By Tanisha Jhaveri
The Big Bang is perhaps one of the most famous and widely-accepted theories about our Universe, but how strong is the evidence for it, really?
A bit of background: in 1922, Alexander Friedmann realized that any Universe that is largely uniform in its distribution of matter and energy would be unstable, and therefore likely to undergo expansion. In 1926, Edwin Hubble discovered that most galaxies are moving away from us, with those further away receding at higher speeds. Eventually, in 1927, Georges Lemaitre unified Hubble's observation with Friedmann's equations and formulated the cosmological theory we know and love today. He concluded that since the Universe is expanding, we can extrapolate this back billions of years to conclude that the Universe was much smaller and denser than it is now.
Granted, the Big Bang theory has been a huge success, with a lot of evidence to back up its claims. However, this model has failed to solve several pressing issues including but not limited to the monopole problem, the horizon problem, and the singularity problem.
The monopole problem stems from the fact that the Big Bang theory predicts the existence of magnetic monopoles, which have never been observed and are difficult to conceive of. A magnetic monopole is a particle that acts as a magnet, but with only one magnetic pole — for example, only a north pole without the south pole, or vice versa. At the high temperatures of the Big Bang's early Universe, monopoles would be produced in large amounts, to the extent that there should be many remaining today that we can detect. However, interestingly, we haven't yet been able to do so.
The horizon problem arises because the opposite “ends” of space are much further apart than the Big Bang theory anticipates. That is, assuming that the Universe’s expansion occurred due to the Big Bang, for its two “ends” to have ever been in “casual contact” (i.e. had any kind of physical interaction or information exchange), information would have had to travel across this large distance at a rate faster than the speed of light, which is impossible. We reach the peculiar conclusion that the two ends of the Universe could not have been in causal contact. However, the uniform temperature of the cosmic microwave background (remaining radiation from the early Universe) suggests that the ends should have been in contact with each other.
Finally, the singularity problem: while Lemaitre shows us the power of extrapolation, there seems to be a limit to how far back in time we can actually extrapolate. This is because, at the very last (or first) moment, we reach a “singularity”—a single point of infinite density and energy. And under these conditions, our known laws of physics break down.
This leads us to arguably the biggest setback of the Big Bang theory: it is not, as often marketed, a theory of the origin of the Universe, because it actually doesn’t describe how and why the Universe came to be in the first place. Rather, it is only a theory of how the Universe grew and developed instantly after its origin, which itself remains unaddressed. Consequently, many alternative theories have been proposed to attempt to explain the beginning of the Universe and to resolve the problems that arise with the Big Bang.
The central question remains: how could the Universe have originated from nothing? In 1973, Edward Tyron attempted to provide an answer. First, let us consider one of the most powerful consequences of quantum mechanics — due to the spontaneous, random emergence of pairs of virtual particles (as permitted by Heisenberg’s Uncertainty principle), no vacuum is completely empty. Virtual particles are short-lived particles that share similar properties with ordinary particles, and are created by quantum fluctuations. In other words, even “empty” space contains energy.
Accordingly, Tyron’s theory of genesis begins with pairs of virtual particles spontaneously appearing in a vacuum. Although, in general, particle pairs like these annihilate immediately to produce radiation, if the virtual particles are separated by virtual forces, they can create what is called a true vacuum bubble which would expand exponentially to form our Universe. In short, spacetime, matter, and energy could have emerged in a vacuum through quantum fluctuations.
On the other hand, the problem in conceiving of an origin to the Universe is avoided entirely by models proposing that the Universe is eternal. One such theory, proposed by Roger Penrose, is Conformal Cyclic Cosmology, which suggests that the Universe is cyclic in nature. Penrose proposes that the Universe goes through aeons, and the end of one aeon marks the beginning of another, which is equivalent to the birth of our Universe.
All in all, while there are a multitude of theories about the origin of the Universe, we don’t really have an answer and perhaps are not particularly close to a specific one. Philosophers and physicists continue to ponder this ambitious question—but for all we know, we could just be living in the Matrix.