What is the error in the measured value of the Hubble Constant? I've found on the internet that the Hubble constant has been measured to be about $2.3\times 10^{-18} \, \mathrm{s}^{-1}$ Does anyone know what the current error bounds are on this value?
 A: The Hubble constant, $H$, has been measured by many experiments. One of the most recently published measurement by the ESA Planck Surveyor indicates,
$$H\approx 2.20 \times 10^{-18} \pm \underbrace{2.50 \times 10^{-20}}_{\text{uncertainty}} \, \, \, \, \, \text{s}^{-1}$$
or equivalently in more common units (for the Hubble parameter),
$$H\approx 67.80 \pm 0.77 \, \, \frac{\text{km}}{\text{s}\,\text{Mpc}}.$$
A: The paper Planck 2013 results. I. Overview of products and scientific results
, dated 6/5/2014 reports at page 38
$67 \pm  1.2$ km/s per megaparsec.
It explains that other techniques have given higher values in conflict with this value.
"Riess et al. (2011) used Hubble Space Telescope observations of Cepheid variables in the host galaxies of eight SNe Ia to calibrate the supernova magnitude-redshift relation. Their ‘best estimate’
of the Hubble constant, from fitting the calibrated SNe magnitude-redshift relation is:
$73.8 \pm  2.4$ km/s per megaparsec.
A value from using "mid-infrared observations with the Spitzer Space Telescope to recalibrate secondary distance methods used in the HST Key Project" is also cited as:
$74.3 \pm  1.5 \pm  2.1$ km/s per megaparsec (the second uncertainty being systematic).
So the various techniques are not within the statistical uncertainty of each other
