Doppler, gravitational and cosmological redshifts

Abstract: Ortiz and Ibarra-Castor (2024) have presented a "Generalized redshift formula" taking account of only energy conservation considerations. Contrary to their claim, we emphasize to invoke both energy and momentum considerations in order to deduce all three types of redshift (Doppler, gravitational and cosmological). We formulate our views on the three physical e ects in a consistent manner in addition to addressing the lack of relevant references in Ref. (Ortiz and Ibarra-Castor, 2024)