"Long before it's in the papers"
January 27, 2015


No shortage of dark matter in Sun’s neighborhood, study claims

Aug. 9, 2012
Courtesy of the Royal Astronomical Society
and World Science staff

A new study us­ing com­put­er sim­ula­t­ions sug­gests there’s plen­ty of “dark mat­ter” in the Sun’s neigh­bor­hood, con­tra­ry to an­oth­er re­cent study that turned up al­most none.

As­tro­no­mers at the Uni­vers­ity of Zürich in Switz­er­land and oth­er in­sti­tu­tions said they ac­tu­ally found un­ex­pectedly high amounts of the in­vis­i­ble ma­te­ri­al.

An im­age from a sim­u­la­tion of the Milky Way used to test the tech­nique for meas­ur­ing dark mat­ter. (Cred­it: Dr A. Hobbs)

The re­sults are con­sist­ent with the the­o­ry that the Milky Way Gal­axy is sur­rounded by a mas­sive “ha­lo” of dark mat­ter, but this is the first study of its kind to use a meth­od rig­or­ously tested against mock da­ta from high qual­ity sim­ula­t­ions, the re­search­ers said. They al­so found what they called tan­ta­liz­ing hints of a new dark mat­ter com­po­nent in our gal­axy. 

The re­sults are to be pub­lished in the jour­nal Monthly No­tices of the Roy­al As­tro­nom­i­cal So­ci­e­ty and are avail­able on­line al­ready.

Dark mat­ter was first pro­posed by the Swiss as­tron­o­mer Fritz Zwicky in the 1930s. He found that clus­ters of ga­lax­ies had to be filled with some sub­stance that kept them from fly­ing apart, al­though just what this stuff was re­mained a mys­tery. At nearly the same time, Jan Oort in the Neth­er­lands con­clud­ed that the amount of ma­te­ri­al near the Sun was nearly twice what could be ex­plained by the pres­ence of known stars and gas alone.

As­tro­no­mers even­tu­ally de­vel­oped a the­o­ry of dark mat­ter and struc­ture forma­t­ion that ex­plains the prop­er­ties of clus­ters and ga­lax­ies in the Uni­verse, but the amount of dark mat­ter in the so­lar neigh­bor­hood has re­mained more mys­te­ri­ous. For dec­ades af­ter Oort’s meas­ure­ment, stud­ies found three to six times more dark mat­ter than ex­pected. Then last year new da­ta and a new meth­od claimed far less than ex­pected. 

The sci­en­tif­ic com­mun­ity was left puz­zled, gen­er­ally be­liev­ing that the ob­serva­t­ions and anal­y­ses simply weren’t sen­si­tive enough to per­form a re­li­a­ble meas­ure­ment.

In the lat­est work, the au­thors said they’re much more con­fi­dent in their meas­ure­ment and its un­cer­tain­ties. This is be­cause they used a state-of-the-art sim­ula­t­ion of our gal­axy to test their tech­nique for meas­ur­ing the amount of ma­te­ri­al be­fore ap­ply­ing it to real da­ta. 

This threw up a num­ber of sur­prises, they went on. They found that stand­ard tech­niques used over the past 20 years were bi­ased, they said, al­ways tend­ing to un­der­es­ti­mate the amount of dark mat­ter. They then de­vised a new “un­bi­ased” tech­nique that reco­vered the cor­rect an­swer from the sim­ulated da­ta. Ap­ply­ing their tech­nique to the po­si­tions and ve­lo­ci­ties of thou­sands of stars known as or­ange K dwarf stars near the Sun, they got a new es­ti­mate of the lo­cal dark mat­ter dens­ity, or com­pact­ness.

“We are 99 per­cent con­fi­dent that there is dark mat­ter near the Sun. In fact, our fa­vored dark mat­ter dens­ity is a lit­tle high. There is a 10 per­cent chance that this is merely a sta­tis­ti­cal fluke. But with 90 per­cent con­fi­dence, we find more dark mat­ter than ex­pected,” said Sil­via Gar­bari of the Uni­vers­ity of Zürich, the stu­dy’s lead au­thor.

The study might be the first ev­i­dence for a “disc” of dark mat­ter in our Gal­axy, as re­cently pre­dicted by the­o­ry and sim­ula­t­ions of gal­axy forma­t­ion, she added. “Or it could be that the dark mat­ter ha­lo of our Gal­axy is squashed.”

Many phys­i­cists are plac­ing their bets on dark mat­ter be­ing a new fun­da­men­tal par­t­i­cle that in­ter­acts only very weakly with nor­mal mat­ter—but strongly enough to be de­tected in ex­pe­ri­ments deep un­der­ground where confus­ing cos­mic ray events are screened by over a kil­o­me­ter of sol­id rock.

An ac­cu­rate meas­ure of the lo­cal dark mat­ter dens­ity is vi­tal for such ex­pe­ri­ments, said study co-au­thor George Lake, al­so at the uni­vers­ity. “If dark mat­ter is a fun­da­men­tal par­t­i­cle, bil­lions of these par­t­i­cles will have passed through your body” by the time your fin­ish read­ing this ar­ti­cle, he said. “Ex­pe­ri­men­tal phys­i­cists hope to cap­ture just a few of these par­t­i­cles each year in ex­pe­ri­ments like XEN­ON and CDMS cur­rently in opera­t­ion. Know­ing the lo­cal prop­er­ties of dark mat­ter is the key to re­veal­ing just what kind of par­t­i­cle it con­sists of.”

* * *

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A new study using computer simulations suggests there’s plenty of “dark matter” in the Sun’s neighborhood, contrary to another recent study that turned up almost none. Astronomers at the University of Zürich in Switzerland and other institutions said they actually found unexpectedly high amounts of the invisible material. The results are consistent with the theory that the Milky Way Galaxy is surrounded by a massive “halo” of dark matter, but this is the first study of its kind to use a method rigorously tested against mock data from high quality simulations, the researchers said. They also found tantalising hints of a new dark matter component in our galaxy. The results are to be published in the journal Monthly Notices of the Royal Astronomical Society. Dark matter was first proposed by the Swiss astronomer Fritz Zwicky in the 1930s. He found that clusters of galaxies had to be filled with some substance that kept them from flying apart, although just what this stuff was remained a mystery. At nearly the same time, Jan Oort in the Netherlands concluded that the amount of material near the Sun was nearly twice what could be explained by the presence of known stars and gas alone. Astronomers eventually developed a theory of dark matter and structure formation that explains the properties of clusters and galaxies in the Universe, but the amount of dark matter in the solar neighborhood has remained more mysterious. For decades after Oort’s measurement, studies found three to six times more dark matter than expected. Then last year new data and a new method claimed far less than expected. The scientific community was left puzzled, generally believing that the observations and analyses simply weren’t sensitive enough to perform a reliable measurement. In the latest work, the authors said they’re much more confident in their measurement and its uncertainties. This is because they used a state-of-the-art simulation of our galaxy to test their technique for measuring the amount of material before applying it to real data. This threw up a number of surprises, they went on. They found that standard techniques used over the past 20 years were biased, always tending to underestimate the amount of dark matter. They then devised a new unbiased technique that recovered the correct answer from the simulated data. Applying their technique to the positions and velocities of thousands of stars known as orange K dwarf stars near the Sun, they got a new estimate of the local dark matter density, or compactness. “We are 99% confident that there is dark matter near the Sun. In fact, our favored dark matter density is a little high. There is a 10% chance that this is merely a statistical fluke. But with 90% confidence, we find more dark matter than expected,” said Silvia Garbari of the University of Zürich, the study’s lead author. The study might be the first evidence for a “disc” of dark matter in our Galaxy, as recently predicted by theory and simulations of galaxy formation, she added. “Or it could be that the dark matter halo of our Galaxy is squashed.” Many physicists are placing their bets on dark matter being a new fundamental particle that interacts only very weakly with normal matter—but strongly enough to be detected in experiments deep underground where confusing cosmic ray events are screened by over a kilometer of solid rock. An accurate measure of the local dark matter density is vital for such experiments, said study co-author George Lake, also at the university. “If dark matter is a fundamental particle, billions of these particles will have passed through your body by the time your finish reading this article. Experimental physicists hope to capture just a few of these particles each year in experiments like XENON and CDMS currently in operation. Knowing the local properties of dark matter is the key to revealing just what kind of particle it consists of.”