Hubble tension between early and late universe

6 problems

• Can pre-recombination physics raise the Hubble constant to 0.73?

  Open

  One proposed fix changes physics in the first 380,000 years so that the standard ruler imprinted in the cosmic microwave background is shorter. The question is whether any such change still fits all the detailed data.

• Do TRGB, JAGB and Cepheid ladders give the same Hubble constant?

  Contested

  Distances to nearby galaxies can be calibrated with pulsating stars (Cepheids), the brightest old red giant stars, or carbon-rich giant stars. These methods should agree if none has a hidden bias.

• Does the tension persist in measurements independent of ladders and rulers?

  Open

  Some methods measure the expansion rate in one step, using gravitational waves from colliding neutron stars or time delays between multiple images of a lensed quasar. They avoid both the distance ladder and the early-universe ruler.

• Does a local underdensity bias the locally measured Hubble constant?

  Contested

  If the Milky Way lies inside a huge region with less matter than average, nearby galaxies recede faster and the measured local expansion rate comes out high. Galaxy counts suggest such a void, while supernova data mostly argue against it.

• Are the oldest stars too old for a Hubble constant of 0.73?

  Open

  The universe cannot be younger than its oldest stars. A faster expansion rate implies a younger universe, so precise ages of the oldest stars test which Hubble constant is right.

• What causes the Hubble tension?

  Contested

  Two careful ways of measuring how fast the universe expands today give different answers. The cause could be a measurement error or new physics.