Collider: The Search for the World's Smallest Particles - Paul Halpern (2009)
Chapter 9. Denizens of the Dark
Resolving the Mysteries of Dark Matter and Dark Energy
I know I speak for a generation of people who have been looking for dark-matter particles since they were grad students. I doubt . . . many of us will remain in the field if the L.H.C. brings home bad news.
—JUAN COLLAR, KAVLI INSTITUTE FOR COSMOLOGICAL PHYSICS (NEW YORK TIMES, MARCH 11, 2007)
There’s an urgency for LHC results that transcends the ruminations of theorists. For the past few decades, astronomy has had a major problem. In tallying the mass and energy of all things in the cosmos, virtually everything that gravitates is invisible. Luminous matter, according to current estimates, comprises only 4 percent of the universe’s contents. That small fraction includes everything made out of atoms, from gaseous hydrogen to the iron cores of planets like Earth. Approximately 23 percent is composed of dark matter: substances that give off no discernable light and greet us only through gravity. Finally, an estimated 73 percent is made of dark energy: an unknown essence that has caused the Hubble expansion of the universe to speed up. In short, the universe is a puzzle for which practically all of the pieces are missing. Could the LHC help track these pieces down?
Anticipation of the missing matter dilemma dates back to long before the issue gained wide acceptance. The first inkling that visible material couldn’t be the only hand pulling on the reins of the universe came in 1932, when Dutch astronomer Jan Oort found that stars in the outer reaches of our galaxy moved in a way consistent with much greater gravitational attraction than observed matter could exert. The Milky Way is in some ways like a colossal merry-go-round. Stars revolve around the galactic center and bob up and down relative to the galactic disk. Oort found that he could measure these motions and calculate how much total gravitational force the Milky Way would need to exert to tug stars back toward its disk and prevent them from escaping. From this required force, he estimated the Milky Way’s total mass, which became known as the Oort Limit. He was surprised to find that it was more than double the observed mass due to shining stars.
The following year, Bulgarian-born physicist Fritz Zwicky, working at Caltech, completed an independent investigation of the gravitational “glue” needed to keep a massive group of galaxies called the Coma Cluster from drifting apart. Because the galaxies within this formation are widely separated, Zwicky estimated a very high figure for the gravity needed. Calculating the amount of mass needed to furnish such a large force, he was astounded to discover that it was hundreds of times that of the luminous matter. Some invisible scaffolding seemed to be providing the support required to hold such a far-flung structure together.
Scientists in the 1930s knew little about the cosmos, aside from Hubble’s discovery of its expansion. Even the concept of galaxies as “island universes” beyond the Milky Way was relatively new. With physical cosmology in such an early stage of development, the astonishing findings of Oort and Zwicky were largely ignored. Decades passed before astronomers acknowledged their importance.
We owe current interest in dark matter to a courageous young astronomer, Vera Cooper Rubin, who entered the field when women were often discouraged from pursuing it. Rubin was born in Washington, D.C., and her childhood hobbies included stargazing from her bedroom window and reading astronomy books—particularly a biography of comet discoverer Maria Mitchell. Much to her frustration, the vocation then seemed a clubhouse with a No Girls Allowed sign prominently displayed.
As Rubin later recalled: “When I was in school, I was continually told to go find something else to study, or told I wouldn’t get a job as an astronomer. But I just didn’t listen. If it’s something you really want to do, you just have to do it—and maybe have the courage to do it a little differently.”1
After earning a B.A. at Vassar, where Mitchell once taught, and an M.A. at Cornell, Rubin returned to her native city to pursue graduate studies in astronomy at Georgetown University. Though not on Georgetown’s faculty, George Gamow, with whom she shared an interest in the behavior of galaxies, was permitted to serve as her thesis adviser. Under his valuable supervision, she received her Ph.D. in 1954.
While raising four children with her husband, mathematician Robert Rubin, it took some time for her to find a permanent position that offered her suitable flexibility. In 1965, the Department of Terrestrial Magnetism of Carnegie Institution in Washington appointed her to its research staff. She soon teamed up with a colleague, Kent Ford, who had built his own telescope. Together they began an extensive study of the outer reaches of galaxies.
Focusing on the Milky Way’s nearest spiral neighbor, the Andromeda galaxy, Rubin and Ford used a spectrograph to record the Doppler shifts of stars on its periphery. A Doppler shift is an increase (or decrease) in frequency of something moving toward (or away) from an observer. The amount of the shift depends on the moving object’s relative velocity. It occurs for all kinds of wave phenomena—light as well as sound. We notice the Doppler effect when fire engines wail at higher and higher pitch when racing closer, and lower and lower when speeding away. For light, moving closer means a shift toward the bluer end of the spectrum (a blue shift, for short), and moving away, a shift toward the redder end (a red shift). Hubble used galactic red shifts in his proof that remote galaxies are receding from us. Doppler spectroscopy has continued to serve as a vital tool in astronomy.
Mapping out the shifts in light spectra of Andromeda’s outermost stars, Rubin and Ford were able to calculate their velocities. They determined how quickly these outliers orbited the galaxy’s center. Plotting stars’ orbital speeds versus their radial distances, the Carnegie researchers produced an impressive graph, called a galactic rotation curve, displaying how Andromeda steered its remotest material.
As Kepler discovered centuries ago, for astronomical situations, such as the solar system, for which the bulk of the material is in the center, objects take much longer to orbit the farther they are from the middle. The outer planets orbit much slower than the inner ones. While Neptune orbits at a tortoiselike 3.4 miles per second, Mercury whizzes around the Sun at an average pace of 30 miles per second. The reason is that the gravitational influence of the Sun drops off sharply at large radial distances and there is not enough mass in the outer solar system to affect planetary speeds to a large degree.
Spiral galaxies such as the Milky Way were once thought to have a similar central concentration of material. Visibly, the densest concentration of stars lies in bulges around their middles. The outer spiral arms and haloes surrounding the main disks seem, in contrast, to be wispy and dilute. But appearances can deceive.
The Carnegie researchers connected the dots on Andromeda’s rotation curve. Fully expecting to see the velocities drop off with radial distances, as in the solar system, they were baffled when their points, even in the outermost reaches, continued along a flat line. Rather than a mountain slope, the curve resembled a level plateau. A flat-velocity profile meant a sprawling out in mass beyond the frontiers of the observed. Something unseen was lending gravitational strength to places where gravity should be puny.
To see if their results were peculiar to Andromeda or more general, Rubin and Ford teamed up with two of their colleagues, Norbert Thonnard and David Burstein, to investigate sixty additional spiral galaxies. Although not all galaxies are spiral—some are elliptical and others are irregular—they chose that pinwheel-like shape because of its simplicity. Unlike other galactic types, the outer stars in spirals generally revolve in the same direction. For that reason, their speeds are easier to plot and analyze.
Relying on data collected by telescopes at Kitt Peak Observatory in Arizona and Cerro Tololo Observatory in Chile, the team members plotted out rotation curves for all sixty galaxies. Amazingly, each exhibited the same leveling off in velocities observed for Andromeda. Rubin and her coworkers concluded that most of the material in spirals is spread out and invisible—revealing nothing about its content except its weighty influence. The mystery that had so troubled Oort and Zwicky was back in full force!
What lies behind the mask? Could dark matter be something ordinary that’s simply very hard to see? Could our telescopes just not be powerful enough to reveal the bulk of material in space?
A one-time leading dark matter candidate carries a name that matches its supposed gravitational brawn: MACHOs (Massive Compact Halo Objects). These are massive bodies in the haloes of galaxies that radiate very little. Examples include large planets (the size of Jupiter or greater), brown dwarfs (stars that never ignited), red dwarfs (weakly shining stars), neutron stars (collapsed stellar cores composed of nuclear material), and black holes. Each of these was formed from baryonic matter: the stuff of atomic nuclei and the like, such as hydrogen gas.
To hunt for MACHOs and other hard-to-see gravitating objects, astronomers developed a powerful technique called gravitational microlensing. A gravitational lens is a massive object that bends light like a prism. It relies on Einstein’s general relativity theory that heavy objects curve space-time, which in turn distorts the paths of light rays in their vicinity. This was verified in the 1919 observations of the bending of starlight by the Sun during a solar eclipse.
Microlensing is a way of using the gravitational distortion of light to weigh potential MACHOs when they pass between distant stars and Earth. If an unseen MACHO happened to move in front of a visible star (from a neighboring galaxy in the background, for instance), the starlight would brighten due to the MACHO’s gravitational focusing. After the MACHO moves on, the light would dim again, back to its original intensity. From this brightness curve, astronomers could determine the MACHO’s mass.
During the 1990s, the MACHO Project, an international group of astronomers based at Mt. Stromlo Observatory in Australia, catalogued thirteen to seventeen candidate microlensing events. The team discovered these characteristic brightness variations during an extensive search of the galactic halo using the Large Magellanic Cloud (a smaller neighboring galaxy) to provide the stellar background. From their data, the astronomers estimated that 20 percent of the matter in the galactic halo is due to MACHOs, ranging from 15 percent to 90 percent of the mass of the Sun. These results point to a population of lighter, dimmer stars in the Milky Way’s periphery that cannot directly be seen but only weighed. Though these objects might add some heft to the galactic suburbs, the MACHO Project has shown that they could account for only a fraction of the missing mass.
There are other reasons to believe that MACHOs could help resolve only part of the dark matter mystery. Using nucleosynthesis (element-building) models that estimate how many protons must have been present in the moments after the Big Bang to produce the elements we see today, astrophysicists have been able to estimate the percentage of baryonic matter in the universe. Unfortunately, these estimates show that only a small fraction of dark matter could be baryonic in nature; the rest must be something else. Made of conventional baryonic matter, MACHOs thereby could not provide the full explanation. Consequently, researchers have turned to other candidates.
The beefy acronym MACHO was chosen to contrast it with another class of dark-matter candidates, the ethereal WIMPs (Weakly Interacting Massive Particles). Unlike MACHOs, WIMPs would not be astronomical objects but rather new types of massive particles that interact exclusively through weak and gravitational forces. Because of their heaviness, they’d be slow moving—enabling their gravitational “glue” to help cement together the large structures in space we observe, such as galaxies and clusters.
Neutrinos would fit the bill if they were heavier and more lethargic, given that as leptons, they ignore the strong force, and as neutral particles, they pay no heed to electromagnetism. The lightness and swiftness of neutrinos, however, seems to rule them out as significant components. This fleeting nature is akin to a featherweight, constantly traveling politician trying to draw support for a local council race. Without setting down robust roots in his community, how could he bring people together? Similarly, neutrinos never hang around long enough or make enough of an impact to serve as uniters.
Particles such as neutrinos that would be too light and quick to create structure are sometimes referred to as “hot dark matter.” Although they might compose a portion of the missing mass in the universe, they could not explain how galaxies maintain such tight holds on their outermost stars nor how they clump together into clusters. Slower, bulkier substances, such as MACHOs and WIMPs, are grouped together into “cold dark matter.” These would offer suitable scaffolding—if we could only find enough of them.
If not neutrinos, then which other neutral, nonhadronic particles could carry enough mass and move at a slow enough pace to steer stars and gravitate galaxies? Unfortunately, the standard model doesn’t call any suitable candidates to mind. Apart from neutrinos, MACHOs, and WIMPs, another option, a hypothetical massive particle called the axion, postulated to play a role in quantum chromodynamics (the theory of the strong force) and tagged by some theorists as a leading dark-matter contender, has yet to be found. The search for the universe’s missing mass has been at an impasse.
Enter the LHC to the rescue. Perhaps somewhere in its collision debris the secret key ingredients of cold dark matter will be revealed. Prime contenders would be the lightest supersymmetric companion particles, such as neutralinos, charginos, gluinos, photinos, squarks, and sleptons. Presuming they have energies on the TeV scale, each would present itself through characteristic decay profiles that would show up in tracking and calorimetry.
If dark matter were the main cosmic mystery, physicists would simply be clenching their teeth, crossing their fingers, and waiting expectantly for results at the LHC or elsewhere to turn up a suitable prospect. It would be like posting a reasonable job description and hoping that the right person will eventually apply. However, a much more nebulous search—the quest for dark energy—has turned out to be far more unnerving. Not only is something seriously missing, but scientists have little idea where to look.
Dark energy first jolted the scientific community in 1998, when two teams of astronomers—a group from Lawrence Berkeley National Laboratory led by Saul Perlmutter and a collaboration based at Mt. Stromlo Observatory that included Adam Riess, Robert Kirschner, and Brian Schmidt—announced startling results about the expansion of the universe. Each used supernovas in remote galaxies as distance gauges to trace the cosmic expansion far back in time. By plotting the distances to these galaxies versus their velocities as found by Doppler red-shifts in their spectral lines, the teams could determine how Hubble’s law of galactic recession has changed over billions of years.
The type of exploding stars examined, called Supernova Ia, has the special property that their energy produced follows a regular progression. Because of this predictability, the teams were able to compare their actual with their observed light outputs and calculate how far away they are. This offered a yardstick to galaxies billions of light years away—recording their distances at the time of the stellar burst.
Astronomical objects with known energy output are called standard candles. Like distant street lamps on a dark road, you can judge their remoteness by how bright or dim they seem—assuming that they put out roughly the same wattage. If, when walking down a street at night, your eyes were dazzled by an intense glare, you would likely deem its light source much closer than if it were so faint that you could barely see it. You would thereby be able to use its relative brightness to estimate its distance. Similarly, astronomers rely on standard candles such as Supernova Ia to gauge distances for which there would be no other measure.
The team led by Perlmutter, called the Supernova Cosmology Project (SCP), has deep connections with the world of particle physics. First of all, along with George Smoot’s Nobel Prize- winning exploration of the cosmic microwave background using the Cosmic Background Explorer satellite, it represents an expansion of the mission of Lawrence’s lab. Given that Lawrence was always looking for connections and applications, such a broad perspective perfectly suits the former Rad Lab. Also, one of the SCP’s founding members is Gerson Goldhaber, who won acclaim for his role in the Stanford Linear Accelerator Center- led group that jointly discovered the J/psi particle. His older brother Maurice Goldhaber worked at Cavendish during the Rutherford/Chadwick era and was the longtime director of Brookhaven National Laboratory. So you could say that cosmology and high energy physics—the sciences of the extremely large and extraordinarily small—have become part of the same family.
When the SCP began its explorations, its researchers hoped to use supernova standard candles as means of pinning down the deceleration of the universe. The attractive nature of gravity means that any set of massive objects moving apart must reduce its outward rate of expansion over time. Simply put, what goes up must come down—or at least slow down. Cosmologists therefore expected that the dynamics of the cosmos would follow one of three different paths, depending on the universe’s density relative to a critical value: brake rapidly enough to reverse course, brake gradually enough not to reverse course, or brake at the precise rate required to remain perpetually on the cusp.
All three scenarios would begin with the standard Big Bang. If the density were high enough, the universe would slow down enough over time that after many billions of years its expansion would transform into a contraction. Eventually, everything would compress back together in a Big Crunch. If the density were lower than the critical value, on the other hand, the cosmic expansion would forever continue to slow down—like a tired runner sluggishly pushing himself forward. Though galaxies would move apart at an increasingly lethargic pace, they’d never muster the will to reunite. This possibility is called the Big Whimper. A third option—the density exactly matching the critical value—would involve a universe slowing down so much that it threatened recollapse but never quite did, like an acrobat carefully poised on a tightrope.
Perlmutter and his team fully expected to encounter one of these three possibilities. Their supernova data surprised them with a different story. Plots of velocity versus distance showed that the cosmic rate of expansion was speeding up, not slowing down. Something was pressing the gas pedal instead of the brakes—and it couldn’t be any of the known forces. University of Chicago theorist Michael Turner dubbed this unknown agent “dark energy.”
While both have mysterious identities, dark energy could not be the same as dark matter. In contrast to dark matter, which would gravitate in the same way as ordinary matter, dark energy would serve as a kind of “antigravity,” causing outward acceleration. If dark matter walked into a party, it would serve as a graceful host introducing people and bringing them together, but if dark energy intruded, it would act like the riot police dispersing the crowd. Indeed, too much dark energy in the cosmos would be no fun at all—the universe would eventually tear itself apart in a catastrophic scenario called the Big Rip.
Some physicists have represented dark energy by restoring Einstein’s once-discarded cosmological constant term to general relativity. Although adding such a constant antigravity term would be a simple move, it could use some physical motivation. Physicists would be loath to add anything to a well-established theory without understanding the need for the new term on a fundamental level. That would mean interpreting the field theory behind it. Current field theories, however, support a much larger value of the vacuum energy that would need to be almost, but not exactly, canceled out to yield a reasonable cosmological constant. Thus, matching experimental bounds for cosmic acceleration has proven a daunting task.
Moreover, if dark energy were a constant throughout space and time, it would never lose its effect. With gravity ceding more and more ground over the eons to dark energy, the Big Rip would be an absolute certainty. Before accepting such an outcome as inevitable, most theorists would like to mull over the alternatives.
Princeton physicist Paul Steinhardt, along with theorists Robert Caldwell and Rahul Dave, has suggested a different way of modeling dark energy, through a wholly new type of substance called quintessence. Quintessence is a hypothetical material with negative pressure that pushes things apart (like an elemental Samson on the Philistines’ columns) rather than pulling them together (like ordinary, gravitating matter). Its name harks back to the four classical elements of Empedocles—with quintessence representing the fifth. The distinction between a cosmological constant and quintessence is that while the former would be as stable as granite, the latter could vary from place to place and time to time like moldable putty.
Findings of the Wilkinson Microwave Anisotropy Probe of the cosmic microwave background support the idea that the cosmos is a mixture of dark energy, dark matter, and visible matter—in that order. The satellite picture has not been able to tell us, however, what specific ingredients constitute the duet of dark substances.
Physicists hope that further clues as to the nature of dark energy, as well as dark matter, will turn up at the LHC. The discovery of quintessence at the LHC, for example, would revolutionize the field of cosmology and transform our understanding of matter, energy, and the universe. Indeed, based upon what is found, the fate of everything in space could be in the balance.
Adding a cosmological constant or postulating a novel kind of material are not the only alternatives. Some theorists see a need to rethink the nature of gravity completely. Could gravitation behave distinctly on different scales—acting one way in the planetary arena and another in the galactic realm? Might Einstein’s equations of general relativity, accurate as far as we can judge, be superseded in the grandest domain by another theory? As Rubin has said, “I suspect we won’t know what dark matter is until we know what gravity is.”2
Radical new gravitational theories propose a fundamental change in its mechanism and scope. They imagine that some of gravity’s properties could be explained through its ability to penetrate unseen extra dimensions impervious to other forms of matter and energy. Conceivably, the dark substances in the universe could be shadows of a higher reality.
Remarkably, some of these novel theories, as strange as they seem, could be tested at the LHC. The extraordinary power of high-energy transformations may well reveal new dimensions in addition to novel particles. Who knows which of nature’s long-held secrets the LHC’s unprecedented energies will divulge?