What Alternatives Are There To The Big Bang Model?

In 1964, physicist Arno Allan Penzias and radio astronomer Robert Woodrow Wilson noticed an unusual noise always humming away in the background of their radio telescope observations.

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The noise – like static on a radio – seemed to be there, no matter what direction they pointed the telescope in the sky. At first, the two believed it was coming from the telescope itself, urban interference, or possibly even pigeons living within the telescope's antenna. However, after removing the pigeons and ruling out all other forms of possible terrestrial interference, the noise remained. 

In fact, the noise had been heard by other astronomers, though they had dismissed it as meaningless interference. The reason it could be heard by teams on multiple telescopes and in all different directions was because that irritating noise was in fact the cosmic microwave background (CMB) – the leftover radiation from the Big Bang that is faintly detectable and permeates all of the known universe. 

Penzias and Wilson realized that they were looking at a signal proposed by Ralph Alpher, and that it was evidence for the Big Bang theory, and won the Nobel Prize in Physics for their work.

While the Big Bang is the best explanation for what we have observed in the universe, it isn't the only explanation we've had to date, nor is it accepted by all cosmologists. On top of this, you don't have to go back too far in history to find a time (pre-observations that make the Big Bang scenario more likely) when the scientific consensus was that we live in a static universe.

Static and eternal universe

We haven't actually known about galaxies for all that long. Evidence had been found pointing towards the idea of "island universes" beyond the Milky Way by measuring the distance to Cepheid variable stars in the early 20th century. But it wasn't until 1924 when Edwin Hubble took a closer look at Andromeda – believed to be a nebula at the time – and found Cepheids and measured their distance that we confirmed there was anything beyond our own galaxy. 

Not too long after that, we discovered that the light from more distant galaxies is more red-shifted, which physicists now believe to be the result of the stretching of light's wavelength as the universe expands. 

Before we had evidence of the expanding universe, many scientists and natural philosophers before them assumed the universe to be infinite in time and space, and neither expanding nor contracting. 

When Einstein wrote his 1917 paper Cosmological Considerations in the General Theory of Relativity, he famously introduced the cosmological constant into his equations. This was an attempt to stop the universe (in the equations, don't worry) from eventually collapsing in on itself due to gravitational interactions, and to keep it in line with observations of a static universe. 

While an eternal and infinite universe is appealing, the idea does not fit in with modern observations of redshift and galaxy distances, nor explain the very observable and ever-present CMB. Alternative explanations for these concepts, such as tired light, do not fit well with the evidence.

Big Bounce

One fairly popular idea, which still gets posited from time to time, is that the Big Bang was not the true beginning, but the latest in a cycle of expansions and crunches. 

Sometimes known as a cyclical universe, the basic idea is that the universe contracts down to a minimum but finite size, at which point quantum gravity effects "bounce" it and it begins to expand again. 

In this idea, the laws of physics may vary as the universe contracts and bounces back again, and may leave unique telltale signals on the CMB. No such signal has been found that would make this idea fit the evidence better than the standard model.

Conformal cyclic cosmology

This idea comes from Nobel Prize-winning physicist Roger Penrose, and sprung from thinking about interesting observations about our universe. The second law of thermodynamics states that entropy (it gets complicated, but for the purposes here think "disorder") can only increase. Things become more disordered as time goes on, and at some point, this will result in the "heat death" of the universe, where matter is uniformly distributed and no more "work" can be done. 

In Penrose's idea, this heat death, where all black holes have evaporated and particles decayed and massless radiation dominates, serves as the beginning of a new "aeon". With no particles or black holes, scale in the universe becomes meaningless, and you can conformally rescale the universe (keeping angles unchanged) and are left with what looks like the hot, dense, and smooth universe that could look like the beginning of our own universe. From there, another Big Bang-like event happens, before that universe too undergoes heat death and rescaling.

The idea provides an explanation for the smooth state of the universe at its beginning without the need for inflation, and according to Penrose and others working on the idea, we could see evidence of it from within our current aeon.

"The Conformal Cyclic Cosmology (CCC) proposes that the universe undergoes repeated cycles of (accelerated) expansion, named 'aeons', where the maximal (or infinite) extension of the previous cycle goes to coincide with the Big Bang stage of the successive cycle. No contraction (big crunch) is required in this model," Penrose explained in a paper.

"This is made possible through the conformal structure that dominates space-time at the beginning and at the end of each aeon. The CCC solves the paradox of the super-special initial conditions required by Second Law at the Big Bang, and among its observational consequences, predicts the presence of 'circular rings' in the temperature fluctuations of the Cosmic Microwave Background spectrum."

While interesting, no robust evidence has been found for these ring structures, or "Hawking points" as of yet. It also isn't clear that protons do decay, and if they don't, then the conformal rescaling could not take place.

Eternal inflation

In the standard Big Bang model, the early universe undergoes a period of extremely rapid inflation, driven by the inflaton field, which smoothed out the universe and kept it "flat". In eternal inflation, this rapid growth goes on in some regions of the universe forever, whilst in certain pockets or "bubble universes", inflation ends locally, and matter can exist.

In this idea, universes may have very different laws of physics, and we are living in the one that can support our type of life, making problems like the "fine-tuning" problem disappear. Models vary, but some predict that we should also be able to see signs of collisions between universes in the CMB. 

Again, we lack evidence to show that this is the case, and the Big Bang plus inflation remains our best model of the universe, so far.