New spacey stories: killer asteroids, stellar DNA, and rogue planets

I meant to post at least once a month, but I think you’ll understand that it’s hard to find the time while juggling new freelance science writing work with a 9-month-old kiddo at home. Anyway, here’s a couple pieces I’ve published over the past month. Enjoy! (See links below for the full articles.) As usual, thanks go to my helpful editors: Heather D’Angelo, Lisa Grossman, Lauren Morello and Jane Lee.


Maybe Dark Matter Didn’t Kill the Dinosaurs after All

Artist’s impression of the Chicxulub impact. Credit: Donald E. Davis, via Wikimedia Commons

A giant asteroid or comet the size of a city smashed into the Yucatán 66 million years ago, likely causing the demise of dinosaurs and many other species. Scientists have wondered: is that a random, unfortunate event, or has life on Earth been subjected to periodic impacts from outer space?

Some researchers proposed that, if the dinosaur extinction — the last of five mass extinctions — had an astronomical origin, rather than being driven by volcano eruptions or global warming, for example, then maybe others did too. And if impacts from huge boulders of rock and ice drove these extinctions, they had to come from somewhere. It’s possible that dark matter could periodically dislodge distant comets from their tenuous orbits beyond Pluto, sending a few of them dangerously in Earth’s direction — thus linking the fates of dark matter and dinosaurs.

But a new study by a team of physicists and geologists from Durham University and Lancaster University in the United Kingdom appears to shoot down that dark matter interpretation. If it were true, extinctions would have happened in cycles. But these scientists pored over the fossil record over the past 500 million years, looking for extinctions occurring periodically, but they didn’t find any significant patterns like that in the data.

“We needn’t search the heavens to find reasons for these extinction events. The vast majority of them are due to Earth processes, not astronomical ones,” says David Harper, lead author of the study.

The dark matter idea, popularized by Lisa Randall’s 2015 book, “Dark Matter and the Dinosaurs,” might sound far-fetched. But before this study, it was more plausible. Our solar system resides in the middle of the Milky Way galaxy, which has a disk-like structure. It turns out that the solar system doesn’t just stay put; gravity pulls it up and down through the disk, like a pendulum. Lurking in the outskirts of the solar system, comets in what’s known as the Oort Cloud slowly orbit the sun, whose gravity barely holds them on their trajectories. A pass through the galaxy does change the gravitational forces on them, but not enough to let comets loose.

Instead, Randall speculates that within the same plane as the Milky Way is a much thinner and denser disk of dark matter that we can’t see. (Most astrophysicists think dark matter particles only clump up in sphere-like conglomerations, but one flavor of dark matter could form disks.) Then as the solar system passes through that disk every 32 million years or so, it’s as if the dark matter’s gravity provides a little extra tug, nudging a few comets out of their orbits.

While some untethered comets get flung away, never to be seen again, others head toward Earth. Over time, these would produce periodic blips in the Earth’s history of both craters and mass extinctions, which wouldn’t occur randomly. It’s a new incarnation of an older idea, where earlier astronomers suggested the possibility of a faraway solar companion dubbed Nemesis, which would provide the extra nudge, but it was never found.

Harper and his colleagues performed something called a time-series analysis, looking for subtle cycles in the data that would corroborate Randall’s hypothesis. After removing a background trend and running statistical tests, they found that the extinctions don’t occur periodically on any time-scale.

The risk of finding a pattern when there’s not one really there is enormous, says Michael Benton, an Earth scientist at the University of Bristol in the UK. “The fossil record is patchy, biased, and incomplete,” he says.

The structure of our Milky Way makes for another complication. The galaxy looks more like a fluffy pinwheel than a compact disk. It has spiral arms jutting and curving out while neighboring stars in the galaxy move to and fro, so as our solar system passes through the galaxy, its periodic motion will vary. Harper argues that this motion would then be too irregular to pull in more wayward Earthbound comets that result in mass extinctions.

Randall agrees that there’s more than one cause to extinctions on Earth. But she argues that if things like volcanic activity, plate tectonics, and climate change can’t explain them all, some may have been triggered by cosmic events. She and her colleagues developed a model of a dark matter disk which she says fits the crater record better than a bunch of random impacts.

As it turns out, another new study, unrelated to Harper’s, looks at the record of crater impacts with a similar kind of analysis. They come to the same conclusion: there’s currently no evidence for asteroids or comets periodically colliding with Earth.

“I actually like the idea of asteroid impacts. But from the data we have, all I can say is that it’s unlikely,” says Matthias Meier, the lead author and a cosmochemist at Swiss Federal Institute of Technology in Zurich.

It would help to have more and better crater data. Meier studied 22 craters, but there are nearly 190 known craters worldwide. Many of them were dated more than 50 years ago with earlier methods and aren’t very precise, but if one limits the sample to only the most accurate crater ages, there may be too few of them. Once scientists discover more craters and estimate more accurate ages of them, they could settle the debate.

“This study may mark the end of the speculation,” Benton says. Then after a pause, “No wait, it won’t. There’ll be plenty more, I’m sure.”

[Read the entire story in, published on 14 March 2017. Update (8 Sep. 2017): is unfortunately now defunct, so I’ve posted the story in full here.]

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Book review: “Dark Matter and the Dinosaurs” by Lisa Randall

On the one hand, we have the elusive dark matter particles, dispersed throughout the universe across billions of light-years; on the other, we have the sorely missed dinosaurs, who lived in our own proverbial backyard but were driven extinct by a mysterious impactor 66 million years ago. What if these fascinating yet disparate phenomena, separated by so much space and time, were somehow related?


That, in essence, is the premise of Lisa Randall’s book, “Dark Matter and the Dinosaurs.” Maybe the “vanilla” cold dark matter model we have isn’t the only possible explanation of observations of the expanding universe and the cosmic web of millions of surveyed galaxies, she argues. It’s more fun to consider other more exotic models, even if they turn out to be wrong.

Dark matter particles don’t interact with each other the way our familiar atoms do. In fact, they hardly interact at all. They mostly just move apart with the growing universe and then clump together as they feel the effects of gravity over time. As a result, we end up with nearly spherical dark matter clumps throughout the universe, and we and the rest of the Milky Way are living inside one of those clumps. But if some dark matter interacts like normal matter, it could form a dense and thin disk—even thinner than the disk of our own galaxy. (Picture a compact disk hidden inside a bagel. Here’s a good composite image of our galaxy, on edge, which would be the bagel.)

If that’s the case, then as our solar system moves up and down through the disk, we’ll experience an extra little gravitational nudge each time we go through. This could periodically dislodge comets traveling in tenuous orbits in the Oort cloud in the distant realms of our solar system, flinging one comet away forever and sending another in an unfortunate Earthbound direction, where the consequences of its destructive impact in the Yucatan kills off the dinosaurs some 66 million years ago, thus finally linking dinosaurs to dark matter.

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Inside Science: Dark Matter Particles, Cosmic Lenses, and Super-Earths

Here’s a few new stories I reported on and wrote for Inside Science News Service over the past couple weeks:


Physicists Look Beyond WIMPs For Dark Matter

Physicists are on the hunt for elusive dark matter, the hypothesized but as yet unidentified stuff that makes up a large majority of the matter in the universe. They had long favored “weakly interacting massive particles,” known as WIMPs, as the most likely dark matter candidate, but after an exhaustive search, some scientists are moving on to more exotic particles.

Most estimates suggest that there’s 5-6 times as much dark matter as there are things that we can see, such as galaxies, stars, and planets. Yet physicists know very little about what the mysterious dark matter particles actually are, as they cannot be directly observed and barely interact with normal matter.

New research leaves dwindling room for WIMPs, motivating a search for other particles that could fit the bill.

“The WIMPs are getting harsh experimental scrutiny, and may get ruled out,” said Kathryn Zurek, a physicist at Lawrence Berkeley National Laboratory in California. [Note: She later clarified that WIMPs may become more “strongly constrained” rather than “ruled out.”]

Physicists have used the Large Hadron Collider's ATLAS experiment to probe for potential dark matter particles. (Credit: CERN)

Physicists have used the Large Hadron Collider’s ATLAS experiment to probe for potential dark matter particles. (Credit: CERN)

Zurek and others presented ongoing work on dark matter alternatives to WIMPs in April at an American Physical Society meeting in Salt Lake City. “We should broaden the searchlight, and the natural place is to go lighter,” Zurek said.

She and her colleagues are looking into less massive particles that interact more weakly with ordinary matter. These include an array of particles with exotic names like “axion,” “sterile neutrino,” and “Higgsino,” a theoretical super-partner of the famous Higgs boson.

Axions are hypothetically abundant particles originally proposed in the 1970s to solve a problem with nuclear physics. In the presence of a powerful magnetic field, these minuscule particles, which are lighter than electrons, are predicted to turn into detectable photons. In spite of years of searching, however, they have yet to be found. But the Axion Dark Matter eXperiment, currently being upgraded, should definitely determine whether the particle exists, said Leslie Rosenberg of the University of Washington in Seattle.

Kevork Abazajian, a cosmologist at the University of California, Irvine, sees a new trend in the field over the past decade. “The new generation of early-career physicists is more open to dark matter other than WIMPs,” he said.

He argued that physicists should consider sterile neutrinos, which interact even more weakly than their neutrino counterparts. As they decay, the particles—which are tinier than electrons—could produce detectable X-ray radiation such as that observed in clusters of galaxies. But scientists struggle to distinguish between X-rays that could be emitted by sterile neutrinos versus traditional astrophysical events. Research along these lines suffered a setback when Japan’s powerful X-ray satellite Hitomi broke into pieces last month. But it may have accumulated limited science data before it was lost…

[For more, check out the entire story in Inside Science, published on 28 April 2016. Thanks to Chris Gorski and Sara Rennekamp for editing assistance.]

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As Galaxies’ Light Gradually Fades, the Universe is Slowly Dying!

The Universe, long past retirement at an age of 13.8 billion years, appears to be gradually “dying.” New observations strongly indicate that galaxies, vast collections of billions of stars such as our Milky Way and neighbors Andromeda and Triangulum, generate much less energy than they used to across the wavelength spectrum, a clear trend revealing the fading cosmos.

This composite picture shows how a typical galaxy appears at different wavelengths in the GAMA survey. The energy produced by galaxies today is about half what it was two billion years ago, and this fading occurs across all wavelengths. (Credit: ICRAR/GAMA and ESO.)

This composite picture shows how a typical galaxy appears at different wavelengths in the GAMA survey. The energy produced by galaxies today is about half what it was two billion years ago, and this fading occurs across all wavelengths. (Credit: ICRAR/GAMA and ESO.)

Scientists with the Galaxy and Mass Assembly (GAMA) survey, led by Simon Driver of the International Centre for Radio Astronomy Research in Australia, extensively and thoroughly examined more than 200,000 galaxies. Driver and his colleagues presented the results of their analysis at the general assembly of the International Astronomical Union (IAU) in Honolulu, Hawaii, which came to a close last weekend. Their announcement coincided with their data release and the submission of their paper to the journal, Monthly Notices of the Royal Astronomical Society. The paper has not yet been peer-reviewed or published, but the authors’ main conclusions are unlikely to change.

“While most of the energy sloshing around in the Universe arose in the aftermath of the Big Bang, additional energy is constantly being generated by stars as they fuse elements like hydrogen and helium together,” Driver said. “This new energy is either absorbed by dust as it travels through the host galaxy, or escapes into intergalactic space and travels until it hits something, such as another star, a planet, or, very occasionally, a telescope mirror.”

Stars of all ages throughout this multitude of galaxies convert matter into energy (remember E=mc2?) in the form of radiation ranging from ultraviolet to optical to infrared wavelengths, and astronomers have long known that the total energy production of the universe has dropped by more than a factor of 1.5 since its peak about 2.25 billion years ago. But GAMA scientists, utilizing the Anglo-Australian Telescope at Siding Spring Observatory in eastern Australia, were the first to document the declining energy output so comprehensively over 21 wavebands.

Check out this fly-through of the volume mapped out by the GAMA survey, which is expected to be approximately representative of the rest of the “nearby” universe, with the galaxies’ images enlarged (video courtesy of ICRAR/GAMA/Will Parr, Mark Swinbank and Peder Norberg (Durham University) and Luke Davies (ICRAR)):

The GAMA astronomers’ results point toward the universe’s continued “gentle slide into old age,” as Driver put it, but there is no need to panic! The time-scales involve billions of years, and we humans have only been around for about 100,000th of the universe’s lifespan so far. (That’s like the incredibly short lifetime of mayflies relative to ours.) We should be careful to note that the scientists’ conclusions come from a statistical assessment of numerous and diverse galaxies, similar to the way pollsters or census takers evaluate a population by studying a large number of its members. Individual galaxies and their stars may be young or old, but the general population continues to age with no indication of deviations from the demographic trend, much like the gradual aging of people in Japan.

Filled with galaxies and much more dark matter and much much more empty space, the universe rapidly expands and pulls objects away from each other, countering gravitational forces. Old stars within galaxies provide the fuel for new stars to form, but eventually it becomes harder and harder to scrape enough fuel together to make those new stars and galaxies, and on average the aging universe becomes fainter and fainter. It’s as if potential parents become increasingly unlikely to meet with random encounters and many ultimately die alone.

The universe will eventually pass away, but long after our sun has exploded in its red giant phase and destroyed the Earth and long after the Milky Way and Andromeda collide. I think the universe—and humans—has many more good years left though.

Happy Birthday to Vera Rubin, Discoverer of Dark Matter

Peering through their powerful telescopes, scientists observe a stunningly diverse array of phenomena, including comets, planets, stars, gaseous nebulae, novae, quasars, galaxies, and numerous other exciting things. But astrophysicists argue that these light-emitting objects only amount to a tiny fraction of the universe. According to the latest measurements from the European Space Agency’s Planck telescope earlier this year, they account for less than 5% of the universe’s matter and energy, while mysterious-sounding “dark matter” accounts for nearly six times as much. Nevertheless, dark matter cannot be seen and does not interact with normal matter, so how did astronomers figure out that so much invisible, intangible stuff exists out there?

Vera Rubin measuring spectra, circa 1970. (Credit: American Institute of Physics)

Vera Rubin measuring spectra, circa 1970. (Credit: American Institute of Physics)

As I recently wrote in a post for the International Year of Light, the story of scientists’ discovery and exploration of dark matter began many decades ago. Physicists had long utilized Newton’s and Einstein’s gravitational laws to estimate our sun’s mass by measuring planets’ distances from it and examining how fast they travel around it. For example, Mercury is very close to the sun and orbits it much faster than Pluto, which takes 248 Earth-years to complete an orbit. (If you’re wondering, the sun has a mass larger than a trillion billion billion kilograms. That’s a lot!) Similarly, it turns out that one can make such calculations for stars within galaxies and infer the enclosed mass, but the results of the analysis are not so simple to understand.

Detailed image of the Andromeda Galaxy, recently surveyed by the Panchromatic Hubble Andromeda Treasury
. (Credit: NASA, ESA, J. Dalcanton et al.)

Detailed image of the Andromeda Galaxy, recently surveyed by the Panchromatic Hubble Andromeda Treasury
. (Credit: NASA, ESA, J. Dalcanton et al.)

In the 1960s and 1970s, American astronomer Vera Rubin measured and analyzed the precise velocities of stars in spiral galaxies and came to a startling conclusion. Most stars at outer radii orbit the center at surprisingly large speeds, much faster than they should be based on the mass of the stars themselves, but the galaxies do not tear themselves apart or fling their stars hurtling away. Studying galaxies, such as Andromeda, as a whole, she found that they rotate too quickly for their stars’ gravity to keep them intact. It was as if the galaxies contain and are surrounded by much more unseen dark matter, which gravitationally binds the galaxies together. Rubin’s crucial discovery has not yet received the recognition it deserves.

This critically important area of research came to be known as galaxy “rotation curves,” in which Rubin became an influential figure. Rotation curve measurements of spiral galaxies from two of her many highly-cited publications appear in the reference, Galactic Astronomy, which every respectable astrophysicist has on their bookshelf. Her measurements from hundreds of galaxies constitute strong evidence for the existence of massive clumps of dark matter extending to many thousands of light-years beyond the edge of the galaxies themselves. Astrophysicists also considered the alternative hypothesis that Newton’s gravitational laws need to be modified for objects separated by large distances, but that approach has been less successful and lacks support among the community.

Rotation curves of three spiral galaxies of varying brightness, adapted from an influential 1985 paper by Rubin. (Credit: Binney & Merrifield, "Galactic Astronomy," Princeton, 1998.)

Rotation curves of three spiral galaxies of varying brightness, adapted from an influential 1985 paper by Rubin. (Credit: Binney & Merrifield, “Galactic Astronomy,” Princeton, 1998.)

Vera Rubin turns 87 years old today. She continues her work at the Department of Terrestrial Magnetism at the Carnegie Institution of Washington, and she still publishes research in galactic astronomy. In addition, she writes popularly such as in Scientific American and Physics Today. Moreover, she inspires, supports, and encourages young people, especially women, in science. This includes her four children, all of whom have earned Ph.D. degrees in the natural sciences or mathematics.

In 1965, Vera Rubin was the first woman permitted to observe at Palomar Observatory. When she applied to graduate schools, she was told that “Princeton does not accept women” in the astronomy program; she went to Cornell instead. As she put it in a recent astronomical memoir, “Women generally required more luck and perseverance than men did.” In her 1996 book, Bright Galaxies, Dark Matters, she wrote

Since the 1950s, opportunities for women in astronomy have increased, but serious problems have not disappeared…The saddest part, of course, is that only about one-fifth of the women who enter college intend to study science. Lack of support and encouragement at an early age has by then taken its toll. A young woman who enters graduate school to study science is a rare creature indeed…but the colleges are often a part of the problem rather than part of the solution.

Now with many years of hard work and persistence, people are making gradual progress. For example, Meg Urry leads the American Astronomical Society, France Córdova is the director of the National Science Foundation, and Marcia McNutt now heads the National Academy of sciences. But much more work needs to be done to reduce gender inequality and underrepresentation throughout science research and education.

Many people argue that Vera Rubin would be a strong contender for a Nobel Prize in Physics, and I join that call. She has already won many other awards, including the National Medal of Science, but the Nobel would officially recognize her enormous contributions to astrophysics and her critical role in illuminating the way to dark matter. Considering that the 2011 Nobel Prize went to Saul Perlmutter, Brian Schmidt, and Adam Riess for discovering dark energy, it’s time for dark matter to have its day.

Nine New Dwarfs Discovered in Our Local Group of Galaxies

Just as astronomers are examining dwarf planets, they’re investigating dwarf galaxies too. Two weeks ago, an international collaboration of scientists with the Dark Energy Survey (DES) peered around the southern hemisphere and announced in a paper in the Astrophysical Journal that they found candidates for nine new “satellite” galaxies around our Milky Way. For those of you keeping count—and many people are—if confirmed, this means that we now have 35 satellites in our Local Group of galaxies, which could even tell us something about the dark matter out there.

An illustration of the previously discovered dwarf satellite galaxies (in blue) and the newly discovered candidates (in red) as they sit outside the Milky Way. (Image: Yao-Yuan Mao, Ralf Kaehler, Risa Wechsler.)

An illustration of the previously discovered dwarf satellite galaxies (in blue) and the newly discovered candidates (in red) as they sit outside the Milky Way. (Image: Yao-Yuan Mao, Ralf Kaehler, Risa Wechsler.)

The smallest known galaxies (as might be inferred from their name), dwarf galaxies are extremely faint and difficult to detect, sometimes only containing a few hundred stars and appearing to blend in with the stars in the disk of the Milky Way. They can also be difficult to distinguish from globular clusters, which are just clumps of stars that evolved with a galaxy and orbit around its core.

Astrophysicists refer to galaxies that travel around a larger galaxy as “satellite” galaxies. In many cases, these galaxies were previously floating through space, minding their own business, until the gravitational force of the massive galaxy pulled them in. Some astronomers think that that is what happened to the Small Magellanic Cloud and Large Magellanic Cloud, the brightest satellites of the Milky Way. (The Persian astronomer Abd-al-Rahman Al-Sufi discovered the LMC in 964 A.D., and it does sort of look like a “cloud.”) To give these satellites some perspective, they’re mostly between 100,000-200,000 light-years away, while the Milky Way’s radius is about 50,000 light-years, which is already much longer than the road to the chemist’s.

Keith Bechtol (University of Chicago) and Sergey Koposov (University of Cambridge) led parallel studies with the DES, which uses an optical/infrared instrument on a telescope at the Cerro Tololo Inter-American Observatory in the Chilean mountains. “The discovery of so many satellites in such a small area of the sky was completely unexpected,” says Koposov. These findings only include the first-year data of the DES though, and the research team stands poised to discover as many as two dozen more satellite galaxies as they continue their survey.

Six of the nine newly discovered dwarf satellite galaxies. (V. Belokurov, S. Koposov. Photo: Y. Beletsky.)

Six of the nine newly discovered dwarf satellite galaxies. (V. Belokurov, S. Koposov. Photo: Y. Beletsky.)

In 2005-2006, Koposov and his colleagues (Vasily Belokurov, Beth Willman, and others) found about half of the previously detected satellite galaxies of the Milky Way with the Sloan Digital Sky Survey (SDSS), the DES’s predecessor in the northern hemisphere. The SDSS and DES are powerful enough to detect and resolve faint dwarf galaxies that hadn’t been observed before, transforming this field and stimulating interest in the Milky Way’s neighborhood.

Dwarf galaxies could reveal new information about dark matter, since their mass in stars is outweighed by thousands of times by the mass of dark matter particles surrounding them. Astrophysicists developing numerical simulations of growing clumps of dark matter, thought to host galaxies within them, have been concerned that more satellite clumps form in the simulations than satellite galaxies have been observed in the Milky Way–a discrepancy referred to as the “missing satellites” problem. It’s not clear yet whether the newly discovered satellite galaxy candidates could solve or complicate this problem. Moreover, astrophysicists continue to worry about other problems, including disagreements between observed galaxies and simulations involving the masses and angular momenta of dark matter clumps. In any case, scientists working with the DES continue to push the debate further, and their ongoing survey will be of great interest to the astronomical community.

A simulated dark matter "halo" with satellites, possibly similar to the Milky Way. (Credit: Volker Springel, Aquarius Simulation.)

A simulated dark matter “halo” with satellites, possibly similar to the Milky Way. (Credit: Volker Springel, Aquarius Simulation.)

For more coverage, check out this article by Monica Young in Sky & Telescope and articles in Wired and Washington Post. If you’re interested, you can also see my own earlier research on satellite galaxies in dark matter models and on the Magellanic Clouds.

New Science at the American Astronomical Society Meeting

I’d just like to summarize some of the exciting new scientific results presented at the American Astronomical Society meeting in Seattle last month. I think it will be interesting to those of you science lovers who’re wondering what all the hubbub was about and for you astronomers who weren’t able to make it.

This is my third and final post in a series about the AAS meeting. The first two dealt with science policy, and diversity and sustainability. As I mentioned in a previous post, I enjoyed attending as both a scientist and science writer, and I was happy to personally meet the journalists writing excellent stories about the meeting (some of which I’ve linked to below).

I’ll start with some special sessions and other sessions focused on interesting science that included results I hadn’t seen before, and then I’ll end with some interesting plenary talks given by great speakers. It was a busy meeting and many of the sessions ran in parallel, so it’s inevitable that I missed some things and that this summary is incomplete. (Plus, I’m usually drawn to the sessions about galaxies, dark matter, and cosmology, and I often miss the other ones.) If you know of interesting announcements or talks that I missed here, you’re welcome to comment on them below.

Sloan Digital Sky Survey (SDSS)

After 15 years of great science, it was exciting to see the SDSS have its final public data release—until SDSS-IV data eventually come out, that is. At the press conference, Michael Wood-Vasey gave an overview, Constance Rockosi spoke about the data release, Daniel Eisenstein spoke about the Baryon Oscillation Spectroscopic Survey (BOSS), Jian Ge spoke about the Multi-object APO Radial Velocity Exoplanet Large-area Survey (MARVELS), and Steven Majewski spoke about the APO Galactic Evolution Experiment (APOGEE). According to Rockosi, more than 6,000 papers have been published using publicly released SDSS data. The SDSS has observed tens of thousands of stars, hundreds of thousands of quasars, and millions of galaxies.

In addition, members of the BOSS collaboration presented (nearly) final results at a session dedicated to the survey. If you’re interested, check out this article I wrote about it for Universe Today. (Thanks to Nancy Atkinson for editing assistance.)

Distribution of galaxies in a slice of the BOSS survey. (Courtesy: SDSS-III)

Distribution of galaxies in a slice of the BOSS survey. (Courtesy: SDSS-III)


Researchers presented newly published results and interesting work-in-progress about the evolution of distant galaxies using spectroscopic data from the 3D-HST survey, which is led by Pieter van Dokkum (Yale Univ.) and Ivelina Momcheva (Carnegie Observatories), combined with imaging data from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS), taking advantage of instruments aboard the Hubble Space Telescope. The figure below shows the spectral features of tens of thousands of galaxies, which indicate star formation activity, active galactic nuclei activity, and stellar age. If you’re interested, I wrote an article about some of these results for Sky & Telescope. (Thanks to Monica Young for editing assistance.)

Spectral features of high-redshift galaxies. (Courtesy: Gabriel Brammer, 3D-HST)

Spectral features of high-redshift galaxies. (Courtesy: Gabriel Brammer, 3D-HST)

Andromeda Galaxy

In a session dedicated to Andromeda—known as M31 by astronomers—as well as in other related sessions, research scientists and Ph.D. students presented studies about the stars, globular clusters, molecular clouds, dust, structure, dynamics, surface brightness profile, and stellar halo of the galaxy. The continued interest in our fascinating neighbor is understandable; Andromeda’s only 2.5 million light-years away from our galaxy! Like our Milky Way, Andromeda is a spiral galaxy, and it’s the most massive galaxy in the Local Group.

Many of these AAS results came from the Panchromatic Hubble Andromeda Treasury (PHAT) Survey, which is led by Julianne Dalcanton (Univ. of Washington), who presented highlights in a press conference as well. Dalcanton and her colleagues released this PHAT panoramic image of the galaxy below, and it received well-deserved press attention, including in NBC and Sky & Telescope.

Map of Andromeda galaxy. (Courtesy: HST, PHAT)

Map of Andromeda galaxy. (Courtesy: HST, PHAT)


Many people were understandably excited about extra-solar planets, or exoplanets, detected by scientists with NASA’s Kepler space telescope. Every day of the meeting included talks and posters about the masses, abundances, dynamics, compositions and other properties of exoplanets as well as those of stars and supernova remnants examined with Kepler. In addition, astronomers’ announcement that they now have more than 1,000 confirmed exoplanets with Kepler and follow-up observations garnered considerable media attention (including these articles in Nature, BBC, and New York Times). They have at least 3,000 more planet candidates, and they will surely identify many more as Kepler continues its mission through 2016.

Of course, astronomers seek to find as many as possible Earth-like planets in or near the habitable regions orbiting Sun-like stars (often referred to as the “Goldilocks” zone). When these are successfully identified, the next step is to characterize their properties and try to assess the likelihood of life forming on them. Astronomers have found at least eight Earth-size planets in the habitable zone, including two of the newly announced ones, Kepler-438b and Kepler-442b. They also released these cool old-school travel posters. If you have a space ship that can travel 500 light-years a reasonable time, you should check out 186f on your next vacation!

Kepler's alien planet travel posters. (Courtesy: NASA)

Kepler’s alien planet travel posters. (Courtesy: NASA)

“Pillars of Creation”

On the 25th anniversary of the launch of the Hubble Space Telescope, astronomers released new images of the iconic star-forming region in the Eagle Nebula in the Serpens Cauda constellation, known as the “pillars of creation.” Journalists at Slate, CBS, and elsewhere shared these amazing images. At first I thought not much science was done with them, but by combining observations at visible and infrared wavelengths, astronomers can investigate what’s happening with the cold gas clouds and dust grains and assess how rapidly new stars are forming and where. For more, you can also see Hubble’s press release, which coincided with the press conference on the first day of the meeting.

Image of "pillars of creation." (Courtesy: NASA and ESA)

Image of “pillars of creation.” (Courtesy: NASA and ESA)

Other Results

I saw many other interesting talks and posters at the meeting, but I don’t have the time/space to get into them here. On galaxies and the large-scale structure of the universe (which I’m interested in), I saw talks involving modeling and measurements with the Galaxy And Mass Assembly (GAMA) survey, the Six-degree Field Galaxy Survey (6dF), and I presented research using the PRIsm MUlti-object Survey (PRIMUS). But the SDSS dominated the field.

In addition, Joss Bland-Hawthorn, Sarah Martell, and Dan Zucker presented some impressive early science results from the GALactic Archaeology with HERMES (GALAH) survey of the Milky Way, which uses an instrument with the Anglo-Australian Telescope. (GALAH is named after an Australian bird.) Astronomers combine GALAH observations with astrometry from Gaia and over the survey’s duration will produce detailed data for 1 million stars in our galaxy! In particular, they utilize a technique called “chemical tagging” to study the abundances of at least 15 chemical elements for each star, allowing for studies of stellar dynamics and merger events from infalling “satellite” galaxies. I look forward to seeing more results as they continue to take data and analyze them; their first public data release is planned for 2016.


I’ll briefly describe a couple of the plenary talks below, but I missed a few others that sounded like they could be interesting, including “The Discovery of High Energy Astrophysical Neutrinos” (Kara Hoffman); “Gaia – ESA’s Galactic Census Mission” (Gerry Gilmore); and “The Interactions of Exoplanets with their Parent Stars” (Katja Poppenhaeger).

Also, Paul Weissman (JPL/Caltech) gave an overview of the Rosetta mission and the comet C-G/67P, and Al Wootten (NRAO) gave an overview of many recent science papers using the Atacama Large Millimeter Array (ALMA). Rosetta and its lander Philae has run a few experiments already, and scientists with the mission have found that the bulk density of the nucleus is less than half the density of water ice and that its D/H ratio is different than the abundance ratio of the Earth’s oceans. More recently, Rosetta detected a crack in the “neck” of the comet, and they’ve abandoned an idea for a close flyby search for the lost lander, which might wake up in a few months when it receives more solar power. And if you’re interested in ALMA science, such as involving the gas kinematics of protostars and protoplanetary disks and the gas and dust clouds of distant galaxies, watch for proceedings from their recent Tokyo meeting, which are due to be published next month.

Cosmology Results from Planck

Martin White (UC Berkeley) gave an excellent talk about cosmological results from the Planck telescope, which he described as having the “weight of a heavy hippo and the height of a small giraffe.” Based on analyses of the power spectrum of the cosmic microwave background (CMB) radiation, so far it seems that the standard model of cold dark matter plus a cosmological constant (ΛCDM) is still a very good fit. Scientists in the collaboration are obtaining tighter constraints than before, and the universe still appears very flat (no curvature). They are planning a second data release this year, including more simulations to assess systematic uncertainties and more precise gravitational lensing measurements. White ended by saying, “I can explain to you what really well, but I can’t tell you why at all.”

White also hinted at, but didn’t reveal anything about, the joint analysis by Planck and BICEP2 astrophysicists. That analysis was completed recently, and now it seems that the detected polarization signal might be at best a mixture of primordial gravitational waves produced by inflation and of Milky Way dust, and they’ve obtained only an upper limit on the tensor-to-scalar ratio. Check out my recent article in Universe Today about this controversy.

Courtesy: ESA

Courtesy: ESA

Inflation and Parallel Universes

Max Tegmark (MIT) has talked and written about both inflation and the multiverse for many years, such as in a 2003 cover article and a recent blog post for Scientific American and in his book, “Our Mathematical Universe.” From the way he presented the talk, it was clear that he has discussed and debated these issues many times before.

Tegmark began by explaining models of inflation. According to inflation, the universe expanded for a brief period at an exponential rate 10-36 seconds after the Big Bang, and the theory could explain why the universe appears to have no overall curvature, why it approximately appears the same in all directions, and why it has structures of galaxies in it. In one entertaining slide, he even compared the expansion rate of a universe to that of a fetus and baby, but then he said, “if the baby kept expanding at that rate, you’d have a very unhappy mommy.”

Expansion rates of a baby (human) and a baby universe

Expansion rates of a baby (human) and a baby universe

He subtitled his talk, “Science or Science Fiction?”, and that question certainly came up. Tegmark argued that inflation seems to imply at least some levels of a multiverse (see his slide below), which makes many astrophysicists (including me) nervous and skeptical, partly because parallel universes aren’t exactly testable predictions. But he made the point that some general relativity predictions, such as about what happens in the center of a black hole, aren’t testable yet we accept that theory today. He discussed “modus ponens” arguments: once we accept “if p then q,” then if p is asserted, we must accept q, whether we like it or not. In other words, if inflation generally predicts parallel universes and if we accept inflationary theory, then we must accept its implications about parallel universes. This is an important issue, and it’s another reason why BICEP2 and Planck scientists are trying to resolve the controversy about polarization in the CMB.

Predictions of different levels of the multiverse.

Predictions of different levels of the multiverse.

The Dark and Light Side of Galaxy Formation

Finally, in another interesting talk, Piero Madau (UC Santa Cruz), who was recently awarded the Heineman Prize for Astrophysics, spoke about galaxy formation and dark matter. In particular, he spoke about difficulties and problems astrophysicists have encountered while attempting to model and simulate galaxies forming while assuming a cold dark matter (CDM) universe. For example, he described: the cusp-core controversy about the inner profiles of dark matter clumps and galaxy groups; the problem of angular momentum, which is conserved by dark matter but not gas and stars; the missing satellites problem, in which more simulated dark matter subclumps (“subhaloes”) than observed satellite galaxies are found; and the “too-big-to-fail” problem, such that simulated subhaloes are much more dense than the galaxies we see around the Milky Way. These problems motivated astrophysicists to rethink assumptions about how galaxies form and to consider warm or self-interacting dark matter.

Madau ended by saying that evidence that the universe conforms to expectations of the CDM model is “compelling but not definitive,” and warm dark matter remains a possibility. Considering all of the exciting work being done in this field, this could be “the DM decade”…but then he said people have been talking of a DM decade for the past thirty years.

Rise of the Giant Telescopes

The biggest telescope ever constructed, the Thirty Meter Telescope (TMT), officially broke ground on Mauna Kea in Hawai’i on Tuesday. Building on technology used for the Keck telescopes, the TMT’s primary mirror will be segmented combining 492 hexagonal reflectors that will be honeycombed together, and it will have an effective diameter of 30 meters, as you’ve probably guessed. (Astrophysicists come up with very descriptive names for their telescopes and simulations.) 30 meters is really really big—about a third the length of an American football field and nearly the size of a baseball diamond’s infield. When it’s built it will look something like this:


(If you’re interested, here’s a shameless plug: we discussed the TMT’s groundbreaking on the Weekly Space Hangout with Universe Today yesterday, and you can see the video on YouTube.)

The groundbreaking and blessing ceremony, which included George Takei hosting a live webcast, didn’t go quite as planned. It was disrupted by a peaceful protest of several dozen people who oppose the telescope’s construction. The protesters chanted and debated with attendees and held signs with “Aloha ‘Aina” (which means ‘love of the land’) and using TMT to spell out “Too Many Telescopes.” There has been a history of tension over what native Hawaiians say is sacred ground in need of protection and is also one of the best places on Earth to place telescopes. This is a longstanding issue, and the tension between them back in 2001 was reported in this LA Times article. According to Garth Illingworth, co-chair of the Science Advisory Committee, “It was an uncomfortable situation for those directly involved, but the way in which the interactions with the protesters was handled, with considerable effort to show respect and to deal with the situation with dignity, reflected credit on all concerned.” In any case, construction will continue as planned.


The TMT’s science case includes observing distant galaxies and the large-scale structure of the early universe, and will enable new research on supermassive black holes, and star and planet formation. The TMT is led by researchers at Caltech and University of California (where I work), and Canada, Japan, China, India. Its optical to near-infrared images will be deeper and sharper than anything else available, with spatial resolution twelve times that of the Hubble Space Telescope and eight times the light-gathering area of any other optical telescope. If it’s completed on schedule, it will have “first light” in 2022 and could be the first of the next generation of huge ground-based telescopes. The others are the European Extremely Large Telescope (E-ELT, led by the European Southern Observatory) and the Giant Magellan Telescope (GMT, led by the Carnegie Observatories and other institutions), which will be located in northern Chile.

Every ten years, astronomers and astrophysicists prioritize small-, medium-, and large-scale ground-based and space-based missions, with the aim of advising the federal government’s investment, such as funding through the National Science Foundation (NSF) and NASA. The most recent decadal survey, conducted by the National Academy of Sciences is available online (“New Worlds, New Horizons in Astronomy and Astrophysics“). For the large-scale ground-based telescopes, the NSF will be providing funding for the Large Synoptic Survey Telescope (which I’ve written about here before) and the TMT. There had been debates about funding either the TMT or the GMT, but not both, though a couple years ago GMT scientists opted out of federal funding (see this Science article). NASA is focusing on space-based missions such as the upcoming James Webb Space Telescope (JWST) and Wide-Field InfraRed Survey Telescope (WFIRST), which will be launched later this decade.

Astrophysicists Gather in Aspen to Study the Galaxy-Dark Matter Connection

I just returned from a summer workshop at the Aspen Center for Physics, and I enjoyed it quite a bit! The official title of our workshop is “The Galaxy-Halo Connection Across Cosmic Time.” It was organized by Risa Wechsler (Stanford) and Frank van den Bosch (Yale) and others who unfortunately weren’t able to attend (Andreas Berlind, Jeremy Tinker, and Andrew Zentner). The workshop itself was very well attended by researchers and faculty from a geographically diverse range of institutions, but since it was relatively late in the summer, a few people couldn’t come because of teaching duties.

photo 1

Since I grew up in Colorado, I have to add that Aspen is fine and I understand why it’s popular, but there are many beautiful mountain towns in the Colorado Rockies. Visitors and businesses should spread the love to other places too, like Glenwood Springs, Durango, Leadville, Estes Park, etc… In any case, when we had time off, it was fun to go hiking and biking in the area. For example, I took the following photo after hiking to the top of Electric Peak (elev. 13635 ft., 4155 m), and lower down I’ve included photos of Lost Man Lake (near the continental divide) and the iconic Maroon Bells.

photo 11

The Aspen Center for Physics (ACP) is a great place for working and collaborating with colleagues. As they say on their website, “Set in a friendly, small town of inspiring landscapes, the Center is conducive to deep thinking with few distractions, rules or demands.” As usual, we had a very flexible schedule that allowed for plenty of conversations and discussions outdoors or in our temporary offices. Weather permitting, we had lunch and some meetings outside, and we had many social events too, including lemonade and cookies on Mondays and weekly barbecues. It’s also family-friendly, and many physicists brought their spouses and kids to Aspen too. I’ve attended one ACP summer workshop on a similar theme (“Modeling Galaxy Clustering”) in June 2007, and it too was both fun and productive. Note that the ACP workshop is very different than the Madrid workshop I attended earlier this summer, which had specific goals we were working toward (and I’ll give an update about it later).

This year’s Aspen workshop connected important research on the large-scale structure of the universe, the physics of dark matter halo assembly, the formation and evolution of galaxies, and cosmology. We had informal discussions about the masses and boundaries of dark matter haloes in simulations, ways to quantify the abundances and statistics of galaxies we observe with telescopes and surveys, and how to construct improved models that accurately associate particular classes of galaxies with particular regions of the “cosmic web”—see this Bolshoi simulation image, for example, and the following slice from a galaxy catalog of the Sloan Digital Sky Survey:


While some of these issues have plagued us for years and remain unresolved, there are some subtle issues that have cropped up more recently. We (including me) have successfully modeled the spatial distribution of galaxies in the “local” universe, but now we are trying to distinguish between seemingly inconsistent but similarly successful models. For example, we know that the distribution of dark matter haloes in numerical simulations depends on the mass of the haloes—bigger and more massive systems tend to form in denser environments—as well as on their assembly history (such as their formation time), but these correlations can be quantified in different ways and it’s not clear whether there is a preferred way to associate galaxies with haloes as a function of these properties. For the galaxies themselves, we want to understand why some of them have particular brightnesses, colors, masses, gas contents, star formation rates, and structures and whether they can be explained with particular kinds of dark matter halo models.


The main purpose of these workshops is to facilitate collaborations and inspire new ideas about (astro)physical issues, and it looks like we accomplished that. The previous workshop I attended helped me to finish a paper on analyzing the observed spatial distribution of red and blue galaxies with dark matter halo models (arXiv:0805.0310), and I’m sure that my current projects are already benefiting from this summer’s workshop. We seem to be gradually learning more about the relations between galaxy formation and dark matter, and my colleagues and I will have new questions to ask the next time we return to the Rockies.

Finally, here are those Maroon Bells you’ve been waiting for:


Comparing Models of Dark Matter and Galaxy Formation

I just got back from the “nIFTy” Cosmology workshop, which took place at the IFT (Instituto de Física Teórica) of the Universidad Autonoma de Madrid. It was organized primarily by Alexander Knebe, Frazer Pearce, Gustavo Yepes, and Francisco Prada. As usual, it was a very international workshop, which could’ve been interesting in the context of the World Cup, except that most of the participants’ teams had already been eliminated before the workshop began! In spite of Spain’s early exit, the stadium of Real Madrid (which I visited on a day of sightseeing) was nonetheless a popular tourist spot. I also visited the Prado museum, which had an interesting painting by Rubens involving the Milky Way.


This was one of a series of workshops and comparison projects, and I was involved in some of the previous ones as well. For example, following a conference in 2009, some colleagues and I compared measures of galaxy environment—which are supposed to quantify to what extent galaxy properties are affected by whether they’re in clustered or less dense regions—using a galaxy catalog produced by my model. (The overview paper is here.) I also participated in a project comparing the clustering properties of dark matter substructures identified with different methods (here is the paper). Then last year, colleagues and I participated in a workshop in Nottingham, in which we modeled galaxy cluster catalogs that were then analyzed by different methods for estimating masses, richnesses and membership in these clusters. (See this paper for details.)

This time, we had an ambitious three week workshop in which each week’s program is sort of related to the other weeks. During the first week, we compared codes of different hydrodynamical simulations, including the code used by the popular Illustris simulation, while focusing on simulated galaxy clusters. In week #2, we compared a variety of models of galaxy formation as well as models of the spatial distributions and dynamics of dark matter haloes. Then in week #3, we’re continuing the work from that Nottingham workshop I mentioned above. (All of these topics are also related to those of the conference in Xi’an that I attended a couple months ago, and a couple other attendees were here as well.)

The motivation of these workshops and comparison workshops is to compare popular models, simulations, and observational methods in order to better understand our points of agreement and disagreement and to investigate our systematic uncertainties and assumptions that are often ignored or not taken sufficiently seriously. (This is also relevant to my posts on scientific consensus and so-called paradigm shifts.)

Last week, I would say that we had surprisingly strong disagreement and interesting debates about dark matter halo masses, which are the primary drivers of environmental effects on galaxies; about the treatment of tidally stripped substructures and ‘orphan’ satellite galaxies in models; and various assumptions about ‘merger trees’ (see also this previous workshop.) These debates highlight the importance of such comparisons: they’re very useful for the scientific community and for science in general. I’ve found that the scatter among different models and methods often turns out to be far larger than assumed, with important implications. For example, before we can learn about how a galaxy’s environment affects its evolution, we need to figure out how to properly characterize its environment, but it turns out that this is difficult to do precisely. Before we can learn about the physical mechanisms involved in galaxy formation, we need to better understand how accurate our models’ assumptions might be, especially assumptions about how galaxy formation processes are associated with evolving dark matter haloes. Considering the many systematic uncertainties involved, it seems that these models can’t be used reliably for “precision cosmology” either.