✺ Radiation: the beginning and the end

What radiation is

Light is radiation: packets of energy called photons. Every warm thing glows — you in infrared, the Sun mostly in visible light — and the colour of the glow gives the temperature. Explaining that glow is where quantum physics began (Planck, 1900). Photons carry entropy as well as energy: many cool photons hold more than a few hot ones with the same energy. Established

The beginning: a universe of light

For its first ~50,000 years the universe's energy was mostly radiation; then matter took over. When it had cooled enough for atoms to form, 380,000 years in, that light was set free. It's still here as the microwave background, stretched from about 3,000 degrees to 2.7 above absolute zero, with about a billion photons for every atom. Established

The middle: matter's time

Gravity spends the free energy stored in the evenness: gas clumps, stars ignite and forge the elements, planets and life form in the flow of starlight. Earth takes in sunlight and sends the same energy out as about twenty times as many infrared photons — and runs on the difference. This is where complexity lives. Established

The end: light again

Stars burn out; over 10¹⁹–10²⁰ years close passes fling most of them out of their galaxies, and the rest slowly spiral inward. Black holes hold the most entropy anything can, but not for ever: they evaporate as Hawking radiation, from 10⁶⁷ years for one the Sun's mass to around 10¹⁰⁰ for the largest. What's left is a thin, ever-colder bath of radiation — "heat death" means even, not empty: nothing left to flow downhill. Established (the processes) · Contested (whether protons and dark matter last for ever)

Where the potential goes

It isn't made again. Free energy is spent at every step — clumping, starlight, black holes, evaporation — and entropy counts the spending. Meanwhile expansion stretches radiation: each photon's wavelength grows and its energy falls. Across an expanding universe, total energy isn't conserved at all — the laws aren't the same from moment to moment, which is what energy conservation needs. In any lab, energy is always conserved. Established (some physicists prefer to say the energy goes into the gravitational field — a matter of bookkeeping they still argue about Contested)

Other kinds of radiation

Gravity radiates too: merging black holes send out gravitational waves, ripples of spacetime itself, first detected in 2015. And a background of neutrinos from the first seconds fills space, not yet seen directly. Established (gravitational waves) · the neutrino background is inferred, not detected

Could the end be a beginning?

Roger Penrose says yes. When the last black holes have evaporated, only light is left, and light has no clocks and no sense of size. A cold, infinitely thin end then looks, to the geometry, like a hot, smooth beginning — and a new aeon starts. This is conformal cyclic cosmology. Speculative

The price is the entropy: a new start must be low entropy, in the gravitational ledger. Penrose's answer is that black holes destroy information as they evaporate, which resets the count. Most physicists now expect information to come back out in the radiation (the Page curve, reproduced in recent calculations: theory, not yet observation), which would leave the entropy in place. Contested

Every cyclic idea meets this wall (Tolman, 1930s: each cycle starts messier than the last) — it's the Past Hypothesis again, hole H3. The ideas that avoid a reset have time's arrows point away from a join, with nothing carried across: the Janus point and a mirror universe before the Big Bang. Speculative