Science

Dark energy remains a mystery as Einstein’s theory of gravity passes another test

Scientists are still looking to no avail for flaws in Einstein’s theory of general relativity that could explain the mysterious force driving the accelerating expansion of the universe.

Researchers have studied 100 million galaxies looking for signs that the strength of gravity has changed throughout the history of the universe or across vast cosmic distances. Any sign of such a change would indicate that Einstein’s general theory of relativity is incomplete or in need of revision. The variation could also shed light on what dark energy is, beyond scientists calling it what causes the expansion of the universe to accelerate.

While no such changes in gravity have been detected, the work will help two future space telescopes – the European Space Agency’s Euclid mission and NASA’s Nancy Grace Roman Space Telescope – also track changes in gravity through space and back through time.

“There is still an opportunity to challenge Einstein’s theory of gravity as measurements become more and more precise,” team member and former NASA Jet Propulsion Laboratory (JPL) scientist Agnès Ferté said in a statement.

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To understand why dark energy and the accelerating expansion of the universe are so troubling for scientists, imagine swinging a child on a swing, watching it slow down and almost come to a complete stop. Then, all of a sudden, the swing suddenly speeds up and continues to move faster without any push.

The scientists’ equivalent is that the expansion of the universe should slow down after the initial push of the Big Bang. But it’s not. It is accelerating, and the term “dark energy” is a substitute for the mysterious force that causes this acceleration.

As a result, dark energy essentially works against the force of gravity, pushing space objects apart when gravity brings them closer together. And since dark energy makes up about 68% of the energy and matter in the universe, it’s a mystery that researchers are eager to unravel.

So the Dark Energy Explorer team used the 4-meter Victor M. Blanco telescope in Chile to look back 5 billion years.

Test of gravity in space and time

Light travels at a constant speed, which means that astronomers see distant space objects as they were in the past.

For example, it takes about seven minutes for light to travel from the Sun to the Earth, so from our planet we see our star as it was seven minutes ago. Moving on, when astronomers look at a Milky Way object one light-year away, they see what it was like a year ago. And for some of the distant galaxies that the James Webb telescope studies, light has been coming to us for tens of billions of years, and we see galaxies as they were when the Universe, which is 13.8 billion years old, was in relative infancy.

However, not observations of the galaxies themselves may hint at changes in the strength of gravity, but what happened to their light during its long journey to the telescope.

Foray into space-time

According to general relativity, mass warps the very fabric of space-time, and objects of greater mass cause more curvature. A common analogy involves placing balls of varying weights on a stretched rubber sheet. A bowling ball leaves a deeper dent in a sheet than a tennis ball; the star distorts space-time more than the planet.

Objects like galaxies warp space-time so much that light passing through a galaxy is bent. When this light reaches the Earth, the object emitting it is shifted to a visible position in the sky. Astronomers call this effect gravitational lensing.

Because light from a background object can take multiple paths past a massive object such as a galaxy, called a lensing object, gravitational lensing can cause the source to be distorted, magnified, or even at multiple locations in the sky. (It was gravitational lensing that smeared distant galaxies in the first image taken by the James Webb Space Telescope.)

However, gravitational lensing effects can be more subtle, and these subtle effects are often caused by dark matter in the lensing object. And since dark matter interacts only with gravity, completely ignoring light and other matter, its shape and structure are determined only by this force.

The first publicly released scientific-quality image from NASA’s James Webb Space Telescope, released on July 11, 2022, is the deepest infrared image of the universe to date. (Image credit: NASA, ESA, CSA and STScI)

Einstein was right (again)

But back to the new study. Scientists at the Dark Energy Research have been looking for these subtle distortions, called “weak gravitational lensing,” in images of distant galaxies. The researchers reasoned that this would reveal changes in the distribution of dark matter in lensing galaxies, which in turn would hint at changes in the strength of gravity across time and space—perhaps shedding light on the mysterious dark energy.

However, observations of the shape of dark matter in 100 million galaxies have shown that everything is still consistent with Einstein’s general theory of relativity.

However, this does not mean that the quest is over. Astronomers will now turn to the Euclid and Roman space telescopes, due to launch in 2023 and 2027 respectively, to look for these gravity variations in even more ancient galaxies, hoping to discover changes that could pave the way for understanding darkness. energy.

While this new study looked at galaxies as they were 5 billion years ago, Euclid will look back 8 billion years ago, and Romanus will look back even further, observing galaxies as they were 11 billion years ago, according to NASA.

“We still have so much to do before we are ready for Euclid and Roman,” Ferte said. “Therefore, it is important that we continue to collaborate with scientists around the world on this issue, as we have done with the study of dark energy.”

The results of the team were presented on 1 August. December 23 at the International Conference on Particle Physics and Cosmology (COSMO’22) in Rio de Janeiro. A document detailing the team’s findings has been posted to the arXiv.org preprint repository.

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