The Big Bang refers to the phenomenon that gave birth to our universe about 15 billion years ago. Briefly, this is what happened. Around 15 billion years ago, there was no such thing as the universe. Instead, there was only a singularity. Our entire universe was compressed within this singularity. At that moment, space and time did not exist. Suddenly, there was an ineffable explosion and the universe came into existence. Now for the details.
Quantum theory suggests that moments after the explosion, the four forces of nature; strong nuclear, weak nuclear, electromagnetic and gravity were combined as one super force. Elementary particles (quarks) begin to bond in trios to form photons, positrons and neutrinos. Their antiparticle counterparts were also formed. Particles and antiparticles collided with each other and were annihilated. However, a few particles avoided annihilation and these make up the majority of our universe today.
During this creation and annihilation of particles, the universe was also expanding at an alarming rate. The universe doubled in size for at least 100 times in a thousandth of a second. The universe at this point was an ionized plasma where matter and radiation were inseparable. The ratio of particles and antiparticles, neutrons and protons is equal. When the universe aged to 100th of a second, old neutrons began to decay on a massive scale. This allows free electrons and protons to combine with other particles. Eventually, the remaining neutrons combine with protons to form deuterium (heavy hydrogen). These deuterium nuclei combine with protons to form helium nuclei. Due to these collisions and annihilations, matter was unable to remain stable for more than a few nanoseconds before being scattered by electrons.
At this point, radiation is so dense that no light is visible. The temperature dropped to 1013K. Strong Nuclear, Weak Nuclear and Electromagnetic interactions are now able to exert their force. One second after the explosion, the temperature dropped to 10 billion degrees and photons no longer have the energy to disrupt the creation of matter or to transform energy into matter. After 3 minutes, the temperature becomes 1 billion degrees. Protons and neutrons slowed down enough to allow nucleosynthesis to take place. Atomic nuclei of helium was produced. For the next 300,000 years, the universe continue to expand and cool to 10,000K. Helium nuclei was then able to absorb free floating electrons and form helium atoms. Meanwhile, hydrogen atoms were bonding together and forming lithium. At this moment, photons that were trapped in matter became free form each other and light can be seen.
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