Growth of structure and the neutrino mass

6 problems

• How much does baryonic feedback suppress small-scale matter clustering?

  Open

  Black holes and exploding stars blow gas out of galaxies, which smooths the matter distribution on small scales. If this effect is stronger than assumed, it could explain the low clumping seen by some surveys.

• Does the growth rate of structure follow general relativity?

  Open

  In general relativity the rate at which clumps grow follows directly from the expansion history. Measuring both separately tests whether gravity on cosmic scales is the same as in the solar system.

• Is the cosmological neutrino mass limit consistent with oscillation data?

  Contested

  Neutrino oscillation experiments prove that the three neutrino masses add up to at least about 0.06 eV. Assuming the standard model of cosmology, cosmology now allows at most about 0.064 eV, and its best fit prefers even less, which is either a fluke, a sign of new physics, or a hidden error.

• Cosmological measurement of the neutrino mass sum to 0.015 eV

  Open

  Cosmology can weigh neutrinos because heavier neutrinos smooth out the clumping of matter. The task is to measure the total mass with cosmological data, not just bound it.

• Is the CMB reionization optical depth underestimated?

  Contested

  Free electrons released when the first galaxies ionized hydrogen scatter microwave background light, an effect measured by a number called the optical depth. If it is larger than the Planck value, several current tensions, including the neutrino-mass one, ease.

• Is the low clustering amplitude from weak lensing a real discrepancy?

  Contested

  Weak gravitational lensing, the slight distortion of distant galaxy shapes by intervening matter, measures how clumpy matter is today. Some surveys find it less clumpy than the cosmic microwave background predicts and others do not.