Dark Matter Notes

Astronomy reveals our diminished importance in the universe.
Earth is a small planet orbiting a typical star in the suburbs of a vast galaxy.
Observable matter is only a fraction of what exists.

Vera Rubin studied spiral galaxy rotation in the 1960s-70s.
She aimed to understand galaxy rotation to learn about their structure and mass.
Johannes Kepler discovered planets farther from the Sun orbit slower.
Isaac Newton quantified this with gravity calculations, allowing mass determination.
The same principles should apply to galaxies.
Measuring gas cloud orbits at galaxy edges allows calculation of total galaxy mass using Doppler shift.

Rubin expected gas clouds farther from the galactic center to move slower.
She observed the opposite: gas clouds moved faster at greater distances.
At best, velocities flattened out instead of declining as predicted.
This indicated constant gravity throughout the galaxy disk.
Visible matter decreases with distance from the center, so there was not enough visible mass to cause constant gravity.
The explanation is dark matter contributes to gravity.
Galaxies are embedded in dark matter halos.
Rubin determined there is five to six times more dark matter than visible matter.

In the 1930s, Fritz Zwicky found galaxies in clusters moving too fast to remain bound by visible matter.
He suggested more gravity than that provided by visible matter was present.
Zwicky's observations had large uncertainties and overestimated dark matter.
Rubin's more accurate work is now credited with the discovery.
The term "dark matter," coined by Zwicky, is still used.

Observations of elliptical galaxies and galaxy clusters confirmed Rubin’s findings.
Initially, the concept of dark matter faced skepticism.
Everything emits light but dark matter does not.

Astronomers systematically considered all potential candidates for dark matter.
Candidates included cold gas, dust, dead stars, rogue planets, and exotic subatomic particles.
Various detection methods were employed, but all yielded negative results.
Normal matter was eliminated as a possibility.

Einstein proposed that space is a "fabric."
Gravity is the warping of space caused by mass and energy.
Light bends when passing through warped space.
The amount of bending depends on the mass causing the warping.
Lenses also bend light.
Gravitational lensing occurs when a massive object bends light from a more distant object.
A galaxy cluster can act as a lens, distorting the images of galaxies behind it.
Einstein's equations allow mass measurement of galaxy clusters via gravitational lensing.

The Bullet Cluster is a collision of two galaxy clusters located $3,500,000,000$ light years away.
Galaxies pass through each other during cluster collisions.
Gas clouds in clusters collide and heat up, emitting X-rays.
Chandra X-ray Observatory maps hot gas distribution.
Optical images reveal background galaxies distorted by Bullet Cluster gravity.

Dark matter clouds pass through each other like galaxies during a cluster collision.
Gravitational lensing maps the mass distribution, revealing dark matter surrounding the subclusters.
Dark matter is separate from the hot gas.

Observations of the Bullet Cluster support the existence of dark matter.
Simpler explanations suggest ordinary matter cannot fully explain the structure and movements we see in the universe.
The exact composition of dark matter remains unknown.
Axions are one possibility, but other particles are also considered.

Dark matter has significantly influenced the universe's evolution.
Smaller objects formed first, then merged into larger structures.
The formation of large structures would have been difficult without dark matter due to high energy output from early stars and galaxies.
Dark matter facilitates the formation of the structures we observe.