• Theory of everything: Is there a singular, all-encompassing, coherent theoretical framework of physics that fully explains and links together all … See more

• Yang–Mills theory: Given an arbitrary compact gauge group, does a non-trivial quantum Yang–Mills theory with a finite mass gap exist? (This problem is also listed as one of the See more

• Hierarchy problem: Why is gravity such a weak force? It becomes strong for particles only at the Planck scale, around 10 See more

• Quantum chromodynamics: What are the phases of strongly interacting matter, and what roles do they play in the evolution of the See more

• Quantum gravity: Can quantum mechanics and general relativity be realized as a fully consistent theory (perhaps as a quantum field theory)? Is spacetime fundamentally continuous or discrete? Would a consistent theory involve a force … See more

• Axis of evil: Some large features of the microwave sky at distances of over 13 billion light years appear to be aligned with both the motion and orientation of the solar system. Is this … See more

• Solar cycle: How does the Sun generate its periodically reversing large-scale magnetic field? How do other solar-like stars generate their magnetic fields, and what are the … See more