Dark Energy (AQA A Level Physics): Revision Note

Dark energy

• Over several decades, astronomers have gathered a wealth of evidence that has furthered the 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 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 $H=68\mathrm{km}{\mathrm{s}}^{-1}{\mathrm{Mpc}}^{-1}$

  • Whereas measurements from type 1a supernovae give a value of $H=73\mathrm{km}{\mathrm{s}}^{-1}{\mathrm{Mpc}}^{-1}$

• 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 rethink some of the 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 reworking

  • it could be that the cosmic distance ladder is incorrect, and current methods of determining extremely great distances may need refining further

  • or maybe the Big Bang Theory is wrong, and a new theory of the origin of the Universe is required

The cosmic distance ladder