On the measurement of cosmological parameters

Rupert Croft


We have catalogued and analysed
cosmological parameter determinations
and their error bars published
between the years 1990 and 2010. Our study focuses on the
popularity of measurements, their precision and their accuracy.
The accuracy of past measurements is gauged by comparison
with the WMAP results of Komatsu et al. (2011).
The 637 measurements in our study are of 12 different
parameters and we place the techniques used to carry them out into 12
different categories.
We find that the popularity of parameter measurements (published
measurements per year) in all 12 cases except
for the dark energy equation of  state parameter $w_0$ peaked between
1995 and 2004. Of the individual techniques, only Baryon Oscillation
measurements were still rising in popularity at the end of the studied
time period. The quoted precision (fractional error)
of most measurements has been declining
relatively slowly, with several parameters, such as the amplitude
of mass fluctutations $\sigma_{8}$ and the Hubble constant $H_{0}$
remaining close to the $10\%$ precision level  for a 10-15 year
The accuracy of recent parameter measurements is generally what would be
expected given the quoted error bars, although before the year 2000,
the accuracy was significantly worse, consistent with an
average underestimate of the error bars by a factor of $\sim 2$.
When used as complement to traditional forecasting techniques, our
results suggest that future measurements of parameters such as fNL,
and $w_{a}$ will have been informed by the gradual improvment in understanding
and treatment of systematic errors and are likely to be accurate. However,
care must be taken to avoid the effects of confirmation bias, which may
be affecting recent measurements of dark energy parameters.
For example, of the 28 measurements of $\Omega_{\Lambda}$ in our sample
published since 2003, only 2 are more than 1 $\sigma$ from the WMAP
results. Wider use of blind analyses in cosmology could help to avoid this.


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