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Did “dark stars” reign in early time?

Dec. 6, 2007
Courtesy University of Utah
and World Science staff

“Twin­kle, twin­kle lit­tle star” is a be­lov­ed nurse­ry rhyme—but per­haps a very wrong de­scrip­tion of the ear­li­est stars, some sci­en­tists pro­pose. Their new study cal­cu­lates that the first stars could have been gi­gan­ti­c and in­vis­i­ble “dark stars,” some of which might still ex­ist.

This artist's con­cep­tion shows what an in­vis­i­ble "dark star" might look like when viewed in in­fra­red light that it emits as heat. The co­re is en­vel­oped by clouds of hy­dro­gen and he­li­um gas. A new Uni­ver­si­ty of Utah study sug­gests the first stars in the uni­verse did not shine, but may have been dark stars. (Cour­te­sy U. of Utah)

These gloomy ob­jects would be pow­ered by the mys­te­ri­ous “dark mat­ter,” un­seen stuff that sci­en­tists be­lieve makes up most mat­ter in the un­iverse, though it’s rec­og­nized only through its gravita­t­ional pull on the oth­er mat­ter.

The study “dras­tic­ally al­ter the cur­rent the­o­ret­i­cal frame­work for the forma­t­ion of the first stars,” said study co-author and as­t­ro­phys­i­cist Pa­o­lo Gon­dolo of the Un­ivers­ity of Utah. The find­ings are to be pub­lished next month in the re­search jour­nal Phys­i­cal Re­view Let­ters.

How long dark stars would live is un­clear, so they may still con­ceivably ex­ist, Gon­dolo added; “we have to search for them.” They may be de­tect­a­ble, he said, through their emis­sions of par­t­i­cles known as gam­ma rays, neu­tri­nos and an­ti­mat­ter; and through their as­socia­t­ion with clouds of cold, mo­lec­u­lar hy­dro­gen that nor­mally would­n’t har­bor these en­er­get­ic par­t­i­cles.

Gon­dolo said his re­search col­leagues de­cid­ed to call the shad­owy ob­jects dark stars, from the ti­tle of a 1967 song by rock band The Grate­ful Dead. Dark star is a “catchier” term, Gon­dolo ad­mit­ted, than the one he him­self pre­ferred: brown gi­ant.

The un­iverse is thought to have formed 13 bil­lion years ago in a sud­den ex­pan­sion or “infla­t­ion” of time and space dubbed the Big Bang. Some time lat­er, some of the ear­li­est mat­ter be­gan clump­ing to­geth­er—with gra­vity’s help—pro­duc­ing stars and ga­lax­ies. But phys­i­cists es­ti­mate that vis­i­ble mat­ter rep­re­sents only four per­cent of the un­iverse, which al­so con­sists of 23 per­cent dark mat­ter and 73 per­cent “dark en­er­gy”—anoth­er un­seen force, which helps the un­iverse ex­pand.

Con­ven­tion­al the­o­ry holds that the first stars ap­peared as hy­dro­gen and he­li­um atoms clumped to­geth­er in­to dense clouds. Even­tu­ally the grow­ing pres­sure in them trig­gered nu­clear fu­sion, the pro­cess that keeps stars lit.

But past stud­ies haven’t con­sid­ered the role of dark mat­ter in this, Gon­dolo said. His group de­cid­ed to ad­dress that—a proj­ect com­pli­cat­ed by the fact that sci­en­tists don’t know what dark mat­ter is. But one lead­ing view is that it con­sists of ent­i­ties called weakly in­ter­act­ing mas­sive par­t­i­cles, or WIMPS. A type of these, called a neu­tra­lino, must ex­ist un­der the­o­ries that seek to ex­plain the or­i­gin of mass, Gon­dolo said.

If form­ing stars con­tained dark-mat­ter neu­tralinos, Gon­do­lo’s group found, these should have in­ter­acted so they an­ni­hi­lated each oth­er, pro­duc­ing sub­a­tom­ic par­t­i­cles called quarks and an­ti­quarks. This would al­so give off heat. Rath­er than cool­ing and shrink­ing like a nor­mal, em­bry­on­ic star, dark mat­ter would keep this ob­ject hot and large, pre­vent­ing fu­sion. 

Dark stars would form some 80 mil­lion to 100 mil­lion years af­ter the Big Bang, Gon­dolo said. And al­though they would con­tain mostly nor­mal mat­ter, they would be vastly larg­er and “fluffier” than fa­mil­iar stars, be­ing some 400 to 200,000 times as wide as our sun.

They would al­so have glowed in­fra­red light, which is heat: “with your bare eyes, you can’t see a dark star. But the radia­t­ion would fry you,” Gon­dolo re­marked. The quarks and an­ti­quarks pro­duced would, in turn, gen­er­ate fur­ther par­t­i­cles in­clud­ing gam­ma rays, neu­tri­nos and “an­ti­mat­ter,” a rare sub­stance con­sid­ered a sort of evil twin of nor­mal mat­ter.

Dark stars could be im­por­tant to as­t­ro­phys­i­cists for sev­er­al rea­sons, Gon­dolo said. First, they could aid the quest to iden­ti­fy dark mat­ter: gam­ma rays, neu­tri­nos and an­ti­mat­ter have char­ac­ter­is­tic en­er­gy sig­na­tures if they come from dark mat­ter. 

Sec­ond­ly, dark stars may ex­plain why black holes—col­lapsed stars so dense that not even light es­capes—formed much faster than theo­ries say they should have. Gon­dolo said black holes ex­isted only a few hun­dred mil­lion years af­ter the Big Bang, yet cur­rent the­o­ries say they took long­er to form. “Dark stars may help,” he pro­posed, be­cause in one sce­nar­i­o, “they could col­lapse in­to black holes very ear­ly.” But anoth­er pos­si­bil­ity, he added, is that they eventually be­come con­ven­tion­al stars, adding a twin­kle to our nights.

* * *

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“Twinkle, twinkle little star” is a beloved nursery rhyme—but perhaps a very wrong description of the earliest stars, some scientists propose. Their new study calculates that the first stars could have been gigantic and invisible “dark stars,” some of which might still exist. These gloomy objects would be powered by the mysterious “dark matter,” unseen stuff that scientists believe makes up most matter in the universe, and that is recognized only through its gravitational pull on the other matter. The new work “drastically alter the current theoretical framework for the formation of the first stars,” said study co-author and astrophysicist Paolo Gondolo of the University of Utah. The findings are to be published next month in the research journal Physical Review Letters. How long these dark stars would last is unclear, so they may still conceivably exist, Gondolo added; “we have to search for them.” They may be detectable, he said, through their emissions of particles known as gamma rays, neutrinos and antimatter, and their association with clouds of cold, molecular hydrogen that normally wouldn’t harbor these energetic particles. Gondolo said his research colleagues decided to call the shadowy objects dark stars, from the title of a 1967 song by rock band The Grateful Dead. The name is “catchier,” Gondolo admitted, than the term he himself preferred, brown giant. Physicists estimate that visible matter represents only four percent of the universe, which also consists of 23 percent dark matter and 73 percent “dark energy”—another unseen force, which helps the universe expand. The universe is thought to have formed 13 billion years ago in a sudden expansion or “inflation” of time and space dubbed the Big Bang. Some time later, some of the earliest matter began clumping together—with gravity’s help—producing stars and galaxies. They contained mostly dark matter but also included hydrogen and helium, which are normal matter. Conventional theory holds that the first stars appeared as hydrogen and helium atoms clumped together into dense clouds. Eventually the growing pressure in the clouds triggered nuclear fusion, the process that keeps stars lit. But past studies haven’t considered the role of dark matter in the first stars, Gondolo said. His group decided to address this—a project complicated by the fact that scientists don’t know what dark matter is. But one leading view is that it consists of entities called weakly interacting massive particles, or WIMPS. A type of these, called a neutralino, must exist under theories that seek to explain the origin of mass, Gondolo said. If forming stars contained dark-matter neutralinos, Gondolo’s group found, these should have interacted so they “annihilated” each other, producing subatomic particles called quarks and antiquarks. This would also give off heat. Rather than cooling and shrinking like a normal, embryonic star, dark matter would keep this object hot and large, preventing fusion. Dark stars would form some 80 million to 100 million years after the Big Bang, Gondolo said. And although they would contain mostly normal matter, they would be vastly larger and “fluffier” than familiar stars, being some 400 to 200,000 times as wide as our sun. They would also have glowed infrared light, which is heat: “with your bare eyes, you can’t see a dark star. But the radiation would fry you,” Gondolo remarked. The quarks and antiquarks produced would, in turn, generate further particles including gamma rays, neutrinos and “antimatter,” a rare substance considered a sort of evil twin of normal matter. Dark stars could be important to astrophysicists for several reasons, he added. First, he explained, they could aid the quest to identify dark matter: gamma rays, neutrinos and antimatter have characteristic energy signatures if they come from dark matter. Secondly, dark stars may explain why black holes—collapsed stars so dense that not even light escapes—formed much faster than expected. Gondolo said black holes existed only a few hundred million years after the Big Bang, yet current theories say they took longer to form. “Dark stars may help,” he proposed, because in one scenario, “they could collapse into black holes very early.” But another possibility, he added, is that they eventually become conventional stars, adding a twinkle to our nights.