Big Bang Theory: Does general relativity accurately model space's birth-growth?

• Clarifying the question

  

  Is General Relativity’s Standard hot “Big Bang” model true, where space came into being from a nothing-point called a singularity? Does General Relativity get the early history of our Universe right?

  Structurally, our Universe is same in all directions and in all locations (i.e. homogenous and isotropic). That means a straightforward derivation from Einstein’s gravity equations to our spacetime will inevitably yield the “Big Bang” model of our Universe. Most broadly,

  “The Big Bang model describes a universe that is dynamic and evolving, one that started from an extremely hot and dense state at a finite time in the past…” [Ralph A. Alpher & Robert Herman, Genesis of the Big Bang (Oxford, 2001), 29.]

  However,

  “If we extrapolate this prediction to its extreme, we reach a point when all distances in the universe have shrunk to zero. An initial cosmological singularity therefore forms a past temporal extremity to the universe. We cannot continue physical reasoning, or even the concept of spacetime, through such an extremity. For this reason most cosmologists think of the initial singularity as the beginning of the universe. On this view the big bang represents the creation event; the creation not only of all the matter and energy in the universe, but also of spacetime itself.” [Paul Davies, “Spacetime singularities in cosmology” in The Study of Time III, 78–9., ed. Fraser (Springer, 1978), 78-9.]

  This is called the Friedmann–Lemaître–Robertson–Walker (FLRW) Big Bang model:¹

  The FLRW Big Bang Model = def. The model, calculated from General Relativity as applied to any homogenous and isotropic universe (like ours), wherein inevitably all space-time reality began at a singularity which ultimately expanded into the Universe we see today. Free floating particles in space with only gravity acting on them would all have a history of traceably moving backwards in time, and ultimately collapsing with space and time itself towards a universal singularity point.² (On this model, no space-time existed outside/before the singularity, so it is meaningless to ask where it happened or what was temporally “before.”)³

  Is a model like this, which contains an initial cosmic singularity, true?

  1. • Stanford Encyclopedia of Philosophy: “If we push backwards far enough, we find that the universe reaches a state of compression where the density and gravitational force are infinite. This unique singularity constitutes the beginning of the universe—of matter, energy, space, time, and all physical laws. It is not that the universe arose out of some prior state, for there was no prior state. Since time too comes to be, one cannot ask what happened before the initial event. Neither should one think that the universe expanded from some state of infinite density into space; space too came to be in that event. Since the Big Bang initiates the very laws of physics, one cannot expect any scientific or physical explanation of this singularity.” [Bruce Reichenbach, “Cosmological Arguments,” in The Stanford Encyclopedia of Philosophy (Nov 2016)]
  2. More technically, all past-directed geodesics terminate a the singularity.
     • Gott et al.: “The universe began from a state of infinite density about one Hubble time ago. Space and time were created in that event and so was all the matter in the universe. It is not meaningful to ask what happened before the big bang; it is somewhat like asking what is north of the North Pole. Similarly, it is not sensible to ask where the big bang took place. The point-universe was not an object isolated in space; it was the entire universe, and so the only answer can be that the big bang happened everywhere.” [1976, p. 65] [3] It is not meaningful to ask what happened before the Big Bang or “where” it took place.
  3. Gott, Gunn, Schramm, & Tinsley: “Space and time were created in that event and so was all the matter in the universe. It is not meaningful to ask what happened before the big bang; it is somewhat like asking what is north of the North Pole. Similarly, it is not sensible to ask where the big bang took place. The point-universe was not an object isolated in space; it was the entire universe, and so the only answer can be that the big bang happened everywhere.” [Gott, J. R. III, Gunn, J. E., Schramm, D. N., and Tinsley, B. M. “Will the universe expand forever?” Scientific American, (March 1976). 65.]

• General relativity makes very accurate predictions

  

  General relativity has made remarkably precise predictions that have been consistently validated through extensive observations and experiments. Its predictive power extends across a wide range of phenomena, from the bending of light by gravity to the expansion of the universe. This enduring accuracy makes it a cornerstone of modern physics.

  These are eight things general relativity accurately predicted:

  • Gravitational Redshift: General relativity predicts that light traveling out of a gravitational well will be redshifted, meaning its wavelength will be stretched and its frequency will decrease. This was confirmed in 1959 by the Pound-Rebka experiment, which measured the redshift of light moving in Earth’s gravitational field.
  • Perihelion Precession of Mercury: The orbit of Mercury around the Sun precesses, or rotates, over time. While Newtonian mechanics could explain most of this precession, there was a small discrepancy that couldn’t be accounted for. General relativity explained this discrepancy precisely, and the observed precession matches the prediction made by Einstein’s theory.
  • Deflection of Light by Gravity (Gravitational Lensing): General relativity predicts that light will bend when it passes near a massive object. This was first confirmed during the solar eclipse of 1919 by Arthur Eddington, who observed the deflection of starlight by the Sun. Today, gravitational lensing is routinely observed and used in astrophysics to study distant galaxies and dark matter.
  • Time Dilation in Gravitational Fields: Time runs slower in stronger gravitational fields, a prediction confirmed by the Hafele–Keating experiment in 1971. Atomic clocks flown on airplanes showed differences compared to those on the ground, consistent with predictions of general relativity. This effect is also crucial for the accuracy of the Global Positioning System (GPS), which needs to account for time dilation due to both gravity and relative motion.
  • Frame-Dragging (Lense-Thirring Effect): General relativity predicts that massive rotating objects will “drag” spacetime around with them. This effect, known as frame-dragging, was confirmed by the Gravity Probe B experiment, which measured the precession of gyroscopes in orbit around the Earth.
  • Gravitational Waves: General relativity predicts the existence of ripples in spacetime caused by accelerating massive objects. Gravitational waves were directly detected for the first time in 2015 by the LIGO and Virgo collaborations, observing waves produced by the merger of two black holes.
  • Black Holes: General relativity predicts the existence of black holes, regions of spacetime where gravity is so strong that not even light can escape. Observations of X-ray emissions from accretion disks around black holes, the motion of stars near the center of the Milky Way (indicating the presence of a supermassive black hole), and the Event Horizon Telescope’s image of the black hole in the M87 galaxy have all confirmed these predictions.
  • Cosmological Predictions: General relativity forms the basis for modern cosmology, predicting the expansion of the universe. This was observationally confirmed by Edwin Hubble’s discovery of the redshift-distance relationship for galaxies. Additionally, the theory predicts phenomena such as the cosmic microwave background radiation and the large-scale structure of the universe, both of which have been confirmed by observations.

  This is relevant because according to general relativity, the universe is dynamic and can expand or contract, and if we extrapolate the observed expansion of the universe backward in time, we reach a point of infinite density and temperature known as the initial singularity.

  So? On quantum scales the laws of physics as we know them break down--General relativity is no longer a reliable guide.¹

  1. At quantum levels, general relativity is no longer a reliable guide because it doesn’t incorporate quantum effects. This singularity could be more of a mathematical artifact indicating the breakdown of the theory rather than a physical reality. But theorems developed by Stephen Hawking and Roger Penrose in the 1960s show that, under reasonable assumptions, general relativity does predict a singularity at the beginning of the universe. These theorems suggest that if general relativity does hold, then the universe must have had a beginning. The idea of a singular beginning aligns with the philosophical principle of Occam’s Razor, which prefers the simplest explanation that accounts for all observations. An absolute beginning avoids the complexities introduced by cyclic or pre-Big Bang models. To date, there is no direct observational evidence for phenomena or remnants from before the Big Bang. While some models propose such scenarios, they remain speculative without empirical support. The standard Big Bang model, with a singular beginning, remains the most robust and simplest model that fits current observational data without requiring additional assumptions.

• Space expanded from a hot dense state

  

  The universe (all of space, time, and matter) really did expand from a single hot dense state, whether or not it began as a singularity.

  See this page to check out these 3 arguments,:

  • Galaxies are diverging (“red shifted”) like dots on a balloon.
  • A “Cosmic Microwave Background Radiation” permeates space like a gas.
  • There is a lot of helium-4, and some deuterium, helium-3, lithium-7.

  This is relevant because it is a very surprising confirmed prediction of General Relativity, and might inductively warrant a tentative belief that the backwards contraction reaches the singularity point based on its clear past trajectory.

  So? Plausibly General Relativity’s success is limited in scope.¹

• General relativity may fail at Quantum Sizes

  

  General relativity “breaks down” in quantum-sized settings insofar as its predictions may no longer apply; we need a theory of quantum-gravity to unify quantum mechanics and general relativity.

  After all…

  • Quantum influences may rub out predictions of General Relativity

  This is relevant because the size at which singularities are predicted and modeled by general relativity is less than the Planck length (i.e. quantum size).

  By way of response, however…

  • We ought to give General Relativity's predictions benefit of the doubt.
  • Independent lines of evidence for a singular beginning supplement General Relativity's prediction, amplifying the likelihood that it's prediction is accurate.