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

RETURN TO THE WORLD SCIENCE HOME PAGE


“Dark matter” may be half what was thought—at least locally

Oct. 9, 2014
Courtesy of the International Centre 
for Radio Astronomy Research
and World Science staff

A new meas­ure­ment of “dark mat­ter” in our gal­axy in­di­cates there is half as much of the mys­te­ri­ous sub­stance as pre­vi­ously thought.

That could have weighty im­plica­t­ions, since as­tro­no­mers be­lieve the in­vis­i­ble stuff makes up most of the ma­te­ri­al sub­stance of the uni­verse. But re­search­ers said it’s un­clear wheth­er the new re­sults are al­so true of oth­er ga­lax­ies, which like ours, are thought to be en­vel­oped in dark mat­ter—in­vis­i­ble stuff de­tected only through its gravita­t­ional pull.

Sci­en­tists used a meth­od de­vel­oped al­most a cen­tu­ry ago to con­clude that the weight of dark mat­ter clus­ter­ing around our gal­axy is equiv­a­lent to 800 bil­lion Suns. They probed the edge of our gal­axy, the Milky Way, look­ing close­ly, for the first time, at its fringes, about five tril­lion kilo­me­ters (three tril­lion miles) from Earth.

We’ve known for a while that most of the uni­verse is hid­den, said as­t­ro­phys­i­cist Pra­jwal Kafle of the Uni­vers­ity of West­ern Aus­tral­ia, which par­ti­ci­pates in a col­la­bora­t­ion called the In­terna­t­ional Cen­tre for Ra­di­o As­tron­o­my Re­search. 

“S­tars, dust, you and me, all the things that we see, only make up about four pe­r­cent of the en­tire uni­verse,” he said. “About 25 pe­r­cent is dark mat­ter and the rest is dark en­er­gy.” Like dark mat­ter, dark en­er­gy is anoth­er mys­tery—some­thing be­lieved to drive an ac­cel­er­ated ex­pan­sion of the uni­verse—but it was not a sub­ject of the new stu­dy.

Kafle meas­ured the mass, or “weight,” of the Milky Way’s dark mat­ter by stu­dying the speed of stars through­out the gal­axy, in­clud­ing the edges, which he said had nev­er been stud­ied to this de­tail. He used what he de­scribed as a re­li­a­ble tech­nique de­vel­oped by Brit­ish as­tronomer James Jeans in 1915—dec­ades be­fore dark matter was known.

Kafle’s meas­ure­ment, pub­lished in the As­t­ro­phys­i­cal Jour­nal, helps solve an al­most two-decade-old mys­tery, ac­cord­ing to Kafle and his co-authors.

“The cur­rent idea of gal­axy forma­t­ion and ev­o­lu­tion, called the lamb­da cold dark mat­ter the­o­ry, pre­dicts that there should be a hand­ful of big sat­el­lite ga­lax­ies around the Milky Way that are vis­i­ble with the na­ked eye, but we don’t see that,” Kafle said.

“When you use our meas­ure­ment of the mass of the dark mat­ter the the­o­ry pre­dicts that there should only be three sat­el­lite ga­lax­ies out there, which is ex­actly what we see; the Large Mag­el­lanic Cloud, the Small Mag­el­lanic Cloud and the Sag­it­ta­ri­us Dwarf Ga­laxy.”

Univers­ity of Syd­ney as­t­ro­phys­i­cist Ge­raint Lew­is, who was al­so in­volved in the re­search, said the mis­sing sat­el­lite prob­lem had been “a thorn in the cos­mo­lo­g­i­cal side for al­most 15 years… Kafle’s work has shown that it might not be as bad as eve­ry­one thought, al­though there are still prob­lems to over­come.”

But the re­search­ers wrote that it is “d­if­fi­cult to com­ment if the con­centra­t­ion [of dark mat­ter] we get is typ­i­cal or atyp­i­cal” of oth­er ga­lax­ies like ours.

The study al­so pre­sented a “holis­tic” mod­el of the Milky Way, which al­lowed the sci­en­tists to meas­ure sev­er­al in­ter­est­ing things such as the speed re­quired to leave the gal­axy.

“Be pre­pared to hit 550 kilo­me­ters [350 miles] per sec­ond if you want to es­cape the gravita­t­ional clutches of our gal­axy,” Kafle said. “A rock­et launched from Earth needs just 11 kilo­me­ters [seven miles] per sec­ond to leave its sur­face, which is al­ready about 300 times faster than the max­i­mum Aus­tral­ian speed lim­it in a car!”


* * *

Send us a comment on this story, or send it to a friend

Sign up for
e-newsletter
   
 
subscribe
 
cancel

On Home Page         

LATEST

  • St­ar found to have lit­tle plan­ets over twice as old as our own

  • “Kind­ness curricu­lum” may bo­ost suc­cess in pre­schoolers

EXCLUSIVES

  • Smart­er mice with a “hum­anized” gene?

  • Was black­mail essen­tial for marr­iage to evolve?

  • Plu­to has even cold­er “twin” of sim­ilar size, studies find

  • Could simple an­ger have taught people to coop­erate?

MORE NEWS

  • F­rog said to de­scribe its home through song

  • Even r­ats will lend a help­ing paw: study

  • D­rug may undo aging-assoc­iated brain changes in ani­mals

A new measurement of “dark matter” in our galaxy indicates there is half as much of the mysterious substance as previously thought. That could have weighty implications, since astronomers believe the invisible stuff makes up most of the material substance of the universe. But researchers said it’s unclear whether the new results are also true of other galaxies, which like ours, are thought to be enveloped in dark matter—invisible stuff detected only through its gravitational effects. Scientists used a method developed almost a century ago to conclude that the weight of dark matter clustering around our galaxy is equivalent to 800 billion Suns. They probed the edge of our galaxy, the Milky Way, looking closely, for the first time, at its fringes, about five trillion kilometers (three trillion miles) from Earth. We’ve known for a while that most of the universe is hidden, said astrophysicist Prajwal Kafle of the University of Western Australia, which participates in a collaboration called the International Centre for Radio Astronomy Research. “Stars, dust, you and me, all the things that we see, only make up about four percent of the entire universe,” he said. “About 25 percent is dark matter and the rest is dark energy.” Like dark matter, dark energy is another mystery—something believed to drive an accelerated expansion of the universe—but it was not a subject of the new study. Kafle measured the mass, or “weight,” of the dark matter in the Milky Way by studying the speed of stars throughout the galaxy, including the edges, which he said had never been studied to this detail. He used what he described as a reliable technique developed by British astronomer James Jeans in 1915—decades before the discovery of dark matter. Kafle’s measurement, published in the Astrophysical Journal, helps solve an almost two-decade-old mystery, according to Kafle and his co-authors. “The current idea of galaxy formation and evolution, called the lambda cold dark matter theory, predicts that there should be a handful of big satellite galaxies around the Milky Way that are visible with the naked eye, but we don’t see that,” Kafle said. “When you use our measurement of the mass of the dark matter the theory predicts that there should only be three satellite galaxies out there, which is exactly what we see; the Large Magellanic Cloud, the Small Magellanic Cloud and the Sagittarius Dwarf Galaxy.” University of Sydney astrophysicist Geraint Lewis, who was also involved in the research, said the missing satellite problem had been “a thorn in the cosmological side for almost 15 years… Kafle’s work has shown that it might not be as bad as everyone thought, although there are still problems to overcome.” But the researchers wrote that it is “difficult to comment if the concentration [of dark matter] we get is typical or atypical” of other galaxies like ours. The study also presented a “holistic” model of the Milky Way, which allowed the scientists to measure several interesting things such as the speed required to leave the galaxy. “Be prepared to hit 550 kilometers per second if you want to escape the gravitational clutches of our galaxy,” Kafle said. “A rocket launched from Earth needs just 11 kilometers per second to leave its surface, which is already about 300 times faster than the maximum Australian speed limit in a car!”