Fresh light illumines search for dark matter

New evidence has emerged for the existence of dark matter -- an invisible constituent of the universe that is believed to play a key role in the formation of galaxies.

By P S Sandhu
Published: Wednesday 31 March 1993

Infrared image of the M81 spir (Credit: NASA)ASTRONOMERS are excited about the recent observations made by the satellite ROSATwhich provide the strongest evidence yet that as much as 90 per cent of the matter in the universe is invisiblewhose presence is indicated by its gravitational pull on matter in space.

Astronomers have been searching for this invisible masswhich they call dark matterever since its existence was postulated in the 1970s. But the amount of dark matter in the universe and the form in which it exists are still uncertain. Malcolm Longair of Cavendish laboratory in Cambridge comments ruefullyI always feel that it is somewhat embarrassing for astronomers to admit they do not know what is the form of most of the mass of the universe.

Their embarrassment may not be for longgoing by the ROSAT observations announced at a meeting in January in PhoenixArizonaof the American Astronomical Society. Research groups worldwide are conducting experiments to detect dark matter and their findingsexpected in the next few yearscould transform the current knowledge of physics (NatureVol. 361No. 6408).
What ROSAT saw ROSATlaunched jointly in June 1990 by GermanyUSA and Britainrecently scanned an immense cloud of hot gasabout 1.3 million light-years in width and positioned near the NGC 2300 group of galaxies.

From their knowledge of the size and temperature of the cloudscientists determined the amount of matter visible in the form of luminous gas was between one-tenth and one-thirtieth of the total mass. The invisible remainderthey heldcan only be accounted for by dark matter.

For the first time,explained Joseph Silkan astronomer at the University of California (UC) at Berkeleyone can conclusively state that diffuse dark matter dominates the gravitational potential of galaxy groups on large scales of distance. The more dramatic consequence is that we can now aspire to ascertain the nature of the dark matter.

Previous evidence for dark matter was far more indirect and inferred from the motions of galactic groups seemingly induced by the gravitational influence of an invisible mass. The cloud probed by ROSAT lies in a small cluster of galaxiesa structure which is far more representative of the universe as a whole.

ROSAT's scan revealed not only the gas temperature but also an abundance of heavy elements in the cloudwhich enabled scientists to determine the gas in the cloud is predominantly primordial and predates galaxy formation. They say the ratio of normal matter to dark matter in the cloudthereforeis likely to reflect the early history of the universewhen dark matter supposedly was formed.

Delhi University astrophysicist N Panchapakesan says the ROSAT scan of the gas cloud was a happy accidentresulting from a six-hour gap in the regular observation schedule of the satellite's X-ray detector. The detector was aimed at the NGC 2300 gas cloud at the suggestion of David Burstein of Arizona State Universitywho suspected the presence of dark matter in the cloud from observations made with a ground-based telescope.

Galaxy formation
The presence of dark matter is an essential feature of current theories on the origin of the universe. According to the standard Big Bang theory of cosmologythe universe originated about 15 billion years ago in an explosion of dense matter and radiation and it has been expanding and cooling since.

Big Bang proponents say that soon after the explosionnormal matter and radiation were largely uniform (to one part in 100As a relic of the Big Bangall parts of the sky are filled with background radiation in the form of microwaves. The recent measurements of this radiation performed by NASA's satellite COBE have proved to be of great importance. They have given the first direct observations of the distribution of matter just 3000years after the Big Bang. Although there is still considerable debate about the interpretation of these resultsmany astrophysicists feel that the COBE findings are consistent with the existence of dark matter. They are convinced that force of gravity could not have integrated matter into the galaxies in their present form unless allowance is made for dark matter.

Dark matter is postulated to consist of weakly interacting particles that decoupled from normal matter and radiation at an early stage in the history of the universe and gathered into lumps. Thenwhen ordinary matter separated from radiation at a later stageit was rapidly attracted to the lumps of dark matter to form galaxies. "Although there is continuing debate as to the precise nature of dark matterthe concept has emerged as one of the leading approaches to explaining galaxy formation at the present timesays Alan Guth of Massachusetts Institute of Technology, who first proposed the inflationary theory of cosmology (See box).Hot" and "cold" dark matter
Theorists describe two different scenarios for galaxy formationbased on the possibility that dark matter could exist in "hot" or "cold" formsdepending on whether it is composed of particleslike neutrinoswhich travel at speeds close to that of lightor of slower-moving particles.

Calculations based on the "hot" scenario indicate a vast difference from the present distribution of galaxies in clusters; the "cold" scenarioon the other handexplains more successfully the clustering of galaxies. Computer simulations of how galaxies form and clusterassuming the existence of cold dark matterhave been performed by astronomers Marc Davis at UC-BerkeleyGeorge Efstathiou at CambridgeCarlos Frenk of the University of Durham and Simon White of the University of Arizona (New ScientistVol. 129No. 1759). Their simulations accurately reproduced the actual distribution of galaxies in the universeproviding compelling support for the cold dark matter hypothesis.

Experimental evidence
Observations of spiral and elliptical galaxies have provided more evidence for the existence of dark matter. In the 1970sVera Rubin and Kent Ford of the Carnegie Institute in USAdiscovered that the outer parts of many galaxies rotate far too fast for the gravitational force of visible matter to be holding them in orbit. Other observations of big clusters of galaxies and of flows of galaxies towards unseen massesfurther suggested the presence of dark matter.

Also supporting the existence of dark matter are estimates of the density of the universe. Scientists say if the density of the universe is less than a certain critical valueit will continue to expand forever; butif it is morethen expansion will eventually cease and the universe will collapse into itself.

That the density of the universe is at the critical value is an essential requirement of the so-called inflation theory of cosmology. But the density due to the total amount of visible matter in the universe accounts for less than 1 per cent of the critical value. Thereforeif the universe is indeed at critical densitythe remaining mass must be dark matter.

Different forms
Part of the dark matteraccording to scientistscould be in the form of ordinary particles called baryonsof which the most common examples are the neutrons and protons that make up the bulk of normal matter. They say baryonic matter in the form of black holesdim starsplanets or asteroids could be a constituent of dark matter. Howeverit is known from calculations based on the proportion of light elements like helium and lithium present in the universe that the density of baryonic matter that exists in the universe is less than 5 per cent of the critical density.

If the universe is indeed at critical density or close to itthe only possibility is that upto 95 per cent of the matter is in the form of non-baryonic dark matterwhich is entirely different from any matter known today.

Density of the universe
Arguments for the existence of non-baryonic dark matter depend crucially on the density of the universe being close to its critical value. ButUniversity of Washington astronomer Craig Hogan contends the density of the universe is only about 20 per cent of the critical value (New ScientistVol. 129No. 1750).

The inflationary theory that predicts the universe is at critical density may be wronghe suggests. Andby studying non-standard models of cosmologydifferent from the inflationary modelHogan has come up with a baryon density of 20 per cent of the critical densitywhich is just enough to account for the observed motions of the galaxies.

Experimental efforts to determine the value of the density have been inconclusive and all that astronomers can say is that the actual density of the universe is between 0.03 and 2 times the critical density.

HoweverK I Kellerman of the US National Radio Astronomy Centre analysed data from 82 radio wave sources in distant galaxies and determined the density of the universe to be close to the critical valuewhich is consistent with the inflationary theory (NatureVol. 361No. 6408).

Kellerman's method was based on a principle suggested in 1958 by English cosmologist Fred Hoylewho found the apparent size of a distant object varies with its redshift (the shift in the frequency of light emitted by it towards the red end of the spectrum) because of the density of the universe. By plotting the size-redshift relation for the radio wave sourcesKellerman obtained the curve expected for a universe at critical density. His findings provide directlarge-scale observational evidence that the universe contains much more mass than can be explained in terms of ordinary baryonic matter.

Physicistsinvestigating this possibility in detailpostulate as candidates for dark matteresoteric entities ranging from cosmic strings and black holes to hypothetical particles like axions and WIMPs (Weakly Interacting Massive Particles).

Researchers at Brookhaven National Laboratory recently launched the first experiments to detect the axion (ScienceVol. 357No. 5070). Their search is based on the fact that in the presence of a magnetic fieldan axion changes to a photonwhich can be detected as a blip of radiation. Although the search is still to yield an axionthe Brookhaven researchers have been able to place an upper limit on its mass.

The unaccounted mass in the universe could be explained in terms of neutrinoswhich are lightneutral particlesprovided they are ascribed a massas several experimental groups have suggested. Even if neutrinos have a mass about one in ten-thousandth of an electronit would be sufficient -- because there are on average about 100 neutrinos per cubic centimetre throughout all space -- to account for all the dark matter in the universe. Howeverthe evidence for neutrinos to have a mass is still weak.

The search for WIMPs
Many current experimental efforts to detect dark matter focus on WIMPspredicted by a theory widely discussed amongst physicists called the supersymmetry theorywhich postulates every known particle has a massive supersymmetric partner. Of thesethe particle corresponding to the photoncalled the photinois the most likely candidate to account for dark matter.

Astronomers estimate from the calculated density of dark matter in the earth's galaxy that there are between 10 and 100 WIMPs in every litre of space. These particles would be moving at one-thousandth the speed of light so that millions of them would be streaming each second through every square centimetre of the earth's surface. Yetthese particles interact so weakly with matter that the majority pass through the earth without leaving the slightest trace. Hencethe task of detecting WIMPs requires experiments with incredible levels of precision.

WIMPs can be detected only by their collisions with an atomic nucleus or electron. About one WIMP is expected to collide with an atom in a kilogram of any material in a day and when the collision occursthe atomic nucleus recoils.

The resulting change in its energy can be detected in a variety of ways. For examplein a semiconductor such as siliconan atom recoiling from collision with dark matter would release free electric charges in a process called ionisation. These charges can be collected and measured with an electronic circuit.

Another method uses certain crystals and liquids that emit recordable light when an atomic collision occurs in an effect called scintillation. A third method is based on the fact that in a crystalif a moving atom slows downit loses energy in the form of vibrations called phonons. This can be detected at temperatures close to absolute zero.

All these methods have been clearly demonstrated in tests with photons and neutrons. Howeverthey cannot be used to detect WIMPs directly because of interference from other sources. Although WIMPs are responsible for about one collision per day in a single kilogram of materialthe same block of material also experiences millions of collisions each day with atomic particles from other sourcessuch as cosmic rays from space and gamma rays and neutrons from radioactivity in surrounding materials.

Thereforeexperiments to detect WIMPs have to be conducted at a depth of at least 10metres to block the most penetrating particles in cosmic rays. Andplacing the measuring apparatus in an enclosure shielded with pure lead or waterprotects it from the radioactivity of the surrounding rock.

Even with these precautionsthe task of building detectors sensitive enough to record as little as a few collision events in a day is formidable. Neverthelessseveral research groups are trying to detect WIMPs experimentally.

A group led by Peter Smith of the Rutherford Appleton Laboratory in the UK is assembling equipment at the bottom of a salt mine in England that is the deepest in Europe and hopes to start their experiments in June (New ScientistVol. 134No. 1818).

At the University of CaliforniaBernard Sadoulet and his colleagues are busy testing detectors with which they will work this winter in an underground facility at nearby Stanford. European researchers are also planning similar experiments in the Apennine mountains in Italy.

Meanwhilescientists are also searching for dark matter in the normal baryonic form -- massive black holes or dead stars that do not emit light. One search method involves the gravitational bending of lighta consequence of Einstein's theory of relativity. If a massiveinvisible object in the earth's galaxy passes close to light from a distant stargravity would act like a lens and concentrate the starlight so it increases in brightness. It is estimated that if a significant fraction of the dark matter in the earth's galaxy is due to such objectsabout 10 to 100 instances of such brightening should be witnessed each year and groups in France and USA have begun to look (New ScientistVol. 134No. 1818). They will continuously monitor about one million stars in a cluster called the Large Magellanic Cloudclose to the earth's galaxy. Should they observe only a fewer number of such eventsit would mean a large part of the dark matter in the galaxy is non-baryonic in nature.

As the search for dark matter intensifies astronomers and astrophysicists are gleefully anticipating a period of exciting work indoors and outdoorsunderground and in space.

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