Astronomy: Dark Matter

Introduction to the Humbling Nature of Astronomy

• Astronomy reveals our smallness in the Universe.
  • Earth is one planet among many.
  • The Sun is one star among hundreds of billions.
  • Our galaxy is one among hundreds of billions.
• New discoveries often show that visible matter is only a fraction of what exists.

Vera Rubin’s Observations (1960s-1970s)

• Studied the rotational velocities of spiral galaxies.
  • Expected: Outer gas clouds should move slower (like planets in the solar system).
  • Found: Outer gas clouds moved faster or at a constant speed.
• Conclusion: There must be additional, unseen mass – Dark Matter.
  • Far more mass than visible matter.

Fritz Zwicky's Earlier Work (1930s)

• Studied galaxy clusters.
  • Found: Member galaxies moved too fast to stay in clusters without extra gravity.
• Miscalculations, but the term "Dark Matter" persisted.

Confirmations and Skepticism

• Additional observations confirmed Vera Rubin’s findings.
• Dark matter behavior: consistent in elliptical galaxies and galaxy clusters.
• Persistent skepticism led to exhaustive checks.
  • All normal matter (gas, dust, stars, rogue planets) was ruled out.

What is Dark Matter?

• Hypotheses: Subatomic particles (e.g., axions).
  • Axions: Mass, low light emission, minimal interaction with normal matter.
  • Detecting axions is challenging.

Gravitational Lensing

• Einstein’s Theory: Gravity warps space, bending light (gravitational lensing).
• Application: Measuring the mass of galaxy clusters via light distortion.

The Bullet Cluster

• A merging collision of two galaxy clusters.
  • Optical images, X-ray observations show the collision's dynamics.
  • Mass map from gravitational lensing shows dark matter presence around subclusters.
  • Confirms existence of dark matter distinct from visible hot gas.

Dark Matter’s Role in Universe Formation

• Impact: Affects the formation of larger cosmic structures.
  • Facilitates clustering under early Universe conditions.
• Estimated: 85% of Universe’s matter is dark matter.

Conclusion

• Dark matter interacts primarily through gravity.
• We can detect dark matter via gravitational effects despite lacking direct observation.
• Dark matter is integral to the Universe’s structure and formation.

Production Credits

• Writer: Phil Plait
• Editor: Blake de Pastino
• Consultant: Dr. Michelle Thaller
• Director: Nicholas Jenkins
• Editor: Nicole Sweeney
• Sound Designer: Michael Aranda
• Graphics Team: Thought Café