From PhysOrg.com: NASA satellite could reveal if primordial black holes are dark matter.
In March of 2009 NASA launched the Kepler satellite. While orbiting the sun, Kepler’s objective was to search for Earth-like planets in a portion of the Milky Way galaxy. Recently a team of physicists proposed that Kepler might also be used to detect or rule out the presence of primordial black holes (PBHs) within a certain mass range. The mass range is a category of PBHs that are primarily made up of dark matter.
Scientists from the university of California, San Diego, University of Pennsylvania, and Academia Sinica (Taipei, Tiawan) have published the study on using the Kepler satellite to detect the PBH dark matter in Physical Review Letters.
Dark matter is one of the biggest unsolved mysteries of science and “an answer would be extraordinary,” Kim Griest, UC-San Diego, said to PhysOrg.com. The two possible outcomes to this study: discovering PBHs are primarily dark matter or nothing is found. According to Griest, the first option would be “totally fascinating,” while the latter would “eliminate…a major contender, but it is not as exciting.”
Since the 1970s PBHs have been believed to be a candidate for dark matter. The PBHs are believed to have formed during the early universe from density changes that resulted from a variety of sources, such as phase transitions and inflation. There is not one single theory for how PBHs formed, so it is unsure how massive they would be. However, previous work in both theoretical and experimental has eliminated most masses, including almost all masses in the range from 10-18 to 1016 solar masses (1.99x1012 kg – 1.99x1046 kg). The only exception is the mass range between 10-13 and 10-7 solar masses. Scientists call these 5 orders of magnitude the “PBH dark matter window.”
Griest and his colleagues think that Kepler data could potentially rule out a significant portion of this window. Currently, one of Kepler’s instruments, the photometer, is measuring the light intensity of stars. The photometer has the ability to measure 150,000 starts every 30 minutes. When analyzing the data, scientists look for specific fluctuations in star light, or stellar flux, since a decrease could signal an Earth-sized planet transiting in front of the star.
Kepler’s photometer has been shown to also detect small amounts of gravitational lensing, also known as microlensing, which is the bending of star light as it travels through nearby space. According to general relativity, the bending is due to the gravity of an invisible mass that acts as a lens and lies between the light source and the observer, in this case the distant star and the Kepler satellite. This lens could be a PBH or another type of massive compact halo object (MACHO) as well as mini halos, all of which are dark matter candidates.
According to calculations done by scientists, Kepler is able to detect microlensing events caused by masses in the range between 5x10-10 and 10-4 solar masses, which means it could rule out 40% of the mass window stated above if it doesn’t detect anything. But if microlensing is detected the implications would be much more exciting; PBH could be dark matter.
Griest and his coauthors have begun looking at the vast amount of data. Analyzing the data is not an easy task, it requires an understanding of the complex light curve data, understanding what constitutes a false positive and background events, and using strict selection criteria.
For more information look to Kim Griest, et al. “Microlensing of Kepler Stars as a Method of Detecting Primordial Black Hole Dark Matter.” Physical Review Letters 107, 231101 (2011).
