What if the cosmos’s most violent deaths might shed light on its darkest secrets? NASA’s Nancy Grace Roman Space Telescope, to be launched by May 2027, is about to do just that. Its daring High-Latitude Time-Domain Survey will scan huge regions of the sky repeatedly every five days for two years, recording the fleeting luminosity of perhaps 100,000 cosmic explosions such as supernovae and kilonovas of several classes as forecast by recent simulations.
At the core of Roman’s approach is the methodical discovery of some 27,000 Type Ia supernovae, ten times the combined total of all past surveys, as reported by NASA. These “standard candles” explosions of stars with incredibly consistent peak luminosity are the cornerstones of measurements of cosmic distances. By tracing their distribution over cosmic history, Roman will map the universe’s expansion history with unprecedented accuracy. “Filling these data gaps could also fill in gaps in our understanding of dark energy,” said Benjamin Rose, a research leader at Baylor University. The reach of the telescope, out to supernovae over 10 billion years old, will enable it to probe eras when the universe was a mere fraction of its present age.
This is timely. The Dark Energy Spectroscopic Instrument collaboration has recently found that the effect of dark energy could be changing, instead of being an unchanging “cosmological constant” as was previously thought. The three-year dataset of DESI, which covers almost 15 million quasars and galaxies, together with supernovae and cosmic microwave background observations, hints that the effect of dark energy might be decreasing with time. “It’s looking more and more like we may need to modify our standard model of cosmology to make these different datasets make sense together and evolving dark energy seems promising,” Will Percival, DESI co-spokesperson, said. Roman’s enormous collection of Type Ia supernovae over enormous distances and times is anticipated to give the statistical power necessary to test these hints and pin down the equation of state for dark energy to much higher accuracy.
Roman’s survey, however, is more than just a standard candles campaign. It is anticipated to reveal ~60,000 core-collapse supernovae, the catastrophic deaths of large stars at least eight times more massive than the Sun. These events, though less valuable for cosmological purposes, are vital to comprehend the stellar life cycle and cosmic dispersal of heavy elements. As Rebekah Hounsell of the Goddard Space Flight Center of NASA described, “By seeing the way an object’s light changes over time and splitting it into spectra… we can distinguish between all the different types of flashes Roman will see.” Machine learning programs, which have learned from the survey’s treasure trove of data, will filter through this cosmic “bycatch” to sort and analyze the many kinds of explosions.
Among the most elusive targets are kilonovas the brief, glowing bursts that are produced by neutron star mergers. Only one has been conclusively confirmed so far, but Roman may be able to see as many as five more. They are events of extreme importance: they are thought to be the universe’s main r-process nucleosynthesis sites, which create the universe’s heaviest elements, like gold and platinum. General-relativistic simulations suggest that even a tiny percentage of neutron-rich matter expelled during a merger can yield the entire set of heavy elements, with infrared emission from the kilonova reaching its peak in days and declining quickly. Roman’s near-infrared sensitivity is perfectly matched to detecting these fleeting signals, illuminating both the mergers’ astrophysics as well as the cosmic origin of noble metals.
Most exciting, perhaps, is Roman’s ability to verify the presence of pair-instability supernovae, the hypothesized explosive demise of the universe’s first, giant stars. Those original giants, several hundred times the Sun’s mass, might have imploded through matter-antimatter pair production, without any remnant remaining. Roman’s broad-field, deep near-infrared imaging might finally see the inescapable chemical signposts of these occurrences, and simulations indicate as many as ten should be detectable. “I think Roman will make the first confirmed detection of a pair-instability supernova,” Rose prophesied.
While DESI further sharpens its 3D map of the universe, and Roman gets ready to release an onslaught of time-domain data, the convergence of these initiatives opens the door to a revolution in our knowledge of cosmic acceleration, the universe’s destiny, and the astrophysical drives that have forged the elements themselves. “Roman’s going to find a whole bunch of weird and wonderful things out in space, including some we haven’t even thought of yet,” Hounsell explained. “We’re certainly looking forward to the unexpected.”
