Dark matter
Bent light and galaxy motions reveal gravity that known stars and gas cannot fully explain. What supplies the additional mass?
Astrophysics
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Black hole information problem
If a black hole evaporates, how is the information about what fell into it represented in what comes out? The answer must reconcile quantum evolution with gravitational calculations.
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?
Coronal heating problem
The Sun’s outer atmosphere is much hotter than its visible surface. Which processes deliver and dissipate enough energy to maintain it?
Solar modelling problem
Measurements of the Sun’s composition and interior do not all agree with standard solar models. Which changes would make the different lines of evidence fit together?
Primordial lithium problem
Old, metal-poor stars contain less lithium than standard early-universe calculations predict. Did the stars destroy it, or is something missing from the calculation or inference?
Core-collapse supernova mechanism
The collapse of a massive star can launch an explosion. Which stars explode, and can models predict the explosion and remnant from the star’s pre-collapse structure?
Diffuse interstellar bands
Starlight passing through interstellar gas picks up many absorption bands. Some have known molecular carriers, but most still lack a secure identification.
Planetesimal formation
Planetary systems start with small solid grains. How do these become gravitationally bound bodies before collisions or inward drift remove them?
Neutron-star equation of state
How does pressure depend on density inside a cold neutron star?
r-process sites
Which events supplied the rapid neutron-capture elements found throughout our Galaxy?
Type Ia supernova progenitors
Which white-dwarf systems produce the different Type Ia supernovae?
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?
Solar dynamo
Where and how does the Sun regenerate the magnetic field underlying its activity cycle?
Diffuse astrophysical neutrino sources
Which populations produce the diffuse high-energy neutrino flux measured by IceCube?
Hot Jupiter radius inflation
Some giant planets close to their stars are larger than ordinary cooling models predict. What keeps them inflated?
Multiple populations in globular clusters
Stars within one globular cluster can have different light-element abundances. Their common origin is still debated.
Pulsar radio emission
A pulsar’s rotating beam is familiar. The plasma process that makes its exceptionally bright radio waves is less certain.
Stellar angular momentum transport
A star’s core and outer layers exchange rotational motion. Which internal processes set that exchange as stars evolve?
Pulsar glitches
Some pulsars suddenly spin faster. What triggers the rapid transfer of rotational motion that produces a large glitch?
Luminous fast blue optical transients
Cow-like transients brighten and fade rapidly while emitting powerful X-rays and radio waves. Which stellar events produce them?