Some Type of Mysterious, Invisible Mass Holds the Universe Together

Tales from the dark side What do we really know about dark matter? Some type of mysterious, invisible mass holds the universe together. Here’s how scientists are searching for it. by Liz Kruesi

Massive galaxy clusters like Abell 2218 have pro- vided scientists with evidence for the existence of dark matter. By analyzing the arcs — warped images of a background galaxy — surrounding the cluster center, astronomers determined the cluster must have a lot more mass than meets the eye. NASA/Andrew Fruchter/ERO Team [Sylvia Baggett (STScI), Richard Hook (ST-ECF), Zoltan Levay (STScI)] (STScI)

he universe doesn’t abide by “what it doesn’t emit X-rays or absorb infrared plotted the velocity versus the distance to Astronomers study the sizes and makes up most of their baryonic mass. Unlike anything we’ve seen you see is what you get.” In fact, radiation). This mysterious stuff is there- create a “rotation curve.” They expected shapes of those arcs to determine a clus- Ordinary matter interacts through elec- For many years astronomers thought dark the stuff we see in space — stars, fore invisible, yet astronomers learned it the velocities to reach a maximum and ter’s mass. By comparing that calculated tromagnetic forces. Thus, as matter col- matter could consist of dead stars, black gas, and dust — accounts for only exists because dark matter interacts with then decrease farther from the center — mass to the mass that comes from only lides it loses energy as radiation (in the holes, and other known objects that emit 10 percent of the universe’s mass. ordinary matter through gravity. but the data showed otherwise. The luminous objects (the galaxies), astrono- form of X-rays, in this case). The hot gas little or no light. They used gravitational TThis visible stuff is ordinary matter, and velocities reach a maximum and then mers can determine how much dark slows during the collision. microlensing to look for these objects. it’s made up of protons, neutrons, and Searching in the dark plateau. With velocities so high at the matter is in a cluster. Astronomers use gravitational lensing This technique is similar to gravitational electrons. Scientists call ordinary matter Swiss astrophysicist Fritz Zwicky first outer edge of galaxies, the stars should Other evidence has turned up in colli- to indirectly map the dark matter distri- lensing except the foreground object is “baryonic matter” because protons and proposed dark matter’s existence in 1933. fling out of their orbits. But they don’t. sions of galaxy clusters, namely the Bullet bution. It turns out that dark matter also much less massive. Instead of light bend- neutrons are subatomic particles called While studying the Coma cluster of gal- Some sort of mass scientists can’t detect cluster. This object is actually the after- passed through the collision unaffected. ing around the object, the body’s gravity baryons. The other 90 percent of the mass axies, he found that the galaxies’ collec- must be holding these outer stars in orbit. math of two galaxy clusters that collided. So images of the Bullet cluster show magnifies the light from behind it. While is “dark matter,” and it likely surrounds tive gravity alone was much too small to A very massive object — such as a gal- Astronomers used a multidetection direct evidence of dark matter. astronomers found some of these MAssive almost every galaxy in the universe. hold the cluster together. axy cluster — can act as a gravitational approach to look at the galaxies, gas, and The evidence is piling up — with 3-D Compact Halo Objects (MACHOs), there Dark matter doesn’t emit, absorb, or The next round of evidence came in lens. Some images of regions around gal- dark matter. When the clusters collided, dark matter maps and other detections. weren’t enough to account for all of the reflect any type of light (so, for example, the 1970s. Astronomers charted the axy clusters show numerous arcs. Those the galaxies’ stars passed through mostly Yet mapping the distribution is one thing; universe’s missing mass. velocities of stars at various distances are background galaxies distorted and unaffected because a lot of space exists knowing the characteristics of this myste- So if dark matter isn’t composed of Liz Kruesi is an associate editor of Astronomy. from the center of a spiral galaxy and magnified by the cluster’s gravity. between the stars. The clusters’ hot gas rious stuff is another story. normal objects, then it likely consists of

28 Astronomy • November© 2014 Kalmbach 09 Publishing Co. This material may not be reproduced in any www.Astronomy.com 29 form without permission from the publisher. www.Astronomy.com that each ordinary particle (such as an Simulating structure electron or quark) has a massive “super- partner” (a selectron or squark) that remains undetected. The leading dark matter candidate is a class of particles that supersymmetry predicts. These particles have mass and interact through the weak nuclear force, but they don’t interact through the elec- tromagnetic force. Because these weakly interacting massive particles (WIMPs) interact via the weak force, they can col- lide with normal atomic nuclei and bounce off them without emitting light

or absorbing radiation. The lightest Klypin (NMSU) Chicago)/Anatoly (U. Kravtsov Andrey WIMP — called the neutralino — is also The universe’s structure seems to evolve as smaller clumps bunch to form larger structures. Astronomers simulate structure evolution with cold dark matter the most popular dark matter possibility. and create models that resemble today’s universe. This simulation shows dark matter distribution, where brighter areas represent more dense regions. Another common CDM candidate is the axion. The axion is also a hypotheti- matter in a detector. A WIMP can collide cal particle, but it arises from a theory with an atomic nucleus and move, or different from supersymmetry. This par- “scatter,” the nucleus. ticle is not a “matter particle” but instead Another method is to indirectly detect a force carrier, similar to the photon dark matter. A WIMP’s antiparticle is

NASA/ESA/CXC/M. Bradac (University of California, Santa Barbara)/S. Allen (Stanford University) (Stanford Allen Santa Barbara)/S. of California, (University Bradac NASA/ESA/CXC/M. (which “carries” the electromagnetic itself, so if two WIMPs interact, they Two massive galaxy clusters collided to form what’s known as the Bullet cluster of galaxies. Ordi- force). It’s much lighter than a WIMP — annihilate each other and produce a nary matter — the hot gas, shown in pink — collided, lost energy, and slowed. The clusters’ dark at least 1 billion times less massive — so shower of secondary particles. Astro- matter (shown in blue) interacted little and passed through the ordinary matter. the universe would need a whole lot physicists can observe many of these sec- more axions than WIMPs to make up all ondary particles — such as electrons, non-baryonic particles — meaning it’s with dark matter,” says Juan Collar of the the invisible matter. positrons (the electron’s antiparticle), not made up of the same stuff as ordinary University of Chicago. “However, these One would expect that with so many gamma rays, and neutrinos. matter (protons and neutrons). Astrono- hypothetical particles turn out to have all CDM particles, WIMPs or axions would Scientists’ methods to locate axions mers split non-baryonic dark matter into of the properties (mass, abundance, life- be easy to find. But because they don’t are “totally different than the direct and two categories: hot and cold. These titles time, probability, and mode of interac- interact through the electromagnetic indirect detection methods used to look have nothing to do with temperature. tion) required to be the dark matter, or at force, detecting them pushes scientists’ for WIMPs,” says Dan Hooper of Fermi Hot means that early in the universe least a fraction of it.” experimental limits. National Accelerator Laboratory in Bata- these particles traveled extremely fast — For decades, physicists have worked to via, Illinois. When an axion traverses a almost at the speed of light. Cold means explain how the four fundamental physi- How to hunt CDM detector that has a magnetic field, it will that early in the universe the particles cal forces fit together. (These forces are The method used to detect dark matter convert into a photon. of Nottingham, U.K.)/ (University Gray Vancouver)/M. of British Columbia, (University NASA/ESA/C. Heymans U.K.)/K. Meisenheimer (Max University, (Oxford Wolf collaboration/C. (Innsbruck)/the STAGES M. Barden collaboration COMBO-17 Heidelberg)/the Astronomy, Institute for Planck traveled more slowly. gravitation, electromagnetism, weak depends on what type of dark matter Instead of trying to detect CDM par- Supercluster Abell 901/902 contains hundreds of galaxies. Astronomers analyzed the distortion of How does particle speed relate to dark nuclear, and strong nuclear.) In the past (WIMP or axion) scientists pursue. Sci- ticles, some scientists aim to create the some 60,000 background galaxies’ shapes to determine the supercluster’s distribution of dark mat- ter. They then combined a visible-light image of the supercluster with the dark matter distribution matter’s composition? Slower particles 30 or so years, they’ve arrived at super- entists looking for WIMPs try to directly particles — WIMPs and axions — in the map (shown as a magenta haze). will bunch up into small structures ear- symmetry theory. This model predicts observe the interaction with ordinary laboratory. To do this, they have to gen- lier in the universe. Those small struc- erate extremely high energies, similar to tures will eventually collide and merge to A rotation curve those shortly after the Big Bang. Only it moves through this dark matter haze. rial. And if they can’t block all of the form larger ones. Astronomers believe particle accelerators have this ability. These particles aren’t the only things noise, they must be able to tell the differ- this is how structure develops and A After the Large Hadron Collider (the Earth collides with as it moves through ence between noise and a WIMP. evolves in our universe: Smaller struc- world’s largest particle accelerator, the haze. Incoming high-energy ordinary Some scientists think about 600 mil- tures eventually merge into the massive B Observed located in Switzerland) comes back particles called cosmic rays bombard lion WIMPs pass through a square meter superclusters we observe today. Astrono- online late this year, scientists should be Earth constantly. Radiation from the Sun of Earth’s surface every second. But mers simulate structure evolution with Velocity able to look for hypothetical particles and more distant sources do, too. remember that they interact weakly. So cold dark matter (CDM) and can create that may make up dark matter. Scientists seek the WIMPs that may how do detectors “see” a WIMP? During Exp models that resemble today’s universe. ected compose the CDM haze by placing most a rare collision, the WIMP will transfer What is CDM? Scientists aren’t sure Bullying the WIMPs dark matter detectors underground and some of its energy to an atom’s nucleus of yet. They have a couple of options that Speed of star A = Speed of star B Astronomers believe a spherical halo of shielding them to block the detector the detector material, and, as a result, the branch from particle physics — but none Distance from center CDM surrounds the luminous galactic material from cosmic rays. The key is to nucleus scatters (think of pool balls). The contrived just to fit into dark matter the- Astronomers were surprised to find that stars A galaxy’s rotation curve compares the disk disk of the Milky Way (and similar halos be able to block signals from “back- amount the nucleus moves (or “recoils”) ories. “Both [options] are generated by far from a galaxy’s core travel at speeds similar stars’ velocities with their distances from the encase most other galaxies). As our solar ground noise” and detect when a dark is related to the WIMP’s energy. Scientists particle theories having nothing to do to those close in. John Smith galaxy’s center. Astronomy: Roen Kelly system travels around the galaxy’s center, matter particle interacts with the mate- detect this recoil a few different ways.

30 Astronomy • November 09 www.Astronomy.com 31 One type of detector uses crystals kept A reliable WIMP detection would be Germanium An incoming WIMP collides at frigid temperatures (only 0.01 degree if the WIMP signal varied as a result of crystal structure with a germanium nucleus above absolute zero). Crystals have a set the time of year. This is because Earth structure, so when a WIMP collides with revolves around the Sun. In June, Earth’s an atomic nucleus, the nucleus recoils and movement is in the same direction as our rams into surrounding structure. In these solar system’s path around the galaxy, so collisions, the scattered nucleus transfers the detected signal should increase. In Germanium atom some of its kinetic energy and slightly December, Earth moves in the opposite heats the material. The frigidity ensures direction, and scientists should detect a that the detected vibrations have resulted signal some 5 to 10 percent smaller. This from only incoming particle interactions. signal difference helps distinguish the Of course, the scientists will likely detect WIMPs from the background noise particles other than just WIMPs. So most because the noise remains the same while ATIC collaboration ATIC : Roen Kelly Astronomy The balloon-borne detector Advanced Thin WIMP detectors use multiple methods to the WIMP signal modulates. Ionization Calorimeter (ATIC) found a nearby determine “on an event-by-event basis if The team of scientists with the DArk source of mysterious cosmic rays. The source what took place looked like a dark matter MAtter (DAMA) experiment claimed could be a dark matter cloud. particle interaction or something more some years ago (and again in 2008) that it mundane,” says Collar. found evidence for the existence of Crumbs along the trail When a WIMP scatters the atomic WIMPs by looking at this modulation. nucleus and hits surrounding atoms, it Unfortunately, DAMA used only one A group using a balloon-borne detector last could knock off electrons, therefore “ion- detection method and therefore may not November disclosed a previously unknown Protons source of high-energy electrons (cosmic izing” these atoms. Certain ionization have been able to discriminate between WIMP rays). Cosmic-ray particles tend to lose much detectors can measure these loose charges. background noise and a WIMP signal. Neutrons of their energy by the time they traverse the In some materials, such as liquid And no other scientific group has repeated Electrons galaxy and Earth’s atmosphere. So scientists xenon, a light flash will indicate a WIMP. DAMA’s discovery. In science, if another typically detect low-energy cosmic rays near After the scattered nucleus rams into group can’t repeat a finding, then there’s a A weakly interacting massive particle (WIMP) occasionally will collide with an Earth’s surface. The high-energy electrons other atoms and frees electrons, the atom distinct possibility that experimental error atomic nucleus. This collision should move, or scatter, the nucleus, which could then that the Advanced Thin Ionization Calorime- emits a light flash called scintillation. and not evidence is responsible. collide with nearby atoms. Scientists may be able to detect heat or light from the interaction. ter (ATIC) group found indicate the electrons Usually if a detector looks for scintilla- are coming from a nearby source — within tion, it will also hunt for ionization. WIMPy signals about 3,000 light-years. Another approach to the direct search So far, direct searches haven’t found The density of neutralinos (or other greater energies. And a larger detector an antenna with amplifier. According to By analyzing the electrons’ detected energies, scientists can determine the is using a bubble chamber — a glass jar WIMPs. Therefore scientists also look for WIMPs) must be high in order for these should find more neutrinos (hopefully of theory, as an axion traverses the cavity, it energy the particles had before traversing filled with a specific type of liquid. the indirect signature of the dark matter particles to meet up and destroy each the WIMP variety). The next-generation will convert into a microwave photon. the atmosphere. That energy matches what When a WIMP hits an atomic nucleus, candidates to complement direct searches. other. This typically happens within neutrino detector IceCube should help in The photon’s frequency will be propor- scientists expect from the products of a pos- it will produce a tiny bubble. Scientists Neutralino annihilations should produce more massive objects. this search. IceCube is currently being tional to the axion’s mass. Scientists, sible cold dark matter particle’s annihilation. then watch the bubble grow. How it electrons, positrons, gamma rays, and A WIMP near the Sun or Earth could built at the South Pole, and will cover a however, aren’t sure what the axion’s mass When two Kaluza-Klein (KK) particles meet, grows depends on whether the interact- neutrinos, along with other particles. Sci- collide with an ordinary particle’s nucleus. very large area — a cubic kilometer. is, which means they’re unsure what fre- they annihilate each other and produce an ing particle was a WIMP or a back- entists can use certain detectors to look (This is similar to what happens within Searches for gamma rays from WIMP quency to search for. Using the antenna electron and its antiparticle, the positron. ground particle. for each product. detector material.) The WIMP will lose annihilations also look promising. The and amplifier, scientists will scan por- (ATIC can’t tell the difference between elec- energy, and its speed could decrease gamma rays should have a specific tions of the microwave region looking for trons and positrons, so its electron detec- below the Sun or Earth’s escape velocity. energy spectrum that depends on how a signal that stands out above the back- tion is a total number of electrons and Look for the secondary particles If that happens, the WIMP cannot escape massive the WIMP is. The Fermi ground noise. positrons.) If the detected particles truly are Neutrino products from KK annihilation, then our the massive object’s gravitational hold. Gamma-ray Space Telescope may be able Detector sensitivity is slowly getting solar system may be passing through or Gamma ray The WIMP can collide with another to detect that particular spectrum and to where it needs to be to pick out axions near a large clump of KK dark matter. Proton nucleus and so on until it settles into the offer an indirect observation of dark mat- — and WIMPs — from background However, these high-energy electrons core of either the Sun or Earth. ter. Says Hooper, “If I had to wager a noise. It’s not there yet, but scientists, could also arise from an undiscovered pulsar At the core, the densities are so high guess, I would say that the best prospects with the help of elegant particle theories, Positron or other object. And more recent observa- Neutralinos that WIMPs collide and produce second- to detect WIMPs in the near future are are throwing everything they have at tions by the Fermi Gamma-ray Space Tele- ary particles and radiation. (As men- with gamma-ray telescopes.” A number searching for (and hopefully detecting) scope cast doubt on the ATIC observations. Electron tioned before, neutrinos and gamma rays of ground-based gamma-ray detectors dark matter. A group using a different detector — the are two such products.) Several experi- are also on the lookout. “Very often in particle physics we have Payload for Antimatter Matter Exploration Antiproton Baryonic ments underground — such as Super- followed such ‘natural’ prescriptions, and Light-nuclei Astrophysics (PAMELA) sat- (ordinary) matter Decay process Kamiokande in Japan — detect neutrinos. Where’s the axion? only to be surprised by nature,” says Col- ellite — reported in 2008 that it saw a WIMP collisions aren’t the only nearby A WIMP may be the leading CDM candi- lar. With more advanced detectors com- higher-than-expected number of positrons. The leading dark matter candidate is the theorized neutralino, the lightest weakly interact- PAMELA looks at energies lower than ATIC. ing massive particle. When two neutralinos collide, they annihilate each other and create a events that release neutrinos — the Sun date, but it isn’t the only one. The axion is ing in the next decade or so, cosmologists Some scientists think that PAMELA may shower of secondary particles. Numerous detectors are looking for these secondary particles, produces them. Neutrino detectors can also a popular possibility. are sure to get a surprise — whether a have observed positrons from another dark which are therefore an indirect signature of dark matter. Astronomy: Roen Kelly decipher WIMP neutrinos from solar An axion detector consists of two hint that they’re on the wrong path or a matter particle’s annihilation. — L. K. neutrinos because WIMP neutrinos have parts: a cavity with a magnetic field and promising detection.

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