NASA's Roman telescope hopes to solve the dark matter mystery

The universe keeps hiding most of its mass, yet it leaves telltale fingerprints. Astronomers are about to read them with NASA’s Nancy Grace Roman Space Telescope, slated to launch in 2027. 

Roman’s survey camera will sweep the sky for more than 160,000 distorted galaxy images, which act as natural zoom lenses forged by gravity.

Lead author Bryce Wedig of Washington University in St. Louis said the haul will “let us measure the small‑scale structure of the cosmos in ways that were impossible before.” 

Understanding dark matter – the basics

According to commonly accepted theory, dark matter is an invisible form of matter that does not emit, absorb, or reflect light, making it undetectable through conventional electromagnetic observations. Yet, it exerts a significant gravitational influence on visible matter in the universe.

Scientists first inferred its existence by noticing discrepancies in the rotation speeds of galaxies. Stars at the outer edges of galaxies move much faster than expected based on the observable mass alone.

To account for this anomaly, astronomers proposed the presence of an unseen mass – dark matter – that makes up approximately 85% of the total matter in the universe.

Despite its abundance, dark matter remains one of the most elusive components in cosmology. Astronomers hope that the Roman Space Telescope will put an end to the search.

Bending light unmasks dark matter

The first trick for the Roman telescope involves gravitational lensing. This is Einstein’s idea that mass curves space so much that it warps passing light.

When a hefty foreground galaxy lines up with a more distant, fainter one, the background glow stretches into arcs, crescents, or even full rings.

Physicists already know that visible stars and gas account for only about one‑sixth of a galaxy’s pull.

The missing piece is dark matter, an unseen substance with a gravitational pull that corrals galaxies and keeps clusters intact.

Roman’s lenses act like laboratory scales. By comparing the brightness and positions of multiple images of the same galaxy, astronomers can infer how much invisible mass is sprinkled through the foreground system.

Roman will detect tiny dark matter clumps

Dark matter is not expected to sit smoothly. Simulations predict countless clumps – some as light as a million Suns, orbiting larger halos like cosmic goose bumps.

Tiny clumps create milliarcsecond kinks in the stretched background arcs, a pattern called substructure.

Spotting enough of those kinks across Roman’s sample lets researchers test whether dark matter is made of sluggish “cold” particles or warmer, faster ones that would erase the smallest clumps.

Wedig’s team estimates that Roman will deliver roughly 500 “gold‑standard” lenses that are bright and sharp enough to weigh individual clumps down to about 108 solar masses.

That is an order of magnitude finer than what the Hubble Space Telescope managed with a few dozen prime systems.

Wide views with sharp resolution

Each Roman frame covers 0.28 square degrees, about 200 Hubble fields stitched together, yet keeps Hubble‑class resolution. The wide‑field instrument achieves this by packing 18 infrared detectors into a honeycomb the size of a door.

During its High‑Latitude Wide‑Area Survey, Roman will take single 146‑second snapshots through four infrared filters.

Simulations show an average of 27 strong lenses hiding in every exposure, enough to amass the 160,000‑lens trove in under two years of sky time.

Wedig’s team also notes that hundreds of the lenses will be perfect for deep follow‑up: stronger distortions, cleaner arcs, and higher signal‑to‑noise.

These choice targets will let scientists trace clumps at even smaller mass scales, tightening the leash on competing dark‑matter models.

Past surveys found fewer and blurrier lenses

Hubble and ground‑based surveys such as SLACS and BOSS Emission-Line Lens Survey (BELLS) uncovered about 1,500 strong lenses over two decades.

Those programs proved the method but lacked volume: only a handful of systems were pristine enough to reveal a lone subhalo.

“Roman will not only significantly increase our sample size, its sharp, high‑resolution images will also allow us to discover gravitational lenses that appear smaller on the sky,” said Tansu Daylan, a co‑author of the new study and faculty fellow at Washington University. 

Better statistics mean astronomers can move from anecdotal detections to population studies, comparing the number of subhalos with the predictions of various particle candidates.

Teaming up with other telescopes

Europe’s Euclid mission, launched in 2023, is already mapping dark matter over a third of the sky at optical wavelengths.

Roman’s deeper infrared reach will pair with Euclid’s breadth, giving each lens a color baseline that sharpens distance estimates for both source and lens.

Closer to home, the Vera C. Rubin Observatory will add nightly optical alerts from the ground, flagging any variable sources like supernovae that pop up inside the arcs.

Those time‑delay measurements can pin down Hubble’s constant while also checking mass models derived from lensing.

Challenges Roman must tame

Roman’s huge focal plane comes with quirks. Wave‑front errors, detector sensitivity shifts, and varying point‑spread functions can masquerade as, or mask, the faint substructure signal.

Wedig’s group built a new pipeline, MEJIRO, to inject realistic telescope noise into synthetic images and test detection algorithms.

They found that ignoring field‑dependent blurring could sway the inferred mass of the smallest clumps by 5 %, which is small but critical when chasing subtle physics. 

Another hurdle is line‑of‑sight clutter: unrelated galaxies between lens and Earth also bend light.

Accounting for those extra tugs requires combining Roman infrared data with ground‑based redshift surveys to map intervening structures.

What dark matter clues will show

If Roman counts as many low‑mass clumps as cold‑dark‑matter theory expects, the prevailing model gains a decisive win.

Should the census come up short, physicists might favor warmer particles or even entirely new forces acting in the dark sector.

Either way, every curved smudge that Roman records is a new piece of evidence. “We won’t see dark matter in the images – it’s invisible – but we can measure its effects,” Wedig noted.

The study is published in The Astrophysical Journal.

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