'Dark matter' doubters not silenced yet
Aug 03,2007 00:00 by Bend_Weekly_News_Sources

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’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.

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])

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.

Courtesy World Science staff