Hubbles Law: Overview, Questions, Preparation

Hubble's law (also known as Hubble's Lemaitre law) helps determine the relationship between the distance of a galaxy to the velocity at which it is moving.

It is stated as:

The velocity of the galaxy, also known as the redshift, is directly proportional to its distance.

Simply put, it says that the larger the distance of a galaxy from the earth, the faster it will move away from it.

The formula for Hubble's law is as follows:

v = H0d

Where,

v represents the velocity of the moving galaxies

H0 represents the Hubble's constant

d represents the distance between the galaxies

The Hubble constant is a unit to measure the pace at which the expansion of the universe is taking place. However, its value keeps on changing

The limitations of Hubble's law are:

• The actual velocity of the galaxy may differ from the observed velocity. It is because the intrinsic motion of the galaxies also influences the velocity.
• Hubble's law also does not take into account the gravitational movements of the galaxies orbiting together.

A redshift of galaxies is a phenomenon where the wavelength of an object increases as the electromagnetic radiation decreases.

The redshift happens because of three reasons. They are:

• Doppler effect: It is the motion of objects in space that causes them to move closer or farther apart from each other.
• Hubble effect: The constant expansion of the universe increases the distance between galaxies without changing their positions. This phenomenon is known as the Hubble or cosmological effect.
• Gravitational redshift: Strong gravitational forces distort the space-time continuum and cause a gravitational redshift.

From the Hubble's constant, the following units were obtained:

• Hubble time

The Hubble time is the time that the universe has taken to expand and reach its present state. It is the reciprocal of the Hubble constant (1/H0)

• Hubble length

The Hubble length is also known as the Hubble distance is a unit of distance. Its measure depends on the speed of light and the Hubble time.

The Hubble length is estimated at approximately 14.4 billion years.

• Hubble volume

The Hubble volume is said to be the volume of the observable universe. It is estimated to be approximately 1031 cubic light-years.

In class 11, the chapter Introduction to motion consists of Hubble's law. It explains the motion of celestial bodies and further states the different laws of motion governing objects on the ground and in space.

Introduction to the motion has a weightage of 8-10 marks in class 11 exams. 

1. For a cluster, velocity is 10^3 km/s. What is the distance? Hο = 70km/s/Mpc

Ans: We know,

v = 10^3 km/s

H0 = 70km/s/Mpc

The formula is:

d = v/Hο

=(10^3) /(70)

=14.28 Mpc

2. A galaxy has a velocity of 1220 km/s. Ho = 70km/s/Mpc. What is d?

Ans: We know,

v = 1220 km/s

Ho = 70km/s/Mpc

Therefore,

d = v/Hο

= 1220/70

= 17.42 Mpc

3. A galaxy is at a distance of 16.42 Mpc. What is its velocity? Ho =70km/s/Mpc 

Ans: We know,

d = 16.42 Mpc

Ho = 70km/s/Mpc 

Therefore,

v = Hod

= 70 × 16.42

= 1150 km/s

Q: Who discovered Hubble's law?

Q: What is the conclusion of Hubble's law?

Q: Why is Hubble's law significant?

Q: What cannot be calculated using Hubble's law?

Q: Is Hubble's constant uncertain?