Einstein’s Gravitational Lenses Could Clear Up Roiling Debate on Expanding Cosmos

In this Hubble Space Telescope view of the distant quasar RXJ1131-1231, a foreground galaxy smears the image of the background quasar into a bright arc (left) and creates a total of four images — a phenomenon known as gravitational lensing.
(Image: © ESA/Hubble/NASA/Suyu et al.)

Warps in the fabric of spacetime can act like magnifying glasses, and that may help solve a cosmic mystery about the rate of the universe’s expansion, a new study found.

This research may one day lead to more-accurate models of the cosmos, which could shed light on the universe’s ultimate fate, the researchers said.

The universe has continued expanding since its birth, about 13.8 billion years ago. By measuring the present rate of cosmic expansion, known as the Hubble constant, scientists can try to learn the fate of the universe, such as whether it will expand forever, collapse upon itself or rip apart completely.

There are currently two primary strategies for measuring the Hubble constant. One involves monitoring nearby objects whose properties scientists understand well, such as stellar explosions known as supernovas and pulsating stars known as Cepheid variables, to estimate their distances. The other focuses on the cosmic microwave background, the leftover radiation from the Big Bang, examining how it has changed over time.

However, this pair of techniques has produced two different results for the value of the Hubble constant. Data from the cosmic microwave background suggests that the universe is expanding at a rate of about 41.9 miles (67.5 kilometers) per second per megaparsec (a distance equivalent to 3.26 million light-years). However, data from supernovas and Cepheids in the nearby universe suggests a rate of about 46 miles (74 km) per second per megaparsec.

This discrepancy suggests that the standard cosmological model — scientists’ current understanding of the universe’s structure and history — might be wrong. Resolving this debate, known as the Hubble constant conflict, could shed light on the evolution of the cosmos.

In the new study, an international team of researchers explored another way to measure the Hubble constant. This strategy depends on the definition of gravity, according to Albert Einstein’s theory of general relativity, as the result of mass distorting space-time. The greater the mass of an object, the more that space-time curves around the object, and so the stronger the object’s gravitational pull is.

That means gravity can also bend light like a lens would, so objects seen through powerful gravitational fields, such as those produced by massive galaxies, are magnified. Gravitational lensing was discovered a century ago, and today, astronomers often use these lenses to see features otherwise too distant and faint to detect with even the largest telescopes.

The new research analyzes gravitational lenses to estimate their distances from Earth, data that could help researchers estimate the rate at which the universe has expanded over time.

“The new method has great potential to provide a unique perspective in measuring the Hubble constant,” study lead author Inh Jee, formerly an astrophysicist at the Max Planck Institute for Astrophysics in Garching, Germany, told Space.com.

One key to estimating the distance of a gravitational lens from Earth depends on an odd feature of gravitational lensing: It often produces multiple images of lensed objects surrounding the lens, resulting in a so-called “Einstein cross.” Because the light that creates these images takes routes of different lengths around the lens, any variation in the brightness of a lensed object will be visible in some of the images before the others. The greater the mass of the lens, the greater the bending of light, and thus the bigger the time difference between observations of the images. Scientists can use these details to estimate the strength of the gravitational field of the lens and thus its mass.

That mass can then feed into calculations used to estimate distance. But scientists first need an additional key measurement.

The other key to estimating the distance of a gravitational lensing galaxy from Earth involves analyzing the positions and velocities of stars within the lens. When these details are combined with estimates of the mass and strength of the gravitational field of the lensing galaxy, scientists can estimate the actual diameter of the lensing galaxy.

They can then compare the actual diameter of a lensing galaxy with its apparent diameter as seen from Earth. The difference between these values can help researchers estimate how far a galaxy of a given size must be in order to appear the size that it does from Earth.

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