The fate of the universe

Some say the world will end in fire, some say in ice” Robert Frost wrote in his famous poem Fire and Ice. While Frost views the end of the world through the lens of human emotions, it inadvertently touches upon a question that has interested scientists and philosophers for centuries. Different scientific hypotheses have suggested various potential outcomes for the universe, ranging from finite to infinite duration, and trying to explain its origins. Edwin Hubble's observations during the 1930s–1950s, which revealed galaxies moving away from each other, led to the Big Bang theory. This theory suggests that approximately 13.787 billion years ago, the universe began as extremely dense and has since expanded, becoming less dense on average. The confirmation of the Big Bang theory relies heavily on understanding the rate of expansion, the average density of matter, and the physical properties of mass energy in the universe.

One prominent theory in cosmology is the “Big freeze”, which aligns closely with Frost’s imagery of a frozen demise. This theory comes from the analogy of a geometrically open universe (a low-density universe). In this scenario, the universe will continue to expand and gradually become colder until it reaches a state of maximum entropy. Energy will be evenly distributed throughout the universe and, no meaningful process will occur. Stars will exhaust their fuel, galaxies will drift apart, and black holes will dominate the universe, which themselves will eventually annihilate. Slowly and inexorably, the universe will become darker and eternally cold. This scenario has gained ground as the most likely fate.

Conversely, there is the “Big crunch” hypothesis, reminiscent of Frost’s vision of a fiery end. This scenario arises from the analogy of a closed universe, where parallel particles will converge just as the lines of longitude at the North and South poles. The big crunch hypothesis is a symmetric view of the ultimate fate of the universe. Galaxies, stars, and even atoms will be drawn together by the pull of gravity. As the universe contracts, the temperatures will rise, culminating in a cataclysmic inferno that consumes everything in its path. A straightforward projection suggests that all matter and space-time could collapse into a dimensionless singularity, mirroring the conditions of the Big Bang that initiated the universe. However, at such extreme scales, quantum effects come into play, introducing complexities that defy simple estimations. Recent observations indicate that this scenario is improbable but not definitively dismissed. Our understanding is limited by the short span of time over which measurements have been conducted, leaving room for future developments.

Some say the world will end in fire, Some say in ice. From what I’ve tasted of desire I hold with those who favor fire. But if it had to perish twice, I think I know enough of hate. To say that for destruction ice Is also great And would suffice. - Robert Frost

Recent observations have introduced a new contender in the cosmic apocalypse narrative: the "Big Rip." This theory suggests an acceleration of the universe's expansion to such a magnitude that it tears apart all existing structures, from galaxies to atoms, in a violent and chaotic rupture. Unlike the gradual decline of the Big Freeze or the collapse of the Big Crunch, the Big Rip is aggressive. While the current rate of acceleration, as defined by the Hubble constant, is insufficient to dismantle local structures like galaxies, it does pave the way for increased spatial distances between them. However, if the Hubble constant keeps increasing, everything in the universe, from galaxies to tiny particles, would break apart. This would eventually lead to a point where energy density, scale factor, and expansion rate become infinite, resulting in a singularity that signifies the universe's end.

The Big Bounce is another hypothesis, which is pretty unpopular. This theory suggests that the universe's origin is part of a cycle, where the collapse of a previous universe led to the beginning of our known universe. One version of the Big Bang theory initially proposed that the universe started from infinite density, which appeared conflicting with established theories like quantum mechanics. In response, an alternative viewpoint emerged from quantum mechanics, suggesting that the universe came into being through a quantum tunnelling event, starting with a density consistent with quantum principles before evolving under classical physics. If the universe is closed, this theory suggests that once our universe collapses, it might give rise to another universe, similar to the Big Bang, either through reaching a universal singularity or due to a repulsive quantum force causing expansion again!

While these theories are fascinating, it’s important to note that our understanding of cosmology is incomplete, and these are just speculations. However, observations, experimentation, and theoretical refinement are being carried out everyday in hope of improving our understanding of the universe. In the end, whether the universe ends in fire, ice, or some other unimaginable fate, one thing remains certain: humanity's quest for understanding will endure.

Victoria Nakafingo