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February 24, 2016

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“Loneliest places” in cosmos may be less empty than thought

Feb. 25, 2016
Courtesy of the Royal Astronomical Society
and World Science staff

Sci­en­tists say the uni­verse is struc­tured like an enor­mous cob­web filled with gi­ant voids, not un­like the holes in Swiss cheese.

Along the cob­web’s many knots and fil­a­ments lie ga­lax­ies. The voids, on the oth­er hand, are al­most emp­ty—or so it was thought, un­til now.

Above:A slab cut from the cu­be gen­er­at­ed by the Il­lus­tris sim­u­la­tion. It shows the dis­tri­bu­tion of dark mat­ter, with a width and height of 350 mil­lion light-years and a thick­ness of 300000 light years. Ga­lax­ies are found in the small, white, high-density dots. Be­low: The same slice, this time show­ing the dis­tri­bu­tion of nor­mal or bary­onic mat­ter. (Cred­it: Markus Haider / Il­lus­tris col­lab­o­ra­t )


Now, a group of as­tro­no­mers claims these voids could con­tain as much as one-fifth of the so-called “nor­mal” mat­ter in the uni­verse, the type of stuff that bas­ic­ally makes up stars and plan­ets. 

Ac­cord­ing to their find­ings, the voids have re­ceived that ma­te­ri­al thanks to the help of black holes at the cen­ters of ga­lax­ies. Black holes are ob­jects so com­pact that their gra­vity be­comes over­whelm­ing and draws in an­y­thing that strays too close, in­clud­ing light. But some of the in­com­ing ma­te­ri­al ends up get­ting spout­ed away—very far away.

The new find­ings don’t sug­gest an­y­thing es­pe­cially ex­cit­ing is go­ing on in the cos­mic voids where that ma­te­ri­al would end up. We would­n’t find much more than a very thin, cold gas float­ing out there. 

On the other hand, the sci­en­tists ar­gue that the find­ings could solve a long­stand­ing prob­lem re­gard­ing why a lot of ma­te­ri­al seems to be mis­sing from the mod­ern uni­verse com­pared to its an­cient self. This is called the “mis­sing bary­on prob­lem.”

The re­search team, led by Markus Haider of the In­sti­tute of Astro- and Par­t­i­cle Phys­ics at the Un­ivers­ity of Inns­bruck in Aus­tria, pub­lish their find­ings in a new pa­per in the journal Monthly No­tices of the Roy­al As­tro­nom­i­cal So­ci­e­ty.

Mod­ern sat­el­lite ob­ser­va­to­ries look­ing at cos­mic mi­cro­wave radia­t­ion—essen­tially a left­o­ver glow from the “Big Bang” thought to have giv­en rise to the uni­verse—have grad­u­ally re­fined our un­der­stand­ing of its make­up. 

The most re­cent mea­sure­ments sug­gest it con­sists of about 5 per­cent “nor­mal” mat­ter, which in­cludes, for ex­am­ple, ev­er­ything we can ac­tu­ally see. Mean­while about 27 per­cent is a mys­te­ri­ous and un­seen “dark” mat­ter, de­tected only through its gravita­t­ional ef­fects, and 68 per­cent is the even more mys­te­ri­ous “dark en­er­gy” that’s push­ing apart the uni­verse.

Ground-based tele­scopes have mapped out many ga­lax­ies and, in­di­rect­ly, the “dark mat­ter” that clings to them, show­ing they lie in this cos­mic web.

Haider and his team in­ves­t­i­gated this in more de­tail, us­ing da­ta from the Il­lus­tris proj­ect, a large com­put­er sim­ula­t­ion of the ev­o­lu­tion and forma­t­ion of ga­lax­ies.

Il­lus­tris sim­ulates a cu­be of space in the uni­verse, meas­ur­ing some 350 mil­lion light years on each side. A light-year is the dis­tance light trav­els in a year. The Il­lus­tris sim­ula­t­ion starts when the uni­verse was “on­ly” 12 mil­lion years old, and tracks how gra­vity and the flow of mat­ter changes the struc­ture of the cos­mos up to now. The sim­ula­t­ion deals with both nor­mal and dark mat­ter, with the most im­por­tant ef­fect be­ing the gravita­t­ional pull of the dark mat­ter.

When the sci­en­tists looked at the da­ta, they found that count­ing all sorts of mat­ter, in­clud­ing dark, only about 6 per­cent of the ma­te­ri­al lies in the voids, which take up four-fifths of the space.

But these voids con­tain a sur­pris­ingly high­er frac­tion of nor­mal mat­ter when one con­sid­ers that on its own—al­most a third of the nor­mal mat­ter, Haider’s team found. And most of that, com­pris­ing about 20 per­cent of the to­tal nor­mal mat­ter, seems to have got­ten in­to the voids thanks the help of the “su­per­mas­sive” black holes found in the cen­ters of ga­lax­ies. 

How would that work? Some of the mat­ter fall­ing to­wards the holes is con­vert­ed in­to en­er­gy. This en­er­gy is de­liv­ered to the sur­round­ing gas, and leads to large out­flows of mat­ter, which stretch for hun­dreds of thou­sands of light years from the black holes, reach­ing far be­yond the ex­tent of their host ga­lax­ies.

“This sim­ula­t­ion, one of the most soph­is­t­icated ev­er run, sug­gests that the black holes at the cen­ter of ev­ery gal­axy are help­ing to send mat­ter in­to the lone­li­est places in the uni­verse. What we want to do now is re­fine our mod­el, and con­firm these in­i­tial find­ings,” Haider said.

The new find­ing, ac­cord­ing to Haider and col­leagues, might help ex­plain the “mis­sing bary­on” prob­lem, where as­tro­no­mers don’t see the amount of nor­mal mat­ter pre­dicted by their mod­els—as if some of the mat­ter formed in the early uni­verse simply dis­ap­peared lat­er.

Il­lus­tris is now run­ning new sim­ula­t­ions, and re­sults from these should be avail­a­ble in a few months, with the re­search­ers keen to see wheth­er for ex­am­ple their un­der­stand­ing of black hole out­put is right. Whatev­er the out­come, it will be hard to see the mat­ter in the voids, as this is likely to be too thin and cold to be de­tect­a­ble, the sci­en­tists not­ed.


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Scientists say the universe is structured like an enormous cobweb filled with giant voids, not unlike the holes in Swiss cheese. Along the cobweb’s many knots and filaments lie galaxies. The voids, on the other hand, are almost empty—or so it was thought, until now. Now, a group of astronomers claims these voids could contain as much as one-fifth of the so-called “normal” matter in the universe, the type of stuff that basically makes up stars and planets. According to their findings, the voids have received that material thanks to the help of black holes at the centers of galaxies. Black holes are objects so compact that their gravity becomes overwhelming and draws in anything that strays too close, including light. But some of the incoming material ends up getting spouted away—very far away. The new findings don’t suggest anything especially exciting is going on in the cosmic voids where that material ends up. We wouldn’t find much more than a very thin, cold gas floating out there. But the scientists argue that the findings could solve a longstanding problem regarding why a lot of material seems to be missing from the modern universe compared to its ancient self. This is called the “missing baryon” problem. The research team, led by Markus Haider of the Institute of Astro- and Particle Physics at the University of Innsbruck in Austria, publish their findings in a new paper in Monthly Notices of the Royal Astronomical Society. Modern satellite observatories looking at cosmic microwave radiation—essentially a leftover glow from the “Big Bang” thought to have given rise to the universe—have gradually refined our understanding of its makeup. The most recent measurements suggest it consists of about 5 percent “normal” matter, which includes, for example, everything we can actually see. Meanwhile about 27 percent is a mysterious and unseen “dark” matter, detected only through its gravitational effects, and 68 percent is the even more mysterious “dark energy” that’s pushing apart the universe. Ground-based telescopes have mapped out many galaxies and, indirectly, the “dark matter” that clings to them, showing they lie in this cosmic web. Haider and his team investigated this in more detail, using data from the Illustris project, a large computer simulation of the evolution and formation of galaxies, to measure the mass and volume of these filaments and the galaxies within them. Illustris simulates a cube of space in the universe, measuring some 350 million light years on each side. A light-year is the distance light travels in a year. The Illustris simulation starts when the universe was “only” 12 million years old, and tracks how gravity and the flow of matter changes the structure of the cosmos up to now. The simulation deals with both normal and dark matter, with the most important effect being the gravitational pull of the dark matter. When the scientists looked at the data, they found that counting all sorts of matter, including dark, only about 6% of the material lies in the voids, which take up four-fifths of the space. But these voids contain a surprisingly higher fraction of normal matter when one considers that on its own—almost a third of the normal matter, Haider’s team found. And most of that, comprising about 20% of the total normal matter, seems to have gotten into the voids thanks the help of the “supermassive” black holes found in the centers of galaxies. How would that work? Some of the matter falling towards the holes is converted into energy. This energy is delivered to the surrounding gas, and leads to large outflows of matter, which stretch for hundreds of thousands of light years from the black holes, reaching far beyond the extent of their host galaxies. “This simulation, one of the most sophisticated ever run, suggests that the black holes at the centre of every galaxy are helping to send matter into the loneliest places in the universe. What we want to do now is refine our model, and confirm these initial findings,” Haider said. The new finding, according to Haider and colleagues, might help explain the “missing baryon” problem, where astronomers don’t see the amount of normal matter predicted by their models—as if some of the matter formed in the early universe simply disappeared later. Illustris is now running new simulations, and results from these should be available in a few months, with the researchers keen to see whether for example their understanding of black hole output is right. Whatever the outcome, it will be hard to see the matter in the voids, as this is likely to be too thin and cold to be detectable, the scientists noted.