Astronomy Picture Of The Day: The Solar System orbits the center of the Milky Way at approximately 220 km/s

 

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The Solar System orbits the center of the Milky Way at approximately 220 km/s, moving through the galaxy's extensive dark matter halo. This halo is modeled as a roughly spherical (or mildly flattened) distribution of non-luminous matter, far more massive than the visible stars and gas.Due to this galactic motion, Earth experiences a steady "wind" of dark matter particles streaming past it, primarily from the direction of the constellation Cygnus—where the Sun is headed in its orbit around the galactic center.This dark matter wind is a foundational assumption in direct detection experiments. Deep underground laboratories shield detectors from cosmic rays and other backgrounds while searching for extremely rare collisions between dark matter particles (often hypothesized as WIMPs—weakly interacting massive particles) and atomic nuclei in the detector material.A key predicted signature is annual modulation: Earth's orbital velocity around the Sun (~30 km/s) adds to or subtracts from the much larger galactic velocity vector. This causes the relative speed—and thus the expected rate of dark matter interactions—to vary slightly over the year. The signal should reach its maximum around early June (when Earth's orbital motion aligns with the Sun's galactic motion) and its minimum around early December, with a modulation amplitude typically expected to be a few percent.This seasonal variation provides a powerful way to distinguish a potential dark matter signal from steady backgrounds, which should not modulate annually in the same http://way.By studying (or attempting to detect) this modulation, scientists can place constraints on dark matter particle properties such as mass, interaction cross-section, and velocity distribution in the halo.Notably, the DAMA/LIBRA experiment in Italy has reported a highly significant annual modulation for over two decades, consistent in phase and period with the expected dark matter signal. However, its interpretation as dark matter remains heavily debated, as more sensitive experiments (such as XENON, LUX-ZEPLIN, and PandaX) have not observed corresponding signals and largely rule out the standard WIMP interpretation of DAMA's result.Nonetheless, the concept of Earth moving through a galactic dark matter wind remains central to the field, elegantly connecting large-scale cosmology and galactic dynamics with precision particle physics experiments conducted deep underground.