Dark matter
Bent light and galaxy motions reveal gravity that known stars and gas cannot fully explain. What supplies the additional mass?
Physics
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Dark energy
The universe's expansion is accelerating. “Dark energy” names the gap in our explanation, while several different physical accounts remain possible.
Quantum gravity
Gravity describes a changing spacetime. Quantum physics describes matter and its possibilities. How should they work together in a single physical account?
Room-temperature superconductivity
The goal is a material that conducts electricity without resistance at room temperature and normal atmospheric pressure. Achieving both conditions together remains a research target.
Matter–antimatter asymmetry
Matter and antimatter can annihilate each other. Yet the observable universe contains enough leftover matter to make stars, planets, and people. What produced that imbalance?
Strong CP problem
The theory of the strong nuclear force permits a kind of asymmetry that experiments find to be extraordinarily small. Why is nature so close to the symmetric case?
Yang–Mills existence and mass gap
A successful framework for particle physics still needs a rigorous four-dimensional construction with a positive gap above its vacuum energy.
Hierarchy problem
The Higgs field sets an important scale in particle physics. Why is that scale so much smaller than other scales that may enter the underlying theory?
Absolute neutrino mass
Neutrino oscillations show that neutrinos have mass differences, but they do not tell us the absolute masses. How light are these elusive particles?
Majorana nature of neutrinos
A neutrino might be its own antiparticle. Establishing whether that is true would change how we understand neutrino mass and a basic particle-number symmetry.
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?
Strange metals
In some materials, electrical transport behaves in ways that the usual picture of independently moving electron-like excitations cannot explain. What replaces that picture?
Atmospheric turbulence
Air contains interacting eddies over an enormous range of sizes. How can their combined effect on heat, momentum, and pollutants be predicted reliably?
Quantum measurement problem
A single photon gives a left or right detector click. Quantum theory predicts the odds, but treating the detectors quantum mechanically produces a superposition of possible records. How should that state account for a definite result?
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?
Proton decay
Protons appear stable in ordinary observations. Could they eventually decay through interactions too rare to have been detected yet?
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?
Jupiter’s interior
Juno’s measurements suggest that Jupiter’s core blends gradually into its hydrogen-rich surroundings. How large is this region, what is it made of, and how did it form?
Earth’s core composition
Earth’s core is mostly iron and nickel, but these alone do not explain its measured properties. Which lighter elements are mixed in, and how are they distributed?
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?
Venus’s climate history
Venus and Earth began as similar rocky planets. Did Venus ever have oceans, or did its water remain steam and escape before oceans could form?