- Why is gravity such a weak force?
- The arrow of time (e.g. entropy’s arrow of time): Why does time have a direction? Why did the universe have such low entropy in the past, and time correlates with the universal (but not local) increase in entropy, from the past and to the future, according to the second law of thermodynamics?
Vacuum energy is underlying background energy that exists in space throughout the entire Universe. This behavior is codified in Heisenberg’s energy-time uncertainty principle. Why does the zero-point energy of the vacuum not cause a large cosmological constant? What cancels it out? There are still some deficiencies in the Standard Model of physics, such as the origin of mass, the strong CP problem, neutrino mass, matter-antimatter asymmetry, and the nature of dark matter and dark energy. Supernovae: What is the exact mechanism by which an implosion of a dying star becomes an explosion?