🌀Riemannian Geometry Unit 13 – General Relativity in Riemannian Geometry

Key Concepts and Foundations

• General Relativity (GR) describes gravity as the curvature of spacetime caused by the presence of mass and energy
• Spacetime combines the three dimensions of space and one dimension of time into a single four-dimensional continuum
• The Equivalence Principle states that gravitational acceleration is indistinguishable from acceleration caused by other forces
  • This implies that the effects of gravity are equivalent to the effects of living in a curved spacetime
• The geometry of spacetime is described using the mathematics of differential geometry and Riemannian geometry
  • This involves concepts such as manifolds, metrics, connections, and curvature
• GR reduces to Newton's theory of gravity in the weak-field limit, where gravitational fields are weak and velocities are much less than the speed of light
• The speed of light is a fundamental constant in GR and is the maximum speed at which information can propagate through spacetime
• GR has been extensively tested and has passed all experimental tests to date, including the precession of Mercury's orbit and the deflection of starlight by the Sun

Riemannian Manifolds Refresher

• A manifold is a topological space that locally resembles Euclidean space near each point
  • Manifolds can be thought of as higher-dimensional generalizations of surfaces
• Riemannian manifolds are smooth manifolds equipped with a Riemannian metric, which allows the measurement of distances and angles
• The metric tensor $g_{ij}$ is a symmetric, positive-definite tensor that defines the inner product between vectors on the manifold
  • In local coordinates, the line element is given by $ds^2 = g_{ij} dx^i dx^j$
• The Christoffel symbols $\Gamma^k_{ij}$ are the components of the Levi-Civita connection, which is the unique metric-compatible and torsion-free connection on a Riemannian manifold
• The Riemann curvature tensor $R_{ijkl}$ measures the intrinsic curvature of the manifold and is constructed from the Christoffel symbols and their derivatives
• The Ricci tensor $R_{ij}$ and scalar curvature $R$ are contractions of the Riemann tensor and provide lower-dimensional measures of curvature
• Geodesics are the shortest paths between points on a Riemannian manifold and are determined by the metric tensor
  • In GR, geodesics represent the paths followed by freely-falling particles in curved spacetime

Einstein's Field Equations

• Einstein's field equations relate the curvature of spacetime to the distribution of matter and energy
• The field equations are given by $G_{ij} = 8\pi T_{ij}$, where $G_{ij}$ is the Einstein tensor and $T_{ij}$ is the stress-energy tensor
  • The Einstein tensor is a function of the Ricci tensor and scalar curvature: $G_{ij} = R_{ij} - \frac{1}{2}Rg_{ij}$
  • The stress-energy tensor describes the density and flux of energy and momentum in spacetime
• The field equations are a set of 10 coupled, nonlinear partial differential equations for the metric tensor $g_{ij}$
• The Newtonian gravitational constant $G$ and the speed of light $c$ appear in the proportionality constant $8\pi$ in the field equations
  • In natural units, where $G = c = 1$, the field equations take the simpler form $G_{ij} = 8\pi T_{ij}$
• The field equations can be derived from the Einstein-Hilbert action, which is a functional of the metric tensor and the matter fields
• The Bianchi identities, $\nabla_i G^{ij} = 0$, are a consequence of the diffeomorphism invariance of the Einstein-Hilbert action and imply the conservation of energy and momentum

Spacetime and Metric Tensor

• In GR, spacetime is a four-dimensional Lorentzian manifold with a metric tensor of signature $(-,+,+,+)$
• The metric tensor $g_{ij}$ determines the geometry of spacetime and encodes the gravitational field
  • In the absence of gravity, spacetime is flat and described by the Minkowski metric $\eta_{ij} = \text{diag}(-1,1,1,1)$
• The line element $ds^2 = g_{ij} dx^i dx^j$ measures the spacetime interval between infinitesimally separated events
  • Timelike intervals ($ds^2 < 0$) connect events that can be causally related, while spacelike intervals ($ds^2 > 0$) connect events that cannot influence each other
• The metric tensor determines the paths of freely-falling particles (geodesics) and the propagation of light (null geodesics)
• The proper time $\tau$ measured by a clock following a timelike path is given by $d\tau^2 = -ds^2$
• The proper distance $\ell$ between simultaneous events along a spacelike path is given by $d\ell^2 = ds^2$
• The metric tensor can be used to raise and lower indices of tensors, and to construct invariant quantities such as the scalar product of vectors

Curvature in General Relativity

• Curvature is a fundamental concept in GR, as it describes the deviation of spacetime geometry from flatness due to the presence of mass and energy
• The Riemann curvature tensor $R_{ijkl}$ is the central object characterizing the intrinsic curvature of spacetime
  • It measures the non-commutativity of parallel transport around infinitesimal loops
• The Ricci tensor $R_{ij}$ and scalar curvature $R$ are contractions of the Riemann tensor and appear in the Einstein field equations
  • The Ricci tensor represents the amount by which the volume of a small geodesic ball deviates from that of a ball in flat space
  • The scalar curvature is the trace of the Ricci tensor and gives a single scalar measure of curvature at each point
• The Weyl tensor $C_{ijkl}$ is the traceless part of the Riemann tensor and describes the tidal gravitational forces that cause geodesic deviation
• The geodesic deviation equation relates the relative acceleration of nearby geodesics to the Riemann tensor
  • This equation describes how tidal forces arise from spacetime curvature
• The singularity theorems of Hawking and Penrose use the concept of geodesic incompleteness to predict the existence of singularities in spacetime under certain conditions, such as the formation of black holes or the origin of the universe in the Big Bang

Geodesics and Free-Fall Motion

• Geodesics are the straightest possible paths between points in a curved spacetime, generalizing the concept of straight lines in Euclidean space
• In GR, freely-falling particles follow geodesics in the absence of non-gravitational forces
  • This is a consequence of the Equivalence Principle, which states that gravitational acceleration is indistinguishable from acceleration caused by other forces
• The geodesic equation describes the motion of a particle in curved spacetime and is given by $\frac{d^2x^i}{d\tau^2} + \Gamma^i_{jk} \frac{dx^j}{d\tau} \frac{dx^k}{d\tau} = 0$, where $\tau$ is the proper time along the particle's path
  • The Christoffel symbols $\Gamma^i_{jk}$ in the geodesic equation represent the connection coefficients and are determined by the metric tensor
• Timelike geodesics ($ds^2 < 0$) represent the paths of massive particles, while null geodesics ($ds^2 = 0$) represent the paths of massless particles like photons
• The path of a freely-falling particle is independent of its mass, a consequence of the Equivalence Principle
• Geodesic deviation describes the relative acceleration of nearby geodesics due to tidal gravitational forces, which are encoded in the Riemann curvature tensor

Solutions to Einstein's Equations

• Solving Einstein's field equations involves finding the metric tensor $g_{ij}$ that describes the geometry of spacetime for a given distribution of matter and energy
• The field equations are a set of 10 coupled, nonlinear partial differential equations, making them difficult to solve in general
  • Exact solutions are known only for highly symmetric situations, such as spherical symmetry or vacuum spacetimes
• The Schwarzschild solution describes the spacetime geometry outside a spherically symmetric, non-rotating mass distribution
  • It is characterized by the Schwarzschild metric, which depends only on the mass $M$ of the central object
  • The Schwarzschild solution predicts the existence of black holes when the mass is concentrated within the Schwarzschild radius $r_s = \frac{2GM}{c^2}$
• The Kerr solution describes the spacetime geometry around a rotating black hole and is characterized by its mass $M$ and angular momentum $J$
  • The Kerr metric reduces to the Schwarzschild metric in the limit of zero angular momentum
• The Friedmann-Lemaître-Robertson-Walker (FLRW) metric describes homogeneous and isotropic cosmological spacetimes
  • It is the basis for the standard Big Bang model of cosmology and depends on the scale factor $a(t)$, which describes the expansion of the universe over time
• Perturbative solutions to the field equations can be obtained by considering small deviations from a known background spacetime, such as the Minkowski or FLRW metrics
  • These solutions are useful for describing gravitational waves, which are ripples in the fabric of spacetime that propagate at the speed of light

Applications and Observational Tests

• GR has been subjected to numerous observational tests since its inception, and it has passed all of them with flying colors
• The precession of Mercury's orbit was one of the first successful tests of GR
  • GR correctly predicted the observed precession rate, which could not be fully accounted for by Newtonian gravity
• The deflection of starlight by the Sun, as observed during solar eclipses, provided another early confirmation of GR
  • The measured deflection angle agreed with the value predicted by GR, which is twice the value predicted by Newtonian gravity
• Gravitational redshift, the stretching of light wavelengths in a gravitational field, has been measured using atomic clocks at different heights on Earth and in space
  • The observed redshift matches the predictions of GR to high precision
• Shapiro delay, the slowing down of light signals as they pass near massive objects, has been detected using radio signals from spacecraft and pulsars
  • The measured delay is consistent with the predictions of GR
• The Hulse-Taylor binary pulsar system has provided indirect evidence for the existence of gravitational waves
  • The observed decrease in the orbital period of the system matches the rate of energy loss due to gravitational wave emission predicted by GR
• The direct detection of gravitational waves by LIGO in 2015 was a major triumph for GR
  • The observed waveforms from binary black hole mergers agree remarkably well with the predictions of numerical relativity simulations based on GR
• Observations of the orbit of stars around the supermassive black hole at the center of the Milky Way have confirmed the existence of an event horizon, as predicted by GR
• Cosmological observations, such as the cosmic microwave background and the large-scale structure of the universe, are consistent with the predictions of the FLRW models based on GR