Unit 12: Universe

The Big Bang Theory, first proposed by Georges Lemaitre in 1927, states that the universe began from a single, extremely hot and dense point called a singularity about 13.8 billion years ago. The entire universe was compressed into this singularity. Then an explosion known as the Big Bang took place. After this explosion, space, time, and matter began to expand and form the vast universe we observe today. It should be noted that the Big Bang was not an explosion in the usual sense — it was rather a rapid expansion of space. As the universe expanded, it cooled, allowing particles to combine into atoms, atoms into stars, and stars into galaxies. Even today, the universe continues to expand, and galaxies are moving farther apart as time passes.

Hubble's Law states that "The velocities at which galaxies are moving apart is directly proportional to the distance between them."

Mathematically: v ∝ d, or v = Hd, where v is the receding velocity of a galaxy, d is the distance between them, and H is Hubble's constant.

Hubble's constant of 73 km/s/Mpc means that for each megaparsec (Mpc) increase in distance from Earth, a galaxy's receding speed increases by 73 km/s. In other words, a galaxy that is 1 Mpc away recedes at 73 km/s, a galaxy 2 Mpc away recedes at 146 km/s, and so on. This shows that more distant galaxies are moving away from us faster, confirming that the universe is expanding.

The expansion of the universe is caused by the outward force of expansion, which always tends to push galaxies farther apart. This outward force is mainly due to two factors: (1) the conditions created during the Big Bang, and (2) a mysterious energy present in space called dark energy. Together, these factors drive the continuous expansion of the universe.

The expansion of the universe is controlled by the inward pull of gravity. Gravity, which is due to the matter present in the universe, tends to slow down and even reverse the expansion. It acts like the cosmic brake to the expansion. The strength of gravity depends on the average density of the universe — the higher the density, the stronger the gravitational pull and the greater its ability to slow or stop expansion.

The universe is expected to have a saddle shape (hyperbolic, negative curvature) when its average density is less than the critical density. In this condition, an open universe is formed. In an open universe, the expansion dominates over gravity, causing space itself to curve negatively and take on a saddle-shaped geometry. In such geometry, the angles of a triangle drawn on the surface of the universe add up to less than 180°.

Universe A (average density is more than the critical density) leads to the Big Crunch. When the average density exceeds the critical density, a closed universe is formed. In a closed universe, gravity is strong enough to stop and eventually reverse the expansion. Galaxies start moving closer together, and the universe grows hotter and denser, ultimately collapsing into an extremely dense point — the Big Crunch. This final state is also called the omega point.

The chapter defines critical density (ρ_c) as the value of average density at which gravity can just stop the expansion of the universe. It is the boundary density that separates the three possible futures of the universe — below it the universe is open, equal to it the universe is flat, and above it the universe is closed. The chapter does not state a specific numerical value for critical density beyond this definition.

The sum of internal angles of a triangle drawn on the surface of the universe depends on the type of universe:

If every celestial object is alone and stationary, this describes the final end state of an open universe, known as the Big Chill or heat death. In an open universe, galaxies drift farther apart over billions of years, stars gradually burn out, and as matter and energy spread over an ever-larger volume, the universe becomes extremely cold and almost empty — a state where each object is essentially isolated and all motion ceases.

Both the open universe and the flat universe expand forever. In an open universe (average density less than critical density), gravity is too weak to stop expansion, so the universe expands forever at an ever-increasing rate. In a flat universe (average density equal to critical density), gravity exactly balances expansion so the universe also expands forever, but the rate of expansion gradually slows down and approaches zero asymptotically.

The interstellar space contains clouds of gas and dust. Gravity slowly pulls these particles together, forming clumps that grow larger and denser over a long period of time. These clumps eventually become stars, planets, moons, and asteroids depending on their mass. Thus, gravity is the creative force behind the formation of all celestial bodies.

The stars in a galaxy are held together by gravity. Each star exerts a gravitational pull on the other stars and on the supermassive black hole at the center of the galaxy. The combined effect of these gravitational forces prevents stars from scattering and maintains the structure of the galaxy. Gravity also pulls galaxies toward one another to create galaxy clusters and superclusters.

Millions of stars are grouped together in a galaxy because of gravitational force. Each star in a galaxy exerts a gravitational pull on every other star and on the supermassive black hole at the center of the galaxy. The combined effect of these gravitational forces prevents the stars from scattering into empty space and holds them together as a structured system. Without gravity, stars would drift apart and no galaxies would exist.

When Hubble plotted the distances of galaxies from Earth against their recession velocities, he observed that the points roughly formed a straight line passing through the origin. This showed a directly proportional relationship between the distance and the recession velocity of galaxies. From this, he concluded that the farther a galaxy is, the faster it is moving away from us, meaning the universe is expanding. Based on these observations, he formulated Hubble's Law in 1929: v = Hd.

The interstellar space contains clouds of gas and dust. Gravity slowly pulls these particles together, forming clumps that grow larger and denser over a long period of time. These clumps eventually become stars, planets, moons, and asteroids depending on their mass. Without gravity, these particles would remain scattered and no celestial bodies would ever form. Thus, gravity is the creative force behind the formation of all celestial bodies.

Gravity holds the stars in a galaxy together by preventing them from scattering, thereby maintaining the structure of each galaxy. It also pulls galaxies toward one another to create galaxy clusters and superclusters. On the largest scales, gravity shapes the entire web-like structure of the universe. Without gravity, galaxies would not exist as organised systems and the large-scale structure of the universe would not take shape.

Gravity pulls together gas clouds in a nebula to form a star, triggering nuclear fusion in its core. Throughout a star's life, the balance between gravity (pulling inward) and the pressure from nuclear fusion (pushing outward) determines the star's stability and size. When a star runs out of hydrogen fuel, gravity causes it to expand into a giant, then explode or collapse into a dense object such as a neutron star or black hole. Thus, gravity determines both the birth and death of stars.

The light from distant galaxies is shifted toward the red end of the spectrum, which indicates that those galaxies are moving away from us. This red shift directly shows that the universe is expanding. If we track this expansion backward in time, it means that all the galaxies and matter were once packed together in a very small, hot, and dense point — which is precisely what the Big Bang theory proposes. Therefore, the observation of redshift is a key piece of evidence supporting the Big Bang.

The inward pull of gravity tends to slow down and even reverse the expansion of the universe. Just as a brake slows a moving vehicle, gravity acts against the outward force of expansion by pulling matter back together. The stronger the average density of the universe, the stronger gravity's braking effect. This opposing force is why gravity is referred to as the cosmic brake in the process of expansion of the universe.

In an open universe, the average density of the universe is less than the critical density. As a result, gravity is too weak to stop the outward force of expansion. Since the expansion force dominates over gravity, nothing can halt or reverse the expansion. Therefore, an open universe continues to expand forever.

In an open universe, the expansion continues indefinitely. Over billions of years, galaxies drift farther apart and stars gradually burn out. As matter and energy spread over an ever-larger volume, the universe becomes colder and darker. Eventually, it may reach a state called the Big Chill or heat death, where the universe is extremely cold and almost empty. Therefore, an open universe may ultimately experience the Big Chill.

In a closed universe, the average density is greater than the critical density, so gravity is strong enough to slow down, stop, and eventually reverse the expansion. After reaching a maximum size, galaxies start moving closer together and the universe grows hotter and denser once again. This contraction may finally end in a state called the Big Crunch, where all matter and energy collapse into an extremely dense point similar to the initial singularity. Therefore, a closed universe may ultimately end in a Big Crunch.

In a flat universe, the average density exactly equals the critical density. Gravity exactly balances the expansion force, so the expansion is never fully halted but only gradually slowed. The rate of expansion decreases over time and approaches zero but never actually reaches zero or reverses. Therefore, a flat universe will continue to expand forever, though at an ever-decreasing rate.

Gravitational force is the most fundamental force shaping the universe at every scale. Its importance can be understood through the following roles:

The interstellar space contains clouds of gas and dust. Gravity slowly pulls these particles together, forming clumps that grow larger and denser over a long period of time. These clumps eventually become stars, planets, moons, and asteroids depending on their mass. Thus, gravity is the creative force behind the formation of all celestial bodies.

The stars in a galaxy are held together by gravity. Each star exerts a gravitational pull on the other stars and on the supermassive black hole at the center of the galaxy. The combined effect of these gravitational forces prevents stars from scattering and maintains the structure of the galaxy. Gravity also pulls galaxies toward one another to create galaxy clusters and superclusters.

Gravitation keeps moons, planets, and stars in their respective orbits. In our solar system, the Sun's gravitational pull holds all the planets, satellites, asteroids, and comets in their paths. As they move in circular orbits, they also experience a centrifugal force that acts outward. The balance between mutual gravitational attraction and the outward centrifugal force prevents them from escaping into outer space or colliding with one another.

Gravity controls the entire life cycle of stars. It pulls together gas clouds in a nebula to form a star where nuclear fusion begins. When a star runs out of hydrogen fuel, gravity causes it to expand, explode, or collapse into a dense object such as a neutron star or black hole. Thus, gravity determines both the birth and death of stars.

When gravity becomes extremely strong, it forms a black hole — a region of space from which nothing, not even light, can escape. Black holes are formed after the death of supermassive stars when gravity overwhelms all other forces and causes an extreme collapse of matter.

In summary, gravitational force is the primary organiser of the universe — from the birth of individual stars to the large-scale structure of the cosmos and the ultimate fate of the universe itself.

Among various hypotheses about the origin of the universe, the Big Bang Theory is the most widely accepted scientific explanation. It was first proposed by Georges Lemaitre, a Belgian physicist, in 1927. The term "Big Bang" was coined later, in 1949.

According to this theory, about 13.8 billion years ago, the entire universe was compressed into a singularity — an infinitely dense and extremely hot point where all matter, energy, space, and time were concentrated. There was no space or time as we know it before this point.

Then, an event known as the Big Bang took place. It should be noted that the Big Bang was not an explosion in the usual sense of the word. It was rather a rapid expansion of space itself. Space, time, and matter all began to expand outward from the singularity.

As the universe expanded, it cooled. This allowed particles to combine into atoms, atoms into stars, and stars into galaxies. During the first few minutes after the Big Bang, it was hot and dense enough for nuclear fusion to occur, producing the lightest elements — mainly hydrogen and helium, along with tiny amounts of lithium. Even today, the proportions of these elements in the universe closely match the predictions of the Big Bang theory.

Even today, the universe continues to expand. Galaxies are moving farther apart as time passes. Edwin Hubble confirmed this in 1929 by observing that the light from distant galaxies is redshifted, showing they are moving away from us — providing direct observational evidence for the Big Bang.

The following are the main evidences which support the Big Bang Theory:

In 1929, Edwin Hubble discovered that galaxies are moving away from us, showing that the universe is expanding. By using a powerful spectrograph at the Mount Wilson Observatory, he observed that the light from almost all galaxies was redshifted. When he plotted the distances of galaxies against their recession velocities, he found a directly proportional relationship, confirming that the universe is expanding in all directions. If we track this expansion backward in time, all the galaxies and matter were once packed together in a very small, hot, and dense point — directly supporting the Big Bang theory.

According to the Big Bang theory, during the first few minutes after the universe began, it was hot and dense enough for nuclear fusion to occur. These reactions produced the lightest elements — mainly hydrogen and helium, along with tiny amounts of lithium. Even today, the proportions of these elements in the universe closely match the predictions of the Big Bang theory. If the Big Bang had not occurred, the universe would not contain these elements in the ratios we observe.

The light from distant galaxies is shifted toward the red end of the spectrum, indicating that they are moving away from us. This phenomenon is called redshift. It occurs because, as a galaxy moves away, its light waves stretch toward the red end of the spectrum — similar to how a siren sounds lower as it moves away from us. This red shift directly shows that the universe is expanding, which is a key piece of evidence for the Big Bang Theory.

Universe A (open universe) lasts longer. Its curve continues to rise indefinitely, meaning it expands forever. Universe C (closed universe) has its expansion reversed and eventually collapses, ending at the omega point. Universe B (flat universe) also expands forever but its expansion rate slows, while Universe A expands at an ever-increasing rate — so Universe A has no endpoint and therefore lasts the longest.

In Universe B (the flat universe), the average density is exactly equal to the critical density. In this condition, gravity exactly balances the expansion force. The expansion slows gradually over time, approaching zero, but never actually stops or reverses. This is the boundary condition between the open and closed universes.

Universe C (closed universe) will experience the Big Crunch. This is because in Universe C, the average density is greater than the critical density. As a result, gravity is strong enough to slow down, stop, and reverse the expansion. After reaching its maximum size, the universe begins to contract — galaxies move closer together, and the universe grows hotter and denser. This contraction ends in the Big Crunch, where all matter and energy collapse into an extremely dense point (the omega point), similar to the initial singularity of the Big Bang.

This condition corresponds to shape C — the flat universe. In a flat universe, the average density is exactly equal to the critical density, so gravity exactly balances the expansion. The universe continues to expand forever, but the rate of expansion decreases over time and approaches zero asymptotically. Over billions of years, stars burn out and matter spreads over an ever-larger volume, causing the universe to become extremely cold and almost empty, while the expansion has nearly (but never fully) stopped.

The reversal of the Big Bang process is expected in shape A — the closed universe (spherical shape). In a closed universe, the average density of the universe is greater than the critical density. As a result, gravity is strong enough to slow down and eventually stop the expansion. After reaching a maximum size, the universe begins to contract under its own gravity. Galaxies start moving closer together, and the universe grows hotter and denser — reversing the conditions that followed the Big Bang. This contraction may finally end in a state called the Big Crunch, where all matter and energy collapse into an extremely dense point (the omega point), which is similar to the initial singularity from which the Big Bang originated. This entire process is considered a reversal of the Big Bang.