Science & Technology · Chapter 12

24 solved questions

Chapter 12: Universe

Every question in this chapter, answered and explained — step-by-step solutions drawn live from the chapter library across 5 question sections.

Very Short Answer Questions8 Give Reason Questions2 Differentiate Between2 Long Answer Questions8 Additional SEE Practice4

Very Short Answer Questions

1SEE Boardqb-q0

Due to the effect of which force are celestial bodies in the universe situated at their position?

Ans.Gravitational force.

2SEE Boardqb-q1

Which theory is considered the most realistic theory about the origin of the universe?

Ans.The Big Bang theory.

3SEE Boardqb-q2

According to the Big Bang theory, how did the universe originate?

Ans.The universe originated from the huge explosion of a highly compressed, energetic single atom.

4SEE Boardqb-q3

According to Hubble's theory, what is the relationship between the distance and speed of galaxies?

Ans.The farther away the galaxies are, the more speed (velocity) they will have.

5SEE Boardqb-q4

Under what condition can an open universe be hypothesized?

Ans.An open universe can be hypothesized when the average density of the universe is less than the critical density.

6SEE Boardqb-q5

What does H represent in Hubble's law, v = Hd?

Ans.H represents Hubble's constant, the constant of proportionality between velocity and distance.

7SEE Boardqb-q6

What is a galaxy?

Ans.A galaxy is a large group of many stars bound together by gravity.

8SEE Boardqb-q7

What is critical density?

Ans.Critical density is the average density of the universe that is just enough for gravity to eventually stop the expansion of the universe.

Give Reason Questions

1SEE Boardqb-q8

As the distance between celestial bodies increases, their separating speed increases. Why?

Ans.According to Hubble's law, v = Hd, the velocity at which galaxies move apart is directly proportional to the distance between them. Hubble's study showed that farther celestial bodies are moving away faster, so as distance increases, separating speed also increases.

2SEE Boardqb-q9

Celestial bodies continue to remain in their own place (orbit). Why?

Ans.Every celestial body has gravity, and there is a mutual gravitational force of attraction between celestial bodies. This gravitational force keeps each body moving in a fixed orbit around another body instead of flying off or colliding with it, so celestial bodies remain in their own place.

Differentiate Between

1SEE Boardqb-q10

Differentiate between Closed Universe and Open Universe.

Closed Universe	Open Universe
The effect of gravity remains nonzero even when the speed of celestial bodies becomes zero.	Celestial bodies retain velocity even when the effect of gravity becomes zero.
Celestial bodies move closer together and the universe collapses to a point (Big Crunch).	Celestial bodies keep moving apart from each other forever.
Average density of the universe is greater than critical density.	Average density of the universe is less than critical density.

2SEE Boardqb-q11

Differentiate between Flat Universe and Closed Universe.

Flat Universe	Closed Universe
Average density of the universe equals the critical density.	Effect of gravity remains greater than zero when the speed becomes zero.
Velocity of celestial bodies and effect of gravity become zero at the same time.	Speed of celestial bodies becomes zero before the effect of gravity does.
Expansion slows down and finally stops; the universe remains stable.	The universe shrinks to a point (Big Crunch) and may explode again.

Long Answer Questions

1SEE Boardqb-q12

What is the universe?

Ans.The universe is the vast region of space that contains all celestial bodies, including planets, stars, satellites, comets, meteors, meteorites, asteroids and galaxies. It is the collective name given to all these celestial bodies together with the space around them.

2SEE Boardqb-q13

State the Big Bang theory.

Ans.The Big Bang theory states that the universe originated from the huge explosion of a highly compressed, energetic single atom. Before the explosion, the four fundamental forces of nature were unified as a single force. Due to excessive pressure, the atom exploded, and all celestial bodies in the universe originated from this explosion. Since then, the universe has been expanding continuously, though its speed of expansion is slowly decreasing due to gravity.

3SEE Boardqb-q14

Write the conclusion of Hubble's study.

Ans.Edwin Hubble studied the velocities of different galaxies using a telescope and found that every galaxy is moving away from every other galaxy. He concluded that the farther a galaxy is, the faster it is moving away, following the relation v = Hd. This showed that the universe is expanding, and since all galaxies were once closer together, Hubble's study supports and confirms the Big Bang theory.

4SEE Boardqb-q15

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

Ans.The expansion of the universe is not necessarily unlimited; it depends on the average density of the universe compared to the critical density. If the average density is less than the critical density, gravity cannot stop the expansion, so the universe will keep expanding forever (open universe). If the average density equals the critical density, the expansion gradually slows down and eventually stops (flat universe). If the effect of gravity remains strong even after the speed of celestial bodies becomes zero, the universe will stop expanding, start contracting, and eventually collapse into a single point (closed universe, ending in a Big Crunch). So whether the expansion has a limit depends on how much matter (density) the universe contains.

5SEE Boardqb-q16

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

Ans.Gravitational force is the mutual force of attraction between all celestial bodies. It keeps every celestial body in its position and correct orbit. Because of gravitational force, the Earth rotates on its axis and revolves around the Sun, and the Moon revolves around the Earth. Gravitational force keeps planets aligned in their orbits around the Sun, preventing them from flying off into space or falling into each other. In short, gravitational force is responsible for holding the whole universe together in an organised structure.

6SEE Boardqb-q17

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

Ans.Although gravitational force pulls the Moon towards the Earth and the Earth towards the Sun, each body also has orbital velocity (a sideways motion) around the other. This orbital velocity balances the pull of gravity: gravity continuously pulls the body inward, while its motion carries it forward, and together these two effects produce a stable curved path (an orbit) instead of a straight-line collision. This is why the Moon keeps revolving around the Earth, and the Earth keeps revolving around the Sun, without colliding.

7SEE Boardqb-q18

Explain the concept of a flat universe.

Ans.A flat universe occurs when the average density of the universe is exactly equal to the critical density. In this situation, the velocity of celestial bodies and the effect of gravity become zero at the same time. The continuously expanding universe gradually slows down and finally stops expanding. The celestial bodies then remain at very large, fixed distances from each other, and since the distances are so great, they are no longer significantly affected by mutual gravitational force. Such a stable, non-expanding, non-collapsing universe is called a flat universe.

8SEE Boardqb-q19

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

Ans.This is an open scientific question, since scientists have not yet been able to precisely measure the actual average density of the universe compared to the critical density. A student may reasonably argue for any of the three possibilities as long as the reasoning is correct: for example, if current observations suggest the average density is lower than the critical density, this would support an open universe that keeps expanding forever; if future measurements show the densities to be equal, a flat universe would be more likely; and if the density is found to be greater than the critical density, a closed universe ending in a Big Crunch would be supported. The key point for SEE is to correctly connect your chosen answer to the relationship between average density and critical density.

Additional SEE Practice

1SEE Boardqb-q20

Define galaxy with an example.

Ans.A galaxy is a large group of many stars bound together by gravity. Example: The Milky Way galaxy, which contains our Solar System.

2SEE Boardqb-q21

What is Hubble's constant and what is its approximate value?

Ans.Hubble's constant (H) is the constant of proportionality in Hubble's law, v = Hd. Its approximate value is 73 km/s/Mpc, meaning two galaxies one megaparsec apart move away from each other at about 73 km/s.

3SEE Boardqb-q22

Draw and label a graph showing the difference between open, flat and closed universe.

Ans.Draw a graph with 'Time since the Big Bang' on the x-axis and 'Radius of the Universe' on the y-axis. Draw three curves starting from the origin: a curve that keeps rising continuously and label it 'Open Universe'; a curve that rises and then becomes almost flat (levels off) and label it 'Flat Universe'; and a curve that rises and then comes back down to zero and label it 'Closed Universe – Big Crunch'.

4SEE Boardqb-q23

What are the four fundamental forces that were unified before the Big Bang explosion?

Ans.The four fundamental forces are gravitational force, electromagnetism, strong nuclear force and weak nuclear force.

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