In 1929 Edwin Hubble published the first solid evidence that the universe is expanding. Drawing upon data from Vesto Slipher and Henrietta Leavitt, Hubble demonstrated a correlation between galactic distance and redshift. The more distant a galaxy was, the more its light appeared shifted to the red end of the spectrum.
The Hubble parameter has a value of about 70 km/s per megaparsec. This means if a galaxy is about 1 megaparsec away , then the galaxy appears to be moving away from us at about 70 km/s. If a galaxy is 2 megaparsecs away, it will appear to recede at about 140 km/s. The greater a galaxy's distance, the greater its apparent speed.
Since the universe is still expanding, with each passing year a galaxy is a bit more distant, and that means its redshift should become slightly larger. In other words,Theoretical redshift drift based on the standard model. Credit: ESO / ELT Science Case This drift is extremely small. For a galaxy 12 billion light-years away, its apparent speed would be about 95% of the speed of light, while its drift would be just 15 cm/s each year. That's much too small for current telescopes to observe. But when the Extremely Large Telescope starts gathering data in 2027, it should be able to observe this drift in time. Estimates are that after 5–10 years of precise observations, ELT should be able to see redshift drifts on the order of 5 cm/s.
While this will become a powerful tool in our understanding of the universe, it will take a lot of data and a lot of time. So a new paper, published on the preprint serverThe authors call this effect redshift difference. Rather than observing the redshift of a galaxy over decades, the team proposes looking for distant galaxies that are gravitationally lensed by a closer galaxy.
This is still beyond our current ability to detect. But while we are waiting for telescopes such as the ELT to come online, we can search for distant lensed galaxies with multiple images. That way when we do have the ability to detectChengyi Wang et al, The Redshift Difference in Gravitational Lensed Systems: A Novel Probe of Cosmology,
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