Origin of the Universe:
The commonly held belief that the Big Bang Theory involved a singular moment of explosive expansion resulting in the formation of stars, galaxies, atoms, and other structures is a misperception of the theory. The theory states that the universe underwent an exponential expansion and that structures formed as temperatures cooled. The Big Bang was not a localized event in space but rather a moment in a time characterized by a hot and dense universe, ubiquitous throughout the universe. Although current scientific understanding cannot explain the existence or cause of the universe, the Big Bang Theory provides a comprehensive explanation for the universe's evolution to its current state, as observed today. This theory is well-supported by evidence, including observations made 13.8 billion years after the moment of the Big Bang.
In the 1980s, the model could not have explained some other observed universe properties. Some of the notable ones are:
The Flatness Problem: The universe seems exceptionally flat, meaning its curvature is nearly zero. This problem is difficult to explain within the framework of the Big Bang theory, which predicts that the universe should either be open (underdense) or closed (overdense), not flat.
When scientists refer to the universe as being "open" or "underdense," they mean that the average density of matter in the universe is less than a critical value so that the overall geometry of the universe is negatively curved, like a saddle. This idea means that the universe's total mass is not enough to cause it to collapse back on itself eventually, and it will continue to expand forever.
When scientists refer to the universe as being "closed" or "overdense," they mean that the average density of matter in the universe is greater than the critical value so that the overall geometry of the universe is positively curved, like a sphere. This idea means that the universe's total mass is enough to cause it to collapse back on itself in a "Big Crunch eventually."
The universe's curvature depends on the balance between the average density of matter and the cosmological constant, representing the energy of the vacuum and acting like a repulsive force. If the average density of matter in the universe is less than the critical value, the universe is open and will continue to expand forever. If the average density of matter in the universe is greater than the critical value, the universe is closed and will eventually collapse. Suppose the average density of matter in the universe is exactly equal to the required value. In that case, the universe is flat and will continue to expand forever, but at a gradually slowing rate.
The Horizon Problem: The cosmic microwave background radiation -faint glow of light that is almost, but not entirely, the same temperature in all directions observed to be uniform in all directions, but this uniformity is difficult to explain if the universe has been expanding for only 14 billion years. The distances between regions of the universe that were in thermal contact at the time of the Big Bang are too large to have been in thermal contact. Cosmic microwave background radiation (CMB) provided a snapshot of the universe when it was only 380,000 years old before the first stars and galaxies had formed. The CMB provides information about the density and distribution of matter in the universe and has been used to determine the universe's age, composition, and evolution.
The Magnetic Monopole Problem: Magnetic monopoles, particles that carry only one magnetic pole, are predicted by some theories of particle physics. However, no magnetic monopoles have been observed, despite extensive searches. This observation challenges the Big Bang theory, which predicts that magnetic monopoles should exist in abundance.
Origin of fluctuations: The origin of fluctuations in the early universe is still a matter of active research. Some theories suggest that they may have been generated by quantum fluctuations during inflation, while others propose that they seeded topological defects such as cosmic strings or textures.
Amplitude problem: The amplitude problem refers to the difficulty in explaining why the observed fluctuations in the early universe have the amplitude they do.
These problems emerged from the theoretical predictions and observational evidence, like:
Cosmic Microwave Background Radiation (CMB): I have already discussed this.
Large-Scale Structure of the Universe: The concept explains the distribution of galaxies and clusters of galaxies observed in the 1970s and early 1980s using radio and optical telescopes.
The abundance of Light Elements: Observations of the abundance of light elements in the universe, such as hydrogen, helium, and lithium. These observations suggest that the universe was highly homogeneous and isotropic in the early stages of its evolution.
Expansion of the Universe: The discovery made by Edwin Hubble in 1929 describes the universe as expanding by measuring the distances and velocities of distant galaxies.
Cosmic Background Neutrino Detection: This concept explains the background radiation of neutrinos produced in the early universe. Neutrinos are weakly interacting particles that appear to permeate the entire universe.
Some theories try to address the problems from the Big Bang Model. Theories like:
Inflationary Universe Theory: This theory proposes that the universe underwent a rapid expansion in the first fraction of a second after the Big Bang
Dark Matter and Dark Energy: The Big Bang model predicts that visible matter makes up only about 4% of the universe. The rest comprises dark matter and dark energy, which cannot be observed directly but inferred through its gravitational effects.
The Holographic Principle: The principle suggests a holographic universe and all its information encoded on a boundary, such as a cosmic horizon. This theory serves as a solution to the black hole information paradox and other issues in quantum mechanics.
Loop Quantum Gravity: This is a theory of quantum gravity that tries to reconcile the theory of general relativity with quantum mechanics. It suggests that space and time are discrete and not continuous and that the Big Bang was not a singular event but a transition between two quantum states.
String Theory: This theory in theoretical physics proposes that the universe's fundamental building blocks are one-dimensional strings rather than point particles. String theory predicts the existence of extra dimensions and offers a potential solution to the problem of unifying the forces of nature.
And others.
The concept of the "moment" preceding the Big Bang raises questions about the nature of time. In subsequent Weekly Writes, we will delve into various theories that address these issues in the Big Bang Model, including the No Boundary Theory by Stephen Hawking and James Hartle and the Conformal Cyclic Cosmology proposed by Roger Penrose. These discussions aim to understand the current research on the universe's origin comprehensively.