The description of how our universe has evolved from the moment of the big bang until today is known as the big bang model. As the universe expanded, it cooled down, and the constantly moving and colliding particles combined and formed atoms that coalesced into stars and galaxies, eventually leading to the universe we see today.
Einstein and His Theory of Relativity
From the time of Isaac Newton in the seventeenth century, most physicists believed the universe was eternal and unchanging. The stars appear to rise and set and go through other apparent cyclic motions, but those could all be explained as effects of the rotation and other motions of the Earth.
Once you took the Earth’s motion into account, the heavens beyond our solar system seemed to be static.
Fast-forward hundreds of years to 1915. That’s when Albert Einstein published his general theory of relativity, which described the nature of space, time, and gravity. Einstein’s theory was confirmed by a number of experimental tests.
But when Einstein applied his theory to the universe on large scales, it made a prediction that didn’t seem to match what astronomers knew about the universe.
This article comes directly from content in the video series The Big Bang and Beyond: Exploring the Early Universe. Watch it now, on Wondrium.
General Relativity and the Cosmological Constant
The equations of general relativity said that a static universe was impossible. According to Einstein’s theory, the universe could be contracting or expanding, but not staying still.
Since all the astronomers of the time believed the universe was static, Einstein modified his theory by adding a new term called the cosmological constant to his equations.
This change was intended to allow for a static universe. But later calculations showed that even with the cosmological constant, the universe according to general relativity still had to contract or expand.
An Expanding Universe
In 1927, a Belgian priest named Georges Lemaître published calculations that described what an expanding universe would look like, based on general relativity. Observers in any galaxy would see other galaxies moving away from them, with distant galaxies moving away faster than nearby ones.
Mathematically, the speed at which we would see another galaxy moving away from us would be proportional to its distance from us. Einstein, however, still believed the universe was static and told Lemaître that his physics was ‘abominable’.
However, two years later, in 1929, Edwin Hubble measured the speeds and distances of 24 galaxies. Hubble found that almost all of them were moving away from us at speeds proportional to their distances. In other words, the farther away from us a galaxy is, the faster it moves away, exactly as Lemaître had described.
That kind of match between mathematical predictions and measured data is what one needs to gain acceptance for a physical theory. Hubble’s data convinced most physicists that the universe is expanding, and Einstein went out of his way to publicly praise Lemaître’s work after that.
The pattern of distant galaxies moving away from us faster than nearby ones came to be called Hubble’s law. But in 2018, the International Astronomical Union voted to rename it the Hubble-Lemaître law, sharing the credit between the person who predicted it and the person who measured it.
Two years after Hubble’s measurements, in 1931, Lemaître published a paper tracing the expansion of the universe backwards to an initial hot, dense state. Lemaître’s term for this cosmic starting point was the primeval atom.
For several decades after Hubble’s discovery, however, a number of sceptical physicists continued to believe that the universe must be static. In fact, the name we now use for Lemaître’s primeval atom was originally coined by one of these sceptics, Fred Hoyle, who in a 1949 radio interview referred derisively to the idea of the universe being created in a ‘big bang’.
As more evidence accumulated, though, the number of sceptics shrank. The decisive turning point was in 1964, when astronomers detected background microwave radiation of exactly the sort predicted by the big bang model. This was a match between theory and experiment that was too strong to ignore.
Since the 1960s, there has been no serious scientific debate about the fact that the big bang model is an accurate description of the history of our universe over the past roughly 14 billion years.
Common Questions about the Big Bang Model and the Evolution of Our Universe
Albert Einstein modified his theory of relativity as all the astronomers of the time believed that the universe was static. Einstein modified his theory by adding a new term called the cosmological constant to his equations.
When Edwin Hubble measured the speeds and distances of 24 galaxies, he found that almost all of them were moving away from us at speeds proportional to their distances. In other words, the farther away from us a galaxy is, the faster it moves away.
The decisive turning point came in 1964, when astronomers detected background microwave radiation of exactly the sort predicted by the big bang model. This was a match between theory and experiment that was too strong to ignore.