Cosmologists have come a long way since Edwin Hubble published that ratty looking plot of galaxy recession velocities versus distance. Hubble wasn’t the first to discover what we now call Hubble’s law, but the name stuck. Extragalactic distance measurements have improved greatly since then. Cosmologists have measured more galaxies, at much greater distances and with greater accuracy and precision.
Of the four forces, only the gravitational force is important for determining motions of matter at large scales. By the 1920s Einstein’s general theory of relativity (GR) had displaced Newton’s theory of gravity. Any theory attempting to describe the dynamics and history of the universe must ultimately depend on GR.
To be sure, there were and continue to be alternative proposals to explain the observations. One early theory was called “tired light.” It was first proposed by Fritz Zwicky in 1929. In this theory, light becomes more redshifted the farther it travels through the intergalactic medium. More on this proposal below.
The Big Bang theory developed from Georges Lemaître’s earlier “primeval atom” or “Cosmic Egg” hypothesis. The Big Bang theory simply says the universe was much smaller, denser, and hotter compared to the present. It has expanded over its history, causing matter on large scales to spread out. On small scales, gravity caused matter to clump together.
Originally, the Big Bang theory was built on the theoretical foundation of GR and the empirical foundation of Hubble’s law. GR has faired extremely well over the past century. It has passed many stringent observational and experimental tests, including, in 2015, the first direct detection of gravitational waves.
Alexander Friedmann, and later independently Georges Lemaître, Howard Robertson, and Arthur Walker, derived the cosmological solution to the GR field equations describing an isotropic and homogeneous dynamic universe. Not only did these equations form the foundation of the Big Bang theory, they also equally supported its main competitor, Steady State. The Steady State theory was proposed by Fred Hoyle, Thomas Gold, and Hermann Bondi in 1948 to do away with the need for a beginning, which the Big Bang theory implied.
Predictions and “Postdictions”
Each cosmological theory makes different predictions, but they “postdict” already available data. Prior to the 1960s, the Big Bang and Steady State models explained the Hubble law within their respective frameworks, and they were both consistent with everything else we could observe in the universe. Continued advancements in observational cosmology, however, soon provided ways of testing them. The first important test came in 1964 with the discovery of the cosmic microwave background (CMB) radiation. Arno Penzias and Robert Wilson of Bell Labs measured the temperature associated with the radiation to be 3.5 +/- 1 degrees K. For an interesting historical overview of the early “near misses” of the discovery of the CMB radiation, see here. The modern value is 2.7 degrees K.
Caught with Its Pants Down
This was an important discovery for several reasons. First, the Big Bang theory required the CMB radiation, while Steady State was silent about it. Steady State was caught with its pants down; its proponents tried to explain the CMB radiation after the fact.
Second, in 1948 Ralph Alpher and Robert Herman actually predicted the temperature associated with the CMB to be 5 degrees K (see here for a detailed history of predictions of the CMB radiation temperature written by Alpher’s son). George Gamow predicted a value of 7 degrees K in 1953 and 50 degrees K in 1961. It is important to note that the predicted values of the CMB radiation temperature depend on knowing such quantities as the value of the Hubble constant and the age of the universe. Neither of these was known very accurately at the time. For example, correcting Gamow’s 1961 temperature estimate for his too-small value for the age of the universe reduces it by a factor of two (see here). Third, the CMB radiation was measured to be uniform in all directions, or isotropic, which also confirms its cosmic status.
The Temperature of Space
However, prior to 1964, Big Bang proponents were not the only ones talking about the “temperature of space.” For example, in 1926 Arthur Eddington calculated a temperature of space due to the energy density of the radiation from stars around the Sun to be about 3.2 degrees K. As explained here, this is just the local radiation field and not the CMB radiation that fills all of space. In addition, this radiation is most intense in the visible region of the spectrum and is far weaker in the radio, where the expected CMB radiation peaks. Eddington never gave his estimate any cosmological significance. The close agreement with the CMB radiation temperature is a coincidence due to our particular location in the Milky Way galaxy.
Others did make explicit predictions of the CMB temperature within the context of a nonexpanding universe. They included a who’s who of scientists: Walther Nernst, Louis de Broglie, and Max Born. They worked within the framework of various tired light models. Unlike the Big Bang and Steady State theories, tired light models were proposed as alternatives to GR to explain the redshifts. Although the “space temperatures” calculated for these models are close to the measured value of the CMB temperature (see here), they never gained much following because they failed to provide a plausible physical mechanism for the light to become tired (see here and here). What’s more, the success of GR over the course of the 20th century chipped away at theories based on alternatives.
Was this the only Big Bang prediction? Not by a long shot! In another post I will discuss the many additional observational tests of the Big Bang theory in the years following Penzias and Wilson’s important discovery.