Scientists are still coming up empty in the hunt for flaws in Einstein's theory of general relativity that could explain the mysterious force driving the accelerating expansion of the universe.
The researchers studied 100 million galaxies looking for signs that the strength of gravity has varied throughout the universe's history or over vast cosmic distances. Any sign of such a change would indicate that Einstein's theory of general relativity is incomplete or in need of revision. Variation could also shed light on what dark energy is, beyond that it's the name scientists give to whatever is causing the expansion of the universe to accelerate.
Despite finding no such variations in gravity's strength, the work will help two forthcoming space telescopes — the European Space Agency's Euclid mission and NASA's Nancy Grace Roman Space Telescope — also hunt for changes in the strength of gravity through space and back through time.
"There is still room to challenge Einstein's theory of gravity, as measurements get more and more precise," team member and former postdoctoral researcher at NASA's Jet Propulsion Laboratory (JPL), Agnès Ferté, said in a statement.
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To see why dark energy and the universe's accelerating expansion is so troubling to scientists, imagine pushing a child on a swing, watching her slow down and come to an almost complete stop. Then suddenly the swing suddenly speeds up and keeps moving faster without any push.
Scientists' equivalent is that the universe's expansion should be slowing after the initial push of the Big Bang. But it isn't. It's accelerating, and the term "dark energy" is a placeholder for the mysterious force driving this acceleration.
As a result, dark energy is, in effect, working against the force of gravity — pushing cosmic objects apart as gravity draws them together. And because dark energy accounts for around 68% of the universe's energy and matter content, this is a mystery that researchers are eager to solve.
So the Dark Energy Survey crew used the Victor M. Blanco 4-meter Telescope in Chile to look 5 billion years back in time.
Testing gravity through space and time
Light travels at a constant speed, meaning that astronomers see distant cosmic objects as they were in the past.
For example, light takes roughly seven minutes to travel from the sun to Earth, so from our planet we see our star as it was seven minutes ago. Moving further afield, when astronomers look at a Milky Way object one light-year away, they see as it was a year ago. And for some of the distant galaxies that the James Webb Telescope is studying, light has been traveling to us for tens of billions of years and we see the galaxies as they were when the 13.8 billion-year-old universe was in its relative infancy.
It isn't the observations of the galaxies themselves that could hint at changes in the strength of gravity, however, but rather what has happened to their light during its long journey to a telescope.
A foray into spacetime
According to general relativity, mass curves the very fabric of spacetime, with objects of greater mass causing more extreme curvature. A common analogy involves placing balls of various weights on a stretched rubber sheet. A bowling ball creates a deeper dent in the sheet than a tennis ball; a star warps spacetime more than a planet.
Objects like galaxies warp spacetime so strongly that as light passes a galaxy, its path is curved. When this light reaches Earth, the object that emitted it shifts in apparent position in the sky. Astronomers call the effect gravitational lensing.
Because light from a background object can take different paths past a massive object like a galaxy — referred to as a lensing object — gravitational lensing can make the source appear distorted, magnified or even in multiple places in the sky. (It's gravitational lensing that smeared distant galaxies in the first image from the James Webb Space Telescope.)
The effects of gravitational lensing can be more subtle, however, and these subtle effects are often caused by dark matter in the lensing object. And because dark matter interacts only with gravity, ignoring light and other matter altogether, its shape and structure are caused by this force alone.
Einstein was right (again)
But back to the new research. The Dark Energy Survey scientists looked 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 would in turn hint at changes in the strength of gravity over time and space — perhaps shedding light on the mysterious dark energy.
However, observations of the shape of dark matter in 100 million galaxies showed everything still in keeping with Einstein's general relativity.
This doesn't mean the quest is over, however. Astronomers will now turn to the Euclid and Roman space telescopes, set to launch in 2023 and 2027 respectively, to search for these variations in gravity in galaxies that are still more ancient, hoping to spot changes that may set a course toward the understanding of dark energy.
While this new study looked at galaxies as they were 5 billion years ago, Euclid will look back 8 billion years, and Roman 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're ready for Euclid and Roman," Ferté said. "So it's essential we continue to collaborate with scientists around the world on this problem as we've done with the Dark Energy Survey."
The team's results were presented on Aug. 23 at the International Conference on Particle Physics and Cosmology (COSMO'22) in Rio de Janeiro. A paper detailing the team's findings has been posted on the preprint repository arXiv.org.