New research could help scientists use gravitational lensing — the warping of light from distant galaxies — to investigate the accelerating expansion of the universe.
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An
artist's interpretation of Einstein's theory of general relativity. (Image
credit: coffeekai via Getty Images) |
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 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.
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.
The first
publicly released science-quality image from NASA's James Webb Space Telescope,
revealed on July 11, 2022, is the deepest infrared view of the universe to
date. (Image credit: NASA, ESA, CSA, and STScI) |
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."
Reference: Research Paper
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