Identifying the dark matter of the Universe is one of the most intruiging problems of modern cosmol- ogy and particle physics. Several Experiments are underway around the world, including tests to detect dark matter candidates known as Weakly Interacting Massive Particles (WIMPs) as they travel through the Earth and at CERN's Large Hadron Collider (LHC), which is trying to produce WIMPs through proton beam collisions.
In presence of self- annihilations, WIMPs captured inside a star can provide an exotic source of energy. Whereas the effects on the Sun are negligible because of the small dark matter density in the solar neighborhood, they may be remarkable on stars living in dense dark-matter environments such as the Milky Way's galactic center (image above), or in galactic halos.
This is the case, for instance, of many asymmetric DM (ADM) models, where DM annihilations may not occur in presence of an asymmetry between DM particles and antiparticles. Still, these particles accumu- lated in a star "can scatter nuclei and transport energy within its bosom. This effect results in a modi- cation of the density and temperature pro les which can lead to detectable changes of the solar neutrino uxes and gravity modes; more compact objects like neutron stars may cumulate enough particles to reach the Chandrasekhar mass and drive the collapse of a central black hole.
The team's reserach showed that the energy transport induced by asymmetric energy transport induced asymmetric dark matter by accumulating inside such stars can provoke dramatic effects on the stellar structure.
The scientists have taken the next step and applied this strategy to main sequence stars in environments with high ADM densities. Main sequence stars have a specific relationship between their temperature (or color) and brightness. In general, the hotter a star, the brighter it is.
When a main sequence star contains ADM, however, it deviates from the standard path. At low densities, the ADM-driven energy transport mechanism makes the star brighter (and bigger) in relation to its temperature, moving the star above the standard path. At high densities, the same mechanism makes the star colder (and smaller) in relation to its luminosity, moving it to the right of the standard path.
As the researchers explain, these deviations result from a complex interaction between the ADM and the star, which begins when ADM particles scatter off the star matter’s nuclei. Because the larger ADM particles can travel further than the nuclei, the dark matter transports the heat energy generated by these interactions out of the star’s core, resulting in a sink of energy in the core.
At low ADM densities, the star’s nuclear energy generation is reduced in the very center of the core, and the star tries to compensate by increasing nuclear energy production outside the core. The increased nuclear reaction rate allows the star to achieve a new equilibrium, but it also makes the star bigger and brighter.
At high ADM densities, the dark matter reduces nuclear generation in the entire nuclear core. The overwhelmed star contracts, becoming smaller and colder.
The scientists calculated that the effects should be greatest for stars with masses around that of our Sun or a little lower. This is good news, considering 60% of the stellar mass in the Milky Way is expected to exist in the range of 0.1 to 1.0 solar masses.
The researchers caution that the chances of clearly observing stars in distant environments such as galactic cores and halos could be difficult. Current observational techniques of stars at these distances involve large uncertainties of the stars’ temperatures and luminosities.
However, the scientists also note that it’s possible that stars could capture ADM in an environment rich with dark matter, and then migrate somewhere else in the galaxy, perhaps closer to us. In this case, the scientists predict that the effects of dark matter would still be visible and more easily detectable.
Elsewhere, in an earlier 2011 study of dark matter in dwarf galaxies, NASA's Fermi Gamma-ray Space Telescope below shows seven dwarf galaxies, circled in white. Observations indicate the dwarf galaxies are full of dark matter because their stars’ motion cannot be fully explained by their mass alone, making them ideal places to search for dark matter annihilation signals.
In 2011, Brown University assistant professor Savvas Koushiappas and graduate student Alex Geringer-Sameth report that dark matter must have a mass greater than 40 giga-electron volts in dark-matter collisions involving heavy quarks. (The masses of elementary particles are regularly expressed in term of electron volts.)
The distinction is important because it casts doubt on recent results from underground experiments that have reported detecting dark matter. Using publicly available data collected from an instrument on NASA’s Fermi Gamma-ray Space Telescope and a novel statistical approach, the Brown pair constrained the mass of dark matter particles by calculating the rate at which the particles are thought to cancel each other out in galaxies that orbit the Milky Way galaxy.
“What we find is if a particle’s mass is less than 40 GeV, then it cannot be the dark matter particle,” Koushiappas said.
The observational measurements are important because they cast doubt on recent results from dark matter collaborations that have reported detecting the elusive particle in underground experiments. Those collaborations say they found dark matter with masses ranging from 7 to 12 GeV, less than the limit determined by the Brown physicists.
“If for the sake of argument a dark matter particle’s mass is less than 40 GeV, it means the amount of dark matter in the universe today would be so much that the universe would not be expanding at the accelerated rate we observe,” Koushiappas said, referring to the 2011 Nobel prize in physics that was awarded for the discovery that the expansion of the universe is accelerating. Independently, the Fermi-LAT Collaboration arrived at similar results, using a different methodology.
Physicists believe everything that can be seen — planets, stars, galaxies and all else — makes up only 4 percent of the universe. Observations indicate that dark matter accounts for about 23 percent of the universe, while the remaining part is made up of dark energy, the force believed to cause the universe’s accelerated expansion.
The problem is dark matter and dark energy do not emit electromagnetic radiation like stars and planets; they can be “seen” only through their gravitational effects. Its shadowy profile and its heavy mass are the main reasons why dark matter is suspected to be a weakly interacting massive particle (WIMP), which makes it very difficult to study.
What physicists do know is that when a WIMP and its anti-particle collide in a process known as annihilation, the debris spewed forth is comprised of heavy quarks and leptons. Physicists also know that when a quark and its anti-quark sibling annihilate, they produce a jet of particles that includes photons, or light.
Koushiappas and Geringer-Sameth in essence reversed the annihilation chain reaction. They set their sights on seven dwarf galaxies which observations show are full of dark matter because their stars’ motion cannot be fully explained by their mass alone.
These dwarf galaxies also are largely bereft of hydrogen gas and other common matter, meaning they offer a blank canvas to better observe dark matter and its effects. “There’s a high signal-to-noise ratio. They’re clean systems,” Koushiappas said.
The pair analyzed gamma ray data collected over a three year period by the Fermi telescope to measure the number of photons in the dwarf galaxies. From the number of photons, the Brown researchers were able to determine the rate of quark production, which, in turn, allowed them to establish constraints on the mass of dark matter particles and the rate at which they annihilate.
“This is the first time that we can exclude generic WIMP particles that could account for the abundance of dark matter in the universe,” Koushiappas said.
Geringer-Sameth developed the statistical framework to analyze the data and then applied it to observations of the dwarf galaxies.
“This is a very exciting time in the dark matter search, because many experimental tools are finally catching up to long-standing theories about what dark matter actually is,” said Geringer-Sameth, from Croton-on-Hudson, N.Y. “We are starting to really put these theories to the test.”