"Long before it's in the papers"
June 03, 2013


“Dark matter” may give off an already-seen signal, physicists say

Sept. 5, 2012
Courtesy of The Niels Bohr Institute
and World Science staff

The mys­te­ri­ous “dark mat­ter” fill­ing the uni­verse may shed its char­ac­ter­is­tic in­vis­i­bil­ity in some situa­t­ions, to give off a form of radia­t­ion that has al­ready been de­tected, some phys­i­cists say.

If the find­ings are cor­rect, sci­en­tists could ex­ploit the ef­fect to help re­solve an in­creas­ingly tan­gled de­bate over wheth­er the dark mat­ter ex­ists at all, and if so, what it is. Re­cent, com­pet­ing stud­ies have con­clud­ed, for example, that in our part of our gal­axy, dark mat­ter is ei­ther plen­ti­ful or—in con­flict with pre­vail­ing dark-mat­ter the­o­ries—prac­tic­ally non­ex­ist­ent.

A di­a­gram of the Planck sat­el­lite, launched in May 2009. The sat­el­lite does not sit still in space, but changes di­rec­tion eve­ry hour as well as ro­tates once a min­ute on its own ax­is. These move­ments mean that it scans the en­tire sur­round­ing uni­verse in the course of six months. (Im­age cour­te­sy Bohr In­sti­tute)

Most main­stream as­tro­no­mers ac­cept the ex­ist­ence of dark mat­ter, based on gravita­t­ional ef­fects that it ex­erts and that seem un­ex­plain­a­ble in any bet­ter way. 

Dark mat­ter is be­lieved to fill the space be­tween ga­lax­ies and be­tween their in­di­vid­ual stars, but its pre­cise na­ture is un­clear. Over the past 70 years, as­tro­no­mers, cos­mol­o­gists and par­t­i­cle phys­i­cists have been look­ing for an­swers to what it could be. 

With new ob­serva­t­ions from the Eu­ro­pe­an Space Agen­cy’s Planck sat­el­lite, re­search­ers from the Niels Bohr In­sti­tute at the Uni­vers­ity of Co­pen­ha­gen, among oth­ers, say they may be clos­er than ev­er to a so­lu­tion to the or­i­gin of the mys­te­ri­ous dark mat­ter.

The sat­el­lite, launched in 2009, has highly sen­si­tive in­stru­ments that can pre­cisely map mi­cro­wave radia­t­ion reach­ing us from across the en­tire sky. The lat­est da­ta re­veals un­usu­al radia­t­ion from our own gal­axy, which opens a new di­rec­tion in un­der­stand­ing the most fun­da­men­tal prop­er­ties of the space, time and mat­ter, said Pa­vel Na­sel­sky, a cos­mol­o­gist at the Bohr In­sti­tute.

“The Planck Sat­el­lite has ob­served a very un­ique emis­sion of ra­di­o radia­t­ion from the cen­ter of our gal­axy, the Milky Way,” he said. “By us­ing dif­fer­ent meth­ods to sep­a­rate the sig­nal for very broad range of wave­lengths [col­ors], the Planck team has been able to de­ter­mine the spec­trum of the radia­t­ion,” he ex­plained.

“The radia­t­ion has a spec­trum which has the same form as that of syn­chro­tron emis­sion, which or­i­ginates from elec­trons and positrons,” or elec­tric­ally charged sub­a­tom­ic par­t­i­cles, he went on. These par­t­i­cles give off that char­ac­ter­is­tic emis­sion when they cir­cu­late swiftly in the cen­ter of the gal­axy along its pow­er­ful mag­net­ic field lines, the path­ways fol­lowed by the pull of the mag­net­ic force, he added.

“I be­lieve that there are quite strong in­dica­t­ions that [the emis­sion] could come from dark mat­ter,” he said.

E­mis­sion from the cen­ter of the Milky Way, de­tected by the Planck sat­el­lite. The black zone "mask" is emis­sion from the ga­lac­tic disk; the blue-red-white zone in the cen­ter of the map is new­found, ab­nor­mal ra­di­a­tion. (Im­age cour­te­sy Bohr In­sti­tute)

Sci­en­tists in­clud­ing Subir Sarkar at the Bohr In­sti­tute have pre­dicted, us­ing cal­cula­t­ions, that dark mat­ter may con­sist of par­t­i­cles that are around 10 times as heavy as the famed Higgs par­t­i­cle, a key build­ing block of mat­ter re­cently thought to have been de­tected with good like­li­hood. The Higgs par­t­i­cle it­self is about 100 times heav­i­er than a hy­dro­gen at­om. 

Dark mat­ter par­t­i­cles are thought to have un­ique prop­er­ties: they don’t in­ter­act with “nor­mal” mat­ter par­t­i­cles or with each oth­er, and are al­so usu­ally very scat­tered.

“We know from the­o­ret­i­cal pre­dic­tions that the con­centra­t­ion of dark mat­ter par­t­i­cles around the cen­ter of ga­lax­ies is very high and we have a strong ar­gu­ment they can col­lide there and in the col­li­sion elec­trons and positrons are formed. These elec­trons and positrons start to ro­tate around the mag­net­ic field at the cen­ter of the gal­axy and in do­ing so pro­duce this very un­usu­al syn­chro­tron radia­t­ion,” Nasel­sky said.

“It has simply not been pos­si­ble to ob­serve this radia­t­ion in such de­tail be­fore, as pre­vi­ous in­stru­ments have not been sen­si­tive enough. But with Planck, this un­usu­al radia­t­ion is seen very clear­ly.

“The radia­t­ion can­not be ex­plained by the struc­tur­al mech­a­nisms in the gal­axy and it can­not be radia­t­ion from su­per­no­va [star] ex­plo­sions. I be­lieve that this could be proof of dark mat­ter. Oth­er­wise, we have disco­vered ab­so­lutely new, and un­known for phys­ics, mech­an­ism of ac­celera­t­ion of par­t­i­cles in the ga­lac­tic cen­ter,” said Nasel­sky, adding he ex­pects ex­cit­ing new re­sults with­in months.

The find­ings so far have been sub­mit­ted to the sci­en­tif­ic jour­nal As­tron­o­my and As­t­ro­phys­ics and posted on­line.

* * *

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The mysterious “dark matter” filling the universe may shed its characteristic invisibility in some situations, to give off a form of radiation that has already been detected, some physicists say. If the findings are correct, scientists could exploit the effect to help resolve an increasingly tangled debate over whether the dark matter exists at all, and if so, what it is. Recent, competing studies have concluded that dark matter is either plentiful or—in conflict with prevailing dark-matter theories—practically nonexistent in our neighborhood of our galaxy. Most mainstream astronomers have accepted the idea of the existence of dark matter, based on gravitational effects that it exerts and that seem unexplainable in any better way. Dark matter is believed to fill the space between galaxies and between their individual stars, but its precise nature is unclear. Over the past 70 years, astronomers, cosmologists and particle physicists have been looking for answers to what it could be. With new observations from the European Space Agency’s Planck satellite, researchers from the Niels Bohr Institute at the University of Copenhagen, among others, say they may be closer than ever to a solution to the origin of the mysterious dark matter. The satellite, launched in 2009, has highly sensitive instruments that can precisely map microwave radiation reaching us from across the entire sky. The latest data reveals unusual radiation from our own galaxy, which open a new direction in understanding the most fundamental properties of the space, time and matter, said Pavel Naselsky, a cosmologist at the Bohr Institute. “The Planck Satellite has observed a very unique emission of radio radiation from the centre of our galaxy, the Milky Way,” he said. “By using different methods to separate the signal for very broad range of wavelengths [colors], the Planck team has been able to determine the spectrum of the radiation,” he explained. “The radiation has a spectrum which has the same form as that of synchrotron emission, which originates from electrons and positrons,” or electrically charged subatomic particles, he went on. These particles give off the characteristic emission when they circulate swiftly in the center of the galaxy along its powerful magnetic field lines, the pathways followed by the pull of the magnetic force, he added. “I believe that there are quite strong indications that [the emission] could come from dark matter,” he said. Scientists including Subir Sarkar at the Bohr Institute have predicted, using calculations, that dark matter may consist of particles that are around 10 times as heavy as the famed Higgs particle, a key building block of matter recently thought to have been detected with good likelihood. The Higgs particle itself is about 100 times heavier than a hydrogen atom. Dark matter particles have unique properties: they don’t interact with “normal” matter particles or with each other, and are also usually very scattered. “We know from theoretical predictions that the concentration of dark matter particles around the centre of galaxies is very high and we have a strong argument they can collide there and in the collision electrons and positrons are formed. These electrons and positrons start to rotate around the magnetic field at the centre of the galaxy and in doing so produce this very unusual synchrotron radiation,” Naselsky said. “It has simply not been possible to observe this radiation in such detail before, as previous instruments have not been sensitive enough. But with Planck, this unusual radiation is seen very clearly. “The radiation cannot be explained by the structural mechanisms in the galaxy and it cannot be radiation from supernova [star] explosions. I believe that this could be proof of dark matter. Otherwise, we have discovered absolutely new (and unknown for physics) mechanism of acceleration of particles in the Galactic centre”, said Pavel Naselsky, adding he expects exciting new results already within the next few months. The results have been submitted to the scientific journal Astronomy and Astrophysics.