Measuring the Expansion of the Universe

The universe is vast and continues to grow, even though its total size remains unknown. Scientists can estimate its rate of expansion, but the process is complex. The farther away an object is, the faster it appears to be moving away from us.

Figure 1. Gravitational Lensing Sheds New Light on the Universe’s Expansion.

On average, for every 3.3 million light-years (one megaparsec) from Earth, objects recede at about 73 kilometers per second. This rate—73 kilometers per second per megaparsec (km/s/Mpc)—is known as the Hubble constant. Figure 1 shows Gravitational Lensing Sheds New Light on the Universe’s Expansion.

Traditional Distance Ladders and a New Approach

For decades, astronomers have estimated the Hubble constant using distance ladders—methods built on well-understood objects like Cepheid variable stars and supernovae. These objects are considered reliable “rungs” because their properties are consistent across galaxies. By studying large samples over many years, researchers have narrowed the range of possible values for the Hubble constant. Yet, some uncertainty has remained, motivating the search for independent techniques.

A recent study led by Project Assistant Professor Kenneth Wong and postdoctoral researcher Eric Paic at the University of Tokyo’s Research Center for the Early Universe introduces such a method: time-delay cosmography. This approach reduces reliance on distance ladders and has broader potential for cosmology.

Using Gravitational Lensing to Track Cosmic Expansion

“To measure the Hubble constant with time-delay cosmography, you need a massive galaxy to act as a gravitational lens,” explained Wong. “The lens’s gravity bends light from background objects, creating distorted images. Under the right conditions, we see multiple images, each traveling along slightly different paths and taking different amounts of time to reach us.”

By tracking identical changes in these images that are slightly out of sync, astronomers can measure these time delays. Combining this with models of the lens galaxy’s mass distribution allows for a precise calculation of cosmic expansion. The resulting Hubble constant aligns well with other modern measurements.

Traditional Distance Ladders and a New Approach

For decades, astronomers have estimated the Hubble constant using distance ladders—methods built on well-understood objects like Cepheid variable stars and supernovae. These objects are considered reliable “rungs” because their properties are consistent across galaxies. By studying large samples over many years, researchers have narrowed the range of possible values for the Hubble constant. Yet, some uncertainty has remained, motivating the search for independent techniques.

Using Gravitational Lensing to Track Cosmic Expansion

“To measure the Hubble constant with time-delay cosmography, you need a massive galaxy to act as a gravitational lens,” explained Wong. “The lens’s gravity bends light from background objects, creating distorted images. Under the right conditions, we see multiple images, each traveling along slightly different paths and taking different amounts of time to reach us.”

By tracking identical changes in these images that are slightly out of sync, astronomers can measure these time delays. Combining this with models of the lens galaxy’s mass distribution allows for a precise calculation of cosmic expansion. The resulting Hubble constant aligns well with other modern measurements.

Source: SciTECHDaily

Cite this article:

Priyadharshini S (2025), Universe’s Expansion Gets a New Twist Through Gravitational Lensing, AnaTechMaz, pp.638