9.3.6 Dark Energy

• Over several decades, astronomers have gathered a wealth of evidence that has furthered our understanding of the Universe
  • Ascertaining a precise value for the Hubble constant H was one of the most important goals of cosmology then, and still is today

• In the past, huge discrepancies between measured values of the Hubble constant and predictions from Hubble's law were ascribed to random and systematic errors

• However, as our telescopes and imaging techniques become more sophisticated, astronomers are finding that different sources at different distances seem to produce fundamentally different values of H
  • For example, CMB observations give a value of H = 68 km s⁻¹ Mpc⁻¹
  • Whereas measurements from type 1a supernovae give a value of H = 73 km s⁻¹ Mpc⁻¹ 

• This discrepancy is known as the Hubble Tension problem, and it’s one of the deepest mysteries in cosmology at the moment

• Measurements of type 1a supernovae, which tend to be very distant, suggest that the Universe is expanding at an increasing rate
• This means that the Universe is not only expanding, but the rate of expansion is accelerating, and astronomers have no idea why  

Dark Energy: A Controversial Solution

• It has been suggested that the acceleration of expansion could be caused by a mysterious energy source called dark energy
• However, dark energy is controversial because:
  • It cannot be detected directly, but there is evidence for its existence
  • Nothing is known about its nature or its origins
  • No mechanisms can currently explain how it drives the accelerated expansion

• If dark energy isn't the solution, then cosmologists may need to re-think some of our fundamental models, for example
  • The mathematics behind Newton's second law and law of gravitation may need modifying to account for observed galactic motion
  • Perhaps there are flaws in Einstein's Theory of General Relativity that need re-working
  • Or maybe the cosmic distance ladder is incorrect and our methods of determining extremely great distances may need refining further

The Cosmic Distance Ladder

Could everything we know about the cosmic distance ladder be wrong? Or are we on the verge of a huge cosmological breakthrough?