The frontier
The most basic metaphysical question: why does a universe exist at all, rather than a complete absence of anything?
The "hard problem": even a complete physical account of the brain seems to leave unexplained why any of it is accompanied by felt experience.
Whether the Big Bang was an absolute beginning or a transition from a prior state.
About a quarter of the universe's energy density behaves gravitationally like matter but emits no light. What is it?
The expansion of the universe is accelerating. What drives it?
The transition from prebiotic chemistry to a self-replicating, evolving system.
The smooth evolution of the wavefunction and the definite outcomes we observe sit uneasily together. What selects an outcome?
Whether the specific laws and constants are necessary, accidental, or selected.
General relativity and quantum field theory are each superbly tested but mutually incompatible in extreme regimes.
Whether life, and intelligent life, exists beyond Earth.
Microscopic physics is nearly time-symmetric, yet macroscopic processes are irreversible. Where does the arrow come from?
Several constants appear to lie in narrow ranges compatible with complexity and life. Is this significant?
Do mathematical truths exist independently of minds, or are they human constructions?
Whether time is a basic feature of reality or arises from a deeper timeless structure.
Whether information is as basic as matter and energy — Wheeler's "it from bit".
Whether physical properties have definite values independent of measurement.
Whether the universe ends in heat death, a big rip, a collapse, or something else.
Whether deliberate agency can be reconciled with deterministic or probabilistic physics.
Whether space and time are smooth at every scale or have a smallest grain near the Planck length.
Why human cognition, evolved for ordinary survival, can grasp deep physical law.