No Dark Matter Near the Sun—Oh Wait, We Found It
Most attempts to detect dark matter from Earth rely on a suitable amount of dark matter in the vicinity of the Sun. One method measures the velocities of local stars to determine how much dark matter exists. Recent corrections to this technique—which has been used over the past 20 years—demonstrates that biases in the method underestimated the true amount of dark matter—leading scientists to suggest there may be more dark matter around the Sun than originally expected.
I work on a dark matter balloon experiment called GAPS (General Antiparticle Spectrometer). The detector looks for antideuterons, cosmically rare particles that can result when dark matter particles collide and annihilate. A great number of factors influence whether GAPS will detect any dark matter, including the detector design and the nature of the dark matter. However, as with many other searches for dark matter, a GAPS detection requires that dark matter exists around the Sun.
In 2012, a team of scientists surveyed over 400 stars in the vicinity of the solar system and used the results to calculate the amount of local dark matter. Surprisingly, the motions of the stars required about ten times less dark matter than expected based on galaxy formation models.1 This result, if it withstands further scrutiny, would severely dampen the hopes of many dark matter searches. Fortunately, new research brings a more optimistic perspective.
Presently, astronomers possess the capacity to measure dark matter’s gravitational influence only. Consequently, any calculation of dark matter quantities from star velocities requires some assumptions—and wrong assumptions introduce biases. One technique to minimize these biases utilizes state-of-the-art galaxy simulations to test how different measurement methods recover the dark matter distribution. This technique allows scientists to know exactly how much dark matter populates the galaxy simulation, unlike in the real world. Such simulations revealed biases in the standard mass-measuring techniques and showed that these biases consistently underestimated the amount of dark matter.
Scientists used these simulations to develop an unbiased method and then applied it to measurements in the neighborhood of the Sun.2 Their results convincingly demonstrate that the Sun resides in a substantial distribution of dark matter. In fact, the estimate derived from this new method indicates that the amount of dark matter may be higher than previously thought.
So, dark matter definitely exists in the solar neighborhood. That gives me hope (and some confidence) that GAPS might find its signature in the near future.
1. C. Moni Bidin et al., “Kinematical and Chemical Vertical Structure of the Galactic Thick Disk. II. A Lack of Dark Matter in the Solar Neighborhood,” Astrophysical Journal 751 (May 1, 2012): doi:10.1088/0004-637X/751/1/30.
2. Silvia Garbar et al., “A New Determination of the Local Dark Matter Density from the Kinematics of K Dwarfs,” Monthly Notices of the Royal Astronomical Society 425 (September 11, 2012): 1445–58.