General Relativity
General vs. Special Relativity
While SR is a theory of spacetime, eponymous with Minkowski space
an Lorentz transforms, GR is a theory of gravitation.
In the limit of velocities small compared to the speed of light
in vacuum, SR becomes classical mechanics with the Galilei transform.
GR, for small masses, becomes Newronian mechanics, while for large
masses gravitation deforms spacetime, leading to phenomena like
gravitational lensing and black holes.
Field Equation
The field equation, as a generalization of Newton's law of gravitation,
has the following form..
$$
\large{
R_{\mu\nu} - \frac{1}{2}\,R\,g_{\mu\nu} + \Lambda\,g_{\mu\nu}
= \kappa\,T_{\mu\nu}}
$$
Cosmological Constant
The cosmological constant $\Lambda$ in the field equation describes
whether a universe subject to the equation is contracting, static
or expanding. Observations by Hubble showed that our universe
is expanding. Observations give the value of $\Lambda$ as
$$\Lambda = 3\,\frac{H_0^2}{c^2}\,\Omega_\Lambda$$
where $H_0 = 66.67\,m\,s^{-1}\,kpc^{-1}$ is the Hubble constant and
$\Omega_\Lambda\approx 0.69$ is the fractional amount of dark energy
present.
Measurements by the Planck collaboration, published in 2018, lead
to a value for $\Lambda$ of
$$\Lambda = 1.1055\times 10^{-52} m^{-2}$$