Cyclic model

A cyclic model is a way scientists think about how the universe might work over very long periods of time. In these models, the universe goes through repeating cycles, like breathing in and out, over and over again without end. One famous idea called the oscillating universe was thought about by the famous scientist Albert Einstein in 1930. This theory suggested that the universe starts with a big explosion called the Big Bang, then grows and spreads out. After a very long time, the force that holds everything together, called gravity, might pull everything back together again in an event called the Big Crunch. After that, the universe could start again with another Big Bang, and the cycle would continue forever.

These ideas help scientists think about big questions, like how old the universe is, how it began, and what might happen in the far future. Even though we now know more about the universe than we did in Einstein’s time, cyclic models are still interesting because they show one possible way the universe could keep going through endless cycles of expansion and contraction. Studying these models helps us understand the forces and rules that shape the cosmos.

Overview

In the 1920s, scientists like Albert Einstein wondered if the universe could go through repeating cycles instead of just expanding forever. They called this the Oscillating Universe Theory. But later, they found a problem: each cycle would need to be bigger and longer than the last, making it hard to explain how everything started.

In the early 2000s, new discoveries about dark energy gave fresh ideas for these cyclic models. One idea, called the brane cosmology model, suggests the universe could explode into existence many times over. This helps explain why the universe is spreading out faster and faster. Other theories, like conformal cyclic cosmology and loop quantum cosmology, also explore the idea of repeating cycles in the universe.

The Steinhardt–Turok model

Main article: Ekpyrotic universe

In this cyclic model, two parallel planes called branes move through a higher-dimensional space and crash into each other from time to time. These crashes mark the shift from the universe shrinking back together to expanding outward again, similar to a Big Crunch followed right away by a Big Bang. The matter and energy we see around us today were created during the most recent crash, following patterns set by tiny quantum changes that happened even earlier.

Over many billions of years, the universe grows and changes until it will eventually start to shrink once more. A special kind of energy, called dark energy, acts as a force between these branes and helps explain some big questions about the universe. This model suggests the universe could continue these cycles forever into the past and future. However, there are still many mysteries, like how exactly these branes behave during their collisions.

The Baum–Frampton model

The Baum–Frampton model is a modern idea about the universe from 2007. It suggests that a special kind of dark energy, called phantom energy, could change how the universe behaves. Normally, this dark energy might cause the universe to end in something called a Big Rip. But in this model, if certain conditions are met, the universe could instead reset itself.

This reset would leave the universe empty, with only dark energy remaining. This helps avoid problems that might happen if the universe shrank with matter still in it. The model uses ideas from string theory but does not completely depend on it.

Other cyclic models

Some ideas about how the universe might go through repeating cycles include Conformal cyclic cosmology, a theory by Roger Penrose. This idea suggests that the universe expands until everything turns to light, allowing it to start a new cycle just like the Big Bang.

Another idea is Loop quantum cosmology, which talks about a "quantum bridge" connecting the end of one cycle to the start of the next. There are also theories, like those by Nikolai Gorkavyi, that explain the universe’s cycles without needing mysterious forces, using the behavior of black holes and gravity.