This blog post examines how the Big Bang theory explains the origin of the universe and why it remains controversial.
We live on Earth. Earth belongs to the solar system, the solar system belongs to our galaxy, and our galaxy belongs to the vast universe. So where did our universe come from? Scientists have proposed many theories to find the beginning and end of this universe. Since Hubble observed the expansion of the universe, the most prominent theory explaining its origin has been George Gamow’s Big Bang theory. According to this theory, the universe began at a single point and underwent repeated rapid expansions, resulting in its current form.
Scientists who first encountered this hypothesis dismissed it as an absurd claim. The name ‘Big Bang’ itself was coined with the intent to mock George Gamow’s idea. Among opposing views, Fred Hoyle’s Steady State Theory gained the most attention. This theory posited that the universe maintains a constant state, independent of space and time, with no beginning or end, and that new matter emerges somewhere to form new universes. However, the discovery of the 2.7K cosmic microwave background radiation, a powerful piece of evidence for the Big Bang theory, led to the abandonment of the Steady State theory. Cosmic microwave background radiation is the microwave radiation emitted when protons and electrons combined in the early universe. The absolute temperature of this microwave radiation gradually decreases over time. Microwave radiation of the same temperature is detected in all directions of the universe, providing strong evidence for the Big Bang theory. Subsequently, theories further solidifying the Big Bang theory continued to emerge. The introduction of the concept of the Planck epoch explained how the four fundamental forces of nature separated after the universe’s birth, and the proven existence of the Higgs boson revealed how fundamental particles like quarks and leptons acquired mass. With the emergence of these supporting theories, humanity has been able to inch closer to understanding the origin of the universe. In this regard, the Big Bang theory is regarded as the most successful theory explaining the universe.
However, as the Big Bang theory expanded its scope, aspects with unclear evidence or difficult-to-grasp concepts emerged. Prime examples are dark energy and dark matter. To explain the density of the universe, scholars calculated its mass and volume. Yet a contradiction was discovered in these calculations. Theoretically, if the calculated mass of the universe was set to 100, the total mass of all observable matter amounted to only 4. Therefore, scientists hypothesized the existence of invisible matter in the universe and named it dark matter. The problem with this theory is that, compared to previous hypotheses, there is no evidence for its existence and no possibility of detection. Although the Big Bang theory explains most currently observed cosmic phenomena well, theories with uncertain foundations, like dark matter, are open to criticism. This article will examine the key concepts of the Big Bang theory and explore its potential criticisms.
First, there are the aforementioned dark matter and dark energy. The biggest problem with these two concepts is that they exist only theoretically. Dark matter is a hypothetical substance devised to solve the cosmic density problem, while dark energy is a hypothetical energy postulated to resolve the force problem caused by dark matter. This issue touches on a core concept of the Big Bang theory. Dark matter possesses mass and thus exchanges gravity with other matter. However, when calculating the total gravity of the entire universe considering all this, a value sufficient to cause the universe to contract is derived. In other words, dark matter leads to a conclusion that contradicts the core concept of the Big Bang theory: the expansion of the universe. Therefore, scientists introduced dark energy, a repulsive force acting on the universe, to maintain its expansion. Yet, evidence supporting either of these concepts still does not exist. Currently, cold dark matter like axions is predicted, but that too is merely a prediction. In other words, dark matter and dark energy are tools devised to explain already observed results, and the biggest problem with these tools is that they are impossible to observe. The term ‘dark’ itself signifies their unobservability. Since concepts introduced to explain the core of the theory are difficult to substantiate, we must be cautious in accepting dark matter and dark energy, the main tenets of Big Bang cosmology.
Next is the problem of the universe’s density. Our universe, following the principle of isotropy, has the same density at every point. This is called the cosmological density. Possible models of the universe within the Big Bang theory include: the ‘Big Freeze’, where the universe expands uncontrollably and eventually cools to a temperature close to absolute zero; the ‘Flat Universe’, which continues to expand indefinitely; and the ‘Big Crunch’, where gravity eventually becomes too strong, causing the universe to contract and ultimately collapse. Which of these states an arbitrary universe will reach depends on its density. If the universe’s density is greater than the critical density, it will undergo a Big Crunch; if it is less than the critical density, it will experience a Big Freeze; and if it is equal to the critical density, it will become a flat universe. The intriguing point is that our universe’s density precisely matches the critical density. Had it deviated from this value, the universe would not have reached its current state and would have perished relatively early. Yet our universe maintains its present form with this exact critical density. The problem is that the probability of any universe coinciding with the critical density is extremely low. Is this merely a coincidence, or does some scientific plausibility exist?
One widely known answer to this is the ‘anthropic principle’. To understand the anthropic principle, one must grasp the concept of ‘multiverse theory’. According to multiverse theory, the universe is not limited to just our own. Like air bubbles rising in water, new universes are constantly being created, and this process repeats infinitely, generating countless universes. These universes possess different densities. Since an infinite number of densities exist, universes with a critical density must inevitably exist. The anthropic principle states that observers existing in such universes must necessarily reside in a universe possessing this critical density.
Because the cosmic density problem addresses the most fundamental question, “Why does it have that value?”, it is difficult to provide a logical answer excluding the anthropic principle. However, the anthropic principle itself has two inherent problems. First, the multiverse theory used in the anthropic principle, like dark matter, lacks evidence. Unlike dark matter, evidence is even harder to find because the laws of nature differ between universes. The multiverse is an argument based solely on imagined properties, making it closer to a philosophical concept. Furthermore, the anthropic principle itself is difficult to regard as a scientific proposition. According to Popper’s theory of science, science must be based on evidence and be falsifiable. However, the anthropic principle states that universes not close to critical density cannot contain observers. Therefore, the proposition that our universe, which contains observers, corresponds to critical density inherently cannot avoid logical fallacy. Nevertheless, these theories are widely accepted because the Big Bang theory successfully explains the universe, and no alternative scientific theory exists to replace it.
We must acknowledge the value of the Big Bang theory for its significant contribution to humanity’s understanding of the cosmos. However, we must not forget to maintain a critical perspective on the limitations of the Big Bang theory and the hypotheses constructed to support it. Science begins with endless questions, is completed through proof, and constantly repeats verification and falsification throughout this process. Therefore, while understanding the Big Bang theory, we must be prepared to ask new questions when we discover areas it cannot address. It is through this attitude that science will advance.
