Diff — Why does matter exist

{{infobox

| title = Why does anything exist?

| image = cmb-timeline.jpg

| caption = The universe's timeline — the Big Bang should have produced equal matter and antimatter, which would have annihilated completely. It didn't (NASA/WMAP Science Team, public domain)

| question = Why did the Big Bang leave behind any matter at all?

| the math = For every ~1 billion antimatter particles, ~1 billion + 1 matter particles were made — the "+1" is everything you see

| required = Sakharov's three conditions (1967): baryon-number violation, C/CP violation, departure from thermal equilibrium

| status = Unsolved — the Standard Model's built-in asymmetry is far too small; something else did it

| suspects = Rare particle decays, neutrino-sector CP violation (leptogenesis), physics beyond the Standard Model

}}

Here is the deepest question in physics, and it is not rhetorical: **the Big Bang should have made equal amounts of matter and antimatter — which should have annihilated each other into pure light. Yet here we are.** Every atom in every star, planet, and person is the leftover of a tiny early imbalance — for every billion antimatter particles, the universe made roughly a billion and one of matter; the billion pairs annihilated, and the "+1" became everything. Explaining that surplus — the problem of **baryogenesis** — is one of the great unsolved problems of physics, and the [[zeptoverse]] program of rare-decay hunting is one of the laboratories built to find it.[[cite: cosmological baryon asymmetry; Sakharov 1967]]

## What we'd need — Sakharov's three conditions

In 1967, Soviet physicist **Andrei Sakharov** — dissident, Nobel laureate, father of the hydrogen bomb — wrote down what any successful matter-making process requires. The list is short and brutal:

1. **Baryon-number violation** — some process must create or destroy matter particles; if baryon number is perfectly conserved, the matter count can never change

2. **C and CP violation** — the physics must treat matter and antimatter *differently*: charge symmetry (C, swapping matter↔antimatter) and charge-parity symmetry (CP) must both be broken, or every matter-making reaction has an antimatter twin that cancels it

3. **Departure from thermal equilibrium** — the process must run in a non-equilibrium moment (like the early universe's expansion); in equilibrium, rates balance and nothing net can be created

All three have been observed — CP violation exists (discovered in 1964 in neutral kaons, earning Cronin and Fitch the 1980 Nobel, and since found in beauty particles — the same quark the [[zeptoverse]] team hunts). The problem is **amount**: the Standard Model's built-in CP violation is orders of magnitude too small to explain the observed surplus. Something else — outside our current theory — did the job.[[cite: Sakharov 1967; Cronin–Fitch kaon discovery; Standard Model CP-violation estimates]]

## The suspects

* **More CP violation in the quark sector** — beauty physics is exactly where tiny rare asymmetries surface; it's why the [[one in ten billion decay]] matters beyond the headline. LHCb already found CP violation in *baryons* in 2025 — evidence the asymmetry runs deeper than the Standard Model accounts for

* **Leptogenesis** — the leading elegant idea: if neutrinos are their own antiparticles (a "Majorana" property) and violate CP in their own sector, a matter-asymmetry in *leptons* could have been converted into the baryon asymmetry — a testable idea being hunted by the neutrino experiments chasing [[neutrino|neutrino mass]]

* **New particles entirely** — any heavy particle that decayed preferentially to matter in the early universe would leave the same fingerprint: a rate measurably off from the Standard Model in today's rare-decay hunts

## How this connects to the smallest signals

The chain is unusually direct. Rare decays — the B⁰ → μμ class the zeptoverse opened — are precision tests of the symmetries Sakharov's conditions require to be broken. A measured rate that deviates from the Standard Model would mean *new particles interfering in the decay* — particles that could be the missing agents of baryogenesis. The reason physicists spent 42 years hunting a one-in-ten-billion decay is that the answer to "why does anything exist" is encoded in processes that happen almost never — the smallest signals answering the biggest question.[[cite: rare-decay new-physics literature; leptogenesis reviews]]

## What we know for sure

* CP violation is real — found in kaons (1964), beauty mesons (2001), and beauty baryons (2025)

* The Standard Model's amount is too small — by many orders of magnitude

* Therefore: **physics we haven't found yet made the universe's matter** — the honest standing of the field

## See also

* [[zeptoverse]] — the frontier hunting the missing physics

* [[one in ten billion decay]] — the golden channel

* [[neutrino]] — the leptogenesis suspect

* [[proton decay]] — if baryon number can be violated at all

## References

- Sakharov — "Violation of CP invariance, C asymmetry, and baryon asymmetry of the universe" (1967)

- Christenson, Cronin, Fitch & Turlay — CP violation in K⁰ decay (1964); Nobel 1980

- Planck collaboration — baryon-to-photon ratio

- LHCb collaboration — CP violation in baryon decays (2025)

[[Category:Science]]

[[Category:Physics]]