3.6.3 The Big Bang Model
As we discussed in section 3 above, the application of General Relativity on the whole Universe initiated the field of relativistic cosmology. In 1927, based on the initial solutions of the Einstein Field Equations, Lemaitre proposed the theory of the expansion of the Universe. Two years after that, Hubble observed that the distances to faraway galaxies were strongly correlated with their redshifts, which meant that they are receding away in all directions, with apparent velocity proportional to their distance. This continuous expansion of the Universe implies that it was denser and hotter in the past. Another two years after that, in 1931, Lemaitre published an article in Nature setting out his theory of the “primeval atom” or the “Cosmic Egg” which later became known as the Big Bang theory of the origin of the Universe, which changed the course of cosmology over the past century.
Before Lemaitre, Friedmann also proposed a similar solution to Einstein’s equations, implying that the radius of the Universe increased over time, but Einstein at first dismissed both, but after Hubble’s discovery, he quickly and publicly endorsed Lemaitre’s theory.
Lemaitre also derived what became known as Hubble’s law, and proposed it as a generic phenomenon in relativistic cosmology. This law makes a direct correlation between the distance to a galaxy and its recessional velocity as determined by the red shift. It can be stated as:
(3.19)
where
is the recessional velocity, typically expressed in
,
is Hubble’s constant, and
is the proper distance from the galaxy to the observer, and the recession velocity is given by
. Lemaitre also estimated the numerical value of the Hubble constant, but the data he used did not allow him to prove that there was an actual linear relation, as Hubble did after two years.
Additionally, Lemaitre proposed that the expansion of the Universe was actually accelerating, which was confirmed observationally in the 1990s through observations of very distant Type IA supernova with the Hubble Space Telescope.
The Big Bang model is not an explosion of matter moving outward to fill an empty Universe. Instead, space itself expands with time everywhere, which increases the physical distance between co-moving points.
The scientific community was once divided between supporters of two different theories, the Big Bang and the Steady-State theory, but a wide range of empirical evidence has strongly favored the Big Bang which is now universally accepted.
In 1964, the cosmic microwave background radiation was discovered, which was crucial evidence in favor of the Big Bang model, since that theory predicted the existence of background radiation throughout the Universe before it was discovered.
Extrapolation of the expansion of the Universe backwards in time, using General Relativity, yields an infinite density and temperature at a finite time in the past. This singularity indicates that General Relativity is not an adequate description of the laws of physics in this regime. It is debated how closely models based on General Relativity alone can be used to extrapolate toward the singularity, but certainly no closer than the end of the Planck epoch.
In 1968 and 1970 Roger Penrose (b. 1931), Stephen Hawking (b. 1942), and George Ellis (b. 1939) published papers where they showed that mathematical singularities were an inevitable initial condition of general relativistic models of the Big Bang. Then, from the 1970s to the 1990s, cosmologists worked on characterizing the features of the Big Bang Universe and resolving outstanding problems.
In 1981, Alan Guth (b. 1947) made a breakthrough in theoretical work on resolving certain outstanding theoretical problems in the Big Bang theory with the introduction of an epoch of rapid expansion in the early Universe he called cosmic inflation, to be described further in section 6.4.
Meanwhile, during these decades, two questions in observational cosmology that generated much discussion and disagreement were over the precise values of the Hubble Constant and the matter-density of the Universe, which was thought to be the key predictor for the eventual fate of the Universe.
In the mid-1990s, observations of certain globular clusters appeared to indicate, that they were about 15 billion years old, which conflicted with estimates of the age of the Universe. This issue was later resolved when new computer simulations, which included the effects of mass loss due to stellar winds, indicated a much younger age for globular clusters. While there still remain some questions as to how accurately the ages of the clusters are measured, globular clusters are of interest to cosmology as some of the oldest objects in the Universe.
Significant progress in Big Bang cosmology has been made since the late 1990s as a result of advances in telescope technology, as well as the analysis of data from satellites such as COBE, the Hubble Space Telescope and WMAP.
The Big Bang theory depends on two major assumptions:
1.The universality of physical laws, that the same known physical laws of nature can be used to calculate the characteristics of the Universe in detail back in time to an initial state of extreme density and temperature. 2.The cosmological principle states that on large scales the Universe is homogeneous and isotropic. This has been confirmed to a level of
via observations of the CMB. Also, the Universe has been measured to be homogeneous on the largest scales at the 10% level.
The science of Time is a noble science, that reveals the secret of Eternity. Only the Elites of Sages may ever come to know this secret. It is called the First Age, or the Age of ages, from which time is emerging.
