In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light). The opposite change, a decrease in wavelength and simultaneous increase in frequency and energy, is known as a negative redshift, or blueshift.

What is a positive redshift?

Astronomers can measure exactly how much redshift or blueshift a galaxy has by looking at its spectrum. A spectrum (the plural is "spectra") measures how much light an object gives off at different wavelengths. ... Positive z values mean the galaxy has a redshift; negative z values mean the galaxy has a blueshift.

What does a redshift of 1 mean?

At redshift z the observed wavelength is larger than that at the source by a factor of 1+z. So z=1 means that the wavelength is twice as long as at the source, z=5 means that the wavelength is 6 times larger than at the source, and so on.