Compared to dark energy or fluctuations in the cosmic microwave background, dark matter is not quite so daunting to explain. It is indirectly detected through its gravitational pull on visible matter – stars and galaxies. There's a lot of it and we don't know what it's made of but scientists have their theories. And so there was some fanfare made over the results of an experiment called AMS meant to detect positrons that would theoretically be emitted if antimatter takes a particular form, called WIMPs (weakly interacting massive particles), and these said WIMPs collide.
The experiment is also interesting because it was conceived by particle physicist Sam Ting, and because it’s flying on the International Space Station. The results were not definitive, but there was enough to work with.
I was disappointed to see little if any explanation for why the experiment flew on ISS and not some unmanned craft. Or why it was so atronomically expensive at $1.6 billion.
Most stories were also weak on the essential background needed to understand why scientists believe dark matter takes the form of some as-yet-undetected kind of particle and not just rocks or planets or dust or other matter that doesn’t show up or why these particles would emit positrons when they collide – it's really not obvious to general readers.
There are good explanations that go back to our understanding of the big bang and limits on how much ordinary matter can exist. There are also good explanations for why some think dark matter takes this particular form that behaves in this peculiar way.
The stories on this latest finding also gave varying estimates for the percentage of our universe made up of dark matter – some said about 80% and others about a quarter. The answer, I am pretty sure, depends on whether you count dark energy as part of the make-up of the cosmos or not.
At the New York Times, Dennis Overbye contributed a detailed, fairly coherent story with one of the most wishy-washy quotes I’d seen for some time:
“I don’t think it makes you believe it must be dark matter, nor do I think it makes you believe it cannot be,” said Neal Weiner, a particle theorist at New York University.
Here’s how he explains the goal:
According to recent measurements by the Planck spacecraft, about 27 percent of the universe, by mass, is composed of some unknown form of matter unlike the atoms that make up us and everything we can see. Astronomers cannot see it, but they can detect its gravitational tug pulling the galaxies and stars around.
I found this paragraph confusing because the Planck results are new and obviously the designer of AMS did not know about them. The story would have made a lot more sense if it had included how we came to understand not just the presence of dark matter but the expectation that it would take the form of some new particle. What were scientists thinking about dark matter when they built AMS and did Planck change anything critical or just offer a refinement?
At AP, Seth Borenstein and John Heilprin employed a sleuthing concept, but the story didn’t explain why this experiment had anything more to do with detective work than any other science project. Read the whole story here.
GENEVA (AP) — It is one of the cosmos' most mysterious unsolved cases: dark matter. It is supposedly what holds the universe together. We can't see it, but scientists are pretty sure it's out there.
Led by a dogged, Nobel Prize-winning gumshoe who has spent 18 years on the case, scientists put a $2 billion detector aboard the International Space Station to try to track down the stuff. And after two years, the first evidence came in Wednesday: tantalizing cosmic footprints that seem to have been left by dark matter.
The story in the Washington Post by Joel Achenbach brought the lofty concepts down to earth. It really is possible to write about dark matter so that regular people can follow you. Read it here.
Here’s how he describes the experiment in one beautifully clear sentence:
It detects cosmic rays, which are particles moving at extraordinary velocity and coming from all over the galaxy. The AMS sorts through the particles, measuring their momentum and charge.
And see how his explanations of positrons and dark matter are simpler and more real than anyone else’s:
A small percentage of the particles that hit the detector are unusual things called positrons, which are like electrons but with the opposite charge. They’re in the class of particles known as antimatter.
Another theorized source of positrons is dark matter. If antimatter seems exotic, dark matter is even more so. No one has ever seen the stuff, and its existence has never been nailed down definitely. Dark matter emits and absorbs no light, and interacts with ordinary matter in a ghostly fashion, primarily through gravity. Dark matter is thought to affect the way galaxies move; they rotate in a manner that suggests that they are carrying some unseen load. In the past two decades, other experiments and detectors have bolstered the idea that dark matter is far more abundant than ordinary matter.
Achenbach makes it look easy. It's hard to write this way, but it sure is nice and easy on the readers. And that’s what we should be striving for – especially in stories about such difficult topics.
David Ewalt at Forbes started with this lede, which confused me because we already had indirect evidence for dark matter from its gravitational pull and we even have estimates for how much is supposed to be there. Read the whole story here.
Researchers working with NASA and the U.S. Department of Energy announced on Wednesday that they may have found evidence of dark matter, an elusive substance that scientists believe constitutes a majority of the mass in our universe, but until now have been unable to actually detect.
At the Los Angeles Times Amina Khan who led this way.
Let that long-held breath out, folks. The Alpha Magnetic Spectrometer has picked up a lot of mysterious antimatter in low Earth orbit – but that doesn’t necessarily mean it’s a sign of dark matter.
There was something weirdly insider-ish about it that I did not get. Did she think regular people on the street were actually worked up about AMS or was it supposed to sarcastic? Read the rest here.