If you are living in one of those villages or towns where the sky is not polluted by artificial light, you can spot the arms of our home galaxy, the Milky Way. Our universe is filled with many wonders, from invisible dark matter to scorching stars, from tiny dust particles to massive galaxies; our universe encompasses them all. Our universe is what we call our home. This article gives a comprehensive overview of our universe’s presumed origin and fate and its constituents.
THE ORIGIN OF OUR UNIVERSE
The Big Bang Theory
Several theories attempt to elucidate the origins of our universe. Many scientists have been trying to develop the idea that accurately describes how our universe came into existence. The most popular and widely accepted theory that explains the genesis of our universe is the big bang theory. According to this theory, more than 13.7 billion years ago, the universe started as a scorching, dense and tiny ball, only a few millimetres across and exploded violently to form space and time as we know it.
Our universe continues to expand and cool down to this day. This rapid expansion of our universe occurred in radiation and matter eras, from a small dense point to infinitesimally vast enough to encompass billions of galaxies and countless stars.
As soon as expansion started from the size of a point, there was a tremendous amount of radiation scattered everywhere. As radiation was ubiquitous in this era, it was known as the radiation era. Space and time were all distorted, and laws of physics did not govern the universe. This era is classified into periods and is spanned for tens of thousands of years.
The four primary forces of nature(electromagnetic, strong nuclear, gravitational and weak nuclear forces) that govern everything today were unified as one superforce, and there was no matter as the temperature was too hot for that to happen(10⁴⁰ K). At the end of this epoch, gravity separated from this bracket of forces.
Grand unification epoch
In the next period (10⁻⁴³ seconds), the universe cooled down to 10³⁶ K, and the strong nuclear force separated from the superforce.
After 10⁻³⁷ seconds, the universe cooled down to 10³³ K, and meteoric expansion of the universe took place almost instantly. Even at the end of this period, the universe was extremely hot and comprised of quarks, electrons etc. This rapid expansion called cosmic inflation occurred tremendously, and there was no light speed barrier.
Approximately at 10 ⁻³⁶ seconds, the electromagnetic force separated from superforce and the four fundamental forces of nature were formed.
In this stage, the expansion stalled briefly, and the universe reheated again to temperatures enough to produce quark-gluon plasma and other elementary particles. Interestingly, the temperature was so hot that these elementary particles constantly collided at tremendously high speeds leading to a reaction called baryogenesis. Physicists think that this phenomenon might be the reason behind the predominance of matter over anti-matter in our universe.
The universe cooled down further to 10¹⁰ K, and its volume increased. The temperature lowered enough to form baryons such as neutrons and protons from quarks and gluons. As fewer antiquarks were present before to form antibaryons, the temperature was no longer hot enough to develop new particle-antiparticle pairs. So, we observe more particles over anti-particles.
About 1 second after the big bang, the temperature cooled down to 10¹⁰ K, and electrons and positrons underwent a similar type of phenomenon as before.
In the last epoch, just after a few minutes, when the universe was cooling down further to 10⁹ K, neutrons and protons combined to form helium nuclei.
In this era, after the formation of helium nuclei, the conditions were favorable for the formation of other atoms. For several hundreds of years, this process went on. Electrons and nuclei combined to form hydrogen atoms. Hydrogen is the most abundant element in the universe. When gravity collected enough matter, it formed nebulae, and stars were finally created.
Hydrogen and helium form an integral part of nebulae, stars and galaxies we observe today. Our solar system was formed 4.6 billion years ago. Over the course of spanning several millions of years, Planets were created in our solar system, and life on earth evolved.
Alternate theories of Universe creation
Cosmologists believe in several alternative theories to the big bang theory to explain the genesis of our universe. The most popular of them is the steady-state theory.
As per this theory, the density of our universe remains constant indefinitely and maintains perfect homogeneity everywhere. The universe has no beginning or an end. But many scientists discard this theory because of the discovery of cosmic microscopic background(CMB) in the 1960s by American astronomers Arno Penzias and Robert Wilson. CMB is the oldest form of electromagnetic radiation formed during the recombination of electrons and nuclei forming hydrogen atoms. This evidence acts as a significant proof for the big bang model of universe creation and disproved the steady-state model.
Also, Edwin Hubble observed that galaxies are moving away from our planet at speeds proportional to their distances from earth(also known as Hubble’s Law), which proved contrary to the assumptions made by the steady-state model. At the same time, Hubble’s observations validated the big bang model of the universe’s origin.
OUR UNIVERSE: INTERESTING FACTS
Some interesting facts about the observable universe:
1) Our universe is assumed to be 13.7 billion years old.
2) Size of the observable universe is diminutive compared to the actual size of our universe. The actual universe is assumed to be infinitesimally big.
3) There are more than 100 billion galaxies in the observable universe.
4) Various models suggest our universe might be flat and homogenous.
5) The universe is continuously expanding, and the expansion rate is increasing every day.
6) Stars, galaxies, planets and other matter constitute a small portion of the universe. The invisible matter is called dark matter, and dark energy includes the majority.
CONSTITUENTS OF OUR UNIVERSE
Our universe is made of normal matter, dark matter and dark energy.
Normal matter includes stars, galaxies, and other objects that are baryonic in nature.
A star is a gigantic ball of hot gases. There are billions and billions of stars of varying sizes and masses in our universe. Numerous fusion reactions occur inside a star’s core to release an enormous amount of energy in a single second, counteracting gravity acting inwards. A star starts its lifecycle as large clouds of gas and dust called a nebula undergoes several transformations and ends its life as a white dwarf, a neutron star, or a black hole. Usually, stars are grouped into constellations or asterisms. A constellation is a collection of stars that appear to have a pattern. Some popular ones are the Orion, the Leo, the Taurus constellation.
A galaxy is a massive group of stars and their systems, gas, dust, held together by the force of gravity. Our solar system is located in a massive galaxy called the Milky Way. Our Milky Way has a diameter of approximately 100,000 to 200,000 light-years. Every enormous galaxy is thought to contain a supermassive black hole. Galaxies come in various shapes and sizes. Hubble came up with a famous classification of galaxies called the Hubble sequence, in which he grouped galaxies into four classes
1) Elliptical galaxies: They are elliptical and have smooth brightness, decreasing from the centre.
2) Spiral galaxies: They have two arms of spiral shape and a flattened disk at the centre.
3) Barred-spiral galaxies: It has a central bulge but no discernable spiral arms. They exhibit the characteristics of both elliptical and spiral galaxies. Our Milky Way and its neighbour Andromeda galaxies are well-known examples of this category.
4) Irregular galaxies: They have no specific structure and are highly irregular in shape.
Similar to stars, a group of galaxies is called a cluster. At the same time, a group of similar clusters is called a supercluster. Our Milky Way is part of the Local Group, which is a part of Virgo Supercluster.
A black hole is a point of infinite density, so dense that even light can not escape. When enough mass is concentrated in a compact area, the density becomes so immense that it bends spacetime. At the end of its lifecycle, a massive star collapses to form a point of infinite density. Black holes are of two types:
1) Stellar black holes
2) Supermassive black holes
Stellar black holes are created during the final stage of a star’s lifecycle. A Supermassive black hole is formed after a stellar black hole devours its surrounding material.
Dark matter and dark energy
Due to the matter present in the universe, scientists thought that the universe had to recollapse due to inward pulling gravitational force. The rate of acceleration has to decrease as time goes on. But, what is happening confounded many theorists. The universe is not collapsing or slowing down; in fact, it’s growing at a rapid pace. To explain this strange phenomenon, scientists have come up with an invisible matter and energy that permeates throughout the universe.
As per the ACDM model, Dark energy is assumed to constitute around 68% of total energy and mass in our universe. It is supposed to have two forms: energy is filled homogeneously throughout the space called the cosmological constant, the other being the variation of energy in space and time.
Einstein predicted that space is filled with invisible energy and contributes to space expansion. In 2011, three scientists, Saul Perlmutter, Brian Schmidt and Adam Reiss, were awarded the Nobel prize in physics to discover the accelerating universe.
It is a hypothetical matter that pervades in space. Much of the properties of dark matter is a mystery in physics. Dark matter is not detected directly, and dark matter particles are non-baryonic in nature. Based on several observations, such as gravitational lensing of distant galaxies, the discrepancy in the rotational curves of spiral galaxies, interaction with CMB, scientists assume that such non-luminous matter exists.
THE FATE OF OUR UNIVERSE
The fate of our universe is assumed to be dependent on its shape.
1) In case 1, if our universe has a closed spherical geometry, the force of gravity halts the expansion process and starts pulling everything inward, eventually collapsing into a singularity. Here, our universe experiences Big Crunch. This phenomenon follows the big bang model of genesis for the next universe. This process repeatedly repeats as a cycle.
2) In case 2, if our universe is open or flat and assumes the shape of a hyperbola, the expansion continues forever, even after gravity tries to slow the process down and the dark energy accelerates the expansion process. The outcome of this process is that the universe will reach thermal equilibrium, and it will cool down to absolute zero. No fuel for star formation, so no new stars are formed. Only Black holes are left behind, even they are annihilated due to hawking radiation, and our universe will approach Heat Death. Our universe may also experience Big Drop, where our universe expands forever, and every constituent in it gets torn apart until the distance between the particles is infinite.