Top 5 theories on End of Universe

We dive into some of the wildest theories on the ultimate fate of the universe. Will it rip itself apart, expand forever, or start a new cycle in the distant future? The most likely prediction is a heat death, but the other predicted outcomes remain possible within different theoretical frameworks. The best understanding of the universe that humans have is known as the standard model, or technically the ΛCDM model. The eventual fate of the universe depends on a number of factors, including the rate at which the universe is expanding, the density of matter, dark matter and dark energy within it, and the curvature of the universe. Dark matter is an invisible substance that has mass and interacts gravitationally with regular or baryonic matter, but does not interact with the electromagnetic spectrum, rendering it invisible and hence dark.

The ΛCDM model of cosmology. (Image Credit: NASA/ LAMBDA Archive / WMAP Science Team).

Despite decades of searching, scientists have not discovered dark matter, humans only know that it exists, but not its characteristics or nature. Since we do not have a handle on the behaviour of dark matter, we cannot predict with certainty the ultimate fate of the universe. Originally scientists believed that the universe expanded rapidly after the Big Bang, and then started slowing down. In 1998, two independent teams of scientists came to the same dramatic conclusion, that the expansion of the universe is accelerating. This expansion is believed to be driven by dark energy. Now that we know the basics, we can explore the different theories on the ultimate fate of the universe.

This is the most probable scenario according to conventional understanding, where the universe continues to expand forever. The entropy of the universe, or disorder keeps increasing till matter and energy is evenly distributed, and there are no dynamic processes. Stars will burn out, exhausting their nuclear fuel, black holes will evaporate through Hawking radiation, and everything cools down to absolute zero, leaving behind a cold, sparse sea of low-energy particles and photons. There is no energy left in the universe for anything. The theory assumes that the expansion of the universe, driven by dark energy, continues unabated. The timescale for this to occur is staggeringly large, trillions of googols of years.

According to the Big Crunch theory, the expansion of the universe will slow down, stop, and then reverse, collapsing back into a hot, dense state. The spacetime continuum, along with all the matter and energy within it would compress back into a singularity, similar to the Big Bang at the dawn of time. Such a theory requires a closed universe with a positive curvature, but the most sensitive instruments indicate that the universe is flat, making the Big Crunch an unlikely outcome. The speculative quantum gravity would dominate near the end of the crunch as spacetime gets packed into a point. This requires a shift in the behaviour of dark energy, or very little of it, both of which are not supported by conventional science. Depending on the density of matter in the universe, the Big Crunch could take place in hundreds of billions of years.

The Big Rip theory can occur in a flat universe as well. This theory indicates that the universe will eventually tear itself apart in a rapid, catastrophic end. Here, dark energy is not a constant but a dynamic field, called phantom energy. The negative pressure of this phantom energy overwhelms all the other forces, causing the expansion of the universe to accelerate exponentially. The diverging expansion rates force galaxies to dissolve, and then the electromagnetic forces tear atoms apart. This outcome depends on the evolution of dark energy, which is not confirmed. If the hypothesised phantom energy exists and dominates the other forces, then the Big Rip could happen in as little as 30 billion years.

Also known as the False Vaccuum Collapse, this is an interesting theory where a bubble of destruction expanding at the speed of light can obliterate everything in the universe. The universe is so vast that this would still take 95 billion years for this to take place. The theory suggests that the Higgs field, responsible for providing mass to everything in the universe is in a metastable false vacuum state that can suddenly decay to a lower-energy true vacuum state at any time, triggering a bubble where physical constants shift drastically. Matter as we know it cannot exist within this bubble, with general relativity ensuring that the edge is a horizon that no one can escape. This eventuality is dependent on the shape of the Higgs potential, which has not been mapped. This sudden death of the universe can happen any time, even right now.

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In a cyclic or ekpyrotic universe, the process of expansion and contraction repeats itself in cycles, with each gravitational collapse or Big Crunch resulting in a subsequent Big Bang. These exotic models combine the Big Crunch with the rebound mechanism, and include various types of quantum gravity which is not understood yet, including theories such as loop quantum gravity. Quantum gravity can prevent an absolute singularity that causes matter to bounce at ultra-high densities. The classical singularity in general relativity is replaced with a finite factor at small scales where quantum effects dominate. Energy and entropy can reset in every cycle.