Final Exam

What is the key idea of general relativity (GR)?

The universe is four-dimensional spacetime; only spacetime separation is absolute.

What are the two postulates of GR?

1. Laws of physics are the same in all reference frames. 2. Gravity is the curvature of spacetime caused by mass and energy.

What experimental evidence supports GR?

Gravitational lensing, precession of Mercury’s orbit, gravitational redshift, GPS corrections.

What is the cosmological principle?

The universe is homogeneous and isotropic on large scales.

Why is the universe expanding but galaxies aren’t?

Spacetime itself expands, but galaxies are bound by gravity.

What is a black hole?

A region where gravity is so strong that not even light can escape.

How can a black hole form?

When a massive star collapses under its own gravity.

How do we observe black holes?

By detecting X-rays from matter falling in or observing effects on nearby stars.

What is the cosmic elemental abundance?

~75% hydrogen, ~25% helium, <1% everything else.

How does the universe’s temperature change as it expands?

Temperature and energy density decrease as the universe expands.

What happens to atoms/nuclei when heated?

They break apart into constituent particles.

When did particle, nuclear, and atomic physics dominate?

Particle: first microseconds; nuclear: first few minutes; atomic: after ~380,000 years.

What are alpha, beta, gamma radiation?

Alpha: helium nuclei, beta: electrons/positrons, gamma: photons; gamma is most dangerous.

How big is an atom vs. nucleus?

Atom: ~10⁻¹⁰ m; nucleus: ~10⁻¹⁵ m.

How do a cloud chamber and Geiger counter detect radiation?

Cloud chamber: visible tracks; Geiger counter: counts ionizing events.

What is the electric force law?

F=kq1q2/r².

What is an isotope?

Same protons, different neutrons.

How to find protons, neutrons, electrons?

Atomic number = protons = electrons; neutrons = mass number - atomic number.

What’s half-life?

Time for half of a radioactive sample to decay.

How old is the Earth? Universe?

Earth: ~4.5 billion years; Universe: ~13.8 billion years.

What binds a nucleus?

The strong nuclear force.

Difference between fission and fusion?

Fission: splitting heavy nuclei; fusion: combining light nuclei.

Elements used in fission and fusion?

Fission: uranium, plutonium; fusion: hydrogen, deuterium, tritium.

What’s a chain reaction?

Released neutrons cause further fissions.

Applications of fission/fusion?

Nuclear power, bombs, medical treatments.

How did the expanding universe produce hydrogen/helium?

Primordial nucleosynthesis during the first few minutes.

What’s the difference between Big Bang vs. Steady State Cosmology?

Big Bang: universe had a beginning; Steady State: eternal, constant density.

Predictions on galaxy ages?

Big Bang: galaxies age over time; Steady State: all ages distributed evenly.

What’s recombination?

Time when electrons and nuclei combined; universe became transparent.

How were heavier elements created?

Stellar nucleosynthesis inside stars.

How do stars form?

Gas clouds collapse under gravity.

How does mass affect a star’s fate?

Low mass → white dwarf; medium mass → red giant, white dwarf; high mass → supernova, neutron star, black hole.

What are a protostar, brown dwarf, red giant, white dwarf, supernova, neutron star, pulsar?

Protostar: forming star; brown dwarf: failed star; red giant: expanded aging star; white dwarf: collapsed core; supernova: exploding star; neutron star: collapsed dense core; pulsar: rotating neutron star emitting beams.

How are supernovae used to measure expansion?

As standard candles to measure distance and acceleration.

What’s equilibrium radiation?

Radiation where energy in = energy out; CMB is an example.

Who predicted and discovered the CMB?

Predicted by Gamow, Alpher, Herman; discovered by Penzias and Wilson; shown to be equilibrium by COBE satellite.

What’s wave-particle duality?

Particles behave as both waves and particles.

What’s the two-slit experiment?

Shows interference pattern, proving wave-like behavior.

What’s the wave function?

Describes probability of a particle’s position or momentum.

Who contributed to quantum mechanics?

Bohr: atomic model; Heisenberg: uncertainty; Schrödinger: wave equation; Pauli: exclusion principle; Dirac: antimatter.

How does wave-particle duality explain atomic energy levels?

Only certain standing wave patterns allowed → quantized levels.

How does Pauli exclusion explain orbitals and supernovae?

No two fermions can occupy same state → degeneracy pressure resists collapse.

What’s antimatter?

Particles with opposite charge to normal matter.

What happens when matter meets antimatter?

Annihilation, releasing energy.

What are protons and neutrons made of?

Quarks.

How do we study small scales?

Use high-energy particle beams.

Size of quarks and nucleons?

Quark: ~10⁻¹⁸ m; nucleon: ~10⁻¹⁵ m.

How does a particle accelerator work?

Uses electromagnetic fields to accelerate particles.

What are the fundamental particles?

Quarks and electrons.

How do particles interact at quantum level?

Via force-carrying particles (messenger particles).

What are the four forces and their messengers?

Strong (gluons), electromagnetic (photons), weak (W/Z bosons), gravity (gravitons, hypothesized).

Why doesn’t electron fall into nucleus?

Strong force only acts on quarks, not electrons.

What’s antimatter made of?

Anti-quarks, anti-electrons (positrons), anti-protons, etc.

How do matter-antimatter collisions create heavy particles?

Energy converts to mass via E=mc².

How were heavier quarks and leptons discovered?

By colliding matter and antimatter at high energy.

What are the six quarks and six leptons?

Quarks: up, down, charm, strange, top, bottom; leptons: electron, muon, tau + their neutrinos.

What’s the strong force’s behavior with distance?

Increases with separation → quark confinement.

What’s the Higgs Field?

Field giving mass to particles.

What’s the Higgs Boson?

Particle associated with the Higgs Field.

What’s an event horizon?

Boundary beyond which events cannot affect an observer.

What’s the horizon problem?

We see ~100,000 causally disconnected regions with similar properties.

Why is a flat universe unnatural?

Small deviations would grow over time; fine-tuning required.

What’s inflation?

Rapid expansion in early universe solving horizon and flatness problems.

Predictions of inflation?

Flat universe, homogeneity, primordial density fluctuations.

What’s a gravity wave?

Ripples in spacetime from accelerating masses.

When were gravity waves first detected?

2015 by LIGO.

How would primordial gravity waves prove inflation?

By polarizing the cosmic microwave background.

What evidence points to dark matter?

Galactic rotation curves, galaxy cluster dynamics, collisions.

Why is it called dark matter?

It doesn’t emit or absorb light.

How much of the universe is dark matter?

~30%.

What are its properties?

Non-luminous, interacts via gravity, unknown particle nature.

Have we detected dark matter particles?

No, only indirect evidence.