Donald Goldsmith- Dark Matter

8/3/2019 Donald Goldsmith- Dark Matter

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dark matter, ar more than all the luminous stu in the galaxys stars. The bulk o it orms a giant,dark halo extending ar beyond the star-strewngalactic expanses that we see.

Astronomers today, applying Zwickys logic,are still detecting vast quantities o dark matter indistant galaxy clusters. Among the clusters, theyhave observed clouds o hot gas, which would haveescaped the clusters gravitational pull billions o

years ago i the clusters had no more mass thanthat o their stars.

Impressive as those observations are, theres evenmore evidence or the unseen presence o dark matter:the phenomenon o gravitational lensing. Becausegravity bends space itsel (Einsteins nest insightinto nature), light passing close by a massive objectdeviates rom a straight-line trajectory. Hence i amassive object happens to lie almost directly alongour line o sight to a more distant source o light,such as a galaxy, the light we see will be bent or even

ocused, much as i the intermediate object were anoptical lens [ see illustration on opposite page ]. A small

amount o light bending, or lensing, can distortthe galaxy into an unusual shape, just as the thickglass bottom o an old Coke bottle distorts the shapeo a light bulb when you look at the bulb th rough thebottle. Stronger lensing can actually create multipleimages o the same l ight source. Gravitational lens-ing enables astronomers to map the distr ibution o all matter, not just visible matter, because all matter can give rise to a lensing e ect.

What, then, is this dark matter that makes up byar the bulk o all the matter in the universe? No

one knows. But cosmologists do know one thingor sure: most o it cannot be anything like the

matter amiliar to us.Cosmologists classi y all matter into two kinds:

baryonic and nonbaryonic, or, basically, the ordinaryand the exotic. Baryon comes rom the Greekroot barys, meaning heavy; the term was coinedto re er to the heavy particles that use together in the nuclei o ordinary atomsneutrons andprotons. They ar outweigh the electrons, whichare leptons, or light particles, not baryons. Withthe realization that matter exists in more exotic

orms, the term nonbaryonic came to denotenot only leptons but also all other particles thatdo not participate in nuclear usion. One o themost important clues to the mystery o dark matter

comes rom the growing evidence that the bulk o itand thus, most o the matter in the un iverseisnonbaryonic matter.

Baryonic matter orms stars, planets, moons,and even the interstellar gas and dust rom whichnew stars are born. Nonbaryonic matter includesneutrinos, tiny particles each having less than a mil-lionth the mass o the already diminutive electron.Neutrinos were once regarded as likely candidates

or dark matter because they exist in such prodi-gious numbers, but they have now been excluded

rom the dark-matter sweepstakes. Detailed studieso how galaxies orm suggest that dark matter ismost likely made o particles whose masses range

rom roughly that o the proton to several hundredtimes as much.

How do astrophysicists in er that such hypotheti-cal particles o dark matter must be nonbaryonic?They can estimate the total amount o matter

rom the e ects o gravitational lensing and thedistribution o cosmic background radiation.The baryonic part o that total then comes romthe current understanding o how the cosmosbehaved during its earliest epochs. The big bang,with which the universe began, opened an erao nuclear- using ury, a time when all particles

crowded together at unimaginably high densitiesand temperatures. All creation then resembledthe cauldron at the core o a st ar, only ar moreso. From the countless nuclear usions that tookplace in those rst ew minutes a ter the bigbang, there emerged the basic ratio o nuclei inthe universe today: almost entirely hydrogenand helium, with only a minute smattering o all heavier nuclear varieties.

By the end o its rst ew minutes, the un iversehad expanded and cooled, dipping below thebillion-degree temperatures needed or nuclear

usion. Only in much later, highly localizedevents did the stars cook up almost all the heavier elements, such as the carbon, nitrogen, oxygen,silicon, and iron that make up our planet andourselves. Those heavier nuclei, however, com-prise no more than 2 percent o the mass o allbaryonic matter. The other 98 percent is stillmade up o hydrogen, helium, and their isotopes, created immediately a ter the bigbang. By measuring the relative a mountso the various isotopes o hydrogen andhelium nuclei, cosmologists can deducehow much baryonic matter took part in thegreat crucible o cosmic nuclear usion inthe rst hal hour o the universe.

Those results, now con rmed by detailedstudies o the cosmic background radiation,

lead to a startli ng conclusion. Baryonic mat-tersome o it in stars, but much more indi use interstel lar gas orms no more thana sixth o all matter in the universe. Theother ve-sixths must be nonbaryonic matter,either in the orm o elementary part icles or clumped into much larger objects.

The ascination with the unruly propertieso dark matterits distr ibution in space, andmost o all its predominantly nonbaryonicnaturehas given rise to a fourishing dark-matter community. Some members can point to achieve-ments such as improved maps o dark matter andits distribution in intergalactic space [ see illustrationon the following two pages]. Others strive to design,build, and operate experiments that may somedaydetermine the nature o nonbaryonic dark mat-ter, or at least eliminate rom contention some o the hypothetical particles that elementary-particlephysicists have proposed.

Be ore surveying those experiments and thehypotheses that motivate them, its worth notingthat a ew ingenious minds will have none o thedark-matter mystery. Instead, they suggest, theobservations show merely t hat physicists dont yet

ully understand g ravity. Suppose that at t he greatestcosmic distances, gravitational orces deviate slightly

rom what Newton proposed and Einstein re ned.In that case, the motions o stars and galaxies mig htnot refect the existence o enormous quantitie s o dark matter, but rather the simple re usal o theuniverse to obey what physicists presume to bethe laws o nature.

The Israeli physicist Mordehai Milgrom o theWeizmann Institute in Rehovot, Israel, proposedthat approach, and or a time his idea seemed toexplain the observational results without recourseto much dark matter. But to many astronomersnow, Milgroms idea seems on the verge o beingdisproved. Increasingly accurate observations o stellar and galactic motions at various distance scalesseem to con rm existing theories o gravity.

I Einsteins theory o gravity is correct, as appearsto be the case, then nonbaryonic mattermatter

orever di erent rom all known matterhas alwaysruled the univer se. And the best hope or discovering

just what orm it takes now rest s on nding some o itat least a tiny amount!here on Earth. But thatposes a dilemma. The basic nature o nonbaryonicdark matter, its extreme unwillingness to engagein any interactions with ordinary particles, makesit extremely di cult to detect.

The dark-matter communit y, ully aware o thedi culties, has concentrated on experiments de-signed to nd the leading dark-matter candidates.The candidates come in two categories: relativelylarge objects, and submicroscopic elementaryparticles, which would have to exist in hugequantities. The sizable dark-matter candidates

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Map o dark matter (light blue), digitally superposed on a photographmade by the Hubble Space Telescope, shows that a giant ring o invis-ible mass surrounds the dense core o a giant cluster o galaxies called ZwC10024+1652, about 5 billion light-years rom Earth. Astronomersmapped the distribution o mass in the galaxy cluster by observing thee ects o gravitational lensing on background galaxies.

Gravitational lensing can occur when light rom a distant galaxy,center le t, passes through a dark-matter halo around a cluster o galaxies. Here the gravitational pull o the dark matter defects thelight in such a way that an observer on Earth sees two additional images o the galaxy. The diagram is highly idealized; the distancesand angles are not drawn to scale.

Virtual imageo galaxy

Distant galaxy

Dark matter

Cluster o galaxies

Virtual imageo galaxy


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have the generic name MACHO s, short or mas-sive compact halo objects. MACHO s might beblack holes with masses something like that o astar; or smaller, more numerous black holes withmasses similar to t hose o planets; or perhaps thecores o burned-out stars that collapsed but didnot orm black holes. Gravitational lensing canreveal MACHO s, and astronomers have even ounda ew with starlike masses. But the results so ar imply that MACHO s cannot supply the bulk o the cosmic mass.

I so, the best hopes lie with nonbaryonic elemen-tary part icles, which exist so ar only in theory. Butsome o the theories predicting their existence displaypromising elegance and symmetry, so particles arethe avored dark-matter candidates. Two kinds o hypothetical particles seem the most appealing.

First is the axion, a particle named a ter a laun-dry detergent, because its hypothetical propertiescleaned up a confict between a theory known asquantum chromodynamics and certain experimen-tal results. Each axion would have an exceedinglysmall massless than a millionth o the electronsown tiny mass.

I axions do exist and throng our gala xy, theymust occasionally be scattered by the magnetic

elds that permeate the Mil ky Way. The scatter-ings would generate radio waves at a requency

that depends on the small (and unknown) masso the axion. The worlds most advanced axiondetector, at the Lawrence Livermore NationalLaboratory in Cali ornia, seeks those radio wavesby searching a wide band o possible requen-cies with supremely sensitive ampli ers. So ar,all axion searches have proven ruitless, but thesearch goes on.

I not axions, why not WIMP s? The namestands or weakly interacting massive part icleaconcise description o the second leading candi-date among hypothetical dark-matter particles.Weakly interacting means interacting mainlyvia the weak orce, the orce responsible or certainkinds o atomic decay and the least ami liar o

the our undamental orcesin nature. Massive in thiscontext means at least a ewdozen times the mass o a pro-ton. (The name MACHO s,

or dark-matter black holes,was chosen to contrast withWIMP s, which was proposed

rst.) Because WIMP s ariserom the predictions within

a class o persuasive theorieso elementary particles calledsupersymmetric, many particletheorists think WIMP s exist.

To nd the elusive WIMP s, ex-perimental physicists are betting onthe likelihood that, once in a blue moon,a WIMP will collide with ordinary matter.Such a collision would lead to a wimp yas inamazingly smalle ect in the bowels o a WIMP detector, so extraordinary measures must be takeni physicists hope to notice it. Experimenters reducethe normal atomic vibrations o the sensors withinthe detector as ar as possible by cooling the sensorsclose to absolute zero. Placing the apparatus deepunderground shields it rom inter erence rom lesspenetrating potential sources o spurious signals,such as the cosmic rays that continuously bombardthe Earth.

At least hal a dozen competing teams o ex-perimenters rom Europe and the United Statesare now operating and improving their WIMP detectors, which build on two basic designs. Inthe rst design, the sensors are several dozencrystals, each weighing about a kilogram, madeo highly puri ed germanium or silicon. Twodetectors employ that design, one inside the GranSasso tunnel, nearly a m ile beneath Italys Apen-nine mountains, and the other at the bottom o a decommissioned mine in northern Minnesota.I a WIMP strikes an atom in one o the crystals,the crystal should ever so slightly heat up andvibrate. So ar the cr ystal detectors have ound noWIMP s, but the crystals may well ail to provide

22 n a t u r a l h i s t o r y September 2007 September 2007 n a t u r a l

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nally September 2007.

a su ciently large target to succeed in only a year or two o operation.

To increase their chances o registering WIMP impacts, the experimenters naturally would like toenlarge the target, but with crystal-based detectors,that poses technological di culties. Enter the second,

and newer, kind o WIMP detector, whose target isa pool o ultra-pure, lique ed inert gas. Ongoingexperiments with lique ed argon or xenon detectorsnow operate within the Boulby mine i n Yorkshire,England, and in the Gran Sasso tunnel.

Like the crystal detectors, the inert-gas detectorshave yet to register a single WIMP . Nevertheless,experimenters have high hopes or success withthe design, because they can scale up inert-gasdetectors with relative ease. The next generationwill deploy not a ew kilogram s but a ew hundredkilograms o target material. Elena Aprile, a physicistat Columbia University who leads the attempt to

nd a xenon-bumping WIMP in the Gran Sassotunnel, hopes or a sizable gain in sensitivity bythe end o 2008.

In short, attempts to nd the elusive particles o dark matter have so ar yielded only hope andconstruction contracts. In more scienti c language,the experimentally established upper limits on thetendency o WIMP s or axions to interact with or-dinary matt er have grown progressively smaller. So

ar those upper limits do not rule out the viabilityo either o these dark-matter candidates. No onecan say, just yet, that axions or WIMP s do not exist

in su cient numbers, and with enough mass per particle, to account or the bulk o the nonbaryonicdark matter.

Experimenters working with both kinds o dark-matter detectors know that a ormidablecompetitor looms on the horizon. The LargeHadron Collider ( LHC ), built at CERN , the Eu-ropean Organization or Nuclear Research, justoutside Geneva, is now scheduled to begin seriou soperation in mid-2008. Once up and running, theLHC will be the worlds biggest particle accelera-tor. Dug several hundred eet under the Swiss andFrench countryside, it will accelerate two clumps,or beams, o particles many times around a ringmore than ve miles across, be ore smashing thetwo beams into each other.

Although the LHC was not designed to search or dark matter, its collisions will g ive birth to the mostmassive particles ever generated in any machine. I the LHC can veri y supersymmetric particle theo-ries, as expected, it will con rm WIMP s as real.Naturally, the builders o dark-matter detectors,currently stymied in their searches or axions andWIMP s, would like nothing better than to nd thedark matter be ore the LHC can make its roundaboutcon rmation-by-implication.

They must hurry. Bernard Sadoulet, a physicistat the University o Cali ornia, Berkeley, who hasbecome the grand old man o dark-matter detec-

tion, thinks physics has a decent chance o rul ingaxions and WIMP s in or out o contention in thenext ve years.

And what i more sensitive experiments showthat neither axions nor WIMP s can explain the darkmatter? Li e will go on, and so will the universe,most o it made o dark matter o unknown orm,

just as it is today. Inventive theorists will suggestnew possibilities, and experimentally minded par-ticle physicists will improve their detectors and their analyses. And both theorists and experimenters willcontinue to work in the hope that they wil l be the

ortunate ones to resolve this undamental cosmicmystery. Two hundred ty years ago, the towno Whitby, in Yorkshire, produced James Cook,arguably the greatest explorer o Earth. Perhaps inthe next ve years the dark-matter experimentersin the Boulby mine, near Whitby, will be able toannounce one o the greatest discoveries about thecosmos: the nature o dark matter. )

Attempts to fnd the elusive particles that orm dark matterhave so ar yielded onlyhopeand construction contracts.

Three-dimensional map o dark matter, derived

rom observations o the e ects o gravitational lensing, reveals vast isles o dark matter

that dominate the large-scale structure o the universe.The map covers a patch o sky 1.6 degrees on a side, out to 8 billion light-years rom Earth. The ancient light romdistant galaxies enables astronomers to reconstruct how dark matter has evolved (right to le t) since early in thehistory o the universe.

Toda y

8 billion years ag

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Donald Goldsmith- Dark Matter