Dr. Binod Adhikari, Nirakar Sapkota
The Big Bang Cosmology is one of the great achievements of the 20th century. Since the Universe was discovered to be expanding, the reversal of the process would be a contraction. If we played the video of the expanding Universe in reverse, a point in time would arrive when the entire Universe was in a single place, billions of times smaller than the width of an atomic nuclei. Even though the reasons remain unclear about the very beginning of the Universe, the subsequent events are very clear- especially after few seconds of the Big Bang. The creation of various elements, the abundance of Hydrogen and Helium, and the Cosmic Microwave Background (CMB) are very effectively explained by the Big Bang Cosmology. However, it has some problems: three problems to be precise.
The first problem of the Big Bang Cosmology is known as the ‘Horizon Problem’. When distant points on the CMB are observed, they seem to be thermalized despite the fact that light couldn’t have connected these points. For instance, when we look on the CMB from two opposite sides of the sky, we are looking at places which are about 28 billion years apart (given that the Universe is about 13.7 billion years). There is no way light could have travelled 28 billion light years in just 13.7 billion years. However, these points seem to be in thermal equilibrium such that the temperature doesn’t vary more than a thousandth of a degree. If the Hubble expansion is considered, these two points are never too close for light to thermalize them. Even when the Universe was very small, the rate of expansion was higher than the speed of light which prevented these points from being thermalized anytime in the history of the Universe .
The second problem of the Big Bang Cosmology is known as the ‘Flatness Problem’. When the CMB is observed, the observations correspond to a flat Universe. If the Universe had a positive curvature, the patches on the CMB had to be bigger than what we observe and if the Universe had a negative curvature, the patches would have to be smaller than the observed ones. However, the observations reveal that the Universe is nearly flat with its density parameter (Ω0) = . This is problematic because in the past, the early Universe had to be finely tuned to have such a perfectly crafted nearly zero curvature that even after 13 billion years, the curvature is still zero. This would require the density parameter to be equal to 1 within one in hundred trillion. Physicists have a problem with such “fine-tuning” as it is against the Copernican Principle. It is due to this reason that they’ve found the Big Bang Theory to be somewhat problematic in this aspect as the theory doesn’t explain the reason behind such fine-tuning.
The third problem of the Big Bang Cosmology is known as the ‘Monopole Problem’. Decades of observations haven’t been successful to detect even a single magnetic monopole, whereas the Grand Unified Theory (GUT) requires relic abundances (like magnetic monopoles) to be an intrinsic feature of the Universe. Provided that their density is inversely proportional to the cube of scale factor, they dilute away at the same rate as ordinary matter. The theories predict that such relic abundances should be dominant in the Universe . However, the lack of detection of such predicted abundances indicate that something must be wrong with the Big Bang Cosmology or that something needs to be modified.
However, these problems are effectively solved by the Inflationary Cosmology which was first introduced by Alan Guth in 1981.  Inflation is a rapid and exponential expansion of the Universe after 10-36 seconds of the Big Bang up to 10-33 seconds. Even though inflation took place for a short period of time, it increased the scale factor of the Universe at least by 1026 times . After that, the normal expansion continued. This very theory solves the ‘Horizon Problem’ by explaining that the Universe was very small than predicted by the extrapolation of the Hubble expansion in its early age and had enough time to be thermalized. Later, the expansion became exponential and increased the size of the Universe just enough to arrive to nearly the current size after 13.7 billion years of Hubble expansion (even though the acceleration in expansion isn’t considered in this scenario). Moreover, the ‘Flatness Problem’ is easily solved by relating simply to an inflated balloon. As a balloon is inflated, its surface appears to be flat. The exponential expansion stretched the Universe to give it a nearly zero curvature which is why it has flat geometry even in the current era. In addition, the relic abundances got diluted rapidly compared to the energy density at the time of cosmic inflation which is why they became negligible very quickly. However, this scenario works only if the energy density during the end of inflation is enough to be converted into conventional matter without recreating the relic abundances .
In this way, the introduction of inflationary cosmology solved the major problems of the Big Bang Cosmology; thus providing a foundation of modern cosmology. However, some different types of theories may very well be published in the future which might be even more promising than cosmic inflation and able to solve not only the problems of the Big Bang Cosmology but also other problems of the modern Cosmology like dark matter and dark energy.
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