Stellar explosions
How stars explode, what they leave behind, and where heavy elements form.
5 problemsThe observed universe
Unseen matter, cosmic expansion, energetic particles, and brief radio flashes.
Subjects: Cosmology · Astrophysics
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Stars and stellar remnants
Rotation, magnetic fields, cluster chemistry and the remnants of stellar evolution.
14 problemsPlanets and rings
Planet formation, atmospheres, thermal histories, distant orbits and Saturn’s moons and rings.
15 problemsProblems in this collection
11 problemsDark matter
Bent light and galaxy motions reveal gravity that known stars and gas cannot fully explain. What supplies the additional mass?
Dark energy
The universe's expansion is accelerating. “Dark energy” names the gap in our explanation, while several different physical accounts remain possible.
Cosmic inflation
A short period of rapid expansion could explain several features of the early universe. What physical process, if any, produced it?
Hubble tension
Different ways of inferring the present expansion rate of the universe give results that do not fit comfortably together. Is the mismatch in the measurements, the model, or both?
Ultra-high-energy cosmic rays
Particles arrive at Earth with energies far beyond those reached by human accelerators. Which cosmic objects accelerate them, and by what process?
Fast radio bursts
Brief radio flashes arrive from across the universe. Magnetars can produce some of them, but what generates their intense emission and explains the range of observed bursts?
Supermassive black-hole seeds
What initial black holes grew into the massive objects seen in the early Universe?
Origin of cosmic magnetic fields
Where did the seed magnetic fields amplified into cosmic magnetism come from?
Ionizing photon escape during reionization
How many ionizing photons escaped early galaxies and reached intergalactic hydrogen?
Diffuse astrophysical neutrino sources
Which populations produce the diffuse high-energy neutrino flux measured by IceCube?
Fermi bubbles
Giant gamma-ray lobes rise above and below the Milky Way’s center. Their formation history and radiation mechanisms remain debated.