Air-shower muon puzzle
Cosmic-ray air showers produce more muons than standard collision models predict. Which particle-production physics explains the excess?
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Common-envelope ejection
Two stellar cores can spiral through one shared envelope. What determines whether that gas escapes and how close the surviving binary becomes?
Collectivity in small collision systems
Particles from some proton collisions have coordinated motion. Which interactions create those patterns in such small systems?
Inner-core seismic anisotropy
Seismic waves travel through Earth’s inner core at different speeds in different directions. What produces the pattern and its changes with depth?
Speciation with gene flow
New species can diverge while exchanging genes. How do reproductive barriers combine to sustain separation?
Arbuscule maintenance
Does nutrient delivery by a fungal branch inside a root influence how long the plant maintains it?
Coral bleaching
Heat stress can disrupt the partnership between corals and their photosynthetic symbionts. Which cellular pathways cause symbiont loss in different conditions?
Biological carbon-pump efficiency
Ocean plankton capture carbon, but much is recycled before it reaches depth. What determines how much carbon from different organisms survives that journey?
Bergmann’s rule
Many birds are larger in colder places, but the pattern varies. Which inherited and developmental mechanisms create these geographic size differences?
Great Ordovician Biodiversification Event
Marine diversity expanded dramatically around the Ordovician. How did ecological change, nutrient cycling and climate interact to produce that diversification?
Nitrogenase catalytic mechanism
Nitrogenase converts nitrogen gas into ammonia that life can use. Which atomic reaction pathway accounts for its observed chemistry?
Amenability of Thompson’s group F
Can Thompson’s group F admit an average that is unchanged by every group translation?
Hartshorne’s complete intersection conjecture
Must a smooth complex projective variety of sufficiently small codimension be defined by exactly that many polynomial equations?
Nagata conjecture
How low can a curve’s degree be when it must pass repeatedly through many very general points?
Zariski multiplicity conjecture
Does a complex hypersurface singularity’s local topological shape determine its multiplicity?
Baum–Connes conjecture
Can the proper-action geometry of a countable discrete group recover the K-theory of its reduced operator algebra?
Grothendieck’s section conjecture
Does the arithmetic fundamental group of a curve identify exactly its rational points?
Bag containment of conjunctive queries
Can an algorithm decide whether one database query always produces each answer no more often than another?
Electrically pumped GeSn lasers
Germanium–tin lasers can work at room temperature when another laser powers them. Can an electric current do the job?
Elastocaloric work recovery
Squeezing and releasing a metal can move heat. Recovering the mechanical work must also reduce the electricity used by a complete cooler.
Solar thermochemical heat storage
Sunlight can dry a salt that releases heat when it takes up water again. Can durable pellets do this reliably inside an outdoor reactor?
Structural battery qualification
A structural battery stores energy while carrying load. How can one test sample reliably measure both jobs at once?
Iodine-compatible hollow cathodes
Iodine can fuel spacecraft thrusters. A hot electron source for higher-power systems must also survive the reactive iodine plasma.
Transitive inference
Learning that A outranks B and B outranks C can let us compare A with C. How do people build and revise this knowledge?