Open problems

What were the first stars (Population III) like?

The first stars formed from pure hydrogen–helium gas with no metals to cool it, so models predict they were massive (tens to hundreds of solar masses) and short-lived, seeding the first heavy elements and possibly the first black holes. None has been directly observed.

What produces fast radio bursts?

Millisecond radio flashes carrying the energy of days of solar output arrive from extragalactic distances. Thousands are now catalogued; some repeat, most don't. Magnetars are a proven source for at least one burst, but whether they explain the whole population — especially the apparently non-repeating majority — is unsettled.

What is the equation of state of matter inside neutron stars?

Neutron-star cores compress matter to several times nuclear density — a regime no laboratory can reach. The equation of state determines the maximum mass, radius, tidal deformability and whether cores contain hyperons, quark matter or new phases.

How did the first supermassive black holes grow so fast?

Quasars powered by ~10^9 solar-mass black holes existed by z > 7, less than 800 million years after the Big Bang. Stellar-remnant seeds accreting at the Eddington limit cannot grow fast enough, pushing theorists toward 'heavy seeds' — direct-collapse black holes or runaway mergers in dense clusters.

What is the nature of dark energy?

Observations of distant supernovae in 1998 showed that cosmic expansion is accelerating. The unknown cause — about 68% of the universe's energy budget — is called dark energy, but we do not know whether it is a cosmological constant, a dynamical field, or a sign that general relativity fails on large scales.

What particle (or object) makes up dark matter?

Galaxy rotation curves, gravitational lensing, the Bullet Cluster and the CMB all require about five times more gravitating mass than visible baryons provide. Whether dark matter is a weakly interacting particle, an axion, a primordial black hole population, or something else remains unidentified.

Why do early- and late-universe measurements of H₀ disagree?

The expansion rate today inferred from the CMB assuming ΛCDM (~67.4 km/s/Mpc) sits ~5σ away from the distance-ladder value measured with Cepheids and supernovae (~73 km/s/Mpc). Either a systematic error persists on both sides, or the standard cosmological model is incomplete.

Why is there more matter than antimatter?

The observable universe contains essentially no primordial antimatter: for every ~10^9 photons there is roughly one baryon, and zero antibaryons. Sakharov showed this requires baryon-number violation, C/CP violation and departure from equilibrium — but the measured CP violation in the Standard Model is far too small.

How do we reconcile gravity with quantum mechanics?

General relativity describes smooth spacetime geometry; quantum mechanics describes probabilistic fields on a fixed background. They collide at the Planck scale — inside black holes and at the Big Bang — and no experiment has yet tested any theory of quantum gravity.

Where do ultra-high-energy cosmic rays come from?

Charged particles detected above 10^18 eV — including events near 10^20 eV, macroscopic energy in a single subatomic particle — must be accelerated by the most extreme objects in the universe. Candidate sources include AGN jets, gamma-ray bursts and magnetars, but deflection by magnetic fields scrambles the directions.

By what pathway did life first emerge from chemistry?

Life appeared on Earth within ~500 million years of the planet becoming habitable — remarkably fast. But the bridge from prebiotic chemistry to the first self-replicating, compartmentalized, metabolizing system remains unbuilt: we can make nucleotides and protocells separately, never together and self-sustaining.

Where is everybody? The Fermi paradox and the search for technosignatures

The galaxy is old enough and large enough that even slow interstellar colonization should have filled it — yet the sky is silent. The paradox can dissolve through a great filter ahead or behind us, through rarity of intelligence, through our looking wrong, or through civilization lifetimes being short.