# Local measurement of *H*_{0}

- Synopsis:
Prototype class for local Hubble parameter measurements

- Author:
Jesus Torrado, Pablo Lemos

This is a simple gaussian likelihood for the latest local \(H_0\) measurements using a combination of different data.

It defines the following likelihoods

`H0.riess2018a`

: A legacy local measurement of \(H_0\), used in the analysis of Planck data for the Planck 2018 release. (arXiv:1801.01120)`H0.riess2018b`

: Updated local measurement of \(H_0\). (arXiv:1804.10655)`H0.riess201903`

: Riess et al. 2019 constraint (arXiv:1903.07603)`H0.riess2020`

: Riess et al. 2020 constraint (arXiv:2012.08534)`H0.freedman2020`

: Freedman et al. 2020 constraint (arXiv:2002.01550)

The above are all based on simple Gaussian posteriors for the value of H0 today, which is not what is directly measured. It can be more accurate to put a constraint on the intrinsic magnitude, which can then be combined with Supernovae constraints to relate to the expansion. An example is provided in

`H0.riess2020Mb`

: Riess et al. 2020 constraint on M_b (arXiv:2012.08534)

which should be run in combination with sn.pantheon with use_abs_mag: True (contributed by Pablo Lemos).

## Using a different measurement

If you would like to use different values for the \(H_0\) constraint, as a mean and a
standard deviation, simply add the following likelihood, substituting `mu_H0`

and
`sigma_H0`

for the respective values:

```
likelihood:
my_H0:
external: 'lambda _self: -0.5 * (_self.provider.get_param("H0") - mu_H0)**2 / (sigma_H0)**2'
requires: ["H0"]
```