The cyclic universe theory is a cosmological model that proposes that the universe undergoes an infinite series of cycles. In this model, the universe is eternal and has no beginning or end.
The concept of the cyclic universe dates back to ancient Hindu, Buddhist, and Jain cosmology (this will be considered in my book in progress, A History of Esotericism), which proposed the idea of an infinite series of cosmic cycles, called kalpas.
In the 1920s, the cyclic universe theory was proposed in 1922 by Russian Physicist Alexander Friedmann, who worked out a set of equations showing that the end of the universe depends on its density. It could either expand or contract rather than stay stable. With enough matter, gravity could stop the universe's expansion and eventually reverse it. This reversal would result in the universe collapsing on itself, not too dissimilar to a black hole. (see Wikipedia - Big Crunch). This led to the scenario of an infinite series of cycles, each one beginning with a big bang and ending with a big crunch. This fits nicely with the Hindu model of kalpas.
This idea of a cyclic Big Bang - Big Crunch was popular throughout the later 20th century, especially in the Eastern-orientated “New Paradigm” movement, as a way of avoiding the problems associated with the once-only Big Bang model. However, it has two problems.
First, work by British astronomer Richard C. Tolman in 1934 showed that these early attempts failed because of the cyclic problem: according to the Second Law of Thermodynamics, entropy can only increase. This implies that successive cycles grow longer and larger. Extrapolating back in time, cycles before the present one become shorter and smaller culminating again in a Big Bang. (Interestingly, this idea of smaller and smaller previous universes resembles in some way both emergent evolution where the earlier and simpler stages give rise to more detailed later stages, and esoteric cosmology such as Gnosticism and Kabbalah, where the initial simple hypostases or archetypes emanate further more detailed (but also ontologically inferior) ones).
And second, recent measurements of the Hubble Constant and distant supernova and galaxies have conclusively shown that not only does the expansion show no signs of slowing, but it is increasing. This has led to the theory of "dark energy” countering gravity and causing the universe to expand at an accelerating rate.
An alternative cyclic universe theory is the ekpyrotic model, proposed in 2001 by Paul Steinhardt of Princeton University and Neil Turok of Cambridge University. This proposes that each big bang is caused by the collision of two "branes" (higher-dimensional objects) in a larger, multidimensional universe.
Another possibility, proposed in 2007 by theoretical physicists Lauris Baum and Paul Frampton, is the theory that "phantom energy" could cause the universe to expand exponentially and lead to a cyclic model. Then there are even more exotic explanations like Loop quantum cosmology.
But the cyclic theory that has currently gained the most attention is Conformal cyclic cosmology (CCC), a general relativity based cosmological model proposed by British theoretical physicist Roger Penrose. In CCC, the universe iterates through infinite cycles, with the end of each previous universe becoming the Big Bang singularity of the next. The whole thing is counter-intuitive, but so is quantum physics. Penrose popularized this theory in his 2010 book Cycles of Time: An Extraordinary New View of the Universe.
The conformal cyclic cosmology hypothesis requires that all particles must eventually disappear. This includes those which become too widely separated from all other particles to annihilate with them. Even electrons have to either decay, or lose their charge and/or mass, and no conventional speculations allow for this. So the end of the old universe follows a sixth or seventh era, that includes the decay of not just hadrons but electrons, after the end of the Dark Era
Once all matter has decayed into photons / radiation, there is nothing that has a time or distance scale associated with it. This permits it, by conformal geometry, to become identical with the Big Bang, so starting the next cycle. This implies either there is no Eternal Inflation or that Eternal Inflation is some sort of transitional state, implying a further unified theory that incorporates these two theories.
One of the key features of all of these cyclic models is the idea that entropy (a measure of disorder in the universe) is reset at the beginning of each cycle. This allows the cyclic universe to avoid the problem of heat death, which is the idea that the universe will eventually run out of usable energy and become a cold, lifeless place.
In 2015, Penrose and fellow mathematician and cosmologist Vahe Gurzadyan discussed the Fermi paradox, the apparent contradiction between the lack of evidence but high probability estimates for the existence of extraterrestrial civilizations. Within conformal cyclic cosmology, the cosmic microwave background provides the possibility of information transfer from one aeon to another, including of intelligent signals within the information panspermia concept, the idea that life forms can travel across the universe by means of transmission of compressed information representing said life forms e.g. via genome coding, which can then enable the recovery of intelligent life. (see Wikipedia - Conformal cyclic cosmology)
Conformal cyclic cosmology is linear only, and does not take into account the multiverse or the branching universe. For this reason, I don’t think it’s a complete theory, although it does provide a very useful explanation.
Page by M Alan Kazlev, 2023