The universe began approximately 13.8 billion years ago with an event called the Big Bang. According to this scientific model, the universe started as an extremely hot, dense point - often described as a singularity. This initial state underwent a rapid and massive expansion. As the universe expanded, it began to cool, allowing the formation of fundamental particles, which would eventually form all the matter we see today. This expansion continues even now, with galaxies moving away from each other as space itself stretches.
As the universe expanded and cooled after the Big Bang, it went through several critical phases of particle formation. Within the first fraction of a second, fundamental particles like quarks and electrons formed from pure energy. By one second after the Big Bang, quarks combined to form protons and neutrons. About three minutes later, protons and neutrons fused to create the first atomic nuclei, primarily hydrogen and helium. However, the universe remained too hot for electrons to combine with these nuclei. It took approximately 380,000 years for the universe to cool enough for electrons to be captured by nuclei, forming the first neutral atoms. This event, known as recombination, allowed light to travel freely through space for the first time.
After the formation of the first atoms, gravity began to play a crucial role in shaping the universe. Vast clouds of hydrogen and helium gas started to clump together in regions of slightly higher density. As these gas clouds grew more massive, they began to collapse under their own gravity. The densest regions at the centers of these clouds became hot enough to trigger nuclear fusion, giving birth to the first stars about 200 million years after the Big Bang. Stars then grouped together through gravitational attraction to form the first galaxies around 1 billion years after the Big Bang. Over time, galaxies themselves clustered together, forming galaxy clusters and even larger structures called superclusters. Inside stars, nuclear fusion created heavier elements like carbon, oxygen, and iron - the building blocks necessary for planets and eventually life. When massive stars exploded as supernovae, they scattered these elements throughout space, enriching future generations of stars and planets.
Two major pieces of evidence strongly support the Big Bang theory. First is the Cosmic Microwave Background radiation, or CMB. This is essentially the afterglow of the Big Bang itself - a faint radiation that permeates the entire universe. Discovered in 1965, the CMB is remarkably uniform in all directions, with only tiny temperature variations of about 1 part in 100,000. These variations represent the seeds that eventually grew into galaxies and galaxy clusters. The second key evidence is the expansion of the universe. In the 1920s, astronomer Edwin Hubble discovered that galaxies are moving away from us in all directions. Moreover, the farther a galaxy is from us, the faster it appears to be receding - a relationship now known as Hubble's Law. This observation led to the realization that space itself is expanding, stretching like the surface of an inflating balloon. Tracing this expansion backward in time leads inevitably to the Big Bang - a moment when all matter and energy were compressed into an incredibly hot, dense state.
To summarize what we've learned about the beginning of the universe: The Big Bang theory states that our universe began approximately 13.8 billion years ago from an extremely hot, dense state that rapidly expanded. This wasn't an explosion in space, but rather an expansion of space itself. As the universe expanded and cooled, fundamental particles formed within the first second, followed by the first atomic nuclei within minutes. It took about 380,000 years for the first atoms to form, allowing light to travel freely through space for the first time - creating the Cosmic Microwave Background radiation we can still detect today. The first stars appeared around 200 million years after the Big Bang, followed by galaxies about 1 billion years later. The universe continues to expand today, with galaxies moving away from each other at an accelerating rate. While the Big Bang theory is supported by substantial evidence, our understanding continues to evolve as new observations and theories develop, particularly regarding what might have existed before the Big Bang and the nature of dark energy driving the universe's accelerating expansion.