Scientists differentiate between the redshift due to distance and the redshift caused by speed using various observational and theoretical methods. Here are a few ways they distinguish between the two:
Hubble's Law: The observation of redshift in distant galaxies led to the formulation of Hubble's Law, which states that the recessional velocity of a galaxy is directly proportional to its distance from us. The redshift caused by the expansion of the universe (distance-related) follows a specific pattern described by this law. By analyzing the relationship between redshift and distance for a large sample of galaxies, scientists can determine whether the redshift is primarily due to the expansion of space or some other factor.
Cosmological Models: Scientists use theoretical models, such as the Big Bang model and general relativity, to describe the behavior of the universe and its expansion. These models allow for predictions about the expected redshift patterns caused by distance and speed. By comparing observed redshifts with the predictions of these models, scientists can evaluate the contributions of distance and speed to the overall redshift.
Multiple Observational Indicators: Redshift caused by speed (also known as Doppler shift) affects the entire spectrum of light from an object, whereas redshift caused by distance (cosmological redshift) affects specific spectral features. By examining the detailed shape and distribution of the redshift across different wavelengths, scientists can determine whether the redshift is primarily due to the motion of the object itself or the expansion of the universe.
Galaxy Clustering: The large-scale structure of the universe exhibits patterns of galaxy clustering. If the redshift is primarily due to distance, the clustering patterns should be consistent across different redshifts. On the other hand, if redshift is mainly caused by the motion of galaxies, the clustering patterns would exhibit distortions or peculiar motions. By studying the clustering of galaxies at different redshifts, scientists can discern the relative contributions of distance and speed to the observed redshift.
It's important to note that redshift can be influenced by both distance and speed, and disentangling their effects can be challenging. Scientists employ a combination of these methods and often rely on statistical analyses to infer the underlying causes of observed redshifts in astronomical observations.