Question: The Big Bang model relies on general relativity and what?
About the article "Physics", section "Astrophysics".

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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. 
 "Up to 10–43 second after the Big Bang" Planck epoch.
The Planck epoch is an era in traditional (non-inflationary) big bang cosmology wherein the temperature was so high that the four fundamental forces—electromagnetism, gravitation, weak nuclear interaction, and strong nuclear interaction—were one fundamental force.
Little is understood about physics at this temperature; different hypotheses propose different scenarios.
Traditional big bang cosmology predicts a gravitational singularity before this time, but this theory relies on general relativity and is expected to break down due to quantum effects.
Physicists hope that such proposed theories of quantum gravitation as string theory, loop quantum gravity, and causal sets, will eventually provide a better understanding of this epoch.
In inflationary cosmology, times before the end of inflation (roughly 10−32 second after the Big Bang) do not follow the traditional big bang timeline.
The universe before the end of inflation is a very cold near-vacuum and persists for much longer than 10−32 second.
Times from the end of inflation are based on the big bang time of the non-inflationary big bang model, not on the actual age of the universe at that time, which cannot be determined in inflationary cosmology.
Therefore, inflationary cosmology lacks a traditional Planck epoch—though similar conditions may have prevailed in a pre-inflationary era of the universe. 
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. 
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. 
While the Big Bang model is well established in cosmology, it is likely to be refined in the future.
Little is known about the earliest moments of the universe's history.
The Penrose–Hawking singularity theorems require the existence of a singularity at the beginning of cosmic time.
However, these theorems assume that general relativity is correct, but general relativity must break down before the universe reaches the Planck temperature, and a correct treatment of quantum gravity may avoid the singularity.
There may also be parts of the universe well beyond what can be observed in principle.
If inflation occurred this is likely, for exponential expansion would push large regions of space beyond our observable horizon.
Some proposals, each of which entails untested hypotheses, are:  Models including the Hartle–Hawking boundary condition in which the whole of space-time is finite; the Big Bang does represent the limit of time, but without the need for a singularity.
Brane cosmology models in which inflation is due to the movement of branes in string theory; the pre-big bang model; the ekpyrotic model, in which the Big Bang is the result of a collision between branes; and the cyclic model, a variant of the ekpyrotic model in which collisions occur periodically.
Chaotic inflation, in which inflation events start here and there in a random quantum-gravity foam, each leading to a "bubble universe" expanding from its own big bang.
Proposals in the last two categories see the Big Bang as an event in a much larger and older universe, or multiverse, and not the literal beginning.