COSMOLOGY /
13.8 billion years, briefly.
A short tour of everything that has ever happened — from a singularity smaller than a proton to a thin, cold sea of photons drifting through eternity.
The Hubble flash: galaxies are running away.
Edwin Hubble plots distance against redshift and finds a line. The farther a galaxy, the faster it recedes. Space itself is stretching.
- Hubble constant H₀ ≈ 70 km/s/Mpc
- Redshift z = Δλ / λ — light wavelengths get longer as space expands
- Implication: rewind the tape, and everything was once in one place.
The Big Bang. Hot, dense, and everywhere at once.
Not an explosion in space — an unfolding of space. In 10-32 s the universe inflates by a factor of 1026. The plasma cools. Quarks bind into protons. Nuclei form in the first three minutes.
Recombination — the universe goes transparent.
Electrons finally bind to protons. Photons stop scattering and stream free. That first light, redshifted by a factor of about 1100, is the cosmic microwave background — the oldest photograph ever taken.
- Temperature today: 2.725 K
- Anisotropies: ± 1 part in 105 — seeds of every galaxy
- Discovered 1965, Penzias & Wilson, Bell Labs
Dark ages → first stars.
For 100 million years the universe is dim and neutral. Then gravity wins. Pristine clouds of hydrogen collapse and ignite as Population III stars — massive, hot, doomed. Their UV light reionizes the cosmos.
- Pop III stars: ~ 100 – 1000 M⊙, no metals, lifetimes a few Myr
- Reionization complete by z ≈ 6 — about a billion years in
- Their supernovae seeded the cosmos with carbon, oxygen, iron — the stuff of us.
Gravity sculpts the cosmic web.
Tiny CMB ripples grow over billions of years into a foam of filaments, sheets, and voids. Galaxies form at the bright knots where filaments cross.
- Voids span 102 Mpc across — mostly empty
- Galaxy clusters live at filament intersections
- Visible galaxies are foam on top of an invisible scaffold.
Our galaxy: the Milky Way.
A barred spiral — one ordinary galaxy among hundreds of billions. We’re about two-thirds of the way out from the center, riding a minor arm.
- Stars: ~ 100 × 109
- Diameter: ~ 100,000 ly · thickness ~ 1,000 ly
- Central black hole Sgr A*: ~ 4.1 × 106 M⊙
- Mass dominated by an unseen dark-matter halo.
Dark matter: the books don’t balance.
Stars at the edges of galaxies orbit too fast for the visible mass alone. Gravitational lensing maps mass that emits no light. Something is there — and there is a lot of it.
- Doesn’t emit, absorb, or scatter light at any wavelength we’ve probed
- Interacts gravitationally and (so far) only gravitationally
- Candidates: WIMPs, axions, primordial black holes — or new physics entirely.
The expansion is accelerating.
Two independent teams measure distant Type Ia supernovae and find them dimmer than expected. The universe is not just expanding — the expansion is speeding up. Something with negative pressure is pushing space apart.
- Cosmological constant Λ — energy density of empty space
- Equation of state w ≈ -1
- 2011 Nobel Prize: Perlmutter, Schmidt, Riess
- We don’t know what it is. ~68% of everything.
Stelliferous → degenerate → black-hole → heat death.
If Λ holds, the cosmos cools and dilutes forever. Stars run out of hydrogen. White dwarfs, neutron stars, and black holes are all that’s left. Eventually even those evaporate via Hawking radiation.
Open questions.
A century of progress has only sharpened our ignorance. The biggest puzzles are also the most basic.
“We are a way for the cosmos to know itself.”
— Carl Sagan, Cosmos, 1980
Read & watch.
- Steven Weinberg — The First Three Minutes (1977)
- Brian Greene — The Fabric of the Cosmos
- Carl Sagan — Cosmos (book + 1980 series)
- Sean Carroll — From Eternity to Here
- Katie Mack — The End of Everything
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