The Milky Way's mass is a cosmic mystery, with dark matter playing a starring role. Astronomers use orbital velocities to uncover the Galaxy's hidden mass, finding that visible matter alone can't explain what they see.

Dark matter forms a massive halo around our Galaxy, dwarfing the visible components. Scientists are on the hunt for its true nature, with exotic particles like WIMPs and axions as prime suspects in this celestial whodunit.

Measuring the Milky Way's Mass

Orbital velocities for dark matter detection

• Astronomers measure orbital velocities of objects (stars, gas clouds) in the Milky Way to infer the Galaxy's mass distribution
  • Kepler's third law connects orbital velocity to the enclosed mass within the orbit $v = \sqrt{\frac{GM}{r}}$
    • $v$ represents orbital velocity
    • $G$ is the gravitational constant
    • $M$ is the mass enclosed within the orbit
    • $r$ is the orbital radius
• Observed orbital velocities in the Milky Way's disk exceed predictions based on visible matter alone
  • Implies the existence of additional unseen mass, attributed to dark matter
• Orbital velocities remain high even at large Galactic radii
  • Suggests dark matter extends well past the visible disk of the Galaxy
  • This pattern is evident in the Galaxy's rotation curve

Distribution of dark matter in Galaxy

• Dark matter believed to form a roughly spherical halo enveloping the Milky Way
  • Halo reaches far beyond the Galaxy's visible disk
• Dark matter halo significantly more massive than the Milky Way's visible components
  • Estimated ~90% of the Galaxy's total mass is dark matter
• Total mass of the Milky Way including dark matter approximated at $1-2 \times 10^{12} M_{\odot}$
  • Considerably higher than the ~$10^{11} M_{\odot}$ mass of visible components
• Dark matter density peaks near Galactic center and decreases with distance
  • Density falls off more gradually than visible matter density
• Extended dark matter halo accounts for the high outer orbital velocities observed in the Milky Way
• The galactic halo's mass distribution can be inferred from these observations

Candidates for dark matter composition

• Baryonic matter (ordinary protons, neutrons, electrons) proposed as dark matter candidates
  • Possibilities include brown dwarfs, black holes, cold gas clouds
• Baryonic dark matter largely ruled out due to observational constraints
  • Cosmic microwave background and Big Bang nucleosynthesis show baryonic matter is a small fraction of total Universal matter
• Non-baryonic matter now favored as primary dark matter constituent
  • Interacts weakly with electromagnetic radiation making direct detection challenging
• Weakly Interacting Massive Particles (WIMPs) are leading non-baryonic candidates
  • Hypothetical particles with mass but feeble ordinary matter interactions
  • Supersymmetric particles like neutralinos are proposed WIMPs
• Axions also suggested as potential dark matter components
  • Predicted extremely low mass and exceedingly weak coupling to ordinary matter
• Dark matter particles remain undetected despite extensive searches
  • Nature of dark matter persists as an open question in astronomy and physics

Additional methods for dark matter detection

• Mass-to-light ratio analysis helps identify dark matter presence in galaxies
• Gravitational lensing observations provide evidence for dark matter in galaxy clusters