Measuring the distance to many distant galaxy with a standard candle technique.
Hubble presented the observational evidence for one of science’s greatest discoveries—the expanding universe. Hubble showed that galaxies are receding away from us with a velocity that is proportional to their distance from us: more distant galaxies recede faster than nearby galaxies.
A redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation.
The standard candle technique is particularly valuable when studying very distant objects. Eventually, the effects of parallax stop working because things get too far away. However, since a standard candle depends on the inverse square law, it can reach much further as a result.
As long as we have instruments that can detect a standard candle and measure its brightness, it can be used to measure a distance.
therefore before measuring the recission velocity one must use standard candle technique to eliminate any parallax
learn more about parallax here:
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