Chapter 12: Universe
What is the Universe?
The universe is the vast region of space that contains all celestial bodies. It includes planets, stars, comets, satellites, meteors, meteorites, asteroids and galaxies.
- Planets, stars, comets, satellites, meteors, meteorites and asteroids are all celestial bodies.
- A large group of stars forms a galaxy.
- Our Solar System lies inside the Milky Way galaxy.
- Celestial bodies can be very large or very small, and their mass can be very large or very small.
- Some celestial bodies are in a gaseous state and some are in a solid state.
SEE Focus: Remember that the universe is the collective name given to all celestial bodies and the space that contains them.
Figure 12.1: Celestial bodies in the universe
Importance of Gravitational Force in the Universe
Every celestial body has mass, so every celestial body has gravity. There is a mutual force of attraction between all celestial bodies. This force is called gravitational force.
- The Sun attracts the Earth towards its centre, and the Earth also attracts the Sun towards its centre.
- The Earth attracts the Moon, and the Moon also attracts the Earth.
- Because of gravitational force, the Earth rotates on its axis and revolves around the Sun.
- The Moon revolves around the Earth because of gravitational force.
- Gravitational force keeps every planet aligned in its orbit around the Sun.
Very Important for SEE: Gravitational force is the reason celestial bodies stay in their position and move in fixed orbits instead of flying away or colliding.
Figure 12.2: Gravitational force between the Sun, Earth and Moon
12.2 Study of the Universe
Universe Science and Astrophysics
Universe science is the branch of science that studies facts about the universe, including its history and future. Astrophysics is a branch of universe science that studies the origin, structure and future of the universe.
The Milky Way Galaxy
| Fact | Value |
|---|---|
| Type of galaxy | Spiral galaxy |
| Distance of Solar System from centre | About 30,000 light years |
| Diameter of Milky Way galaxy | About 100,000 light years |
| Approximate number of stars | About 1.5 billion |
Big Bang Theory
Many theories have been proposed about the origin of the universe, but the Big Bang theory is considered the most reliable. It was supported by the observation that all galaxies are moving away from each other at great speed.
- 1Before the explosion, the four fundamental forces (gravitational force, electromagnetism, strong nuclear force and weak nuclear force) were unified as a single force.
- 2The universe existed in a highly compressed, very energetic state, about the size of a single atom.
- 3Due to excessive force and pressure, a huge explosion of this atom took place.
- 4All celestial bodies in the universe are believed to have originated from this explosion.
- 5The universe has been expanding since the explosion, and this expansion is slow but continuous.
- 6The speed of the expansion is gradually decreasing due to the effect of gravity.
SEE Focus: Big Bang theory states that the universe originated from the explosion of a highly compressed single atom.
Figure 12.3: Huge explosion of a single atom (Big Bang)
Hubble's Study
In 1929, American astronomer Edwin Hubble used the 100-inch Hooker telescope at Mount Wilson to calculate the velocities of different galaxies. He found that every galaxy is moving away from every other galaxy.
Hubble found that galaxies which are farther away appear smaller and move with greater velocity.
Hubble's Law: v = Hd
| Symbol | Meaning |
|---|---|
| v | Velocity at which galaxies move away from each other |
| d | Distance between the galaxies |
| H | Hubble's constant |
The value of Hubble's constant (H) is about 73 km/s/Mpc (kilometre per second per megaparsec). This means two galaxies that are one megaparsec apart are moving away from each other at about 73 km/s.
Analogy: Just as dots marked on a balloon move farther apart as the balloon is inflated, galaxies in the universe move farther apart as the universe expands.
Very Important for SEE: Hubble's study proved that galaxies are moving away from each other, and the farther a galaxy is, the faster it moves away. This conclusion supports the Big Bang theory.
Figure 12.4: Hubble's relation between velocity and distance
12.3 Future of the Universe
Newton's Law of Gravitation and Critical Density
According to Newton's universal law of gravitation, every mass exerts a gravitational force on every other mass. This force is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centres.
The net gravitational force on any celestial body depends on the average density of the universe. The density of the universe that is just enough to stop the expansion of the universe is called the critical density.
Open Universe
If the average density of the universe is less than the critical density, gravity cannot stop the expansion. Celestial bodies retain velocity even when the effect of gravity becomes zero, so they continue moving apart forever and the universe keeps expanding. This is called an open universe.
Flat Universe
If the average density of the universe is equal to the critical density, the velocity of celestial bodies and the effect of gravity become zero at the same time. The expansion gradually slows down and finally stops, and celestial bodies remain at very large, stable distances from each other. This is called a flat universe.
Closed Universe
If the effect of gravity remains greater than zero even when the speed of celestial bodies becomes zero, celestial bodies will start moving closer together until the universe shrinks to a single point. This collapse is called the Big Crunch. If the universe explodes again after the Big Crunch, this repeating cycle is called a closed universe.
Figure 12.5: Types of universe (open, flat and closed)
Important Definitions
| Term | Definition |
|---|---|
| Universe | The vast space that contains all celestial bodies, including stars, planets, satellites, comets, meteors, meteorites and galaxies. |
| Galaxy | A large group of many stars bound together by gravity. |
| Gravitational force | The mutual force of attraction that exists between every two masses in the universe. |
| Big Bang theory | The theory that states the universe originated from the huge explosion of a highly compressed, energetic single atom. |
| Hubble's law | The law stating that the velocity at which a galaxy moves away is directly proportional to its distance, expressed as v = Hd. |
| Hubble's constant (H) | The constant of proportionality in Hubble's law, with a value of about 73 km/s/Mpc. |
| Critical density | The average density of the universe that is just enough for gravity to eventually stop the expansion of the universe. |
| Big Crunch | The collapse of the universe into a single point due to gravity, as proposed in the closed universe model. |
Important Differences
Open Universe vs Closed Universe
| Open Universe | Closed Universe |
|---|---|
| Average density is less than critical density. | Effect of gravity remains nonzero even when speed becomes zero. |
| Celestial bodies keep moving apart forever. | Celestial bodies eventually move closer and collapse (Big Crunch). |
| The universe keeps expanding continuously. | The universe shrinks to a point and may explode again. |
Flat Universe vs Closed Universe
| Flat Universe | Closed Universe |
|---|---|
| Average density equals critical density. | Effect of gravity remains greater than zero when speed becomes zero. |
| Velocity and gravity become zero at the same time. | Speed becomes zero before gravity's effect does. |
| Expansion slows and stops; the universe remains stable. | The universe collapses inward (Big Crunch), it does not stay stable. |
Important Formula
Hubble's Law: v = Hd
| Symbol | Meaning | Approximate Value |
|---|---|---|
| v | Velocity of separation of galaxies | — |
| H | Hubble's constant | 73 km/s/Mpc |
| d | Distance between galaxies | — |
Common Mistakes in SEE
- Do not confuse 'universe' with 'galaxy' — a galaxy is only one part of the universe.
- Do not write that celestial bodies collide because of gravity; gravity actually keeps them in stable orbits.
- Do not mix up open, flat and closed universe — compare average density with critical density carefully.
- Always write Hubble's law correctly as v = Hd, not d = Hv.
- Remember the correct value and unit of Hubble's constant: 73 km/s/Mpc.
Quick Revision
- Universe = collective name for all celestial bodies and the space around them.
- Gravitational force keeps celestial bodies in their position and orbit.
- Big Bang theory: universe originated from the explosion of a compressed single atom.
- Hubble's law: v = Hd, where H ≈ 73 km/s/Mpc.
- Farther galaxies move away faster — this supports the Big Bang theory.
- Critical density decides the future of the universe.
- Density less than critical density leads to an Open universe (keeps expanding).
- Density equal to critical density leads to a Flat universe (expansion stops, stays stable).
- Gravity's effect remaining nonzero at zero speed leads to a Closed universe (Big Crunch).