This blog post explores how cosmology quantitatively explains the history and structure of the universe. Based on inflationary theory and the conditions of the early universe, it helps us understand the processes through which the universe evolved into its present state.
Introduction
The topic I aim to explain in this blog post is cosmology. Cosmology refers to the study that quantitatively (primarily mathematically) explores the entire universe and, further, humanity’s place within it. To redefine it based on what I will cover today, cosmology is the discipline that deals with the history, structure, and evolving nature of the universe. Hypotheses and equations concerning the universe exist in great variety. While modern cosmology often involves complex concepts and intricate mathematical methods, this blog post aims to explain the subject by excluding such elements as much as possible.
Inflation Theory
Inflation theory is a concept within physical cosmology that explains why the universe is so flat by proposing an exponential expansion of the early universe. Here, the term “flat” refers to one of three classifications describing the overall state of the universe.
The expressions describing the state of the universe are flat, closed, and open. A flat universe means the universe accelerates its expansion until it reaches a constant size. An open universe is one that continues to accelerate its expansion, while a closed universe is one where the accelerating expansion gradually slows down and eventually reverses into contraction. The method for determining the state of the universe involves comparing the energy driving its expansion with the energy driving its contraction.
Returning to the topic, the expansion of a flat universe is thought to be driven by dark energy from a yet-undiscovered field called the inflaton, and is estimated to have occurred in an extremely brief moment, roughly a fraction of a second, immediately after the Big Bang. The strength of the inflationary theory lies in its resolution of several previously raised problems. These three problems are: the flatness problem, stemming from the observation that the universe is extremely flat; the horizon problem, where the universe exhibits isotropy despite the impossibility of information exchange between its two extremes, thus lacking causality; and the magnetic monopole problem, explaining why magnetic monopoles, expected to have been produced in large numbers in the early universe, are now so diluted as to be scarcely observable.
The Three Problems Cosmology Must Solve and Their Solutions
A fundamental characteristic of any theory is its ability to explain actual phenomena. This holds equally true for cosmology; hypotheses must account for the phenomena observed in the universe. The three problems I present below are precisely the items demanding such explanations.
The first problem is the horizon problem. Understanding this requires comprehension of the cosmic microwave background radiation. Cosmic background radiation refers to light uniformly distributed throughout the entire universe, characterized by its extremely low temperature. The fact that the universe is expanding has been proven by Hubble’s law. Therefore, if a single wave formed in the universe and encountered no collisions, it would continue moving indefinitely. During this process, as the space of the universe expands, the length of the wave, or its wavelength, increases. We know that for the same amplitude, the higher the frequency, the greater the energy. Therefore, shorter wavelengths carry higher energy, while longer wavelengths have lower energy. Consequently, waves originating in the universe lose energy over time, resulting in extremely low temperatures. This is precisely what cosmic background radiation is. The existence of cosmic background radiation has already been confirmed through observation.
The horizon problem stems from this. Since the first light would have had the lowest temperature wavelength, the cosmic microwave background radiation we observe is the result of light traveling at the speed of light from the far reaches of the universe. However, light also exists traveling at the speed of light from the opposite direction, and the temperatures of these two signals were measured to be exactly identical. Despite being separated by a distance far greater than the distance light could have traveled during the age of the universe, these two points exhibit identical physical information. This is the horizon problem.
The theory of inflation resolves this problem. The early universe was compressed to a state nearly as small as a single point, allowing information to be shared. Subsequently, rapid expansion caused regions to move apart, yet the initial values remained identical, resulting in the same temperature. Furthermore, the abrupt expansion explains why various variables acting in opposite directions did not have sufficient time to influence each other.
The second problem is the flatness problem. This stems from the requirement to theoretically explain why the current universe is flat. Astrophysicists have already proven through observational data that the universe is indeed flat. Therefore, theory must now explain why this is so.
The inflation theory logically presents the reason why the universe is flat. Scientists currently explain the force causing the universe to contract as gravitational force, or gravity. This means objects tend to pull each other and gradually contract. Conversely, the cause of the universe’s expansion is defined as dark energy. According to the inflation theory, as space expands, dark energy decreases at a rate steeper than the decrease in gravitational force. Therefore, initially, dark energy dominates over gravity, driving expansion. However, as the universe expands, dark energy diminishes more rapidly, leading the two forces to reach a similar ratio. Ultimately, this results in the formation of a flat universe with a constant expansion rate.
The third problem is the magnetic monopole problem. According to theory, the early universe should have contained a vast number of magnetic monopoles. Yet, the fact that they are scarcely observed in the current universe has been pointed out as problematic. The inflation theory explains this as follows: The early universe was so small it could be viewed as a single point, so while the density was high, the total amount of magnetic monopoles was not large. As the universe subsequently expanded to its current size, the density decreased significantly, falling far below observable levels. This process also explains the magnetic monopole problem.
Conclusion
This blog post focused solely on the theory of inflation, but in reality, there are far more diverse theories and a greater abundance of observational results. Therefore, to understand cosmology, it is necessary to carefully examine the new observations and theories that will continue to emerge. Cosmology is a discipline that grows based on constantly changing data, demanding continuous attention and exploration.
