NASA's Nancy Grace Roman Space Telescope launches to solve universe's dark energy mystery

In the late 90s, a space telescope helped flip our understanding of the universe on its head.

Data from the newly minted Hubble Space Telescope, along with a range of ground-based telescopes, showed the universe's expansion was not slowing down as previously thought.

It was speeding up, driven by a shadowy phenomenon dubbed "dark energy".

The astrophysicists who discovered this acceleration, including Australian Brian Schmidt, won the 2011 Nobel prize for their efforts.

But data from our latest galactic maps suggest even the small amount we think we know about dark energy could be wrong.

Nearly 30 years after the dark energy discovery, NASA's new telescope, which was launched overnight, may help solve the mystery.

The Nancy Grace Roman Space Telescope may even drive the next dramatic shift in our understanding of the universe.

"Roman will be a discovery machine that will bring us closer than ever before to answering humanity's most profound questions about our cosmic history," Nicky Fox, associate administrator for NASA's Science Mission Directorate, says in a statement.

NASA astrophysicist Ami Choi says scientists will be able to use Roman to check their results from other telescopes.

If the results are confirmed, scientists will need to toss out their working model for the universe and come up with a new one.

Elusive dark energy permeates the universe

Studying dark energy and its equally mysterious counterpart, dark matter, is a major objective of the Roman mission.

Neither interacts with ordinary matter in ways that make them easy to observe.

But physicists know both phenomena exist because they can see their effects in how galaxies move through space and time.

Cullan Howlett, an astrophysicist at the University of Queensland, says galaxies are distributed across the universe in clusters, connected by filaments across empty voids of space.

"That pattern encodes the ingredients that you need to make a universe, and also how fast the universe has expanded over time since the big bang."

Researchers have been building three-dimensional maps of the universe that show how the cosmic web has formed.

The more detailed the map, the easier it is to judge how dark energy and dark matter have influenced it.

Are we wrong about dark energy?

For the past 20 years, astrophysicists believed dark energy was constant from the big bang to today.

But Dr Howlett says a few recent experiments, including the Dark Energy Spectroscopic Instrument (DESI) and the Dark Energy Survey, hint otherwise.

Instead of being constant, driving the universe into ever-accelerating expansion, dark energy might be getting weaker.

"What we predict the universe might look like in the future will change very dramatically based on what dark energy actually is," Dr Howlett says.

"If dark energy is weakening, eventually the expansion rate of the universe slows down. And if it accelerates faster, eventually the universe could potentially rip itself apart."

If these results pan out, physicists would need to discard their current model of the universe's origin, called the Lambda-CDM model, and develop something new.

Dr Choi compares dark energy research to huge shifts in our understanding of gravity as we went from Newton's laws of motion to Einstein's theory of general relativity.

These dramatic upheavals in scientific thought are known as "paradigm shifts".

But the paradigm on dark energy hasn't shifted yet.

"It could be that the results that we found are due to some quirk of how DESI's measurements have been made," Dr Howlett says.

While the hundreds of astrophysicists working on these projects have been thorough, it's possible the instruments they've used, or the portions of the sky they've studied, are misleading them.

"If you detect something really exciting with one telescope or one experiment, you want to be able to confirm that with an independent dataset," Dr Howlett says.

Enter Roman.

Roman's tools for probing the dark universe

While it's not as powerful as the James Webb Space Telescope, Roman's infrared camera is designed to capture much larger images so it can see many more distant and ancient galaxies in one shot.

It will send back its first images and begin science operations in about three months.

Once it's operating, Dr Howlett wants to add Roman's data to observations from other ground and space telescopes to understand the cosmic web.

"We take as much data as we can from all these different telescopes and stitch them together and make, hopefully one day, the complete map of the universe and all the galaxies it contains."

Brad Tucker, an Australian National University astrophysicist, says Roman's wide-angle camera will help spot a specific type of supernova.

These stars explode in a very consistent way, so observing them can help to calibrate distances; this was how Professor Schmidt and his colleagues originally realised the universe was accelerating.

But since the explosions are rare and fleeting, finding them can be hard. Roman's large shots have a much higher chance of spotting them.

"For supernovas and dark energy, it's a numbers game," Dr Tucker says.

Roman will also allow researchers to make more observations with "gravitational lensing": using the gravity of massive objects to distort light from distant things.

Confirming or refuting the findings on dark energy

Roman will use its space vantage point to see further back into time than ground-based telescopes can.

Its results will help physicists confirm or refute their other findings in its first five years of operation.

Dr Choi is excited for both options.

"Anytime you get a different answer from another experiment, you learn something," she says.

"Either you learn that there's something that you didn't quite understand before about the ongoing experiment, or the one that came before."