What is Hubble Tension?

About Hubble Tension:

• It refers to a discrepancy between the measurements of the rate of expansion of the universe, known as the Hubble constant.
• The Hubble constant, denoted as H0, describes the rate at which galaxies are moving away from each other due to the expansion of the universe.
• If a researcher wants to estimate the Hubble constant, they have two main avenues. These are the cosmic distance ladder and the cosmic microwave background (CMB).
• Cosmic microwave background (CMB):
  • It is a sea of photons, the particles of light, present throughout the universe. They are left over from the Big Bang, its afterglow.
  • Scientists have measured temperature changes in the CMB and studied its large-scale properties using complicated trigonometry.
  • Based on these studies, cosmologists have estimated space to be expanding at around 68 kilometres per second per megaparsec ((km/s)/Mpc).
• Cosmic distance ladder:
  • It is used to measure the distance to objects that are close, further away, or very far away from the earth. One object in particular is the Cepheid variable star.
  • The Cepheid variables have a unique feature: their brightness varies in a predictable way over time.
  • Based on how bright a Cepheid variable is, scientists can estimate how far away it is. Using this, cosmologists have estimated based on various Cepheid variables (and other such objects) is 73 (km/s)/Mpc.
• Thus, these measurements have yielded slightly different values for the Hubble constant. This discrepancy is known as the Hubble tension.
• The significance of the Hubble tension is that it could potentially indicate unknown physics or systematic errors in the measurements. Resolving the tension is crucial for refining our understanding of the universe's expansion and its underlying physics.