Zusammenfassung
Redshifts have been so easy to measure for so long that we tend to neglect
the fact that they too have uncertainties and are susceptible to systematic
error. As we strive to measure cosmological parameters to better than 1% it is
worth reviewing the accuracy of our redshift measurements. Surprisingly small
systematic redshift errors, as low as 0.0001, can have a significant impact on
the cosmological parameters we infer, such as $H_0$.
Here we investigate an extensive (but not exhaustive) list of ways in which
redshift estimation can go systematically astray. We review common theoretical
errors, such as adding redshifts instead of multiplying by (1+z); using v=cz;
and using only cosmological redshift in the estimates of luminosity and
angular-diameter distances. We consider potential observational errors, such as
rest wavelength precision, air to vacuum conversion at altitude, and
spectrograph wavelength calibration. Finally, we explore physical effects, such
as peculiar velocity corrections, galaxy internal velocities, gravitational
redshifts, and overcorrecting within a bulk flow.
We conclude that it would be quite easy for small systematic redshift errors
to have infiltrated our data and be impacting our cosmological results. While
it is unlikely that these errors are large enough to resolve the current $H_0$
tension, it remains possible, and redshift accuracy may become a limiting
factor in near future experiments. With the enormous efforts going into
calibrating the vertical axis of our plots (standard candles, rulers, clocks,
and sirens) we argue that it is now worth paying a little more attention to the
horizontal axis (redshifts).
Beschreibung
Can redshift errors bias measurements of the Hubble Constant?
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