Scientists help probe dark energy by testing gravity

Scientists help probe dark energy by testing gravity

This image – the first released by NASA’s James Webb Space Telescope – shows galaxy cluster SMACS 0723. Some of the galaxies appear smeared or stretched due to a phenomenon called gravitational lensing. This effect can help scientists map the presence of dark matter in the universe. Credit: NASA, ESA, ASC, STScI

Could one of the biggest puzzles in astrophysics be solved by reworking Albert Einstein’s theory of gravity? A new study co-authored by NASA scientists says no.

The universe is expanding at an accelerating rate, and scientists don’t know why. This phenomenon seems to contradict everything researchers understand about the effect of gravity on the cosmos: it’s like throwing an apple in the air and it keeps going up, faster and faster. The cause of the acceleration, called dark energy, remains a mystery.

A new study from the International Dark Energy Survey, using the 4-meter Victor M. Blanco Telescope in Chile, marks the latest effort to determine if it’s just a misunderstanding: that expectations about how the gravity on the scale of the entire universe are defective or incomplete. This potential misunderstanding could help scientists explain dark energy. But the study – one of the most accurate tests to date of Albert Einstein’s theory of gravity on a cosmic scale – reveals that the current understanding still seems correct.

The results, written by a group of scientists including some from NASA’s Jet Propulsion Laboratory, were presented on Wednesday August 23 at the International Conference on Particle Physics and Cosmology (COSMO’22) in Rio de Janeiro. The work helps set the stage for two upcoming space telescopes that will probe our understanding of gravity with even higher precision than the new study and perhaps finally solve the mystery.

Over a century ago, Albert Einstein developed his theory of general relativity to describe gravity, and so far he has accurately predicted everything from Mercury’s orbit to the existence of black holes. . But if this theory can’t explain dark energy, according to some scientists, then maybe they need to change some of its equations or add new components.

To find out if this is the case, members of the Dark Energy Survey looked for evidence that the strength of gravity has varied throughout the history of the universe or over cosmic distances. A positive discovery would indicate that Einstein’s theory is incomplete, which could help explain the accelerating expansion of the universe. They also looked at data from other telescopes besides Blanco, including ESA’s (European Space Agency) Planck satellite, and came to the same conclusion.

This video explains the phenomenon called gravitational lensing, which can cause distorted or smudged images of galaxies to appear. This distortion is caused by gravity, and scientists can use this effect to detect dark matter, which neither emits nor reflects light. Credit: NASA Goddard Space Flight Center

The study reveals that Einstein’s theory still works. So no explanation yet for dark energy. But that research will feed into two upcoming missions: ESA’s Euclid mission, slated for launch no earlier than 2023, which is benefiting from contributions from NASA; and NASA’s Nancy Grace Roman Space Telescope, slated for launch no later than May 2027. Both telescopes will look for changes in the force of gravity over time or distance.

Blurred vision

How do scientists know what happened in the universe’s past? Looking at distant objects. A light year is a measure of the distance light can travel in a year (about 6 trillion miles, or about 9.5 trillion kilometers). This means that an object a light-year away appears to us as it was a year ago, when light first left the object. And galaxies billions of light-years away appear to us as billions of years ago. The new study looked at galaxies dating back around 5 billion years in the past. Euclid will look 8 billion years in the past, and Roman will look back 11 billion years.

The galaxies themselves do not reveal the force of gravity, but what they look like seen from Earth. Most of the matter in our universe is dark matter, which does not emit, reflect, or interact with light. Although scientists don’t know what it’s made of, they do know it’s there, because its gravity reveals it: the great reservoirs of dark matter in our universe are warping space itself. As light travels through space, it encounters these distorted portions of space, causing curved or blotchy images of distant galaxies to appear. This was displayed in one of the first images released by NASA’s James Webb Space Telescope.

Dark Energy Survey scientists are looking for more subtle distortions in images of galaxies due to dark matter bending space, an effect called weak gravitational lensing. The force of gravity determines the size and distribution of dark matter structures, and the size and distribution in turn determines how distorted these galaxies appear to us. This is how images can reveal the force of gravity at different distances from Earth and at distant times throughout the history of the universe. The group has now measured the shapes of over 100 million galaxies, and so far the observations match what is predicted by Einstein’s theory.

“There is still room to challenge Einstein’s theory of gravity, as measurements become more and more precise,” said study co-author Agnès Ferté, who led the research as as a postdoctoral researcher at JPL. “But we still have a long way to go before we’re ready for Euclid and Roman, so it’s essential that we continue to collaborate with scientists around the world on this issue, as we did with the Dark Energy Survey.”

NASA’s Roman mission will test competing theories of cosmic acceleration

More information:
Dark Energy Survey Year 3 Results: Constraints on Weak Lens ΛCDM Extensions and Galaxy Clustering, arXiv:2207.05766 [astro-ph.CO]

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