Home / Topics / Particle Physics & Cosmology
Particle Physics & Cosmology
Elementary particles, forces, the Standard Model, stars and the history of the universe. 51 topics, from beginner to advanced. Open the interactive version with labs and quizzes
Particle physics and cosmology study the most fundamental constituents of matter and the largest structures in the universe. This area describes leptons, quarks and hadrons, the four fundamental forces, the Standard Model, antimatter, and how stars and the early universe evolved. It shows how the smallest scales and the largest scales are connected.
How the topics connect
Particle physics and cosmology describe the smallest constituents of matter and the largest structure of the universe. The Standard Model explains quarks, leptons and forces, while the study of stars, galaxies and the Big Bang shows how matter was made, organised and continues to evolve.
Real-world uses
Use antimatter in positron emission tomography (PET) scans, where positrons annihilate with electrons to produce gamma rays that map metabolism in the body.
Understand the aims of particle colliders such as the LHC, which search for new particles and test the Standard Model.
Explain the redshift of distant galaxies as evidence for an expanding universe and use it to estimate cosmic distances and ages.
Use particle detectors, from cosmic-ray telescopes to neutrino observatories, to study particles that pass through matter with little interaction.
Physicists use detectors such as those at CERN to identify particles, while similar technology is used in medical imaging.
Astronomers use stellar physics, cosmic distance measurements and telescopes to study planets, galaxies and dark matter.
Elementary particles
Quarks, leptons and gauge bosons are the basic building blocks.
Particles with no known substructure: quarks, leptons, gauge bosons and the Higgs. All ordinary matter is made from up and down quarks and electrons.
A proton is made of uud quarks.
'Elementary' means no internal structure has been found.
Four fundamental forces
Gravity, electromagnetism, weak and strong.
Gravity (weakest, infinite range), electromagnetism (infinite range), the weak force (10⁻¹⁸ m, beta decay) and the strong force (1 fm, binds quarks). Relative strengths 10⁻³⁸ : 1/137 : 10⁻⁶ : 1 roughly.
Gravity is 10³⁶ times weaker than electromagnetism between protons.
Gravity isn't yet part of the Standard Model.
Leptons
Electron, muon, tau and their neutrinos.
Six leptons: electron, muon, tau and their neutrinos. They don't feel the strong force. Electron stable; muon lives 2.2 μs; tau 0.29 ps.
Muons from cosmic rays reach the ground.
Charged leptons come in three heavier copies.
Quarks
Up, down, charm, strange, top and bottom.
Six flavours: up, down, strange, charm, bottom, top, with charges +2/3 or −1/3. They carry colour and are never seen alone.
Top quark: 173 GeV, as heavy as a gold atom.
Charge fractions of e.
Hadrons
Particles made of quarks: baryons and mesons.
Composite particles of quarks held by the strong force: baryons (three quarks: proton, neutron) and mesons (quark–antiquark: pion, kaon). Hundreds exist.
π⁺ = u d̄.
Pentaquarks and tetraquarks were found by LHCb.
Antimatter
Every particle has an antiparticle with opposite charge.
Every particle has an antiparticle with the same mass and opposite charges (positron, antiproton). Matter and antimatter annihilate into energy.
PET scans detect positron annihilation gamma rays.
Mostly absent in the universe.
Gauge bosons
Photon, W, Z and gluon carry forces.
Force carriers: photon (EM), W⁺, W⁻, Z⁰ (weak, ~80–91 GeV), gluons (strong), and the hypothetical graviton. Exchange explains forces.
W and Z discovered at CERN in 1983.
Massless photons give infinite range.
Higgs boson
Field that gives mass to elementary particles, discovered in 2012.
The excitation of the Higgs field, which gives mass to W and Z and (through Yukawa couplings) to fermions. Discovered at the LHC in 2012 with mass 125 GeV.
Nobel Prize 2013 for Englert and Higgs.
Doesn't explain most of the proton's mass.
Standard Model
Theory of all known particles and three of the four forces.
Quantum field theory of strong, weak and EM interactions with 17 fundamental particles. Its predictions match experiments to many digits but it excludes gravity, dark matter and neutrino mass.
Gauge group SU(3)×SU(2)×U(1).
Has ~19 free parameters.
Colour charge and QCD
Quarks carry colour; gluons bind them.
Quarks carry one of three colours; gluons carry colour too, so they interact among themselves. Quantum chromodynamics describes this and only colourless combinations are observed.
Proton: red + green + blue = white.
Colour has nothing to do with real colour.
Asymptotic freedom
The strong force weakens at short distances.
The strong coupling weakens at high energy (short distance), so quarks behave almost free in high-energy collisions, but at larger distances it grows. Nobel Prize 2004.
Allows perturbative calculations at colliders.
Opposite of QED behaviour.
Confinement
Quarks are never seen in isolation.
Pulling quarks apart stretches a flux tube whose energy grows with distance until it creates a new quark–antiquark pair, so isolated quarks are never seen.
Separating a quark and antiquark produces jets of hadrons.
Proven only numerically (lattice QCD).
Weak interaction
Responsible for beta decay and neutrino interactions.
Mediated by W and Z bosons; changes quark flavour, causes beta decay, and acts on all fermions. It violates parity maximally.
Neutron decay n → p + e⁻ + ν̄ₑ.
Responsible for fusion in the Sun's first step.
Electroweak unification
Electromagnetic and weak forces unite at high energy.
At energies above ~100 GeV the electromagnetic and weak forces merge into one electroweak interaction (Glashow–Weinberg–Salam). The Higgs mechanism breaks the symmetry at lower energies.
Photon and Z are mixtures of the original fields.
Predicted W and Z masses.
Conservation laws in particles
Charge, baryon number and lepton number are conserved.
Charge, energy, momentum, baryon number, lepton number are always conserved; strangeness and parity are conserved in the strong but not the weak interaction.
n → p + e⁻ + ν̄: lepton number 0 = 1 − 1.
Conservation laws guide allowed reactions.
Feynman diagrams
Pictorial representation of particle interactions.
Lines and vertices diagrams represent particle interactions; each diagram corresponds to a term in the perturbation series for the amplitude. Time runs horizontally, wavy lines are bosons.
Electron scattering by exchange of a photon.
Not literal paths.
Particle accelerators
Linear and circular machines reach high energies.
Linear accelerators and circular synchrotrons use electric fields to accelerate and magnetic fields to steer charged particles to high energy. Colliders give the maximum available energy.
SLAC 3 km linear accelerator; LHC 27 km ring.
Synchrotron radiation limits electron rings.
Large Hadron Collider
27-km ring colliding protons at 13.6 TeV.
27 km ring near Geneva with 1232 superconducting dipole magnets at 8.3 T and 1.9 K. Proton beams collide at 13.6 TeV, 40 million times a second.
Discovered the Higgs in 2012.
ATLAS and CMS detectors.
Particle detectors
Tracking and calorimetry reconstruct collisions.
Layered devices: tracker (curved paths in B give momentum), calorimeters (energy), muon chambers and particle ID. Software reconstructs collisions from billions of readouts.
CMS is 14,000 tonnes.
Triggers pick 1 in a million events.
Neutrino oscillations
Neutrinos change flavour, indicating mass.
Neutrinos change flavour as they travel, which requires mass differences; discovered at Super-Kamiokande and SNO (Nobel 2015). The Standard Model originally had massless neutrinos.
Solar electron neutrinos arrive as other flavours.
Absolute masses are below 0.1 eV.
CP violation
Matter and antimatter behave slightly differently.
Matter and antimatter behave slightly differently in weak decays (kaons, B mesons), required to create more matter than antimatter. The observed amount is far too small to explain the universe.
Discovered in 1964.
Phase in the CKM matrix.
Matter–antimatter asymmetry
Why the universe contains more matter.
The universe contains almost no antimatter, with baryon-to-photon ratio ~10⁻⁹. Sakharov's conditions: baryon number violation, C and CP violation, and non-equilibrium.
For every billion antiquark-quark pairs one extra quark survived.
No complete explanation known.
Grand unified theories
Attempts to unify the strong and electroweak forces.
Attempt to merge strong, weak and EM forces into one at ~10¹⁶ GeV; predict proton decay (T½ > 10³⁴ yr, not seen) and magnetic monopoles.
SU(5) model.
Couplings nearly meet only with supersymmetry.
Supersymmetry
Proposed symmetry connecting bosons and fermions.
Proposes a partner for each particle differing in spin by ½ (selectron, gluino). It could solve the hierarchy problem and provide dark matter, but the LHC hasn't found it so far.
Neutralino as a WIMP candidate.
Constrained by experiments.
String theory
Particles as vibrating strings in extra dimensions.
Fundamental objects are tiny vibrating strings (~10⁻³⁵ m) in 10 or 11 dimensions; different modes appear as different particles, including the graviton. Not yet testable.
Offers quantum gravity.
Needs extra compactified dimensions.
Our Sun
A main-sequence star powered by fusion.
A G-type main-sequence star of 2×10³⁰ kg, 5772 K surface, luminosity 3.8×10²⁶ W; about 4.6 billion years old with ~5 billion left. Fusion at the core (15 million K).
Light takes 8.3 minutes to reach Earth.
99.86% of the solar system's mass.
Stellar life cycle
Stars are born, fuse elements and die based on mass.
Stars form from collapsing gas clouds, burn hydrogen on the main sequence for most of their lives, then swell into red giants. Low-mass stars become white dwarfs; massive stars explode as supernovae and leave neutron stars or black holes.
Lifetime ∝ M⁻²·⁵: massive stars live millions of years.
Mass determines fate.
Hertzsprung–Russell diagram
Plots luminosity against temperature.
Plots luminosity against surface temperature (or colour); stars lie along the main sequence, with giants above and white dwarfs below. It reveals stellar evolution and distance.
The Sun sits in the middle at 5772 K.
Temperature axis runs backward.
Stellar nucleosynthesis
Stars forge elements up to iron.
Stars fuse elements: H → He, He → C, O, then Ne, Mg, Si up to iron; heavier elements come from neutron capture in supernovae and neutron-star mergers. 'We are stardust'.
Gold forms in neutron-star collisions.
Iron fusion doesn't release energy.
Supernovae
Massive stars explode, seeding space with heavy elements.
A core-collapse supernova happens when an iron core exceeds the Chandrasekhar limit and collapses within a second, releasing 10⁴⁶ J, mostly in neutrinos. Type Ia (white dwarf explosions) serve as standard candles.
SN 1987A neutrinos detected on Earth.
Outshines a galaxy for weeks.
White dwarfs
Remnants supported by electron degeneracy pressure.
Remnant cores of Sun-like stars: Earth-sized, 10⁹ kg/m³, supported by electron degeneracy pressure. They cool slowly over billions of years.
Sirius B.
Mass limited to 1.4 solar masses.
Chandrasekhar limit
Maximum white dwarf mass of 1.4 solar masses.
M ≈ 1.4 M☉Above about 1.4 M☉ electron degeneracy can't support a white dwarf; it collapses to a neutron star or explodes. Derived by Chandrasekhar in 1930.
Type Ia supernovae occur near this mass.
Depends on composition.
Neutron stars and pulsars
Ultra-dense remnants emitting periodic radio pulses.
After a supernova, a core of 1.4–2.2 M☉ collapses to a ball ~20 km across made of neutrons. A pulsar is a rapidly rotating magnetised neutron star emitting beams like a lighthouse, with pulses from ms to seconds.
Crab pulsar spins 30 times per second.
Magnetars have 10¹¹ T fields.
Black holes in astrophysics
Stellar and supermassive black holes.
Stellar black holes (3–100 M☉) form from massive star collapse; supermassive ones (10⁶–10¹⁰ M☉) sit in galaxy centres; imaged by the Event Horizon Telescope (M87*, Sgr A*).
Cygnus X-1 about 21 M☉.
Accretion discs make them shine.
Galaxies
Collections of billions of stars.
Gravitationally bound systems of 10⁶–10¹⁴ stars, gas, dust and dark matter: spirals, ellipticals and irregulars. There are about 2 trillion in the observable universe.
Andromeda is 2.5 million light years away.
Collisions and mergers shape them.
Milky Way
Our barred spiral galaxy.
Our barred spiral galaxy: 100–400 billion stars, 100,000 light years across; the Sun orbits the centre at 230 km/s every 230 million years, 26,000 ly from the centre.
Contains a 4 million M☉ black hole.
The band of light we see is its disc.
Hubble's law
Galaxies recede at speeds proportional to distance.
v = H₀dGalaxies recede at speed proportional to distance: v = H₀d, with H₀ ≈ 70 km/s/Mpc. It's evidence of cosmic expansion.
A galaxy at 100 Mpc recedes at 7000 km/s.
Values from different methods disagree (Hubble tension).
Expansion of the universe
Space itself stretches.
Space itself expands, stretching light wavelengths and carrying galaxies apart; there's no centre. The expansion accelerates today due to dark energy.
Like dots on an inflating balloon.
Galaxies aren't flying through space.
Big Bang theory
The universe began hot and dense 13.8 billion years ago.
The universe began about 13.8 billion years ago from a hot, dense state and has been expanding and cooling since. Supported by the CMB, light element abundances and the Hubble flow.
Not an explosion in space.
Doesn't describe t = 0 itself.
Cosmic microwave background
Afterglow of the early universe at 2.7 K.
Uniform 2.725 K blackbody radiation from 380,000 years after the Big Bang when atoms formed and the universe became transparent. Tiny anisotropies (10⁻⁵) seeded galaxies.
Discovered by Penzias and Wilson in 1965.
Part of 'static' on old TVs.
Big Bang nucleosynthesis
Formation of light elements in the first minutes.
In the first 3 minutes protons and neutrons fused into about 75% hydrogen, 25% helium-4 by mass, with traces of deuterium and lithium. Predictions match observed abundances.
Neutron-to-proton ratio of 1:7.
Lithium is a puzzle.
Inflation
Rapid exponential expansion in the early universe.
A proposed exponential expansion at 10⁻³⁶ to 10⁻³² s that solved the horizon and flatness problems and generated density seeds from quantum fluctuations.
Expanded by a factor ≥ 10²⁶.
Gravitational-wave imprints sought in the CMB.
Dark matter
Invisible matter inferred from gravity.
About 27% of the universe's energy content; it interacts gravitationally but not electromagnetically. Evidence: rotation curves, lensing and CMB. Candidates: WIMPs, axions.
Direct-detection experiments haven't found it.
Not antimatter or black holes (mostly).
Dark energy
Mysterious cause of accelerating expansion.
About 68% of the universe, with negative pressure causing accelerated expansion, discovered via distant supernovae in 1998 (Nobel 2011). The cosmological constant is the simplest model.
Its density is about 6×10⁻²⁷ kg/m³.
Different from dark matter.
Critical density
Density separating open and closed universes.
ρ_c = 3H²/8πGρ_c = 3H²/(8πG) ≈ 9×10⁻²⁷ kg/m³ is the density for a spatially flat universe. Observations give Ω ≈ 1.
About 5 hydrogen atoms per cubic metre.
Total matter + energy matches ρ_c.
Fate of the universe
Heat death, big rip or big crunch.
Possible endings: heat death (expanding forever to cold, dark state), Big Rip (dark energy tears all apart), Big Crunch (recollapse), or vacuum decay. Current data favour eternal expansion.
Stars burn out in 10¹⁴ years.
Depends on dark energy's nature.
Exoplanets
Planets orbiting other stars found via transit and radial velocity.
Planets around other stars, found by transits (dip in brightness), radial velocity (stellar wobble), microlensing and direct imaging. Over 5000 are known.
Kepler found thousands.
Many are 'hot Jupiters'.
Habitable zone
Orbit range allowing liquid water.
The orbital region where liquid water could exist on a planet's surface; depends on the star's luminosity. Not a guarantee of life.
For the Sun about 0.95–1.4 AU.
Atmosphere and magnetic field also matter.
Astronomical distances
Light-years, parsecs and astronomical units.
1 AU = 1.5×10¹¹ m; 1 light-year = 9.46×10¹⁵ m; 1 parsec = 3.26 ly. Nearest star Proxima Centauri: 4.24 ly.
Milky Way diameter ~100,000 ly.
Distances are time-delayed.
Parallax and standard candles
Methods to measure cosmic distances.
Parallax gives distances to nearby stars: d (pc) = 1/p (arcsec). Standard candles (Cepheid variables, Type Ia supernovae) with known luminosity give greater distances via the inverse-square law.
Gaia measured a billion stars.
A distance ladder links the methods.
Multiverse ideas
Speculative proposals of many universes.
Hypotheses such as eternal inflation or string landscape suggest many universes with different constants, possibly explaining fine-tuning; no direct test is known.
Anthropic reasoning.
Debated whether it's science.
Frequently asked questions
What is the Standard Model?
It is the current theory describing the known elementary particles, which include quarks, leptons and force-carrying bosons, and the electromagnetic, weak and strong forces. It does not include gravity or dark matter.
Why is gravity not part of the Standard Model?
Gravity is described by general relativity, which has not yet been combined with quantum field theory in a way that works at the smallest scales. Gravity is far weaker than the other forces at the particle scale, so it is hard to test there.
What is the Big Bang?
It is the model in which the universe expanded from an extremely hot, dense early state about 13.8 billion years ago. It describes the expansion and cooling of the universe, not an explosion at a point in existing space.
Why are particle physics and cosmology studied together?
Particles produced just after the Big Bang shaped the early universe, and cosmic observations test particle physics at energies no lab can reach.
Is the universe still expanding?
Yes. Measurements of distant galaxies show that the expansion is speeding up, which is attributed to dark energy.
Other areas
Measurement & Math Tools · Kinematics · Newton's Laws & Forces · Work, Energy & Power · Momentum & Collisions · Rotational Motion · Gravitation & Orbits · Oscillations & SHM · Waves & Sound · Fluids & Elasticity · Heat & Thermodynamics · Kinetic Theory & Statistical Physics · Electrostatics · Current Electricity & Circuits · Magnetism & Induction · AC Circuits & EM Waves · Optics · Quantum Mechanics · Atomic & Nuclear Physics · Relativity