Physics of the Cosmos

Where every inference starts. Cosmic distances, times, masses and densities, with the units built for them: astronomical unit, light-year, parsec, solar mass, solar luminosity. Angular size. Flux, luminosity, and the inverse-square law that separates how bright a thing looks from how bright it is. Order-of-magnitude estimation, used as a working tool rather than an occasional exercise.

Almost everything we know arrived as photons. The electromagnetic spectrum and photon energy. Thermal radiation: the blackbody spectrum, Wien’s displacement law, the Stefan-Boltzmann law, and the temperature they hand you. Atomic spectra in emission and in absorption, and the composition they reveal. Spectral classification. The Doppler shift and radial velocity.

The instrument is part of the physics. Telescopes: aperture and light gathering, diffraction and angular resolution, why radio telescopes are enormous and why some telescopes go to space. Detectors and data: pixels, exposure, photon statistics, signal-to-noise. Reading a real image or spectrum, and telling measurement, noise, and interpretation apart.

How to weigh something you cannot touch. Newtonian gravity, circular motion, and Kepler’s laws. Masses measured from orbits, including binary stars. Escape velocity and gravitational potential energy. The virial theorem, and the characteristic dynamical times that come with it.

Distance first, then everything else. The cosmic distance ladder: parallax, standard candles such as Cepheid variables and Type Ia supernovae, and the distance modulus. Colour and surface temperature. The relation between luminosity, radius and temperature. The Hertzsprung-Russell diagram. Hydrostatic equilibrium: pressure against gravity, and the central pressure and temperature it implies for the Sun. Energy transport by radiation and convection. Nuclear binding energy, the proton-proton chain and the CNO cycle, and the lifetime a star’s fuel supply buys it.

Where the elements come from. Interstellar gas and dust, molecular clouds, extinction and reddening. Star formation as gravity beating pressure, and the collapse that follows. How mass decides a star’s fate. Cluster ages from the main-sequence turnoff. Nucleosynthesis and supernovae. White dwarfs and electron degeneracy. Neutron stars and pulsars. Black holes, the Schwarzschild radius, and accretion.

Structure on the largest scales, and planets around other stars. The Milky Way: disk, bulge, halo, stellar populations. Galactic rotation, and the evidence for dark matter. Spiral, elliptical and irregular galaxies, clusters, and large-scale structure. Quasars, active galactic nuclei, supermassive black holes and their jets. Exoplanets by radial velocity and by transit, and the selection effects that come with each method. Comparative planetology alongside them: what makes a planet rocky or gaseous, and why a world holds on to an atmosphere or loses it.

How we know the universe has a history. Redshift and Hubble’s law. The scale factor, the Friedmann equation qualitatively, and the age of the universe. The thermal history: early hot plasma, primordial nucleosynthesis, recombination, and the cosmic microwave background. Dark matter, dark energy, cosmic inflation, and structure formation, with a line drawn between what is observed and what is assumed. Multi-messenger astronomy: neutrinos, gravitational waves, cosmic rays, and pulsars used as clocks.