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The only way to detect black holes is through their influence on their surroundings. Black holes can be detected when their extreme gravity effects nearby luminous gas and stars. They can also be detected from the gravitational waves generated when two black holes merge. Both of these methods require that the black hole is in close proximity to other objects - gas, stars, or another black hole in a binary system. None of these methods can be used to find free-floating, isolated black holes. Luckily, general relativity gives us another way that these dark objects can be detected.
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Finding Black Holes with Gravitational Lensing
We are searching for black holes in the Milky Way using gravitational lensing. Our Galaxy likely contains 100 million stellar-mass black holes. The number and mass statistics of black holes can provide important constraints on the star formation history, the stellar mass function, supernova physics and how BHs form, the equation of state of nuclear matter, and the existence of primordial black holes. To date, isolated stellar-mass black holes have never been definitively detected and only two dozen black holes in our Galaxy have measured masses – all in binaries.
Microlensing events, where a black hole’s gravity lenses the light of a background star as observed from Earth, provide a way to detect isolated black holes and measure their masses. Black hole lensing events produce a photometric magnification that has a long-duration (>3 months) and an astrometric signature that can be >1 mas. However, the astrometric signature has only recently become detectable with technological advancements in high-resolution imaging, including adaptive optics. Only the combination of photometry and astrometry can be used to precisely measure the mass of the lensing object and determine if it is indeed a black hole or a chance slow-motion lensing event between two normal stars.
We aim to find the first isolated stellar−mass black holes by detecting the astrometric signature of microlensing. Finding just a few black holes would already reduce the orders of magnitude uncertainty on the total number of black holes in the Galaxy and constrain theories of black hole formation and evolution. The proposed development of astrometry techniques, needed for astrometric microlensing, will also lay a foundation for new explorations in many areas of astrophysics.
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Principles of Gravitational Microlensing
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