How do the different theories of the universe stack up? Photo credit: Arnaud Mariat via Unsplash
The question of the fate of the universe is a profound one, exploring the underlying mysteries of space and time. To answer it, cosmologists must be willing to confront philosophical queries about the nature of science itself. The resulting theories end up sounding suspiciously like the back of a Rice Krispies box: rip, crunch, freeze, slurp. Understanding where each of these came from, and why one is yet to be conclusively accepted requires a journey back to the beginning of time.
All of space and time was condensed into an infinitely dense point known as a singularity, violating all physical laws.
The most fundamental problem in physics occurred 14 billion years ago when the universe began. All of space and time was condensed into an infinitely dense point known as a singularity, violating all physical laws. Within a second, the universe then expanded at an immense rate in an explosion that astronomer Fred Hoyle coined ‘The Big Bang’. Hoyle himself opposed this theory and came up with the term to mock the idea that the universe had a sudden beginning. He favoured the Steady State theory that, as a committed atheist, allowed him to omit the role of a creator. The Steady State theory is based on the perfect cosmological principle that the universe does not evolve over time. By the 1970s, Hoyle’s model of an infinitely old and infinitely constant universe was no longer accepted.
Hoyle himself opposed this theory and came up with the term to mock the idea that the universe had a sudden beginning.
Overwhelming evidence combatted the Steady State theory to show there is no doubt that the universe is not the same as it always has been. In fact, it is still expanding now. Edwin Hubble reached this conclusion when he measured the rate at which distant galaxies race away from earth. He inferred that the further away a galaxy is from earth the faster it recedes, which would not happen if the universe was static. Very few know that this evidence would have been impossible to acquire without a scientist named Henrietta Leavitt, who developed a ‘cosmic yard stick’. Despite not being allowed near a telescope at Harvard Observatory, her work processing and computing numbers from stars known as Cepheid variables inspired a stroke of genius. These stars vary in brightness periodically, meaning that when Leavitt compared their luminosity output to the sun, she was able to calculate how far away they were. Once the distance to Cepheid stars was measured, they could be used as markers for measuring relative distances to galaxies or any other cosmic object. The collaborative evidence Hubble and Leavitt provided for the inflationary theory of the universe was particularly important for supporting The Big Bang. When cosmic expansion is reversed in time, the origin of all space can be traced back to a single point .
…her work processing and computing numbers from stars known as Cepheid variables inspired a stroke of genius.
Edwin Hubble (1889-1953) and Henrietta Leavitt (1868-1921)
The universe immediately after The Big Bang was a hot plasma of energetic free electrons, protons and neutrons. Now that space has cooled, there remains an afterglow known as cosmic microwave background radiation (CMBR). When NASA released their highly detailed map of this radiation taken by the Wilkinson probe in 2012, it confirmed fundamental properties of the universe. One such property encoded by CMBR is called isotropy, which essentially states that the distribution of temperature looks the same no matter which direction you view it from. This is because as the universe expanded, it smoothed itself out, much like blowing up a balloon. A few variations in radiation remained, reminiscent of ripples in a pool of water. The ripples had a profound effect, later forming the structures of matter seen today as stars and galaxies. The CMBR encompasses the history of the universe and, along with Hubble’s receding galaxies, acts as a pillar of evidence supporting The Big Bang.
The Cosmic Microwave Background, as taken by the Wilkinson Microwave Anisotropy Probe (WMAP)
The Big Rip
The theory that follows on most closely from Hubble’s cosmic inflation is The Big Rip. This states that the rate at which the universe is expanding means it will eventually tear itself apart. The expansion would first pull planets and stars away from each other, then tear apart the celestial bodies themselves, before finally ripping apart even atoms and nuclei. Space and time would dissolve into nothing. For this theory to be true, a dark energy known as phantom energy would have to exist in order to keep the universe expanding at an accelerating rate. Phantom energy is paradoxically hypothesised to have negative kinetic energy. Very little is known about it aside from this and since it has never been observed, The Big Rip rests on shaky ground.
…the rate at which the universe is expanding means it will eventually tear itself apart.
The Big Crunch
The Big Crunch is the exact opposite. This theory demands some sort of boundary to the universe that the expanding space will eventually recoil against. The Big Bang would be reversed as the universe shrinks back down into a singularity at the end of time. Many cosmologists think that this crunch would trigger The Big Bang again, and the universe would bounce back and forth between expanding and contracting. Roger Penrose developed a theory called Conformal Cyclic Cosmology (CCC) which conflates The Big Bang and the end of the universe, discarding the need for a defined beginning of time. When everything has been consumed by expanding black holes, Penrose interprets this death of the universe as the rebirth of a new one. Anyone who has puzzled over the work of MC Escher will understand why his endlessly and impossibly cyclic art played a key part in inspiring Penrose’s innovative concept.
The Big Bang would be reversed as the universe shrinks back down into a singularity at the end of time.
Escher’s Ascending and Descending
The Big Freeze
This theory is a scenario of absolute heat death. As the universe continues to expand, its temperature will approach absolute zero (-273 degrees Celsius). The Big Freeze is a logical extension of the Second Law of Thermodynamics because as entropy or disorder increases, all the energy will spread out evenly. If the universe is a closed system, eventually there will be no free thermodynamic energy to sustain life. Stars will stop shining, leaving nothing but cold, empty space.
As the universe continues to expand, its temperature will approach absolute zero.
The Big Slurp
The most terrifying theory of all concerns the concept of true empty space, stating that the universe exists in a false vacuum. This means the energy state of the universe is unstable and it could collapse into a true vacuum instantaneously at any time. The Big Slurp was discussed qualitatively in a 1982 paper by theoretical physicists M Turner and F Wilczek, which included the worrying premise that ‘without warning, a bubble of true vacuum could nucleate somewhere in the universe and move outwards at the speed of light, and before we realized what swept by us our protons would decay away.’ This theory is underdeveloped in comparison with the others because of the unpredictability of the collapse. So far, it cannot be anticipated or described by calculation.
…the universe exists in a false vacuum.
The rip, crunch, freeze and slurp theories are pitted against each other by measuring the universe. Similarly to CMBR providing crucial evidence for The Big Bang, balances of matter and radiation can quantitatively be used to support or disprove each prediction. As an example, Roger Penrose searched for remnants of radiation left over from previous universes that would have survived through The Big Bang, anticipating this in the form of uniform rings of temperature. He found no definitive evidence of these in the map of cosmic microwave background radiation, so his cyclic universe theory became obsolete. These theories shine light on the true nature of science as informed guess work that could be falsified at any time. Philosopher Karl Popper envisaged the scientific world as swampy ground: ‘The empirical basis of objective science has thus nothing “absolute” about it. Science does not rest upon solid bedrock’. He compares making any theory to cobbling together stable platforms that could at any time sink and be replaced. The fate of the universe qualifies as the ultimate swamp of ambiguity, despite our best efforts to provide solid theories. At the end of time, none of us will be around to know if the universe goes snap, crackle or pop.
