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


“Dark matter” possibly seen destroying itself

April 3, 2014
Courtesy of NASA
and World Science staff

Some of the gam­ma-ray light com­ing from the cen­ter of our gal­axy may come from “dark mat­ter,” an un­known sub­stance mak­ing up over 80 per­cent of the ma­te­ri­al uni­verse, phys­i­cists say.

This an­i­ma­tion zooms in­to an im­age of the Milky Way, shown in vis­i­ble light, and su­per­im­poses a gamma-ray map of the ga­lac­tic cen­ter from NA­SA's Fer­mi. Raw da­ta tran­si­tions to a view with all known sources re­moved, re­veal­ing a gamma-ray ex­cess hint­ing at the pres­ence of dark mat­ter. (Cred­it: NA­SA God­dard; A. Mel­ing­er, CMU; T. Lin­den, Univ. of Chi­ca­go)

Us­ing da­ta from NASA’s Fer­mi Gamma-ray Space Tel­e­scope, sci­en­tists have de­vel­oped maps show­ing that the ga­lac­tic cen­ter pro­duces more high-en­er­gy gam­ma rays than known sources can ex­plain. 

The ex­cess, they said, is con­sist­ent with some forms of dark mat­ter—in par­tic­u­lar, the mu­tu­al an­ni­hila­t­ion of dark mat­ter par­t­i­cles. Al­most any type of par­t­i­cle in na­ture can an­ni­hilate with its “anti-par­t­i­cle,” a par­ticle that is iden­ti­cal but op­po­site in cer­tain re­spects.

“While the case is not yet closed, in the fu­ture we might well look back and say this was where we saw dark mat­ter an­ni­hila­t­ion for the first time,” said Tra­cy Slatyer, a the­o­ret­i­cal phys­i­cist at Mas­sa­chu­setts In­sti­tute of Tech­nol­o­gy in Cam­bridge, Mass., and a co-au­thor of the stu­dy.

“The new maps al­low us to an­a­lyze the ex­cess and test wheth­er more con­ven­tion­al ex­plana­t­ions” can ac­count for it, added Dan Hoop­er, an as­t­ro­phys­i­cist at Fer­mi Na­t­ional Ac­cel­er­a­tor Lab­o­r­a­to­ry in Ba­ta­via, Ill., and a lead au­thor of the work. “The sig­nal we find can­not be ex­plained by cur­rently pro­posed al­ter­na­tives and is in close agree­ment with the pre­dic­tions of very sim­ple dark mat­ter mod­els.”

The con­ven­tion­al ex­plana­t­ions that were ruled out, he said, in­clud­ed the pres­ence of un­disco­vered pul­sars, a type of rap­idly ro­tat­ing, highly com­pact star, and cos­mic rays strik­ing clouds of gas.

The ga­lac­tic cen­ter teems with sources of gam­ma-ray right, a high-en­er­gy form of light. It’s al­so where as­tro­no­mers ex­pect to find the gal­ax­y’s high­est con­centra­t­ion of dark mat­ter, which is gen­er­ally in­vis­i­ble but pos­sesses gravita­t­ional at­trac­tion. Large amounts of dark mat­ter are thought to at­tract nor­mal mat­ter, form­ing a founda­t­ion up­on which vis­i­ble struc­tures, like ga­lax­ies, are built.

No one knows the true na­ture of dark mat­ter, but a lead­ing class of can­di­dates con­sists of par­t­i­cles called WIMPs, for Weakly In­ter­act­ing Mas­sive Par­t­i­cles. The­o­rists have en­vi­sioned a wide range of WIMP types, some of which may ei­ther mu­tu­ally an­ni­hilate or pro­duce an in­ter­me­diate, quickly-dis­int­e­grat­ing par­t­i­cle when they col­lide. Both of these path­ways end with the pro­duc­tion of gam­ma rays—the most en­er­get­ic form of light—at en­er­gies with­in the de­tec­tion range of Fer­mi’s Large Ar­ea Tel­e­scope.

When as­tro­no­mers sub­tract all known gam­ma-ray sources from LAT ob­serva­t­ions of the ga­lac­tic cen­ter, a patch of lefto­ver emis­sion re­mains. This ex­cess ap­pears most prom­i­nent at a spe­cif­ic en­er­gy range, be­tween 1 and 3 bil­lion elec­tron volts, a bil­lion times great­er than the en­er­gy that char­ac­ter­izes vis­i­ble light.

Hoop­er and col­leagues con­clude that an­ni­hila­t­ions of cer­tain the­o­rized types of dark mat­ter par­t­i­cles pro­vide a re­mark­a­ble fit for the ex­cess based on its spec­trum or en­ergy distri­bution; sym­me­try around the ga­lac­tic cen­ter; and overall bright­ness. Writ­ing in a pa­per sub­mit­ted to the jour­nal Phys­i­cal Re­view D, the re­search­ers say these fea­tures are hard to rec­on­cile with oth­er ex­plana­t­ions pro­posed so far. But they note that plau­si­ble al­ter­na­tives not re­quir­ing dark mat­ter may yet ma­te­ri­alize, and it will take other experiments to confirm the dark mat­ter in­ter­preta­t­ion.

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Some of the gamma-ray light coming from the center of our galaxy may come from “dark matter,” an unknown substance making up over 80% of the material universe, physicists say. Using data from NASA’s Fermi Gamma-ray Space Telescope, scientists have developed maps showing that the galactic center produces more high-energy gamma rays than known sources can explain. The excess, they said, is consistent with some forms of dark matter—in particular, the mutual annihilation of dark matter particles. Almost any type of particle in nature can annihilate with its “anti-particle,” which is identical yet opposite in certain respects. “While the case is not yet closed, in the future we might well look back and say this was where we saw dark matter annihilation for the first time,” said Tracy Slatyer, a theoretical physicist at MIT in Cambridge, Mass., and a co-author of the study. “The new maps allow us to analyze the excess and test whether more conventional explanations” can account for it, added Dan Hooper, an astrophysicist at Fermi National Accelerator Laboratory in Batavia, Ill., and a lead author of the work. “The signal we find cannot be explained by currently proposed alternatives and is in close agreement with the predictions of very simple dark matter models.” The conventional explanations that were ruled out, he said, included the presence of undiscovered pulsars, a type of rapidly rotating, highly compact star, and cosmic rays striking clouds of gas. The galactic center teems with sources of gamma-ray right, a high-energy form of light. It’s also where astronomers expect to find the galaxy’s highest concentration of dark matter, which is generally invisible but possesses gravitational attraction. Large amounts of dark matter are thought to attract normal matter, forming a foundation upon which visible structures, like galaxies, are built. No one knows the true nature of dark matter, but a leading class of candidates consists of particles called WIMPs, for Weakly Interacting Massive Particles. Theorists have envisioned a wide range of WIMP types, some of which may either mutually annihilate or produce an intermediate, quickly decaying particle when they collide. Both of these pathways end with the production of gamma rays—the most energetic form of light—at energies within the detection range of Fermi’s Large Area Telescope. When astronomers subtract all known gamma-ray sources from LAT observations of the galactic center, a patch of leftover emission remains. This excess appears most prominent at a specific energy range, between 1 and 3 billion electron volts, a billion times greater than the energy that characterizes visible light. Hooper and colleagues conclude that annihilations of certain theorized types of dark matter particles provide a remarkable fit for the excess based on its gamma-ray spectrum, its symmetry around the galactic center, and its overall brightness. Writing in a paper submitted to the journal Physical Review D, the researchers say that these features are hard to reconcile with other explanations proposed so far, although they note that plausible alternatives not requiring dark matter may yet materialize. The researchers caution that it will take multiple sightings—in other astronomical objects, the Large Hadron Collider or in some of the direct-detection experiments now being conducted around the world—to validate their dark matter interpretation.