What string theory proposes — fundamental objects are 1D strings vibrating in 10 or 11 dimensions

How many types of fundamental forces are represented in the Standard Model of particle physics and which gauge bosons mediate each of these forces?

Four fundamental forces: electromagnetic (photon), weak (W and Z bosons), strong (gluons), gravity (graviton, not yet confirmed)

What baryon acoustic oscillations are — sound waves frozen in the early universe that set a standard ruler

Baryon acoustic oscillations: Early universe's sound waves, cosmic standard ruler

What the hierarchy problem asks — why is gravity so much weaker than the other three forces

The hierarchy problem questions the vast difference in strength between gravity and other fundamental forces

What gauge symmetry is — local phase transformations that leave physics invariant, the basis of all fundamental forces

Gauge symmetry: Local phase transformations preserving physics, underlying fundamental forces

What the electroweak unification achieved — Glashow, Salam, and Weinberg showed EM and weak force are one

Unified electroweak theory: EM and weak force as one interaction

What the cosmological constant problem is — quantum field theory predicts vacuum energy 10^120 times too large

Quantum field theory overestimates vacuum energy by 10^120 times, causing the cosmological constant problem