Type Ia supernovae reveal the expansion of the Universe is accelerating, pointing to dark energy

The Hubble tension

• Ascertaining a precise value for the Hubble constant $H$ has been one of the most important goals of cosmology.
• In the past, discrepancies between measured values and predictions were attributed to random and systematic errors.
• However, as telescopes and imaging techniques have improved, different measurement methods produce fundamentally different values of $H$:
• CMB observations give $H \approx 68$ km s$^{-1}$ Mpc$^{-1}$
• Type Ia supernovae measurements give $H \approx 73$ km s$^{-1}$ Mpc$^{-1}$
• This discrepancy is known as the Hubble tensionThe unresolved disagreement between different methods of measuring the Hubble constant, which produce values that differ by more than can be explained by measurement uncertainty., and it remains one of the deepest mysteries in cosmology.

Accelerating expansion

• Measurements of Type Ia supernovaeExplosions of white dwarf stars that always reach the same peak luminosity, making them excellent standard candles for measuring distances to far-away galaxies., which tend to be very distant, suggest that the Universe is expanding at an increasing rate.
• The key part is this: the Universe is not only expanding, but the rate of expansion is accelerating. This was completely unexpected.

Dark energy: a controversial solution

• Dark energy is controversial because:
• It cannot be detected directly, although there is indirect evidence for its existence.
• Nothing is known about its nature or its origins.
• No mechanism can currently explain how it drives the accelerated expansion.

If dark energy is wrong, what else could explain it?

• If dark energy is not the answer, cosmologists may need to re-examine fundamental physics:
• Newton's second law and the law of gravitation may need modifying to account for observed galactic motion.
• There may be flaws in Einstein's Theory of General Relativity that require revision.
• The cosmic distance ladderThe chain of overlapping methods used to measure distances in the Universe: parallax for nearby stars, Cepheid variables for nearby galaxies, Type Ia supernovae for distant galaxies, and Hubble's law for the most distant objects. may be incorrect, meaning our methods for determining extremely large distances need further refinement.