Cosmology is the scientific study of the origin, evolution, and ultimate fate of the Universe. Observational discoveries over the past century—specifically galactic redshift and cosmic microwave background radiation—have transformed theoretical cosmology into a quantitative physical science within Cambridge IGCSE Physics.

The Universe is not static; it is undergoing metric expansion. Space itself is stretching, causing galaxies that are not gravitationally bound together to move away from one another.

Galactic Redshift

When light emitted by distant galaxies is analyzed using spectroscopy, characteristic absorption line patterns are shifted toward longer, redder wavelengths relative to laboratory reference spectra on Earth. This phenomenon is known as redshift.

Redshift occurs because as electromagnetic waves travel through expanding space, the wavelength ($\lambda$) of the photons is stretched during transit:

The greater the redshift observed in a galaxy's spectrum, the faster that galaxy is receding from Earth.

1. Hubble's Law

In 1929, Edwin Hubble discovered a direct linear relationship between the recession velocity ($v$) of a distant galaxy and its distance ($d$) from Earth:

where $H_0$ is the Hubble Constant. The Hubble constant measures the rate of expansion of the Universe.

• The currently accepted value for the Hubble Constant in SI units is:

2. Estimating the Age of the Universe

If the Universe has been expanding at a uniform rate since its inception, tracing the motion of receding galaxies backward in time indicates that all matter in the Universe was concentrated at a single point (a gravitational singularity) at time $t = 0$.

Using $v = H_0 d$, the time $t$ elapsed since the Big Bang (the age of the Universe) can be calculated as:

Substituting $H_0 = 2.2 \times 10^{-18}\text{ s}^{-1}$:

3. Cosmic Microwave Background Radiation (CMBR)

The Big Bang Theory is further confirmed by Cosmic Microwave Background Radiation (CMBR).

• CMBR is isotropic microwave radiation filling all points in space around us.
• It represents thermal remnant radiation emitted roughly 380,000 years after the Big Bang when the Universe cooled sufficiently for neutral atoms to form (recombination).
• Originally emitted as high-energy gamma and ultraviolet radiation, the wavelengths of these photons have been stretched by a factor of 1100 over 13.8 billion years of spatial expansion into the microwave region, matching a blackbody spectrum at $T \approx 2.725\text{ K}$.

Modern cosmological measurements rely on space telescopes such as the James Webb Space Telescope (JWST) and the Planck satellite. JWST observes deep-space galaxies in high-resolution infrared to detect ultra-high redshift light ($z > 10$) emitted over 13 billion years ago, allowing astronomers to observe the earliest galaxy formations following the Big Bang.

Galactic redshift and Cosmic Microwave Background Radiation provide compelling empirical proof of an expanding Universe. Utilizing Hubble's Law ($v = H_0 d$) allows physicists to calculate the Hubble constant and accurately estimate the age of the cosmos at 13.8 billion years.