AQA GCSE HIGHER PHYSICS - Understanding Redshift and the Expanding Universe

Redshift and the Expanding Universe

Introduction:

Redshift is a phenomenon observed in light from distant galaxies, where the light is shifted towards the red end of the electromagnetic spectrum. This shift is a direct consequence of the Doppler effect, which we experience in everyday life with sound waves. This tutorial will explore the concept of redshift and how it provides evidence for the expanding universe.

The Doppler Effect:

Imagine a stationary ambulance. You hear the siren at a certain frequency. Now, as the ambulance moves towards you, the sound waves are compressed, causing the frequency to increase and the pitch to rise. As the ambulance moves away, the sound waves are stretched out, lowering the frequency and making the pitch drop. This is the Doppler effect.

Redshift and the Expanding Universe:

Light, like sound, is a wave. When a source of light is moving away from an observer, the wavelengths of light are stretched, causing a shift towards the red end of the spectrum (redshift). Conversely, if the source is moving towards the observer, the wavelengths are compressed, causing a blueshift.

The redshift observed in light from distant galaxies is a key piece of evidence for the expanding universe. Astronomers have found that the farther away a galaxy is, the greater its redshift. This suggests that all galaxies are moving away from each other, indicating an expansion of the universe.

Measuring Redshift:

Astronomers use spectroscopy to measure redshift. Light from distant galaxies is passed through a prism, which separates the light into its different wavelengths. By comparing the wavelengths of spectral lines in the galaxy's light to those of known elements on Earth, astronomers can determine the amount of redshift.

The Hubble Law:

Edwin Hubble, in the 1920s, discovered a direct relationship between the redshift of a galaxy and its distance from Earth. This relationship, known as Hubble's Law, states that the farther away a galaxy is, the faster it is moving away from us.

Implications of an Expanding Universe:

The expanding universe has significant implications for our understanding of the cosmos. It suggests that the universe had a beginning, likely in the form of the Big Bang. It also raises questions about the ultimate fate of the universe: will it continue to expand forever, or will it eventually collapse back on itself?

Summary:

Redshift is a crucial phenomenon that supports the theory of an expanding universe. By observing the redshift of distant galaxies and using Hubble's Law, astronomers have gained valuable insights into the evolution and nature of the cosmos. Understanding redshift is essential for comprehending the vastness and dynamic nature of the universe.