Chapter 12 Universe

Introduction to the Universe

The vast area around us is called the universe. It contains many celestial bodies, including planets, stars, comets, satellites, meteors, meteorites, asteroids, etc. A group of many stars forms a galaxy. Our Solar System is located in the Milky Way galaxy.

The universe is a huge region. Science has not yet been able to obtain concrete information about the size, origin, and spread of the universe. There are some very large celestial bodies with very large mass, as well as some very small celestial bodies with less mass. Some are in a gaseous state, while others are in a solid state.

Due to the gravitational force of attraction, every celestial body, including planets, satellites, and stars, is held in its position. The collective name for all these celestial bodies is the universe. Astronomers at different times have proposed various theories about the origin of the universe. Among them, the Big Bang theory is considered the most reliable. According to this theory, all celestial bodies in the universe are in motion. Hubble’s study of the movement of these celestial bodies is important. Due to the gravitational force that exists between these moving bodies, different hypotheses such as the open universe, closed universe, and flat universe have been proposed about the future of the universe.

12.1 Importance of Gravitational Force in the Universe

The Earth is a planet with its own mass, which is why it has gravity. The Sun, Earth, and Moon each have their own gravity. There is a mutual force of attraction between them. The sun attracts the earth towards its center, and the earth also attracts the sun towards its center. Similarly, the Earth attracts the moon towards its center, and the moon also attracts the earth towards its center.

This mutual force of attraction exists among all celestial bodies in space and is called gravitational force. Due to this force, the earth rotates on its axis and revolves in its orbit around the sun. The moon also revolves around the Earth. Similarly, every celestial body in the solar system revolves around the sun. Gravitational force is responsible for maintaining the alignment of the planets in their orbits and making them revolve around the sun. This force prevents planets and satellites from colliding with each other while moving in their orbits.

12.2 Study of the Universe

Universe science is the branch of science that studies various facts about the universe, including its history and future. One of its branches, Astrophysics, studies the origin, structure, and future of the universe.

The Big Bang Theory of the Universe

Our solar system is located at a distance of about 30,000 light-years from the center of a spiral galaxy named the Milky Way, which has a diameter of about 100,000 light-years. It is estimated that there are about 1.5 billion stars in this galaxy alone. While many theories about the origin of the universe have been proposed, the Big Bang theory is considered the most reliable. This theory is believed to have been born from the observation that all galaxies, including the Milky Way, are moving away from each other at tremendous speeds.

According to this theory, the universe is believed to have originated from the big explosion of a single atom. Before the explosion, the four fundamental forces in the universe—gravitational force, electromagnetism, strong nuclear force, and weak nuclear force—are considered to have been unified as a single force. Therefore, in its first stage of existence, the universe is considered to have been in a very compressed and energetic state, the size of a small single atom. Due to excessive force and pressure, a huge explosion of this atom took place, and the universe originated. All celestial bodies in the universe are believed to have originated from this explosion. Just as particles scatter and move away from each other in an explosion, all celestial bodies are moving away from each other after the huge explosion of that dense atom. Therefore, the size of the universe is also increasing day by day. Its expansion is slow but continuous. However, the speed of these bodies is decreasing due to the force of gravity.

Hubble’s Study of the Universe

It has been discovered that the galaxies in space are moving away from each other. In 1929, the American astronomer Edwin Hubble calculated the velocities of various galaxies and discovered that every galaxy is moving away from other galaxies. His estimation showed that the farther the celestial bodies are from the earth, the smaller they appear and the greater their velocity. He plotted the velocities of the constellations against their distances on a graph and explained the relationship, which is known as Hubble’s Law.

This relation is represented by the equation: v = Hd

Where ‘v’ is the velocity with which galaxies are moving away from each other, ‘d’ is the distance between them, and ‘H’ is Hubble’s constant. The value of this constant is 73 km/s/Mpc (Kilometer per second per Megaparsec). This means if any two galaxies are at a distance of one megaparsec, they are separating from each other with a velocity of 73 km/s. It can be known from Hubble’s equation that the farther the galaxies are, the faster they are separating. It is clear that the constellations in the universe are moving away from each other. Therefore, it can be assumed that they were close to each other in the past, which means they were all at the same point a long time ago. Thus, Hubble’s study helps to confirm the Big Bang theory.

12.3 Future of the Universe

All the galaxies in the universe are moving away from each other at a certain velocity, and the size of the universe is increasing. However, due to the gravitational pull of the masses in the universe, the velocity of all galaxies is decreasing. As the distance between bodies increases, the effect of gravity also decreases. The future of the universe depends on whether the velocity of expansion becomes zero before the effect of gravity, the effect of gravity becomes zero before the velocity, or both become zero at the same time.

The net magnitude of the gravitational force depends on the average density of the universe. The average density required to stop the expanding universe is called the critical density.

a) Open Universe: If the celestial bodies retain some velocity when gravity becomes zero, they will continue to move away from each other forever, and the size of the universe will continue to increase. This situation is considered an open universe. This will happen if the average density of the universe is less than the critical density, as gravity cannot stop the expansion process.

b) Flat Universe: If gravity and the velocity of the celestial bodies become zero at the same time, the celestial bodies will remain in the same position, and the universe will remain stable. This state is called a flat universe. This will happen if the average density of the universe is equal to the critical density. The expansion rate will gradually decrease and finally stop, with an infinite distance between each celestial body.

c) Closed Universe: If the gravitational effect remains when the speed of the celestial bodies becomes zero, they will start to come closer, and the universe will contract to a single point. The phenomenon in which the vast universe shrinks and collapses is called the Big Crunch. If this happens, the universe created by the explosion of a single point will end by becoming a point again, potentially exploding to form a new universe. This suggests the universe has a life cycle of expansion and contraction (Big Bang and Big Crunch).

1. Choose the correct option for the following questions:

2. Differentiate between:

(a) Closed Universe and Open Universe

Feature	Closed Universe	Open Universe
Ultimate Fate	Ends in a “Big Crunch” (collapse).	Expands forever (“Big Freeze”).
Density	Average density is greater than critical density.	Average density is less than critical density.
Expansion	Eventually halts and reverses.	Continues indefinitely, possibly accelerating.
Geometry	Spherical (positive curvature).	Hyperbolic (negative curvature).
Governing Force	Gravity is strong enough to halt and reverse the expansion.	The initial expansion is too strong for gravity to ever stop it.

(b) Flat Universe and Closed Universe

Feature	Flat Universe	Closed Universe
Ultimate Fate	Expansion stops after an infinite time.	Collapses into a “Big Crunch.”
Density	Average density is equal to critical density.	Average density is greater than critical density.
Expansion Rate	Approaches zero asymptotically.	Becomes negative after halting.
Geometry	Flat (zero curvature).	Spherical (positive curvature).
Stability	Becomes static after infinite time.	Unstable, leads to collapse.

3. Give reason:

(a) As the distance between celestial bodies increases their separating speed increases.

This occurs due to the expansion of space itself, as described by Hubble’s Law. The law shows a direct proportionality between the recessional velocity (v) of galaxies and their distance (d). It’s not that galaxies are flying through space away from each other, but rather that the fabric of space between them is stretching, carrying the galaxies along with it. The more space there is between two galaxies, the faster they will appear to move apart.

(b) Celestial bodies continue to remain in their own place.

Celestial bodies remain in their stable positions and orbits primarily due to gravitational force. This force provides the necessary centripetal pull that balances the object’s forward momentum (inertia). For instance, the Sun’s gravity keeps the Earth in a stable orbit, preventing it from flying off into space. This balance between gravity and motion is what allows planets, moons, and stars to maintain their positions relative to each other within their systems.

4. Answer the following questions:

(a) What is the universe?

The universe is the entirety of space and time and all of its contents, including all celestial bodies like planets, stars, and galaxies, as well as all forms of matter and energy.

(b) State the Big Bang theory.

The Big Bang theory posits that the universe originated from a cataclysmic explosion of a very small, extremely hot, and dense point called a singularity. This event marked the beginning of both space and time. Ever since this initial explosion, the universe has been continuously expanding and cooling down, leading to the formation of the stars and galaxies we observe today.

(c) Write the conclusion of Hubble’s study.

Edwin Hubble’s primary conclusion was that the universe is expanding. His observations showed that galaxies are moving away from our own, and he discovered a direct correlation between a galaxy’s distance and its speed of recession: the farther away a galaxy is, the faster it is moving away from us. This relationship is now known as Hubble’s Law.

(d) According to the Big Bang theory, the universe is continuously expanding. Is there no limitation to the expansion of the universe? Explain your arguments.

The expansion of the universe may be limited. Its ultimate fate depends on the average density of matter and energy within it. There are three main scenarios based on a value called the “critical density“: Closed Universe (density > critical), Open Universe (density < critical), and Flat Universe (density = critical). Thus, the expansion is not necessarily limitless; it is governed by the total density of the universe.

(e) Write the importance of gravitational force in the position of celestial bodies in the universe.

Gravitational force is crucial for maintaining the structure and order of the universe. It is the force that holds matter together to form stars, planets, and galaxies. It dictates the orbits of planets around stars and moons around planets, ensuring they remain in stable positions and do not collide or drift away. In essence, gravity is the architect of the cosmic structures we see.

(f) Earth revolves around the Sun; Moon revolves around the Earth. They have their force of gravity as well as the gravitational force between them, but they never collide with each other. Clarify this with reasons.

These bodies do not collide because they are in stable orbits, which represent a balance between their forward velocity and the gravitational pull of the larger body they orbit. The Earth is constantly being pulled toward the Sun by gravity, but its high sideways speed causes it to constantly “miss” the Sun, resulting in a continuous circular/elliptical path. The same principle applies to the Moon’s orbit around the Earth. This balance prevents them from either falling into each other or flying off into space.

(g) Explain the concept of a flat universe.

A flat universe is a cosmological model where the average density of the universe is precisely equal to the critical density. In this scenario, the universe has just enough mass to stop expanding, but not enough to cause a collapse. The expansion rate will gradually decrease over an infinite amount of time, eventually approaching zero. The geometry of space in such a universe is considered flat, like an infinite plane.

(h) In your opinion, what might be the future of the universe: open, flat or closed? Explain your answer with suitable arguments.

Based on current scientific evidence, the future of the universe appears to be most similar to the open universe model. Observations of distant supernovae have shown that the universe’s expansion is not slowing down; it is actually accelerating. This acceleration is attributed to a mysterious force known as “dark energy.” If this trend continues, the universe will expand forever at an ever-increasing rate, becoming progressively colder and darker. This fate is sometimes referred to as the “Big Freeze” or “Heat Death.” For more in-depth information, you can explore resources from NASA on Dark Energy.