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Article by: Andacs Robert Eugen, on 22 August 2023, at 08:23 am PDT

Edwin Hubble's work in 1929 was a significant moment in the history of cosmology. He provided the first concrete evidence of the universe's expansion, thanks to the data meticulously collected by Vesto Slipher and Henrietta Leavitt. Hubble discovered a relationship between the distance of galaxies and their spectral shift, known as redshift. This correlation revealed that farther away galaxies exhibited a more prominent shift towards the red end of the spectrum.

Today, we understand that the universe's expansion is the result of cosmic expansion, where space itself expands, giving the illusion of galaxies receding from our point of view. The Hubble parameter is the metric that measures the pace of this expansion. Although we have a reasonably precise estimate of this value, there remains some tension in the results obtained from different approaches.

The issue lies in our current limitation to only measure cosmic expansion in the present moment. This limitation restrains our ability to determine whether cosmic expansion follows the principles of general relativity or embodies a more nuanced extension of Einstein's foundational model. However, new telescopic instruments offer a potential solution by enabling us to track the evolutionary trajectory of cosmic expansion through the "redshift drift effect."

Currently, the Hubble parameter is around 70 km/s per megaparsec. This means that for every megaparsec a galaxy distances itself from us, it appears to recede at a velocity of about 70 km/s. The rate increases with distance, resulting in a more significant apparent speed for galaxies positioned farther away. The universe's inexorable expansion means that a galaxy's distance compounds every year, leading to an increase in its redshift. In essence, cosmic expansion dictates that the redshifts of galaxies should gradually shift towards the red end of the spectrum over time.

The movement of galaxies is difficult to detect, especially those 12 billion light-years away. Even though the apparent velocity of such galaxies is about 95% of the speed of light, the actual movement is only 15 cm/s per year which is too small for telescopes to detect. However, the Extremely Large Telescope (ELT) is expected to commence its mission in 2027. It is projected to see redshift drifts of 5 cm/s after 5-10 years of observation.

While this development is exciting, it requires a lot of time and data to unravel the mysteries of the universe. A recently published paper on the preprint server arXiv proposes an alternative approach using gravitational lensing. This approach avoids the need for decades-long observations of single galaxies by identifying distant galaxies subject to gravitational lensing by closer galactic neighbors.

The innovative approach is called the "redshift difference" effect. When a closer intervening galaxy aligns between a distant galaxy and us, the distant galaxy is lensed. It appears as a single, distorted arc adjacent to the foreground galaxy. But on certain occasions, gravitational lensing produces multiple images of a distant galaxy, each covering varying distances and exhibiting a subtly distinct redshift. By comparing these redshifts, we can unveil the elusive redshift drift.

Although this approach is beyond our observational capabilities, it holds great promise. The search for distant lensed galaxies bearing multiple images is an endeavor that can bear fruit in the interim. At the same time, we wait for the deployment of advanced telescopes like the ELT. Ultimately, this approach will help us uncover the mysteries of the universe without being beholden to the protracted passage of time.