Astrophysics: Exploring the Universe Through Physics

Stellar Physics

Stars form from collapsing molecular clouds (Jeans instability: M > M_J ≈ 1-100 M☉ depending on T and ρ). Protostellar phase: Kelvin-Helmholtz contraction heats core to ~10⁷ K → hydrogen fusion ignites via pp-chain (low-mass) or CNO cycle (>1.3 M☉). Main sequence: hydrostatic equilibrium between gravity and radiation pressure, governed by mass-luminosity relation L ∝ M^3.5. Stellar lifetimes: τ ∝ M/L ∝ M⁻²·⁵ (Sun: 10 Gyr, 10 M☉: 20 Myr, 0.1 M☉: trillions of years). Post-main sequence: shell hydrogen burning → red giant, helium flash (for low mass), triple-alpha process → carbon. Massive stars: onion-shell structure (H-He-C-O-Ne-Si-Fe), iron core collapse → supernova, neutron star or black hole.

Cosmology

Big Bang: universe began 13.8 billion years ago from an extremely hot, dense state. Cosmic Microwave Background (CMB): thermal radiation at T = 2.725 K, redshifted from z ≈ 1100 (recombination epoch, 380,000 years). Hubble's Law: v = H₀d, H₀ ≈ 67-73 km/s/Mpc (tension between CMB and local measurements). ΛCDM model: ~68% dark energy (Λ), ~27% dark matter, ~5% baryonic matter. Inflation: exponential expansion in first 10⁻³⁶ s, explains flatness, horizon, and monopole problems. Baryogenesis: matter-antimatter asymmetry (1 in 10⁹ excess baryons). Large-scale structure: cosmic web of filaments, voids, clusters — formed by gravitational instability from primordial density fluctuations.

Dark Matter

Evidence: galaxy rotation curves (Rubin & Ford, 1970s), gravitational lensing (Bullet Cluster), CMB anisotropies, structure formation simulations. Properties: non-baryonic, non-luminous, gravitationally interacting, cold (non-relativistic). Candidates: WIMPs (Weakly Interacting Massive Particles, 1-1000 GeV), axions (10⁻⁵ eV), sterile neutrinos. Direct detection: xenon-based experiments (LZ, XENONnT, PandaX) — no signal yet, cross-section limits < 10⁻⁴⁷ cm². Indirect: gamma-ray excess from Galactic Center (Fermi-LAT, debated). Collider: LHC searches for missing transverse energy. Alternative: MOND (Modified Newtonian Dynamics) — explains rotation curves without dark matter, but fails for clusters and CMB.

Gravitational Waves and Black Holes

Gravitational waves: ripples in spacetime predicted by Einstein's GR (1916), first detected by LIGO (GW150914, 2015). Sources: binary black hole mergers (most common), binary neutron star mergers (GW170817 with kilonova), continuous waves from pulsars. Black holes: Schwarzschild radius r_s = 2GM/c², event horizon, singularity. Kerr black holes: rotating, ergosphere allows Penrose process for energy extraction. Hawking radiation: T = ℏc³/(8πGMk_B) — evaporation timescale for solar-mass BH > 10⁶⁷ years. Supermassive BH: 10⁶-10¹⁰ M☉ in galactic centers, M-σ relation (mass correlates with bulge velocity dispersion). EHT: first image of M87* (2019) and Sgr A* (2022) — ring-like structure confirms GR predictions.

Open Questions

Nature of dark energy: cosmological constant, quintessence, or modified gravity? Hubble tension: H₀ = 67.4 (Planck) vs. 73.0 (SH0ES) km/s/Mpc — systematic error or new physics? Baryon asymmetry: why matter dominates over antimatter (Sakharov conditions met but insufficient in SM). Black hole information paradox: unitarity vs. information loss at the event horizon. Quantum gravity: reconciling GR with quantum mechanics (string theory, loop quantum gravity, causal set theory). Multi-messenger astronomy: combining GW, EM, neutrinos, cosmic rays for complete picture. Future facilities: LISA (space-based GW), Einstein Telescope, Vera C. Rubin Observatory (LSST), James Webb Space Telescope discoveries.

Frequently Asked Questions

What is astrophysics?

Astrophysics applies physics principles to understand celestial objects and phenomena, including stars, galaxies, black holes, and the universe as a whole.

What is dark matter?

Dark matter is an invisible form of matter that doesn't emit or absorb light but interacts gravitationally, making up about 27% of the universe's total energy content.

How old is the universe?

The universe is approximately 13.8 billion years old, determined from CMB measurements and the ΛCDM cosmological model.

What are gravitational waves?

Gravitational waves are ripples in the fabric of spacetime caused by accelerating massive objects, first directly detected by LIGO in 2015.

What happens inside a black hole?

Beyond the event horizon, all paths lead to the singularity. Current physics breaks down at the singularity, and a theory of quantum gravity is needed to fully describe it.

Try it live

Everything above runs in your browser — open Astrophysics: Binary Black Hole Merger Simulator and change the parameters while it is running. Nothing is installed, nothing is uploaded, the whole model lives in one tab.