Nasa is about to launch a space observatory that could help settle some of the biggest mysteries in modern astronomy. The Nancy Grace Roman space telescope will study how the universe began, what it is made of and how it has evolved over time.
Author
- Noelia Noël
Senior Lecturer, School of Mathematics and Physics, University of Surrey
The observatory is named after Nasa's first chief of astronomy and its first female executive. Nancy Grace Roman , who died in 2018 aged 93, made important contributions to our understanding of stars and played a crucial role in shaping Nasa's space-based astronomy programme.
The telescope is undergoing final preparations ahead of a launch set to take place from Florida's Kennedy Space Center in late August . It is eight months ahead of its original schedule, and under budget - a rare feat for a Nasa flagship mission.
Unlike the James Webb space telescope , which is designed to look at individual objects in extraordinary detail, Roman is built to survey the sky at scale. It will take in vast areas of the cosmos, allowing astronomers to build up a much broader picture of the universe and how it changes.
Roman has an unusual history. It began life as a "spare" spy satellite , donated to Nasa by the US government agency responsible for gathering intelligence from space.
Roman carries two main instruments, added by Nasa. These are the wide field instrument and the coronagraph . The wide field instrument will detect infrared light coming from the universe. Infrared light has wavelengths that are slightly longer than those of visible light. This allows the instrument to see through cosmic dust and look deeper into the cosmos.
Its field of view - how much sky it sees when pointed in a given direction - is about 100 times that of the Hubble space telescope's infrared camera, but with similar sharpness. The wide field instrument's capabilities also mean it can survey the cosmos about 1,000 times faster than Hubble. Observations that would take Hubble centuries could be completed by Roman in a fraction of the time.
The coronagraph is designed to block the overwhelming glare of nearby stars. The idea is a bit like placing a hand in front of the Sun to make it easier to see something faint beside it.
By suppressing starlight, the coronagraph will allow astronomers to test methods for directly imaging faint planets and discs of dust and gas around other stars.
Over the course of its mission, Roman is expected to detect billions of galaxies and tens of billions of stars. Roman's combination of scale and precision gives the telescope its power. It will see enough of the sky to reveal broad cosmic patterns, while measuring galaxies and stars accurately enough to detect small but important effects.
Cosmic evolution
Modern cosmology - the study of how the universe originated and evolved - rests on a remarkably successful model . According to this model of the universe, ordinary matter - the stuff that makes up stars, planets and people - accounts for just a small fraction of everything out there. The rest of the cosmos is made up of two unseen, mysterious components.
The first is dark matter , which makes up about 25% of the universe. This form of matter neither reflects nor absorbs light, yet its gravitational pull helps hold galaxies together. The mission could help shed light on the nature of dark matter by tracking how it has shaped the formation of galaxies across different epochs of cosmic history.
The second unseen component is dark energy , which makes up around 70% of the universe. Its exact nature remains elusive, but dark energy is causing the expansion of the universe to accelerate. For decades, astronomers assumed that it is constant, or uniform. However, recent results hint that dark energy could be changing over time . Roman will put these competing ideas to the test.
If dark energy is constant, it points to a universe that keeps expanding forever. If dark energy changes, it could point to new physics beyond our current theories.
Roman may help address a longstanding puzzle : why we observe the universe at a moment when the densities of matter and dark energy are roughly comparable. In the early universe, matter dominated.
In the far future, dark energy is expected to dominate completely. The present era appears to be a transition between the two - a cosmic intermission lasting billions of years, but brief in the lifespan of the universe.
This could be a simple coincidence or a clue to new insights about the cosmos. For example, it could be that dark energy is changing over time, that gravity behaves differently on cosmic scales or that some ingredient is missing from our model of the universe. Distinguishing between these scenarios would require observations across huge spans of cosmic time - exactly the kind of data that Roman will provide.
Beyond cosmology
Roman's science extends well beyond dark energy. It will carry out the largest survey yet of planets beyond our solar system , detecting thousands of new worlds using a technique called gravitational microlensing.
Microlensing happens when the gravity of a foreground star briefly bends and magnifies the light from a more distant background star - acting like a natural magnifying lens passing between us and a distant object. If the foreground star has a planet, that planet can create a small extra brightening in the signal, revealing its presence.
Many of these exoplanets would be difficult to find with the two most widely used techniques to discover exoplanets - the transit and radial velocity methods. Using microlensing, Roman could find planets in wide orbits, low mass planets and even free floating worlds that are not bound to any star.
Roman is also part of a new era of survey astronomy. Its wide-field capabilities place it alongside other major missions. The European Space Agency's Euclid space mission is mapping billions of galaxies to study dark matter and dark energy. The Vera C. Rubin Observatory , built on a mountaintop in Chile, will repeatedly scan the southern sky.
Together, these observatories will help astronomers map the universe across different wavelengths, distances and timescales.
Roman will also work together with the James Webb space telescope, identifying large-scale patterns and unusual objects across the sky, which Webb can then study in detail. This combination of breadth and depth is increasingly important in modern astronomy.
Roman's greatest impact may come from the unexpected. With repeated observations of large areas of sky, it will detect rare events and previously unseen phenomena. For astronomers, this is one of the mission's most exciting aspects.
By mapping the universe in unprecedented detail, the Roman telescope will provide a powerful test of our current understanding of cosmology - and potentially point the way to new physics.
Noelia Noël does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
