Dark Matter Notes

Dark Matter: An Introduction

  • Astronomy reveals our diminishing importance in the universe.
    • Earth is a planet among many.
    • Our sun is a star among billions.
    • Our galaxy is one among billions.
  • What we observe is not all there is.

Vera Rubin and Galaxy Rotation

  • In the 1960s and 1970s, Vera Rubin studied spiral galaxy rotation.
  • Johannes Kepler (1600s): Planets farther from the sun orbit slower.
  • Isaac Newton: Quantified gravity's strength, allowing calculation of the sun's mass.
  • Galaxy rotation and mass are related.
    • Measuring gas cloud orbit speeds reveals galaxy mass.
  • Doppler shift measures nebulae velocity.
  • Expected: Gas clouds farther from the galactic center would move slower.
  • Observed: Gas clouds maintained or increased speed with distance.
    • Gravity was constant throughout the disk.
    • Images showed mass decreasing with distance from the center.
  • Explanation: Dark matter contributes to gravity.
    • Galaxies are embedded in dark matter halos.
    • Dark matter is 5-6 times more abundant than visible matter.

Fritz Zwicky and Galaxy Clusters

  • 1930s: Fritz Zwicky studied galaxy cluster speeds.
  • Galaxies moved too fast to stay in clusters.
  • Conclusion: More gravity existed than visible matter accounted for.
  • Zwicky's initial observations had high uncertainty.
  • Rubin's observations were more accurate.
  • The term "dark matter" stuck, coined by Zwicky.

Confirmation and Skepticism

  • Later observations confirmed Rubin's findings.
    • Elliptical galaxies show similar behavior.
  • Galaxy cluster velocities confirm dark matter presence.
  • Initial skepticism due to the lack of light emission from dark matter.

The Search for Dark Matter's Composition

  • Astronomers systematically considered potential dark matter candidates.
    • Cold gas, dust, dead stars, rogue planets, subatomic particles.
  • Detection methods were employed for each candidate.
    • Cold gas emits radio waves.
  • Candidates made of normal matter (protons, electrons, neutrons) were eliminated.

Axions: A Potential Candidate

  • Axions are theoretical subatomic particles.
  • Properties align with dark matter characteristics:
    • Massive, providing gravity.
    • Minimal light emission.
    • Weak interaction with normal matter: They'd pass right through you.
  • Axion clouds could envelop galaxy clusters.

Gravitational Lensing

  • Einstein's theory: Space is a fabric.
  • Gravity: A warping of space.
    • Analogy: Bowling ball on a mattress bending the surface.
  • Light bends when passing through warped space.
    • More massive objects warp space more.
  • Lenses bend light; gravitational lensing occurs due to massive objects.

Observing Dark Matter with Gravitational Lensing

  • Galaxy clusters have significant mass.
  • Light from galaxies behind a cluster is bent and distorted.
  • Distortion reveals cluster mass and mass distribution.

The Bullet Cluster

  • A collision of two galaxy clusters 3.5 billion light-years away.
  • Galaxies pass through each other during collisions.
  • Gas clouds collide and heat up, emitting X-rays.
  • Chandra X-ray Observatory maps hot gas location.
    • Gas located between the galaxies, slowed by the collision.
    • Bow shock formation.
  • Background galaxies' light is distorted by the Bullet Cluster's gravity.
  • Dark matter is mapped using this distortion.
    • Dark matter is centered around the subclusters, not the hot gas.
    • Consistent with axion behavior.
  • Other clusters show similar behavior.

Implications of Dark Matter

  • The simplest explanation: dark matter exists.
  • Dark matter's nature is still unknown.
  • Axions are a possibility, and experiments are underway to detect other candidates.
  • Dark matter significantly impacts the universe.

Dark Matter and the Formation of the Universe

  • Smaller objects formed first, merging into larger structures.
    • Stars, then galaxies, then clusters.
  • Early universe: Energy from newborn stars hindered structure formation.
  • Dark matter enables the formation of large-scale structures.
  • Dark matter makes up approximately 85% of the matter in the universe.
  • Normal matter is a minority.
  • Dark matter surrounds and penetrates us, binding galaxies together.