NASA's upcoming Nancy Grace Roman Space Telescope, planned for launch no later than May 2027, with teams working diligently on a possible launch as early as fall 2026, represents a new era in astronomical research. This observatory, equipped with advanced instruments, is envisioned as a kind of discovery machine, primarily thanks to its extremely wide field of view that will generate an unprecedented amount of scientific data. Its key instrument, the Wide Field Instrument (WFI), which operates in the near-infrared spectrum, is capable of capturing an area of the sky that is 200 times larger than what the infrared camera on the legendary Hubble Space Telescope can record, with the same image sharpness and sensitivity. As part of its five-year primary mission, the Roman telescope will dedicate approximately 75% of its observing time to conducting three core community surveys, which have been defined through extensive collaboration within the scientific community. One of these key surveys will be dedicated to a systematic search of the sky for cosmic events that appear, flash, and change over time, such as stellar explosions or neutron star collisions.

In Search of Transient Cosmic Phenomena

The program called the High-Latitude Time-Domain Survey will direct its gaze outside the plane of our Milky Way galaxy, specifically to areas of high galactic latitudes. The goal is to study objects whose brightness changes over time. The main scientific driver of this ambitious survey is the detection of tens of thousands of a specific type of stellar explosion known as Type Ia supernovae. These supernovae are of exceptional importance in modern cosmology because they serve as "standard candles" – tools with which astronomers can precisely measure distances in the universe and map the history of its expansion. By analyzing these cosmic cataclysms, scientists aim to unravel some of the greatest mysteries of the universe, including the nature of the enigmatic dark energy.

Masao Sako from the University of Pennsylvania, who co-chaired the committee responsible for defining this survey, points out: "Roman is designed to find tens of thousands of Type Ia supernovae at greater distances than ever before. Using them, we can measure the expansion history of the universe, which depends on the amount of dark matter and dark energy. Ultimately, we hope to learn more about the nature of dark energy itself." It is the data from Roman that will provide key insights into whether dark energy is a constant or if its strength has changed throughout cosmic history.

Supernovae as the Key to Understanding Dark Energy

Type Ia supernovae are extremely useful as cosmological probes because astronomers know their intrinsic brightness at the moment of peak explosion with great certainty. They always explode with nearly identical maximum brightness. By comparing this known intrinsic brightness with their observed, apparent brightness from Earth, scientists can calculate how far away they are. The dimmer the supernova appears, the farther away it is from us. At the same time, the Roman telescope will also be able to measure the speed at which these supernovae are apparently receding from us by analyzing the redshift of their light. By tracking the recession speed at different distances, scientists will be able to precisely reconstruct the history of cosmic expansion through different epochs of the universe. This is crucial for understanding the role of dark energy, the force driving the accelerated expansion of the universe.

Only the Roman telescope will have the ability to find the faintest and most distant supernovae that illuminated the early cosmic epochs. It will thus complement the work of ground-based telescopes, such as the Vera C. Rubin Observatory in Chile, which are limited by the absorption of light in the Earth's atmosphere and other effects. The greatest strength of the Rubin Observatory will be in finding supernovae that occurred within the last 5 billion years. On the other hand, Roman will extend this collection to much earlier periods in the history of the universe, reaching as far back as 11 billion years into the past, to a time when the universe was only about 3 billion years old. This will more than double the measured timeline of the universe's expansion history, providing unprecedentedly detailed insight. Recently, the Dark Energy Survey revealed hints that dark energy might be weakening over time, rather than being a constant expansive force. Roman's investigations will be crucial for testing this intriguing possibility.

Observation Strategy and the Hunt for Rare Events

To detect transient objects, whose brightness changes, Roman must revisit the same areas of the sky at regular intervals. The High-Latitude Time-Domain Survey will dedicate a total of 180 days of observing time to these observations, spread over a five-year period. Most of the observations will take place during a two-year period in the middle of the mission, when the same fields will be re-imaged every five days. An additional 15 days of observation will be conducted at the beginning of the mission to establish a baseline, reference image of the sky.

"To find things that change, we use a technique called image subtraction," explains Sako. "You take a new image and subtract an image of the same part of the sky taken much earlier – as early as possible in the mission. In this way, you remove everything that is static, and you are left with only the objects that are new or have changed in brightness."

The survey will also include an extended component that will revisit some of the observation fields approximately every 120 days to search for objects that change on longer timescales. This will help in the detection of the most distant transient phenomena that existed even 13 billion years ago, or just one billion years after the Big Bang. These objects vary more slowly due to the time dilation caused by the expansion of the universe.

"We really benefit from observing over the entire five-year duration of the mission," says Brad Cenko of NASA's Goddard Space Flight Center, the other co-chair of the survey committee. "This allows you to catch these very rare, very distant events that are otherwise hard to come by, but which tell us a lot about the conditions in the early universe."

The Quest for the Most Exotic Phenomena in the Universe

This extended component of the survey will collect data on some of the most energetic and longest-lasting transient phenomena. Among them are Tidal Disruption Events, spectacular phenomena in which a supermassive black hole literally tears apart a star that gets too close with its gravity. The survey will also search for predicted but as yet unseen events known as pair-instability supernovae. These are theoretically predicted explosions of extremely massive stars that are so powerful they leave behind no remnant, such as a neutron star or a black hole, but are completely dispersed into space. The discovery of such an event would represent a huge step forward in understanding the evolution of the most massive stars.

Survey Details and Global Collaboration

The High-Latitude Time-Domain Survey will be divided into two imaging tiers: a wide tier that will cover a larger area of the sky and a deep tier that will focus on a smaller area, but with longer exposure times to detect fainter and more distant objects. The wide tier, which will cover just over 18 square degrees (an area of the sky as large as 90 full Moons), will target objects within the last 7 billion years, i.e., in the second half of the universe's history. The deep tier, covering an area of 6.5 square degrees, will reach fainter objects that existed as long as 10 billion years ago. The observations will take place in two areas, one in the northern and one in the southern sky, to ensure comprehensive coverage.

This survey will also be joined by a spectroscopic component, which will be limited to the southern sky. "We have a partnership with the ground-based Subaru Observatory, which will perform spectroscopic follow-up in the northern sky, while Roman will do spectroscopy in the southern sky. With spectroscopy, we can confidently determine what type of supernova it is," explains Cenko. Spectroscopy breaks down an object's light into its constituent colors, revealing its chemical composition and other physical characteristics, which is crucial for confirming that it is a Type Ia supernova. Together with the other two core surveys of the Roman telescope, the High-Latitude Wide-Area Survey and the Galactic Bulge Time-Domain Survey, this survey will help map the universe with a clarity and depth never before achieved.