General relativity

• What Is the Second Law of Black Hole Thermodynamics?
  The black-hole second law says horizon area cannot shrink classically, while generalized entropy—including outside matter—cannot decrease.
• Does the Universe Have a Schwarzschild Radius?
  A Schwarzschild-radius calculation can be made for a chosen cosmic region, but it does not show that the expanding universe is a black hole.
• How Kaluza–Klein Reduction Produces a Dilaton Field
  Kaluza–Klein reduction on a circle turns higher-dimensional gravity into lower-dimensional gravity, a gauge field, and a scalar radion.
• Is There a Relationship Between Gauge Fields and Spin Connections?
  A spin connection is a gauge-like connection for local Lorentz frames. See how it compares with internal gauge fields, covariant derivatives, and curvature.
• How Does Pressure Affect the Expansion of the Universe?
  In cosmology, pressure contributes to gravity: positive pressure slows expansion, while sufficiently negative pressure can accelerate it.
• How Is the Second Covariant Derivative of a Scalar Computed?
  Compute a scalar's second covariant derivative by differentiating its gradient as a covector and subtracting the Christoffel-symbol term.
• Does Time Dilation Cause Gravity? What General Relativity Says
  Gravitational time dilation is linked to gravity and can help describe it, but it is not the whole cause: gravity is spacetime geometry.
• Does Dark Energy Have Gravity?
  Dark energy gravitates through its energy density and negative pressure: in general relativity it drives accelerated cosmic expansion, though its local effects are tiny.
• How Is the Strong Equivalence Principle Stronger Than the Weak One?
  The strong equivalence principle extends universal free fall to self-gravitating bodies and gravitational experiments; here is how it differs from the weak principle.