The prevailing theory that describes the origin and evolution of the universe is known as the Big Bang theory. According to this theory, the universe began as a hot, dense, and extremely small singularity around 13.8 billion years ago. The Big Bang theory is supported by a wide range of observational evidence and has become the standard cosmological model.
Here is a simplified overview of the prevailing theory of how the Big Bang played out:
Singularity: The universe was in an incredibly hot and dense state, with all matter and energy concentrated into an infinitesimally small point called a singularity. At this point, the laws of physics as we currently understand them cease to be applicable.
Expansion: The universe began to rapidly expand and cool down, undergoing a phase of exponential expansion known as cosmic inflation. During inflation, the universe grew from an incredibly small size to a size many times larger than the observable universe in a fraction of a second.
Formation of Elementary Particles: As the universe continued to cool, the energy present in the early universe started to convert into various elementary particles, such as quarks, electrons, and neutrinos.
Nucleosynthesis: A few minutes after the Big Bang, the temperature dropped enough for protons and neutrons to combine through nuclear reactions, resulting in the formation of light atomic nuclei like hydrogen, helium, and a trace amount of lithium. This process is known as nucleosynthesis.
Matter-Dominated Era: After nucleosynthesis, the universe entered a phase where matter (including both elementary particles and the atomic nuclei formed) dominated the energy content. Over time, the universe continued to expand and cool, allowing matter to clump together due to gravitational attraction.
Formation of Cosmic Structures: As matter clumped together under gravity, regions of higher density formed, eventually leading to the formation of galaxies, stars, and other cosmic structures. Gravity played a crucial role in the growth of these structures, shaping the large-scale distribution of matter we observe today.
Cosmic Microwave Background (CMB): About 380,000 years after the Big Bang, the universe had cooled sufficiently for electrons to combine with protons, forming neutral atoms. This allowed light to travel freely, resulting in the release of a vast amount of energy in the form of cosmic microwave background radiation. The CMB is a faint, uniform glow that permeates the entire universe and serves as an important source of information about the early universe.
Dark Energy and Expansion Acceleration: Observations made in the late 20th century revealed that the expansion of the universe is accelerating. This acceleration is thought to be caused by an enigmatic form of energy called dark energy, which constitutes a significant portion of the universe's total energy content.
The Big Bang theory, along with subsequent developments in cosmology, provides a comprehensive framework for understanding the origin, evolution, and large-scale structure of the universe. However, there are still open questions and areas of ongoing research, such as the nature of dark matter and dark energy or the precise details of inflation, which scientists are actively investigating.