In May 2027, NASA’s Nancy Grace Roman Space Telescope will launch to space. Appropriately named after the “Mother of Hubble,” the telescope will use its 2.4-meter (7.9 ft) wide field of view primary mirror and advanced instruments to investigate the deeper mysteries of the cosmos. Roman will spend 75% of its observing time over its five-year primary mission conducting three core community surveys selected by the scientific community. Among them, Roman will conduct a High-Latitude Time-Domain Survey (HLTDS) to detect tens of thousands of type Ia supernovae.
Astronomers will use these “standard candles” to measure the rate of cosmic expansion and test theories regarding Dark Energy. This mysterious force was first theorized by astronomers and cosmologists in the 1990s to explain the accelerating expansion of the Universe. The survey will utilize Roman’s Wide-Field Instrument (WFI), a 300-megapixel multi-band visible and near-infrared camera that will capture an area 200 times larger than the Hubble Space Telescope and with the same image sharpness and sensitivity.
Measuring the Cosmos
To measure distances on cosmological scales, scientists rely on what is known as the “Cosmic Distance Ladder,,” where each wrung corresponds to a different measuring technique. For objects located within a few hundred thousand to a few million light-years, astronomers will use variable stars (Cepheid Variables or RR Lyrae Variables) as a “standard candle” to conduct parallax measurements. For objects that are a few dozen to a few hundred million light-years distant, nothing less than a Type Ia supernova will suffice for a standard candle.
Type Ia supernovae are especially useful because astronomers know how inherently bright they are at their peak (aka. intrinsic luminosity). By comparing this to their observed brightness, scientists can determine how distant they are. By measuring their redshift, the extent to which it is elongated as it passes through space, scientists can measure the rate of cosmic expansion. In addition, Roman’s sensitivity and high resolution will allow astronomers to observe supernovae that occurred up to 10 billion years ago (ca. 3 billion years after the Big Bang), expanding the observed timeline of cosmic expansion by more than twice.
Masao Sako, the Arifa Hasan Ahmad and Nada Al Shoaibi Presidential Professor of Physics and Astronomy at the University of Pennsylvania, was co-chair of the committee that defined the High-Latitude Time-Domain Survey. As he indicated in a Space Telescope Science Institute (STScI) press release:
Roman is designed to find tens of thousands of type Ia supernovae out to 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 understand more about the nature of dark energy. We have a partnership with the ground-based Subaru Observatory, which will do spectroscopic follow-up of the northern sky, while Roman will do spectroscopy in the southern sky. With spectroscopy, we can confidently tell what type of supernovae we’re seeing.
Recent observations by the James Webb Space Telescope (JWST) revealed a huge population of particularly bright and red galaxies that existed during Cosmic Dawn (ca. less than 1 billion years after the Big Bang). These “little red dots” (LRDs), as they’ve come to be known, surprised astronomers since they were brighter and more plentiful than accepted cosmological models would predict. Webb’s early observations also revealed that the Universe expanded faster than these models predicted, prompting new theories about “Early Dark Energy” (EDE).
Moreover, recent findings from the Dark Energy Survey (DES) suggest that the influence of Dark Energy may be weakening over time. If true, this will have serious implications for our current cosmological models, which predict that cosmic expansion will continue until the Universe experiences a scenario known as “heat death,” where the last of the stars die. By detecting type Ia supernovae up to 11 billion light-years away, Roman could test these and other theories regarding this mysterious, theoretical force.
This infographic describes the High-Latitude Time-Domain Survey that will be conducted by NASA’s Nancy Grace Roman Space Telescope. Credit: NASA’s Goddard Space Flight Center
Finding Supernovae
The HLTDS will be split into two imaging “tiers” in the northern and southern skies, consisting of a wide tier covering a larger area of more than 18 square degrees, targeting objects within the past 7 billion years of cosmic history. There will also be a deep tier that will focus on smaller areas (6.5 square degrees) for longer time intervals to detect fainter objects that existed up to 10 billion years ago. To detect transient objects, the HLTDS will begin with a 15-day observation period where Roman will visit many cosmic fields to establish a baseline for comparison.
This will be followed by 180 days of observing the same fields at regular intervals, largely during the middle part of its 5-year primary mission. This process, said Sako, is called image subtraction, where images are taken of a field, and anything static or unchanging is subtracted from new images of the same field to isolate new things. The survey will also include an extended component where observing fields will be revisited every 120 days to search for objects that change over longer periods.
This will allow Roman to observe some of the most energetic and longest-lasting transient events and objects that existed up to one billion years after the Big Bang.
These latter supernovae vary in brightness more slowly due to the time dilation caused by cosmic expansion. “You really benefit from taking observations over the entire five-year duration of the mission,” said survey co-chair Brad Cenko of NASA’s Goddard Space Flight Center. “It allows you to capture these very rare, very distant events that are really hard to get at any other way but that tell us a lot about the conditions in the early Universe.”
The HLTDS is one of three core community surveys, the others being the High-Latitude Wide-Area Survey (HLWAS) and the Galactic Bulge Time-Domain Survey (GBTDS). Together, these surveys will help map the Universe with a clarity and depth that has never been achieved. Roman’s achievements will also complement those of the ESA’s Euclid mission, which is currently observing objects in our Universe from up to 10 billion years ago, also for the purpose of measuring the influence of Dark Energy.
Further Reading: STScI