Astrophysics Professional

Modern astrophysics is driven by multi-messenger, multi-wavelength observations with sophisticated instruments.

Imaging and photometry

CCDs (charge-coupled devices) revolutionised optical astronomy: linear, sensitive, calibratable. Modern surveys use mosaic focal planes: Rubin Observatory's LSST camera has 3.2 gigapixels. Photometry measures fluxes in calibrated bandpasses; the AB magnitude system: m_AB = −2.5 log₁₀(f_ν) − 48.6 (for f_ν in erg/s/cm²/Hz).

Spectroscopy

Dispersing light with a grating or prism gives a spectrum: emission/absorption lines identify elements, Doppler shifts give velocities (v/c = Δλ/λ). Multi-object spectrographs (e.g. DESI) take 5,000 spectra simultaneously. Integral field units (IFUs) provide spectra for every spatial pixel.

Radio and mm astronomy

Single-dish: sensitivity ∝ A_eff/T_sys. Interferometry (VLBI): angular resolution λ/D_baseline, achieving micro-arcsecond resolution.

Space-based IR and X-ray

JWST (6.5 m, L2 orbit, 0.6–28 μm) revolutionises high-z galaxy science. Chandra (sub-arcsecond X-ray imaging) and eROSITA (all-sky X-ray survey) map hot gas and AGN.

The interstellar medium (ISM) is the matter between stars — gas, dust, cosmic rays, and magnetic fields — the raw material for star formation and the repository of stellar ejecta.

ISM phases

• Cold neutral medium (CNM): T ~ 100 K, n ~ 30 cm⁻³, 21-cm HI emission
• Warm neutral medium (WNM): T ~ 8,000 K, n ~ 0.4 cm⁻³
• Warm ionised medium (WIM): T ~ 8,000 K, ionised by UV from hot stars
• Hot ionised medium (HIM): T ~ 10⁶ K, n ~ 0.003 cm⁻³, X-ray emitting, shock-heated by supernovae
• Molecular clouds: T ~ 10 K, n ~ 10³–10⁶ cm⁻³, CO/H₂; sites of star formation

Dust

Dust grains (silicates, carbonaceous) make up ~1% of ISM mass but cause extinction (absorption + scattering) and thermal emission in the IR. Polarised dust emission traces magnetic field geometry.

On scales > 100 Mpc the universe is homogeneous; on scales of 1–100 Mpc it is structured in a cosmic web of filaments, sheets, and voids.

Correlation functions and power spectra

The two-point correlation function ξ(r) = ⟨δ(x)δ(x+r)⟩ measures clustering. The power spectrum P(k) is its Fourier transform. The matter power spectrum P(k) has a characteristic shape set by matter-radiation equality and baryon acoustic oscillations (BAO scale ~150 Mpc).

BAO as a standard ruler

The acoustic scale imprinted in the CMB (θ_s) also appears in the galaxy correlation function. Measuring it at different redshifts probes the expansion history D_A(z), constraining dark energy.

Galaxy surveys

SDSS mapped ~3 million galaxies spectroscopically. DESI (ongoing 2023–) aims at 40 million. Euclid (2023 launch) will survey 1.5 billion galaxies photometrically and 30 million spectroscopically — precision dark energy via BAO + weak lensing.

The growth of structure from small primordial fluctuations to today's cosmic web is described by perturbation theory in an expanding universe.

Linear growth

In the matter-dominated era, density perturbations δ = δρ/ρ grow as δ ∝ a(t) (scale factor). The growth rate f = d ln δ / d ln a ≈ Ω_m^{0.55}(z) — measurable from galaxy redshift-space distortions.

Transfer function

The transfer function T(k) encodes how modes of different scales evolve from the primordial power spectrum P(k) = A_s k^{n_s-1} through matter-radiation equality, recombination, and baryon diffusion damping (Silk damping).

Non-linear regime

At δ > 1, perturbations go non-linear and collapse into dark matter halos. The Press-Schechter formalism predicts the halo mass function n(M,z); modern calibrations use N-body simulations (Millennium, IllustrisTNG).

The most energetic processes in the universe produce ultra-high-energy cosmic rays and gamma-ray bursts.

Cosmic rays

Charged particles (mostly protons and heavy nuclei) arriving at Earth with energies up to ~10²° eV — the GZK limit. Below ~10¹⁵ eV (the knee): likely Galactic (supernova remnant shock acceleration). Above ~10¹⁸ eV (the ankle): extragalactic. Pierre Auger Observatory and Telescope Array map arrival directions. The GZK (Greisen-Zatsepin-Kuzmin) cutoff: protons above ~5 × 10¹⁹ eV photopion-produce on the CMB; sources must be within ~50 Mpc.

Gamma-ray bursts (GRBs)

The most luminous transients: peak luminosity ~10⁵¹ erg/s. Two classes: short GRBs (< 2s) from NS mergers (GW170817 was accompanied by GRB 170817A); long GRBs (> 2s) from collapsars (BH formation during the core collapse of a rapidly rotating massive star). Prompt emission from internal shocks in a relativistic jet (Γ ~ 100–1000); afterglow from the jet interacting with the surrounding ISM.

Inflation provides the initial conditions for the hot Big Bang and generates the primordial spectrum of perturbations.

Slow-roll inflation

A scalar field φ (inflaton) with potential V(φ) rolls slowly: ε = (M_P²/2)(V'/V)² ≪ 1, η = M_P² V''/V ≪ 1. The scale factor grows quasi-exponentially: a ∝ e^{Ht}, H² = V/(3M_P²).

Quantum fluctuations as seeds

Each mode of the inflaton field δφ_k satisfies a harmonic oscillator equation. In the de Sitter background, quantum fluctuations are squeezed to become classical perturbations at horizon crossing. The scalar power spectrum: P_s(k) = H²/(8π²ε M_P²), evaluated at k = aH.

Tensor modes

Inflation also generates a primordial gravitational wave background with P_T = 2H²/(π²M_P²). The tensor-to-scalar ratio r = P_T/P_s = 16ε. Detecting r via B-mode CMB polarisation is the primary goal of next-generation CMB experiments (CMB-S4, LiteBIRD).

LIGO/Virgo opened the Hz–kHz band; future detectors will probe nHz–mHz, covering the full GW spectrum.

Ground-based detectors (Hz–kHz)

LIGO A+ (O5, 2027), Einstein Telescope (EU, 2030s), Cosmic Explorer (US, 2030s) — 10–40 km triangular detectors with 10× LIGO sensitivity. Will detect stellar-mass mergers to z > 5.

Space-based (mHz band)

LISA (launch ~2035): 2.5 Gm arm triangular array. Sources: SMBH mergers (10⁵–10⁷ M_☉) to z ~ 20, extreme mass ratio inspirals (EMRIs), mHz stochastic background, ultracompact binaries in the Milky Way.

Pulsar timing arrays (nHz band)

PTAs use an ensemble of millisecond pulsars as arms of a GW detector. NANOGrav (2023) reported evidence for a stochastic GW background at nHz — likely from SMBH binary inspirals. IPTA combines NANOGrav, PPTA, EPTA, InPTA.

Multi-messenger astronomy

GW + electromagnetic + neutrino observations of the same source (neutron star mergers, core-collapse supernovae) unlock unprecedented physics.

Despite enormous progress, astrophysics faces deep open questions.

The nature of dark matter

All direct detection experiments (LUX-ZEPLIN, XENONnT, PandaX) have found nothing. Either WIMPs are lighter or heavier than expected, or dark matter is a different particle entirely. Axion searches (ADMX, ABRACADABRA, CASPEr) are ongoing. Primordial BHs as dark matter are constrained but not ruled out in certain mass ranges.

Dark energy and the Hubble tension

H₀ measured from the CMB (67 km/s/Mpc, Planck) disagrees at >5σ with local measurements (73 km/s/Mpc, SH0ES). Either systematic errors remain, or new physics beyond ΛCDM (early dark energy, additional radiation, modified gravity) is at play.

Reionisation epoch

How and when did the first stars and quasars reionise the neutral hydrogen in the universe (z ~ 6–12)? JWST is finding surprisingly massive galaxies at z > 10, challenging simple models.

Neutron star interiors

What is the equation of state of dense nuclear matter? Is there a quark-hadron transition in NS cores? Multi-messenger observations (GW tidal deformabilities + X-ray NS radii from NICER) are rapidly narrowing the uncertainties.