Damn you little h! – Darren Croton

Title: Damn you little h!


Speaker: Darren Croton, Swinburne University (dcroton AT astro.swin.edu.au)


Date: 3pm (AEST) Friday 19th July 2013


Abstract: The Hubble constant, H_0, or its dimensionless equivalent, “little h”, is a fundamental cosmological property that is now known to an accuracy better than a few percent. Despite its cosmological nature, little h commonly appears in the measured properties of individual galaxies. This can pose unique challenges for users of such data, particularly with survey data.

In this talk I show:

  • how little h arises in the measurement of galaxies,
  • how to compare like-properties from different datasets that have assumed different little h cosmologies, and
  • how to fairly compare theoretical data with observed data, where little h can manifest in vastly different ways.

This last point is particularly important when observations are used to calibrate galaxy formation models, as calibrating with the wrong (or no) little h can lead to disastrous results when the model is later converted to the correct h cosmology.

I’ll argue that in this modern age little h is an anachronism, being one of least uncertain parameters in astrophysics, and propose that observers and theorists instead treat this uncertainty like any other.

I’ll conclude with a “cheat sheet” of nine points that should be followed when dealing with little h in data analysis.


Additional Material: Talk slides can be found here. The accompanying paper is available on the archive here.


Link to Lecture: You can find the lecture on YouTube here, or on Google+ here.