The visible
universe.
A guide to what the eye, the dish, and the mirror have seen — from a 4.6 Gyr-old yellow dwarf, to galaxies whose photons left before Earth had oceans.
Eight planets and a star.
The Sun contains 99.86 % of the system's mass. Around it, four rocky terrestrials, two ice/gas giants, and beyond Neptune the Kuiper Belt and the Oort Cloud — a roughly spherical reservoir whose comets perturb our skies.
Bode's pattern is no longer law, but distance still matters: the snow line, around 4 AU in our young solar nebula, separated rock-builders from gas-grabbers.
- Mercury — 88 d orbit, no atmosphere
- Venus — 92 bar CO₂, runaway greenhouse
- Earth — only known biosphere
- Mars — Olympus Mons, 22 km tall
- Jupiter — 318 M⊕, 79+ moons
- Saturn — A/B/C rings, 1 m thick
- Uranus — 98° axial tilt
- Neptune — 5.4 hour winds, 2,100 km/h
How stars live and die.
Molecular Cloud Collapse
Cold H₂ regions (10–30 K) fragment under self-gravity. The Jeans mass sets the threshold. Protostars accrete from a disk; bipolar jets clear the envelope.
Hydrogen Fusion
Core p-p chain or CNO cycle converts 4H → He. Mass dictates everything: a 0.5 M☉ red dwarf burns for ~80 Gyr, a 25 M☉ blue giant for only ~5 Myr.
White Dwarf · Neutron Star · Black Hole
Below ~8 M☉: planetary nebula, white dwarf supported by electron degeneracy. Above: core-collapse supernova, leaving neutron star or — beyond Tolman–Oppenheimer–Volkoff limit — a black hole.
Three equations that opened the cosmos.
Newton's Law of Gravitation, 1687
Every mass attracts every other mass.
G = 6.674 × 10⁻¹¹ N·m²/kg². The first universal law — pulled the apple and held the Moon.
Einstein Field Equations, 1915
Mass–energy curves spacetime.
Predicted gravitational lensing, Mercury's perihelion shift, gravitational waves (LIGO, 2015), and the expanding universe itself.
Friedmann Equation, 1922
The universe's expansion history.
From it we get the Hubble flow, the critical density, and — once Λ > 0 — accelerated expansion driven by dark energy.
Hubble's tuning fork.
Edwin Hubble in 1926 sorted the "nebulae" into ellipticals (E0–E7), spirals (Sa, Sb, Sc) and barred spirals (SBa–SBc), with irregulars off to the side. Modern surveys (SDSS, GAMA) refine but preserve the scheme.
The Milky Way is an SBbc spiral, ~100 kly across, ~10¹¹ stars, with a 4.15 × 10⁶ M☉ supermassive black hole — Sagittarius A* — at the heart, imaged by the Event Horizon Telescope in 2022.
Galaxies inhabit a cosmic web: filaments and walls of dark matter, voids the size of a hundred Mpc.
From the Big Bang to now.
Quantum gravity dominates; physics as we know it does not yet apply.
Alan Guth (1980): exponential expansion by factor ≥ 10²⁶ smooths and flattens the cosmos, seeds quantum fluctuations.
Quarks confine into protons and neutrons.
~75 % H, 25 % He by mass, traces of Li forged. Predicted by Gamow, Alpher, Herman; confirmed.
Universe cools to 3,000 K; electrons bind to nuclei; photons stream free as the CMB.
Pop III stars ignite; reionize the neutral fog. JWST is now finding their successors.
Sun and planets form from a collapsing molecular cloud.
Dark energy dominant; accelerated expansion; cosmic web fully formed.
The oldest photograph.
Discovered by accident in 1964 by Arno Penzias and Robert Wilson at Bell Labs, the CMB is a near-perfect 2.725 K blackbody bathing the sky from every direction. Tiny anisotropies — one part in 10⁵ — encode the seeds of every galaxy we see.
COBE (1992), WMAP (2003), and Planck (2013, 2018) measured those wrinkles, fixing the universe's geometry as flat and its composition: 68.5 % dark energy, 26.5 % dark matter, 5 % ordinary matter.
peak λ ≈ 1.06 mm · Tγ = 2.7255 ± 0.0006 K
Deep field.
A long-exposure analog. JWST's actual NIRCam fields show galaxies at z ≥ 13, photons emitted ~325 Myr after the Big Bang.
Who saw further.
Hipparchus
~150 BCE
Catalogued ~850 stars; discovered precession of equinoxes.
Copernicus
1473–1543
De revolutionibus moved Earth from the cosmic centre.
Galileo
1564–1642
Moons of Jupiter, phases of Venus, Milky Way as stars.
Kepler
1571–1630
Three laws of planetary motion from Tycho's data.
Hubble
1889–1953
Showed galaxies recede; the universe expands.
Henrietta Leavitt
1868–1921
Cepheid period-luminosity relation — the cosmic ruler.
Jocelyn Bell Burnell
b. 1943
Discovered radio pulsars in 1967; rotating neutron stars.
Vera Rubin
1928–2016
Galaxy rotation curves implied invisible halos.
Spacetime's vanishing point.
The Schwarzschild radius — rs = 2GM/c2 — defines the event horizon: the surface beyond which nothing escapes. For one solar mass, ~2.95 km. For Sgr A*, 12 million km.
Stellar-mass black holes are the corpses of massive stars. Supermassive ones (10⁶–10¹⁰ M☉) anchor every large galaxy, tied to host bulge mass via the M–σ relation.
Hawking (1974): black holes radiate, with T = ħc3/(8πGMk) — vanishingly small for stellar mass, but profound for theory.
EHT imaged M87* (2019) and Sgr A* (2022). LIGO detected the GW150914 binary merger in 2015 — confirming Einstein's last prediction.
Other worlds.
Transit
Star dims by 0.01–1 % as planet crosses face. Kepler (2009–18): 2,778 confirmed. TESS continues all-sky survey.
Radial Velocity
Star wobbles. 1995: Mayor & Queloz find 51 Pegasi b — first exoplanet around a sun-like star (Nobel 2019).
Direct Imaging
Coronagraph blocks starlight. JWST has resolved HR 8799 system, sniffed CO₂ in WASP-39b's atmosphere.
"Astronomy compels the soul to look upward, and leads us from this world to another."— Plato, Republic, Book VII
The infrared revolution.
Launched on Christmas Day 2021, parked at the L2 Lagrange point, the James Webb Space Telescope's 6.5 m segmented gold-coated beryllium mirror collects ~6× the light of Hubble's, in wavelengths 0.6–28 μm.
In the infrared, light from the very early universe is shifted into view — letting JWST see galaxies at z > 13, less than 330 Myr after the Big Bang. It also peers through dust into stellar nurseries (the Pillars of Creation, Carina) and analyzes exoplanet atmospheres by transmission spectroscopy.
- Diameter: 6.5 m (18 hexagons)
- Sun-shield: tennis court sized, 5 layers
- Operating temperature: ~50 K
- Mission lifetime: 20 yr+ propellant
What we're chasing now.
The Hubble Tension
Local distance-ladder gives H₀ ≈ 73 km/s/Mpc; CMB-derived gives 67.4. The 5σ disagreement may signal new physics — early dark energy? Modified gravity?
Dark Matter Detection
XENONnT and LZ probe WIMPs with ever-better sensitivity. Axion searches (ADMX) underway. Or perhaps modifications to gravity à la MOND/MOG?
Multi-messenger Astronomy
GW170817 — neutron-star merger seen in gravitational waves and gamma rays. IceCube neutrinos pin down blazar TXS 0506+056 as a cosmic-ray source.
Habitable Worlds Observatory
NASA's planned ~6 m UV/optical/IR telescope (2040s) — designed to image and spectrally characterize ~25 nearby Earth-like exoplanets.
What we still don't know.
What is dark energy?
Cosmological constant Λ? Dynamical scalar field? An artefact of inhomogeneity? It drives 68 % of the cosmos.
Why is the universe flat & smooth?
Inflation explains it — but what is the inflaton field? Is there a multiverse of pocket universes?
Are we alone?
Drake equation, Fermi paradox. Biosignatures (O₂+CH₄), technosignatures (SETI). No detection yet.
What lies beyond the cosmological horizon?
~46.5 Gly comoving — but the universe likely extends far further. Maybe infinitely.
How did supermassive black holes get so big, so early?
JWST sees ~10⁹ M☉ black holes at z ≈ 7. Direct collapse? Mergers?
Information paradox
Hawking radiation seems thermal. Where does the infallen information go? Holography? Soft hair? Firewalls?
Watch & read.
PBS Space Time — Cosmology Playlist
Matt O'Dowd's accessible-but-rigorous channel — the gold standard for theoretical physics on YouTube.
References
- Weinberg — Cosmology (2008)
- Carroll — Spacetime and Geometry (2003)
- Peebles — Cosmology's Century (2020)
- Planck Collaboration — A&A 641, A6 (2020)
- Riess et al. — ApJ Letters 934 (2022) on H₀
- JADES Collaboration — JWST early-universe survey
- EHT — ApJL 875 (2019); 930 (2022)