Updated: Aug 4
Author: Sharika Dhakappa
The Big Bang is the most widely accepted theory of how the universe originated. Most physicists believe that the tremendously large universe we observe today began as a tiny, dense point. If the evolution of the universe till today were to be depicted as a movie, the Big Bang would be the beginning of it. We do not yet know what came before the Big Bang or whether that is even a meaningful question to ask. The cosmic movie would run for 13.8 billion years which is the current age of the universe as estimated by the WMAP satellite.
When we think of the Big Bang, we often imagine an enormous explosion. The Big Bang, however, was not an explosion but rather an expansion of space itself with time. Space and time were created in the Big Bang and hence it does not make sense to ask where the Big Bang occurred.
The theory of the Big Bang was born when a Belgian priest named Georges Lemaitre theorized that the universe came from a single primordial atom. It gained support when Hubble observed that galaxies are moving away from us at speeds proportional to their distances from us which means that distant galaxies are moving away faster - the universe is expanding. And if it is expanding, it must have been smaller in the past. It must have been concentrated in a single infinitely dense point called a singularity.
This does not mean that we are at the center of the universe. A common example is that of a raisin bread. The raisins are analogous to the galaxies while the dough represents the space between them. When the dough expands on cooking, each raisin appears to be moving away from every other raisin.
The Beginning of Everything
The cosmic movie begins with a hot, dense universe in which the four fundamental forces today: the gravitational, electromagnetic, strong and weak nuclear forces are united as a single force. The expansion of the universe begins at time t = 0 and progresses rapidly. In only 10^(-43) seconds, the universe has grown 10^(-35) meters across. This era called the Planck era is characterized by extreme conditions. The expansion cools the universe, but the universe is still at a temperature of 10^(32) Kelvin (i.e., 1 followed by 32 zeros!) by the end of the Planck era. Our current understanding about the Planck era is limited.
The period that follows immediately after is called the Grand Unification epoch, when one of the unified forces - gravity separates out from the electronuclear forces. Physical attributes such as mass and charge are still meaningless. By the end of this epoch, the temperature of the universe has dropped by a factor of a hundred thousand Kelvin.
One by one the initially unified forces separate out. This time it is the strong force that leaves the electromagnetic and weak force unified into an electroweak force. This electroweak epoch marks the beginning of the formation of W and Z bosons. These bosons are fundamental particles that are together responsible for the weak force (“fundamental” meaning they are indivisible).
Ten billionths of a second after the Big Bang, W and Z bosons are no longer being created and they decay away. The electromagnetic and weak forces separate out finally and particles are created. The universe is a sea of quarks, antiquarks, leptons, and other fundamental particles. Quarks are the basic building blocks of protons and neutrons, and antiquarks are antiparticles of quarks (i.e., particles that have some properties similar to those of quarks in magnitude but have the opposite signs).
The interaction of quarks with antiquarks releases energy. If the number of quarks and antiquarks are equal, they annihilate each other and no more of these particles are left behind. But for some reason that we do not know yet, some quarks were left behind indicating the number of quarks had been greater than the number of antiquarks.
The particle era ends at t = 1 millisecond and the universe is no longer hot enough to create more quarks. The quarks are drawn together by the strong nuclear force to form hadrons. Hadrons are just subatomic particles made of two or more quarks. The most familiar hadrons are protons and neutrons which both contain three quarks each.
The process of annihilation repeats itself first with hadron-antihadron pairs and then at t = 1 second with electron-positron pairs (positrons are the antiparticles of electrons). Again, electrons are left behind because of some mysterious asymmetry.
Protons combine with other protons and neutrons forming the first atomic nuclei and eventually lighter atoms such as hydrogen, helium and trace quantities of lithium, deuterium, and tritium. Then, for around another 400,000 years some free electrons continue to wander in the universe until it cools to 3000 Kelvin at which the electrons combine with atomic nuclei and complete the formation of atoms. In this process of “recombination”, light is emitted, and this light is detectable even today as we shall see later.
The universe goes on expanding and cooling. Eventually lumps of matter form and gravitate towards each other. Galaxies are formed, each containing hundreds of billions of stars powered by thermonuclear reactions in their cores. Each star at the end of its life explodes scattering different elements in space.
The Birth of the Solar System
Throughout all these events, interstellar dust and hydrogen gas accumulates to form a structure called a molecular cloud. Somewhere in the Milky Way galaxy, a little over 9 billion years after the Big Bang, fragmentation of one such cloud led to the formation of a protostar which would later develop into the Sun as we know it. In its accretion phase, some of the dust and ice particles collided to form clumps which eventually developed into planetesimals.
The Sun’s heat pushed volatiles such as water and ammonia slightly away in a zone where they cooled enough to condense and accrete until they formed four rocky planets: Mercury, Venus, Mars, and our home planet- the Earth. Cooler regions gave rise to the gas giants- Jupiter, Saturn, Uranus and Neptune. Life arose from the primordial oceans on the surface of the Earth, full of organic and inorganic compounds.
Our cosmic movie up to the present is almost complete, but an important event remains. The Earth in its orbit crashes into an object half as wide as itself. The collision increases Earth's spin, melts its outer layers and debris is flung into orbit around the Earth. Clumps of this debris come together, and the Moon is formed. As much as we admire the Moon, such collisions are disastrous and cause adverse changes in the ecosystem of our planet.
Another such collision that took place some sixty-five million years ago when a massive asteroid hit the Earth and is said to have caused the extinction of the dinosaurs. Again, this was catastrophic, but it allowed our ancestors to fill up vacant niches. These ancestors - the mammals eventually developed into Homo sapiens, their brains gradually developed enough to question the origin of the universe.
The Evidence for the Big Bang
The Big Bang theory is supported majorly by two scientific discoveries. One is Hubble’s observation of galaxies moving away from us leading to the discovery that the universe is expanding. The other was an accidental discovery.
In 1964, at Bell Labs, Arno Penzias and Robert Wilson who were conducting experiments with the Holmdel Horn Antenna discovered a strange buzzing noise coming from all parts of the sky. They tried to get rid of all possible sources of interference that were causing this unexplainable hum but failed to eliminate it. Only then did they realize that they were observing the radiation left over from an early very hot and dense universe- a relic of the Big Bang. With the expansion of the universe over time, this radiation, known as the Cosmic Microwave Background (CMB), cools and is observable currently in microwaves.
Other evidence in favor of the Big Bang theory has been found through the relative abundances of light elements formed in the first few minutes after the Big Bang.
Today, precise and accurate measurements of many of the parameters of the Big Bang model have been made including the age of the universe. We are still not sure of how it will all end- we have our theories but that is a topic for another day. However, we do know that we have a long time before it does. Till then we proceed in our quest to study the evolution of the universe, peering back to the beginning of time with advancing technology and instruments such as the James Webb Space Telescope (JWST).
Neil deGrasse Tyson - Astrophysics for People in a Hurry
The Big Bang and the Expanding Universe - Durham University URL: https://www.icc.dur.ac.uk/~tt/Lectures/Galaxies/LocalGroup/Back/bigbang.html
Origins of the Universe, Explained - National Geographic. URL: https://www.nationalgeographic.com/science/article/origins-of-the-universe
WMAP Big Bang CMB Test, WMAP Age of the Universe - NASA. URL: https://wmap.gsfc.nasa.gov/universe/bb_tests_cmb.html, https://wmap.gsfc.nasa.gov/universe/uni_age.html
The Early Universe - University of Oregon. URL: https://pages.uoregon.edu/jimbrau/astr123/Notes/Chapter27.html
The Start of Everything - University of Arizona. URL: https://ircamera.as.arizona.edu/NatSci102/NatSci102/lectures/eraplanck.htm
Formation of the Solar System: Birth of Worlds - NASA. URL: https://mobile.arc.nasa.gov/public/iexplore/missions/pages/yss/november.html