Dark matter speed
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Dark Matter Speed Distribution Near the Solar System
Recent studies using advanced simulations of Milky Way-like galaxies have provided new insights into the speed distribution of dark matter near the Sun. The local dark matter speed distribution is best described by a modified Maxwellian profile, peaking at a most probable speed of about 257 km/s and sharply truncating above 470 km/s. This peak is higher than what the standard halo model predicts, and there are fewer very-high-speed particles than previously assumed. The local dark matter velocity dispersion is estimated to be around 280 km/s, tightly correlated with the observed circular speed of the Milky Way at the Solar radius .
High-Speed Dark Matter Particles and Their Origins
The highest-speed dark matter particles in the Solar neighborhood are not native to the Milky Way halo. Instead, they predominantly originate from the Large Magellanic Cloud (LMC) or are Milky Way halo particles accelerated by the LMC's recent passage. These particles can reach speeds of 700–900 km/s relative to Earth, which is near or even above the local escape speed. Such high-speed particles are especially relevant for direct detection experiments, as they can dominate the signal for low-mass dark matter candidates .
Impact of Velocity Distribution on Detection and Astrophysical Processes
The shape of the dark matter velocity distribution, especially its high-speed tail, significantly affects the capture rate of dark matter by the Sun and the sensitivity of direct detection experiments. Most fitting formulas for the local velocity distribution struggle to accurately describe both the peak and the high-velocity tail, but including the local escape velocity improves the fit. Variations in the velocity distribution can change the dark matter capture rate by the Sun by up to 15–20%, and uncertainties grow for higher dark matter masses . Additionally, anisotropic components in the velocity distribution can be probed using daily modulation effects in sensitive detectors, which can help identify non-standard velocity structures for sub-GeV dark matter .
Mechanisms for Speeding Up Dark Matter
Novel mechanisms have been proposed where solar neutrinos can accelerate sub-GeV dark matter to velocities well above 10^-3 times the speed of light. In certain models, this process can produce a dark matter velocity distribution that closely resembles the solar neutrino spectrum, potentially enhancing the detectability of light dark matter in direct detection experiments .
Constraints on Cold Dark Matter Speeds
Despite these high-speed populations, the bulk of cold dark matter in the universe remains extremely slow-moving. Observational constraints from the cosmic microwave background and matter power spectrum limit the present-day velocity dispersion of cold dark matter (outside collapsed structures) to less than 54 m/s, confirming that cold dark matter is indeed "cold" on cosmological scales .
Conclusion
The speed of dark matter near the Solar System is shaped by both local galactic dynamics and external influences like the LMC. While most dark matter moves at moderate speeds, a significant high-speed tail exists, primarily due to interactions with satellite galaxies. The precise shape of the velocity distribution is crucial for interpreting direct detection experiments and understanding astrophysical processes such as dark matter capture by the Sun. Ongoing research continues to refine our understanding of these distributions and their implications for dark matter searches 1234+2 MORE.
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