Question: General relativity and what other theoretical foundation underlie the Big Bang model?
About the article "Physics", section "Astrophysics".

Candidate passages:
General relativity is used to predict that at the beginning of the Universe, a body containing all mass, energy, and spacetime in the Universe would be compressed to an infinitely dense point.
The use of only general relativity to predict what happened in the beginnings of the Universe has been heavily criticized, as quantum mechanics becomes a significant factor in the high-energy environment of the earliest Universe, and general relativity on its own fails to make accurate predictions.
In response to the inaccuracy of considering only general relativity, as in the traditional model of the Big Bang, alternative theoretical formulations for the beginning of the Universe have been proposed, including a string theory-based model in which two branes, enormous membranes much larger than the Universe, collided, creating mass and energy.
It is impossible to see the singularity or the actual Big Bang itself, as time and space did not exist inside the singularity and, therefore, there would be no way to transmit any radiation from before the Big Bang to the present day.
However, evidence for the existence of an initial singularity, and the Big Bang theory itself, comes in the form of the cosmic microwave background and the continued expansion of the Universe. 
The conformal cyclic cosmology (CCC) is a cosmological model in the framework of general relativity, advanced by the theoretical physicists Roger Penrose and Vahe Gurzadyan.
In CCC, the universe iterates through infinite cycles, with the future timelike infinity of each previous iteration being identified with the Big Bang singularity of the next.
Penrose popularized this theory in his 2010 book "Cycles of Time: An Extraordinary New View of the Universe". 
The history of the Big Bang theory began with the Big Bang's development from observations and theoretical considerations.
Much of the theoretical work in cosmology now involves extensions and refinements to the basic Big Bang model. 
The Big Bang theory is the prevailing cosmological model for the early development of the universe.
According to the theory, the Big Bang occurred approximately 13.798 ± 0.037 billion years ago, which is thus considered the age of the universe.
At this time, the universe was in an extremely hot and dense state and began expanding rapidly.
After the initial expansion, the universe cooled sufficiently to allow energy to be converted into various subatomic particles, including protons, neutrons, and electrons.
Though simple atomic nuclei formed within the first three minutes after the Big Bang, thousands of years passed before the first electrically neutral atoms formed.
The majority of atoms that were produced by the Big Bang are hydrogen, along with helium and traces of lithium.
Giant clouds of these primordial elements later coalesced through gravity to form stars and galaxies, and the heavier elements were synthesized either within stars or during supernovae.
The Big Bang is the scientific theory that is most consistent with observations of the past and present states of the universe, and it is widely accepted within the scientific community.
It offers a comprehensive explanation for a broad range of observed phenomena, including the abundance of light elements, the cosmic microwave background, large scale structure, and the Hubble diagram.
The core ideas of the Big Bang—the expansion, the early hot state, the formation of light elements, and the formation of galaxies—are derived from these and other observations.
As the distance between galaxies increases today, in the past galaxies were closer together.
The consequence of this is that the characteristics of the universe can be calculated in detail back in time to extreme densities and temperatures, while large particle accelerators replicate such conditions, resulting in confirmation and refinement of the details of the Big Bang model.
On the other hand, these accelerators can only probe so far into high energy regimes, and astronomers are prevented from seeing the absolute earliest moments in the universe by various cosmological horizons.
The earliest instant of the Big Bang expansion is still an area of open investigation.
The Big Bang theory does not provide any explanation for the initial conditions of the universe; rather, it describes and explains the general evolution of the universe going forward from that point on.
Georges Lemaître first proposed (in 1927) what became the Big Bang theory in what he called his "hypothesis of the primeval atom".
Over time, scientists built on his initial ideas of cosmic expansion and that this expansion could be traced back to the cosmic origin – a starting point for space and time that Lemaitre called "the day without yesterday" -- to form the modern synthesis.
The framework for the Big Bang model relies on Albert Einstein's general relativity and on simplifying assumptions such as homogeneity and isotropy of space.
The governing equations were first formulated by Alexander Friedmann and similar solutions were worked on by Willem de Sitter.
In 1929, Edwin Hubble discovered that the distances to far away galaxies were strongly correlated with their redshifts—an idea originally suggested by Lemaître in 1927.
Hubble's observation was taken to indicate that all very distant galaxies and clusters have an apparent velocity directly away from our vantage point: the farther away, the higher the apparent velocity, regardless of direction.
Assuming that we are not at the center of a giant explosion, the only remaining interpretation is that all observable regions of the universe are receding from each other.
While the scientific community was once divided between supporters of two different expanding universe theories—the Big Bang and the Steady State theory, observational confirmation of the Big Bang scenario came with the discovery of the cosmic microwave background radiation in 1964, and later when its spectrum (i.e., the amount of radiation measured at each wavelength) was found to match that of thermal radiation from a black body.
Since then, astrophysicists have incorporated observational and theoretical additions into the Big Bang model, and its parametrization as the Lambda-CDM model serves as the framework for current investigations of theoretical cosmology. 
The Friedmann–Lemaître–Robertson–Walker metric that is necessary for the Big Bang and Steady State models emerged in the decade after the development of Einstein's general relativity and was accepted as a model for the universe after Edwin Hubble's discovery of his eponymous law.
It was not clear early on how to find a "universe solution" to Einstein's equations that allowed for a universe that was infinite, unending, and immutable (scientists of the time assumed for philosophical reasons the universe should have such a character).
Even after the development of expanding universe theories, people would engage in this exercise from time to time when looking for a replacement for general relativity.
Any alternative theory of gravity would imply immediately an alternative cosmological theory since current modeling is dependent on general relativity as a framework assumption.
What is included are a number of models based on alternative gravitational scenarios as well as early attempts to derive cosmological solutions from relativity.