Evolution of Dark Matter

How ideas have changed

Outline

• Timeline– Hints– People– Some Explanations

• right and wrong

What is Dark Matter?

• Matter that isn’t directly visible– Doesn’t have to be “weird” matter

• Discovered accidentally (i.e. not predicted)

• Still much debate– May or may not exist!

Matter in the Universe

Regular Matter15%

Dark Matter85%

The first hints…1933

• Fritz Zwicky– Knew galaxies in

clusters move around due to gravity

– Decided to “measure” this gravity

Zwicky & Coma Cluster 1933

#1: Calculated mass from velocity of galaxies

– Mass of cluster = Mgravity

#2: Calculated mass from brightness & number of galaxies

– Mass of cluster = Mvisible

He performed 2 separate measurements of cluster’s mass:

Zwicky & C.C. 1933

• Two independent measurements of mass

• What did he find?

Mgravity >>> Mvisible

(much, much, much greater than)

Zwicky & C.C. 1933

In words: Galaxies appear to be moving faster than their visible matter would allow.

(His result: 98% of the mass is hidden

Mgravity >>> Mvisible

Sinclair Smith & Virgo Supercluster 1936

• Three years after Zwicky

• Found the same result for the Virgo Supercluster

Smith Sinclair, not Sinclair Smith

What’s going on?

• Two different masses measured

• There can only be one correct mass for a cluster of galaxies.– What’s the deal?

• Easiest explanation: must be invisible matter in clusters.

Does this make sense?

• Facts: – Galaxies move faster

than visible matter would allow

– Gravity makes things move faster

•Unseen mass = more gravity = higher velocity

More Evidence: Rotation Curves

• 1939– Completely different method for detecting dark

matter– NOTHING to do with clusters

Rotation Curves 1939

• A graph of orbital speed versus distance from center

• Our solar system: close means fast.

• Can we do the same for whole galaxies?

How do the orbital velocities differ as we go away from the center?

Babcock & Andromeda 1939

• Measured rotation curve for Andromeda Galaxy.

• Key idea: greater mass means faster orbital speed

Observed

Expected**If mass only comes from visible matter

Note the difference in the shapes of curves;

Rotation curve is FLAT at large radii.

Rotation Curves: Many Galaxies

THEY’RE ALL FLAT!

Rotation Curve: Our Galaxy

• Key Idea: greater mass means faster orbit.

• We are orbiting faster than we should be!

• Easiest explanation: visible matter isn’t the only matter.

Some ideas 1940

• Two observations that support dark matter:– Clusters of Galaxies– Galactic Rotation Curves

•1940 astronomers skeptical of data–Bad observing techniques?–Dust blocking light?

Jumping Ahead 1970

• 1970s Astronomers:– Believe the data– No idea what it was

• Theories:– Black holes– Hot gas between galaxies– Free-Range Jupiters

Jumping Ahead Today

Visible mass: 10% Hot gas mass: 20%

Today

• 15% Visible Matter; 85% Dark Matter (5%)• Theories: Hot Dark Matter & Cold Dark Matter

– Slow-moving or Fast-moving?

• Cold Dark Matter seems to be the best theory today.

• Other theory: DM doesn’t exist!!! We don’t understand gravity on large scales.

DM doesn’t exist

Best theory

Hot gasWeird matter Non-weird matter