🌀Riemannian Geometry Unit 3 – Connections and Covariant Derivatives

Key Concepts and Definitions

• Manifolds are topological spaces that locally resemble Euclidean space and provide a framework for studying curved spaces
• Tangent spaces $T_pM$ at each point $p$ on a manifold $M$ consist of tangent vectors that represent infinitesimal displacements
• Vector fields assign a tangent vector to each point on the manifold, allowing for the study of smooth functions and differential operators
• Connections define a way to transport vectors along curves on the manifold while preserving certain properties (e.g., parallelism, length, or angle)
• Covariant derivatives $\nabla_XY$ measure the change of a vector field $Y$ along the direction of another vector field $X$, taking into account the curvature of the manifold
  • Generalizes the concept of directional derivatives from Euclidean space to curved manifolds
• Curvature quantifies the extent to which a manifold deviates from being flat (Euclidean) and is related to the non-commutativity of covariant derivatives
• Torsion measures the failure of a connection to be symmetric, i.e., the difference between $\nabla_XY$ and $\nabla_YX$

Connections on Manifolds

• Connections provide a way to compare vectors in different tangent spaces and define parallel transport
• Affine connections are linear maps $\nabla: \Gamma(TM) \times \Gamma(TM) \to \Gamma(TM)$ satisfying certain properties (linearity, Leibniz rule, and compatibility with the tangent bundle structure)
  • $\Gamma(TM)$ denotes the space of smooth vector fields on the manifold $M$
• Christoffel symbols $\Gamma^k_{ij}$ are the components of a connection with respect to a chosen coordinate system
  • They describe how the basis vectors of the tangent space change as one moves along the manifold
• Levi-Civita connection is a unique torsion-free connection that is compatible with the metric (preserves inner products)
  • Fundamental in Riemannian geometry and general relativity
• Connections can be extended to act on tensor fields of arbitrary rank, allowing for the definition of covariant derivatives on more general objects
• Parallel transport along a curve using a connection preserves the direction and magnitude of vectors, but may result in a different vector due to curvature

Parallel Transport and Geodesics

• Parallel transport is the process of moving a vector along a curve on a manifold while maintaining its "parallelism" according to a given connection
  • Generalizes the notion of parallel lines in Euclidean space to curved manifolds
• Geodesics are curves on a manifold that parallel transport their own tangent vectors
  • Intuitively, they are the "straightest" possible paths between two points on the manifold
  • In Riemannian geometry, geodesics are locally length-minimizing curves
• Geodesic equation $\nabla_{\dot{\gamma}}\dot{\gamma} = 0$ characterizes geodesics as curves with vanishing covariant acceleration
  • $\gamma(t)$ is the parametrized curve and $\dot{\gamma}$ is its tangent vector
• Exponential map $\exp_p: T_pM \to M$ sends a tangent vector $v$ at point $p$ to the point reached by following the geodesic starting at $p$ with initial velocity $v$ for a unit time
  • Provides a local diffeomorphism between the tangent space and the manifold near $p$
• Geodesic deviation measures the relative acceleration between nearby geodesics and is related to the curvature of the manifold
• Parallel transport and geodesics play a crucial role in understanding the geometry of curved spaces and have applications in physics (e.g., general relativity)

Covariant Derivatives

• Covariant derivatives extend the concept of directional derivatives to vector fields on manifolds, taking into account the curvature of the space
• For vector fields $X$ and $Y$, the covariant derivative $\nabla_XY$ measures the change of $Y$ along the flow of $X$
  • Generalizes the Euclidean directional derivative $X(Y) = \sum_i X^i \frac{\partial Y}{\partial x^i}$
• In local coordinates, the covariant derivative is expressed using Christoffel symbols: $\nabla_XY = \sum_i X^i \left(\frac{\partial Y^k}{\partial x^i} + \sum_{j} \Gamma^k_{ij} Y^j\right) \frac{\partial}{\partial x^k}$
• Covariant derivatives satisfy several properties:
  • Linearity: $\nabla_{aX+bY}Z = a\nabla_XZ + b\nabla_YZ$ and $\nabla_X(aY+bZ) = a\nabla_XY + b\nabla_XZ$
  • Leibniz rule (product rule): $\nabla_X(fY) = (Xf)Y + f\nabla_XY$ for a smooth function $f$
  • Compatibility with the metric: $X\langle Y, Z\rangle = \langle \nabla_XY, Z\rangle + \langle Y, \nabla_XZ\rangle$ (for Riemannian manifolds)
• Covariant derivatives can be extended to act on tensor fields of arbitrary rank using the Leibniz rule and linearity
  • Allows for the study of differential equations and geometric properties of tensor fields on manifolds
• The covariant derivative of the metric tensor $g$ vanishes identically for the Levi-Civita connection: $\nabla g = 0$
  • Ensures that parallel transport preserves inner products and lengths of vectors

Curvature and Torsion

• Curvature measures the non-commutativity of covariant derivatives and the deviation of a manifold from being flat (Euclidean)
• Riemann curvature tensor $R(X, Y)Z = \nabla_X\nabla_YZ - \nabla_Y\nabla_XZ - \nabla_{[X, Y]}Z$ quantifies the curvature of a Riemannian manifold
  • $[X, Y] = XY - YX$ is the Lie bracket of vector fields $X$ and $Y$
  • Measures the difference between parallel transporting a vector along two different paths
• In local coordinates, the components of the Riemann curvature tensor are given by $R^l_{ijk} = \frac{\partial \Gamma^l_{jk}}{\partial x^i} - \frac{\partial \Gamma^l_{ik}}{\partial x^j} + \sum_m (\Gamma^l_{im}\Gamma^m_{jk} - \Gamma^l_{jm}\Gamma^m_{ik})$
• Ricci curvature $\operatorname{Ric}(X, Y) = \operatorname{tr}(Z \mapsto R(X, Z)Y)$ is a contraction of the Riemann curvature tensor
  • Measures the average curvature in different directions at a point
• Scalar curvature $S = \operatorname{tr}(\operatorname{Ric})$ is the trace of the Ricci curvature tensor
  • Provides a single scalar value that characterizes the curvature at each point
• Torsion tensor $T(X, Y) = \nabla_XY - \nabla_YX - [X, Y]$ measures the failure of a connection to be symmetric
  • Vanishes identically for the Levi-Civita connection
• Curvature and torsion provide important geometric insights and have applications in various areas of mathematics and physics
  • Einstein field equations in general relativity relate the curvature of spacetime to the presence of matter and energy

Applications in Physics

• General relativity describes gravity as the curvature of spacetime, with the metric tensor encoding the geometry
  • Geodesics in spacetime correspond to the paths of freely falling particles and light rays
  • Einstein field equations $G_{\mu\nu} = 8\pi T_{\mu\nu}$ relate the Einstein tensor $G_{\mu\nu}$ (constructed from the Ricci curvature and scalar curvature) to the stress-energy tensor $T_{\mu\nu}$ (describing matter and energy)
• Parallel transport and covariant derivatives are essential for formulating physical laws in curved spacetime
  • Conservation laws and equations of motion must be expressed using covariant derivatives to ensure their validity in arbitrary coordinate systems
• Geodesic deviation equation $\frac{D^2\xi^{\mu}}{d\tau^2} = -R^{\mu}_{\nu\rho\sigma}u^{\nu}u^{\rho}\xi^{\sigma}$ describes the relative acceleration of nearby geodesics due to spacetime curvature
  • $\xi^{\mu}$ is the deviation vector, $u^{\mu}$ is the tangent vector to the geodesic, and $\frac{D}{d\tau}$ denotes the covariant derivative along the geodesic
  • Explains tidal forces and the stretching and squeezing of matter in gravitational fields
• Curvature singularities, such as those found in black holes and at the Big Bang, are regions where the spacetime curvature becomes infinite
  • Indicate the breakdown of classical general relativity and the need for a quantum theory of gravity
• Connections and covariant derivatives also play a role in gauge theories, such as electromagnetism and the Standard Model of particle physics
  • Gauge fields (e.g., the electromagnetic potential) can be interpreted as connections on principal bundles, with the curvature corresponding to the field strength (e.g., the electromagnetic field tensor)

Examples and Calculations

• Sphere $S^2$ with the induced metric from $\mathbb{R}^3$:
  • Christoffel symbols: $\Gamma^{\theta}_{\phi\phi} = -\sin\theta\cos\theta$, $\Gamma^{\phi}_{\theta\phi} = \Gamma^{\phi}_{\phi\theta} = \cot\theta$, others zero
  • Geodesics: great circles (e.g., equator, meridians)
  • Riemann curvature tensor: $R_{\theta\phi\theta\phi} = \sin^2\theta$, others zero or related by symmetries
  • Scalar curvature: $S = \frac{2}{r^2}$, where $r$ is the radius of the sphere
• Hyperbolic plane $\mathbb{H}^2$ with the Poincaré metric $ds^2 = \frac{dx^2 + dy^2}{y^2}$:
  • Christoffel symbols: $\Gamma^x_{yy} = -\frac{1}{y}$, $\Gamma^y_{xy} = \Gamma^y_{yx} = \frac{1}{y}$, others zero
  • Geodesics: vertical lines and semicircles orthogonal to the boundary at infinity
  • Riemann curvature tensor: $R_{xyxy} = -\frac{1}{y^2}$, others zero or related by symmetries
  • Scalar curvature: $S = -2$
• Schwarzschild spacetime describing a non-rotating, uncharged black hole:
  • Metric: $ds^2 = -\left(1-\frac{2M}{r}\right)dt^2 + \left(1-\frac{2M}{r}\right)^{-1}dr^2 + r^2(d\theta^2 + \sin^2\theta d\phi^2)$
  • Christoffel symbols: $\Gamma^t_{tr} = \frac{M}{r^2}\left(1-\frac{2M}{r}\right)^{-1}$, $\Gamma^r_{tt} = \frac{M}{r^2}\left(1-\frac{2M}{r}\right)$, $\Gamma^r_{rr} = -\frac{M}{r^2}\left(1-\frac{2M}{r}\right)^{-1}$, $\Gamma^r_{\theta\theta} = -r\left(1-\frac{2M}{r}\right)$, $\Gamma^r_{\phi\phi} = -r\sin^2\theta\left(1-\frac{2M}{r}\right)$, $\Gamma^{\theta}_{r\theta} = \Gamma^{\theta}_{\theta r} = \frac{1}{r}$, $\Gamma^{\theta}_{\phi\phi} = -\sin\theta\cos\theta$, $\Gamma^{\phi}_{r\phi} = \Gamma^{\phi}_{\phi r} = \frac{1}{r}$, $\Gamma^{\phi}_{\theta\phi} = \Gamma^{\phi}_{\phi\theta} = \cot\theta$
  • Geodesics: orbits of test particles and light rays in the gravitational field of the black hole
  • Ricci curvature: $R_{tt} = -\frac{2M}{r^3}$, $R_{rr} = \frac{2M}{r^3}\left(1-\frac{2M}{r}\right)^{-1}$, $R_{\theta\theta} = 2M$, $R_{\phi\phi} = 2M\sin^2\theta$
  • Scalar curvature: $S = 0$ (vacuum solution)

Further Reading and Resources

• "Riemannian Geometry" by Manfredo P. do Carmo - Classic textbook covering the foundations of Riemannian geometry, including connections, covariant derivatives, curvature, and geodesics
• "Riemannian Manifolds: An Introduction to Curvature" by John M. Lee - Comprehensive introduction to Riemannian geometry, with a focus on curvature and its applications
• "General Relativity" by Robert M. Wald - Textbook on general relativity that extensively uses the language of differential geometry, including connections and covariant derivatives
• "Gauge Fields, Knots and Gravity" by John Baez and Javier P. Muniain - Introduces the geometric foundations of gauge theories and their applications in physics, emphasizing the role of connections and curvature
• "Lectures on Differential Geometry" by Shlomo Sternberg - Lecture notes covering a wide range of topics in differential geometry, including connections, covariant derivatives, and curvature
• "Geometry, Topology and Physics" by Mikio Nakahara - Explores the interplay between geometry, topology, and physics, with sections devoted to connections, fiber bundles, and gauge theories
• Online resources:
  • MathWorld: Covariant Derivative, Christoffel Symbols, Riemann Curvature Tensor
  • Wikipedia: Connection (mathematics), Parallel transport, Geodesic, Curvature
  • nLab: Covariant Derivative, [Connection on a Bundle](