The Big Bang

As he considered the expansion of the universe, Lemaitre rederived Einstein's general theory of relativity for an expanding universe. Only a mathematical genius could perform this derivation, which involved complex matrices and equations. Friedmann had previously derived it, but his work was ignored and lost. Lemaitre determined that the general theory of relativity and the linear relationship between galactic velocity and distance supported each other and led to the conclusion that the universe expanded from a single point (Figure 1-37). Many scientists had difficulty accepting this concept. However, as with the heliocentric model, data kept confirming it, and the Big Bang model of the universe eventually became irrefutable.

Lemaitre realized that an expanding universe must be homogenous (uniform) in order to steadily expand for billions of years. Prior to measurements by Hubble, Lemaitre predicted that even though the local universe is not homogenous, with galaxies and clusters of galaxies, that the universe is homogenous (uniform) at large scales. By 1931, Hubble had peered much farther out into space (30 Mpc) and confirmed that the speed of galaxies is proportional to their distance from us and that this was true in all directions.[3] [4] When Hubble discovered large-scale homogeneity (uniform expansion rate in all directions), this was compelling evidence in support of Lemaitre’s hypothesis. As a result, Roger Eddington invited Lemaitre to republish his 1927 French paper in the renowned English language journal, Monthly Notices of the Royal Astronomical Society, in 1931.

In a subsequent letter to the journal Nature in 1931, Lemaitre proposed that all the observable matter and energy in the universe began from a single quantum or bundle of energy, which was the first explicit formulation of the Big Bang theory.

“We can conceive of space beginning with the primeval atom and the beginning of space being marked by the beginning of time.”

When Lemaitre and Einstein were both visiting Cal Tech in January 1933 (Figure 1‑38), Einstein told Lemaitre that he could not accept the beginning at a singularity because it resembled creation; however, Einstein later stood up and applauded after Lemaitre gave a presentation on his theory and said, “This is the most beautiful and satisfactory explanation of creation to which I have ever listened.” [5] [6] Even though Einstein publicly complimented Lemaitre’s theory, he worked privately on alternative explanations for receding galaxies. He gave up when Hubble’s Law contradicted his attempts at alternative mathematical and physical models of the universe.[7]

Scientists discovered conclusive evidence for the Big Bang by accident. Arno Penzias and Robert Wilson from Bell Laboratories developed a horn antenna for satellite communications (Figure 1‑39). When the satellite project became obsolete in 1963, Penzias and Wilson asked to use the massive antenna to study radio signals in the universe. As they began analyzing their data, they found a persistent, low-frequency signal, which was in the microwave part of the electromagnetic spectrum, and it appeared uniformly in all directions. At the same time, Jim Peebles and Robert Dicke from Princeton, unaware of Alpher and Herman’s earlier work, had just calculated that the relic radiation of the Big Bang should be low energy microwave radiation and that this would result in a hiss in a radio telescope like the noise found by Penzias and Wilson. Princeton was near Bell Labs, and when the scientists heard about each other’s work, they realized that this background microwave radiation was confirmation of the Big Bang.

The Cosmic Microwave Background Radiation (CMBR) convinced the majority of the scientific community that the universe had a hot beginning and had expanded since the beginning of time, which is the Big Bang model of the universe. After the discovery of the CMBR, scientists began working on a telescope that would sample the spectrum (different wavelengths) of the CMBR. This was an extremely challenging task. Finally, in 1988, scientists sent the COBE (Cosmic Background Explorer) satellite into orbit. The COBE radiation pattern aligned exactly (Figure 1‑40) with the theoretical radiation curve that the Big Bang would have produced. The theoretical and observed spectra perfectly overlap. When John Mather presented this radiation curve at an American Astronomical Society meeting in January 1993, scientists were so excited that they rose and gave a standing ovation.[9]