In 1929, Edwin Hubble published the first convincing evidence that the universe is expanding. Based on data from Vesto Slifer and Henrietta Leavitt, Hubble showed a relationship between galactic distance and redshift. The further away the galaxy was, the more its light seemed to be shifted towards the red end of the spectrum. We now know that this is due to cosmic expansion. Space itself is expanding, so distant galaxies seem to be moving away from us. The rate of this expansion is known as the Hubble parameter, and while we have a good idea of ​​its meaning, there are still some inconsistencies between the different results.

One of the difficulties in resolving this tension is that so far we have only been able to measure cosmic expansion as it now appears. This also means that we cannot tell whether cosmic expansion is the result of general relativity or a more subtle extension of Einstein’s model. But as powerful new telescopes are built, we can observe the evolution of cosmic expansion thanks to what’s known as the redshift drift effect.

The Hubble parameter has a value of about 70 km/s per megaparsec. This means that if a galaxy is about 1 megaparsec (about 3 million light-years) away, the galaxy appears to be moving away from us at about 70 km/s. If a galaxy is 2 megaparsecs away, it will recede at about 140 km/s. The greater the distance to the galaxy, the greater its apparent speed. Since the universe is still expanding, the galaxy is getting further and further away each year, which means its redshift must increase slightly. In other words, cosmic expansion means that the redshift of galaxies should shift more redshifts over time.

This shift 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 only 15 cm/s each year. This is too small for observations with modern telescopes. But when the Extremely Large Telescope (ELT) starts collecting data in 2027, it will be able to observe this shift in time. It is estimated that after 5-10 years of accurate observations, the ELT will be able to see redshift shifts of the order of 5 cm/s. While this is a powerful tool for our understanding of the universe, it will take a lot of data and a lot of time. So the new paper proposes another method using gravitational lensing.

The authors refer to this effect as the redshift difference. Rather than observing a galaxy’s redshift over decades, the team proposes to look for distant galaxies that are gravitationally lensed by a nearby galaxy. Many distant galaxies are lensed by a closer galaxy between us and a distant galaxy, but most lensed galaxies appear as a single warped arc next to the foreground galaxy.

But sometimes gravitational lensing can produce multiple images of a distant galaxy. Since each image of a distant galaxy takes a slightly different path to reach us, each path’s distance is also slightly different. So instead of waiting decades for the galaxy to move away from us, we can take snapshots of the galaxy divided over years or decades. Each image will have a slightly different redshift, and by comparing them we can measure the redshift shift. Still can’t detect it. But while we wait for telescopes like the ELT to kick in, we can look for distant lensed galaxies with a few images. That way, once we can detect the redshift shift, we won’t have to wait decades for results. Source