Aspect

Description

Examples/Key Points

Definition

The phenomenon where the universe increases in size over time leading to galaxies moving away from each other.

Described by Edwin Hubble in the 1920s.

Theoretical Foundation

Based on the Big Bang Theory suggesting the universe started as a singular point and has been expanding since.

Singularities, cosmological models, inflationary universe.

Evidence

Observational data supporting expansion of the universe.

Redshift (Hubble’s Law): Galaxies show redshift proportional to their distance.

As the universe expands the wavelengths of light stretch causing them to shift towards longer, redder wavelengths. This is called redshift.

Cosmic Microwave Background Radiation (CMB). It is the cooled remnant of the first light that could ever travel freely throughout the Universe. This 'fossil' radiation the furthest that any telescope can see was released soon after the Big Bang. Scientists consider it as an echo or 'shockwave' of the Big Bang.

Hubble’s Law

A law stating that the velocity of galaxies moving away is proportional to their distance from the observer.

Formula: v=H0×dv=H0​×d, where H0H0​is the Hubble constant.

Hubble Constant (H₀)

A measure of the rate of expansion of the universe.

Current estimates vary between 67–74 km/s/Mpc depending on measurement techniques.

Dark Energy

The mysterious force driving the acceleration of the universe's expansion.

Constitutes about 68% of the universe; linked to the cosmological constant (ΛΛ).

Role of General Relativity

Einstein’s theory describes the dynamic nature of spacetime, forming the basis for models of an expanding universe.

Field equations include the cosmological constant to account for expansion.

Observable Universe

The part of the universe we can observe, determined by the speed of light and the age of the universe.

Diameter: Approx. 93 billion light-years.

Accelerated Expansion

Discovery that the expansion rate is increasing over time not slowing as previously assumed.

Nobel Prize in Physics 2011 to Perlmutter, Schmidt and Riess for supernova observations.

Cosmological Models

Friedmann-Lemaître-Robertson-Walker (FLRW) metric: Describes homogeneous, isotropic universe.

Standard models include open, closed and flat universes based on curvature.

Measurements

Techniques used to determine expansion parameters.

Standard candles (e.g., Type Ia supernovae).
Standard rulers (e.g., Baryon Acoustic Oscillations).

Impact on Future Universe

Scenarios based on the rate and nature of expansion.

Big Freeze: Heat death due to continuous expansion.
 Big Crunch: Universe collapses on itself (unlikely).

PRACTICE QUESTION

Q.Which of the following statements regarding the expansion of the universe is/are correct?

1. The redshift observed in the light from distant galaxies provides evidence for the expansion of the universe.
2. The rate of expansion of the universe is uniform and does not change over time.
3. The expansion of the universe causes the galaxies themselves to expand.

Options:
A. 1 only
B. 1 and 2 only
C. 2 and 3 only
D. 1, 2 and 3 

Answer: A.

Explanation:

Statement 1 is correct. The redshift observed in the spectra of light from distant galaxies (discovered by Edwin Hubble) indicates that galaxies are moving away from us. This phenomenon supports the idea of an expanding universe. The further a galaxy is the faster it appears to move away consistent with Hubble's Law.

Statement 2 is incorrect. Observations such as those involving Type Ia supernovae suggest that the expansion rate of the universe is not uniform. It is accelerating due to the influence of dark energy.

Statement 3 is incorrect. The expansion of the universe affects the space between galaxies not the galaxies themselves. Gravitational forces within galaxies prevent them from expanding.