The turn of the 21st century saw many advances in science. In the field of cosmology, various applications of existing theories were questioned, while new theories also emerged. Very few theories, however, came close in impact, acceptance, and even controversy as did the Big Bang theory.
The Birth of The Big Bang Theory
Big Bang cosmology was born out of both theory and observation.
Albert Einstein, after developing the theory of general relativity, attempted to apply it to the universe as a whole, an idea that had hitherto disturbed him, as he felt the universe to be inherently unstable. Now, he tried to dispel this notion by introducing a cosmological constant to his field equations, something to hold the universe stable.
Meanwhile, the astronomer Edwin Hubble, while trying to discover galaxies that lay beyond the Milky Way, noticed variability of the redshift in these galaxies: the farther away a galaxy was, the faster it appeared to be moving away. This led to the geometrical implication that the universe was expanding.
The idea that the universe was expanding came with the direct corollary that it had been smaller in the past, positing the possibility of an event with which this process began. There were many opponents of this new theory, and one of them, in an unwitting attempt at derisiveness, gave it the name of ‘Big Bang’; a name thought to be so cute that it stuck. The subsequent discovery of background radiation, correctly predicted by this theory, began to cement it as a standard part of the cosmological discourse on the beginning of the universe.
However, the fact that the theory is now almost universally embraced does not mean that it did not bring about its own set of problems. The Big Bang theory came with some inconsistencies that began to be identified in the 1970s.
Learn more about Big Bang cosmology.
An Interaction of Scientists
The potential anomalies arising out of the Big Bang theory did not emerge by chance. They developed because of a system change in the scientific paradigm of the time. Science had always been a field that involved hyper-specialization. Each area was so detailed and technical that it was all a scientist focused on from graduate school onwards.
The Big Bang theory broke down these scientific boundaries. A smaller universe would have meant greater energy density. The farther back in time one went, the higher the density became; going back far enough meant that there would be a time when energy and heat would be so high that things would work very differently than they do now. This observed behavior called for the particle physicists to work with the astrophysicists, which is where the problems with the theory began. Particle physicists started to raise questions that the Big Bang theory couldn’t answer on its own. As a result, three big questions began to appear.
This is a transcript from the video series Redefining Reality: The Intellectual Implications of Modern Science. Watch it now, on The Great Courses Plus.
Missing Magnetic Monopoles
The first problem raised was that of missing magnetic monopoles. Magnets, like electricity, have two charges, referred to as north and south. And, just like electric charges, opposite magnetic charges attract and are governed by equations with the same forms.
The difference, however, is that while single electric charges, such as a particular negative charge, can exist, magnetic monopoles cannot. For instance, if a bar magnet is broken in half, that does not produce two separate north and south poles. Instead, it creates two smaller dipolar magnets.
The problem here is that the energy density of the early universe that was posited by the Big Bang theory should create a lot of magnetic monopoles. Yet, none have been found to exist.
The problem became worse when particle physicists understood that magnetic monopoles, if they existed, would be massive compared to other particles; so big, in fact, that they would have hampered the creation of other particles as well. So, the creation of magnetic monopoles, which would follow from the Big Bang would have wreaked havoc on the creation of matter.
The Uniformity Conundrum
Another problem was highlighted by Albert Einstein’s cosmological principle, which posits that the universe is homogenous and isotropic. Local deviations may occur, but an overall picture is the same from every angle.
This is similar to how pictures of different points on Earth are vastly different from each other, but a picture from space would not reflect the differences between these points. A differently shaped Earth would not have allowed this phenomenon.
The problem is, that this constant curvature of space-time, determined by the density of mass and energy, means that there is equilibrium over all space-time. But why should this be? Any process to create this connection would require it to be faster than the speed of light. In other words, the vastness of the universe means that there could not possibly be a physical process that brought the far reaches of it into equilibrium, that is, without violating the theory of relativity.
Learn more about the theories of relativity.
The Problem of the Flat Universe
The third concern raised by the Big Bang theory is similar to the second one. Looking at the universe reveals that it is geometrically flat. The geometry of Euclid is binding on the structure of the universe. ‘Flat’ is, in fact, a unique point that lies halfway between a positive curvature and a negative one. The odds of it coming up randomly across all space-time are next to nothing. In such a context, how did the universe end up being flat? Why is the energy density equally distributed? Any smallest deviation from equilibrium would actually have multiplied over time, also changing the distribution of energy density over time.
These problems were plaguing the Big Bang theory when the American Alan Guth and the Russian Andrei Linde came up with the notion of inflation, which posited than an expansion, a size increase of 1050, happening early in the history of the universe, 10-35 of a second after the Big Bang, for a time of only 1030th of a second, would account for all these problems.
If the early universe did, in fact, create magnetic monopoles, the rapid expansion would have moved them so far from each other that their density would effectively approach zero. This expansion would also have caused a rapid cooling that would have changed the environmental conditions to keep magnetic monopoles from being formed. Thus, the universe would have ended up creating many fewer monopoles than first thought, and the ones created would be so scattered that they might never be found.
This expansion could explain the cosmological principle. The problem here—that of the universe being homogenous without areas being able to influence others—would be solved, because if the universe was tightly packed before expansion, parts would have been close enough to be causally connected, thereby eliminating the need for a faster than light process. After the expansion, these parts would be so far isolated, and in equilibrium, that no further influences would occur.
Finally, the effect of the inflation, by working out the mathematical form, would have been to flatten out space, creating a uniform energy density.
While the inflation postulated by Guth and Linde does solve the three basic problems with the Big Bang theory, it has been said to be nothing more than an abductive inference. It is not the only possible explanation, but as of now, it is the best one offered, and therefore accepted widely.
Common Questions about the Big Bang Theory
The Big Bang theory is the most widely accepted explanation of the creation of the universe, starting from a small singularity, and then rapidly expanding.
Magnetic monopoles refer to the existence of singular magnetic poles, such as north, or south, independent of the other pole. According to the Big Bang theory, there should have been a lot of magnetic monopoles existing in the universe, but none have been found so far.
The cosmological principle, first presented by Albert Einstein, posits that the universe is uniform, homogenous and isotropic. The Big Bang theory, however, offers no explanation for this principle, which would require a process faster than light, in other words, one which defies the general theory of relativity, to effectively create this homogeneity.