path) in this composite image released Oct. 4, 2017, by the European Space Agency. The astronomy club at the agency’s European Space Astronomy Centre near Madrid, Spain, took the photo in 2013.

Talk about timing! Photographers with the European Space Agency hoping to spot the International Space Station crossing the face of the Sun got more than they bargained for in this sunny snapshot.

In the photo, which was taken in 2013 but only released by ESA on October 4, 2017), a bird crosses the Sun at the same exact time as the space station. The entire photo session lasted just 1.2 seconds.

“It requires planning, patience and a measure of luck,” ESA officials wrote in an image description. “The camera must be set up at the right time in the right place to capture the Space Station as it flies past at 28,800 km/h [17,900 mph]. At such speeds the photographer has only seconds to capture the transit, and if any clouds block the view, [they’d have] to wait for another opportunity weeks later.”

While the skies were clear of clouds for this station-sun photo, at least one feathered flyer was in the viewing area.

“The station flies around Earth at [a distance of] around 400 kilometers [about 250 miles], allowing the astronomy club to calculate that the bird was flying 86 meters [280 feet] from the camera lens,” ESA officials wrote. “The difference in size and distance makes both the bird and the space station appear the same size.”

When the space station crosses the face of the Sun, it is known as solar transit. Such transits can also occur for other planets or stars, or with Earth’s Moon, as in this case of this stunning view at left of the station crossing the nearly full Harvest Moon captured by photographer Alexander Krivenyshev.

[1] Tariq Malik, “Bird Photobombs The Space Station And Sun in Awesome Photo,” (October 10, 2017), accessed at . Tariq Malik is Managing Editor. Email Tariq Malik at or follow him @tariqjmalik and Google+. Follow us @Spacedotcom, Facebook and Google+. Original article on