Is there a formulation of general relativity in terms of differential forms instead of tensors with indices and sub-indices? If yes, where can I find it and what are the advantages of each method? If not, why is it not possible?
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Short answer. Example: The Palatini action, where the action is a functional of a tetrad/vierbein $e$ and a spin connection $\omega$.
$$S(e, \omega) = \int \epsilon_{abcd} e^a \wedge e^b \wedge \Omega^{cd}$$
where $\Omega$ is the curvature associated to the connection form $\omega$:
$$\Omega = D\omega = d\omega + \omega \wedge \omega$$
Other examples listed here (with references):
Samuel-Jacobson-Smolin action
Goldberg action:
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differential forms. It's not 100% clear how to read the question so I'll fire in all directions.
In general relativity, the field of interest is the symmetric metric tensor $g$ which you see written $g_{\mu\nu}$ or e.g. $g_{ab}$. The latter often implies abstract index notation.
The base vectors on the cotangential space are $\mathrm dx^i$ so there you have forms, but it's not like you want to write the symmetric tensors like the energy-momentum tensor $T$ as image of $\mathrm d$. You can drop the indices from objects like $g_{\mu\nu}$ any time you want. E.g. You can write down Einsteins field equations as $G=\kappa\ T$ and then it's in your face covariant per definition.
In this answer someone rants about why you wouldn't want to dismiss index notation anyway. (But the person has governed zero reputation on this site yet, so I would take his opinions with a grain of salt.)
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It was Cartan who developed General Relativity in his book "ON MANIFOLDS WITH AN AFFINE CONNECTION AND THE THEORY OF GENERAL RELATIVITY " relying only on "Affine Connections", it is not clear to me what to be called a "formulation of General relativity in terms of differential forms", but I take it granted from the question that one is trying to develop a theory using index free notation and keeping in mind "Christoffel Symbols" are fundamental building blocks in deriving Field equations. Actually "bundle of linear forms" - what Cartan mentioned, gives rise to a variant of Christoffel symbols (hence in some loose sense FORMS), and torsion of the Geometrical space considered (actually Cartan gives more, he actually predicts "spin" like systems that are absent in Einstein's formulation of GTR as Einstein considered Manifolds with ZERO torsion and torsion is a FORM also, made precise in the above mentioned text), this is also the birth place of modern day "Fiber Bundle" theory ( see his book "Riemannian Geometry in an Orthogonal Frame") this fiber bundle theory has the power to accommodate Yang-Mills theory into solid Geometrical Ground and brings GTR and YM having a common mathematical background, both of this book contains enough material to satisfy the approach of the seeker. Now this theory is called Cartan-Einstein theory, and Einstein's theory is contained in this Cartan-Einstein theory as sub-theory.
One can consult the following letters (Elie Cartan - Albert Einstein Letters on Absolute Parallelism 1929-1932 ) between Einstein and Cartan, to have a taste of how far this theory can be treated as "physical"