"Long before it's in the papers"
January 28, 2015


“Dark matter” doubters not silenced yet

Aug. 2, 2007
Special to World Science  

As­tro­no­mers have be­lieved for dec­ades that most of the mat­ter in the cos­mos is un­seen. It be­trays it­self only through its gravita­t­ional pull on vis­i­ble ob­jects, whose move­ments are of­ten hard to ex­plain with­out this “dark mat­ter.”

And the past year has seen in­creas­ingly bold claims that as­tro­no­mers have “proved” the stuff’s ex­ist­ence.

This Hub­ble Space Tel­e­scope com­pos­ite im­age shows a ghost­ly "ring" of pre­sumed dark mat­ter in the gal­axy clus­ter Cl 0024+17. It was called one of the strongest pieces of ev­i­dence to date for the ex­ist­ence of dark mat­ter, an un­known sub­stance that per­vades the uni­verse. (Cred­its: NASA, ESA, M.J. Jee and H. Ford [JHU])

De­spite that, there’s a core of doubt­ers who are­n’t go­ing away. Many of them are stick­ing by an al­ter­na­tive the­o­ry that holds that tweak­ing our un­der­stand­ing of gra­vity could ex­plain things bet­ter than in­vok­ing some un­seen sub­stance un­like any we know.

One such modified-gra­vity the­o­ry has been “re­markably re­silien­t,” wrote as­tron­o­mer Stacy Mc­Gaugh of the Un­ivers­ity of Mar­y­land in Col­lege Park, Md. in the Aug. 3 is­sue of the re­search jour­nal Sci­ence. That the­o­ry, known as Mod­i­fied New­to­nian Dy­nam­ics or MOND, was pro­posed in 1983 by the Is­rae­li phys­i­cist Morde­hai Mil­grom.

As­tro­no­mers have sus­pected dark mat­ter’s ex­ist­ence since the 1930s, when the Dutch as­tron­o­mer Jan Oort found that ga­lax­ies did­n’t con­tain nearly enough vis­i­ble mass for their own gravita­t­ional force to hold them to­geth­er. Oort sug­gested there must be more, un­seen mat­ter.

Re­search­ers still haven’t been able to find it. But sev­er­al cos­mo­lo­g­i­cal the­o­ries sug­gest par­t­i­cles that could form it. And ev­i­dence for it has been mount­ing, ac­cord­ing to many as­tro­no­mers.

A year ago, re­search­ers claimed that “proof” of dark mat­ter could be found in a stup­en­dous crash be­tween two gal­axy clus­ters. The event had wrenched apart “dark” and “nor­mal” mat­ter—which nor­mally hang around close to­geth­er—letting as­tro­no­mers de­tect each sep­a­rate­ly. The dark mat­ter was again de­tecta­ble through its gra­vity, they said, which subtly al­ters the paths of light rays from ga­lax­ies in the back­ground.

Oth­er si­m­i­lar find­ings ap­peared more re­cent­ly, in the June 1 is­sue of The As­t­ro­phys­i­cal Jour­nal. This time re­search­ers us­ing NASA’s Hub­ble Space Tel­e­scope said they found a huge, ghostly “ring” of dark mat­ter in a gal­axy clus­ter called Cl 0024+17, five bil­lion light-years from Earth, again thanks to a col­li­sion. NASA billed the find­ing as some of “the strongest ev­i­dence yet” for dark mat­ter.

Yet prob­lems keep crop­ping up, Mc­Gaugh wrote. A study in the May 10 is­sue of Sci­ence found that the bi­zarre gravita­t­ional ef­fects at­trib­ut­ed to dark mat­ter were show­ing up even where they weren’t ex­pected to un­der dark mat­ter the­o­ry it­self. Mod­i­fied gra­vity the­o­ry—ac­cord­ing to which gra­vity is stronger on in­ter­ga­lac­tic scales than the stand­ard laws of gra­vity sug­gest—ex­plained the puz­zle neat­ly, he wrote. 

The Sci­ence stu­dy, by Frédéric Bour­naud, of the AIM Lab­o­r­a­to­ry in Gif-sur-Yvette, France, and col­leagues, ex­am­ined a “d­warf gal­ax­y” that had formed from a larg­er gal­ax­y’s ejected de­bris in a crash. Com­put­er sim­ula­t­ions had found that only nor­mal mat­ter should be ejected in these col­li­sions; yet in con­flict with this, the dwarf gal­axy seemed to con­tain the same pro­por­tions of dark mat­ter seen else­where. 

This sug­gests the “dark mat­ter” there­in is just some hid­den form of or­di­nary atoms, re­search­ers said—not the un­fa­mil­iar, ex­ot­ic sub­stance that as­tro­no­mers tra­di­tion­ally pos­tu­late most dark mat­ter is, based on sev­er­al the­o­ret­i­cal con­sid­era­t­ions. Con­ven­tion­al mod­els hold that of the 27 per­cent of the un­iverse that con­sists of mat­ter, more than 98 per­cent is un­seen; about five-sixths of that in turn is non­bary­onic, mean­ing not or­di­nary atoms.

Bour­naud pro­posed the stuff in the dwarf gal­axy is just hard-to-see hy­dro­gen. The find­ings may “pose an ex­is­ten­tial cri­sis for non­bary­onic,” or ex­ot­ic, dark mat­ter, Mc­Gaugh wrote. 

On the oth­er hand, awk­wardly for MOND, cer­tain gal­axy clus­ters still seem not to have enough mass even af­ter that the­o­ry is ap­plied. So “MOND ap­pears to re­quire dark mat­ter it­self—a con­sid­er­able em­bar­rass­ment for a the­o­ry that seeks to sup­plant the need for invis­i­ble mass,” Mc­Gaugh wrote.

MOND re­search con­tin­ues, though it’s less pop­u­lar than dark mat­ter stud­ies. Arx­iv.org, an on­line reposito­ry of phys­ics pa­pers, lists 12 pa­pers with MOND in the ti­tle for this year to date, four of which are ac­cept­ed for pub­lica­t­ion in sci­en­tif­ic jour­nals. By com­par­i­son there are 39 pa­pers with “dark mat­ter” in the ti­tle. 

Fu­ture ex­pe­ri­ments could find dark mat­ter in the lab­o­r­a­to­ry, if it ex­ists: phys­i­cists be­lieve the Large Had­ron Col­lider, a par­t­i­cle ac­cel­er­a­tor to be built near Ge­ne­va, might do so. “Re­gard­less of how these ex­pe­ri­ments play out, there is clearly a great deal of fun­da­men­tal phys­ics left for us to learn,” Mc­Gaugh wrote.

* * *

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Astronomers have believed for decades that most of the matter in the cosmos is unseen. It betrays itself only through its gravitational pull on visible objects, whose movements are often hard to explain without this “dark matter.” And the past year has seen increasingly bold claims that astronomers have “proved” the stuff’s existence. Despite that, there remains a core of skeptics who dispute it. Many of them are sticking by an alernative theory that holds that tweaking our understanding of gravity could explain things better than invoking some unseen substance unlike any we know. One such modified-gravity theory has been “remarkably resilient,” wrote astronomer Stacy McGaugh of the University of Maryland in College Park, Md. in the Aug. 3 issue of the research journal Science. That theory, known as Modified Newtonian Dynamics or MOND, was proposed in 1983 by the Israeli physicist Mordehai Milgrom. Astronomers have suspected dark matter’s existence since the 1930s, when the Dutch astronomer Jan Oort found that galaxies didn’t contain nearly enough visible mass for their own gravitational force to hold them together. Oort suggested there must be more, unseen matter. Researchers still haven’t been able to find it. But several cosmological theories suggest particles that could form it. And evidence for it has been mounting, according to many astronomers. A year ago, researchers claimed that “proof” of dark matter could be found in a stupdendous crash between two galaxy clusters. The event had wrenched apart “dark” and “normal” matter—which normally hang around close together—letting astronomers detect each separately. The dark matter was again detectable through its gravity, they said, which subtly alters the paths of light rays from galaxies in the background. Other similar findings appeared more recently, in the June 1 issue of the Astrophysical Journal. This time researchers using NASA’s Hubble Space Telescope said they found a huge, ghostly “ring” of dark matter in a galaxy cluster called Cl 0024+17, five billion light-years from Earth, again thanks to a collision. NASA billed the finding as some of “the strongest evidence yet” for dark matter. Yet problems keep cropping up, McGaugh wrote. A study in the May 10 issue of Science found that the bizarre gravitational effects attributed to dark matter were showing up even where they weren’t expected to under dark matter theory itself. Modified gravity theory—according to which gravity is stronger on intergalactic scales than the standard laws of gravity suggest—explained the puzzle neatly, he wrote. The Science study, by Frédéric Bournaud, of the AIM Laboratory in Gif-sur-Yvette, France, and colleagues, examined a “dwarf galaxy” that had formed from a larger galaxy’s ejected debris in a crash. Computer simulations had found that only normal matter should ejected in these collisions; yet in conflict with this, the dwarf galaxy seemed to contain the same proportions of dark matter seen elsewhere. This suggests the “dark matter” therein is just some hidden form of ordinary atoms, researchers said—not the unfamiliar, exotic substance that astronomers traditionally postulate most dark matter is, based on several theoretical considerations. Conventional models hold that of the 27 percent of the universe that consists of matter, more than 98 percent is unseen; about five-sixths of that in turn is nonbaryonic, meaning not ordinary atoms. Bournaud proposed the stuff in the dwarf galaxy is just hard-to-see hydrogen. The findings may “pose an existential crisis for nonbaryonic,” or exotic, dark matter, McGaugh wrote. On the other hand, awkwardly for MOND, certain galaxy clusters still seem not to have enough mass even after that theory is applied. So “MOND appears to require dark matter itself—a considerable embarrassment for a theory that seeks to supplant the need for invisible mass,” McGaugh wrote. MOND research continues, though it’s less popular than dark matter studies. Arxiv.org, an online repository of physics papers, lists 12 papers with MOND in the title for this year to date, four of which are accepted for publication in scientific journals. By comparison there are 39 papers with “dark matter” in the title. Future experiments could find dark matter in the laboratory, if it exists: physicists believe the Large Hadron Collider, a particle accelerator to be built near Geneva, might do so. “Regardless of how these experiments play out, there is clearly a great deal of fundamental physics left for us to learn,” McGaugh wrote.