Physic Labs

Cosmology and astrophysics

Dark matter

Explore galaxy rotation curves and the role of the dark-matter halo. Increase the halo mass to see the curve flatten, comparing Vc2(r)=GM(<r)/rV_c^2(r) = GM(<r)/r with the Keplerian expectation Vc∝r−1/2V_c \propto r^{-1/2}.

Advanced

Equipment

  • Spiral-galaxy model with a dark-matter halo
  • Sliders for halo mass and viewing angle
  • Rotation-curve plot V(r) and readout

Procedure

  1. Observe the rotation curve

    Set "Halo mass" low (≈1) and inspect the V(r) curve: beyond the luminous disk, velocity falls nearly as Vc∝r−1/2V_c \propto r^{-1/2} since most mass lies inside that radius. Use the readout to note values at several radii.

  2. Increase the halo mass

    Slowly raise "Halo mass" and watch the curve flatten at large radii. With a halo where M(<r)∝rM(<r) \propto r, Vc2=GM(<r)/rV_c^2 = GM(<r)/r gives Vc≈V_c \approx constant. Compare the curve's outer endpoint at three halo settings — this is precisely the "dark-matter evidence" in galaxies.

  3. Change the viewing angle and synthesize

    Drag "Viewing angle" to see the galaxy tilted/edge-on: the Doppler-measured velocity is the projection vsin⁡iv\sin i, so a face-on galaxy yields no rotation curve. Press "Pause" and predict: if dark matter were luminous matter (faint stars, gas), would the curve flatten? Conclude that mass and light distributions differ.

Simulation

Experiment history

In 1933 Fritz Zwicky measured galaxy velocities in the Coma cluster and found them moving too fast for the visible mass — he called the missing part "dunkle Materie" (dark matter). The finding lay dormant for decades until Vera Rubin and Kent Ford (1970s) measured rotation curves of dozens of spiral galaxies: velocities at the outer edge stay flat instead of falling — mass must extend far beyond the light. Independent evidence followed: gravitational lensing, baryon acoustic oscillations in the cosmic microwave background (WMAP, Planck), and the Bullet Cluster showing mass (via lensing) separated from hot X-ray gas. The Λ\LambdaCDM model now estimates ~85% of matter is electromagnetically non-interacting; underground detectors are still hunting dark-matter particles.

Related physicists

Albert Einstein

Related library topics

Dark matter