banner credit: NASA/GSFC
Roman will use type Ia supernovae to measure cosmic distances, which will help us understand how the universe has expanded over time.
Credit: NASA's Goddard Space Flight Center/CI Labs
Roman’s supernova survey will help clear up clashing measurements of how fast the universe is currently expanding, and even provide a new way to probe the distribution of dark matter, which is detectable only through its gravitational effects. One of the mission’s primary science goals involves using supernovae to help pin down the nature of dark energy – the unexplained cosmic pressure that’s speeding up the expansion of the universe.
Roman will use multiple methods to investigate dark energy. One involves surveying the sky for a special type of exploding star, called a type Ia supernova.
Many supernovae occur when massive stars run out of fuel, rapidly collapse under their own weight, and then explode because of strong shock waves that propel out of their interiors. These supernovae occur about once every 50 years in our Milky Way galaxy. But evidence shows that type Ia supernovae originate from some binary star systems that contain at least one white dwarf – the small, hot core remnant of a Sun-like star. Type Ia supernovae are much rarer, happening roughly once every 500 years in the Milky Way.
In some cases, the dwarf may siphon material from its companion. This ultimately triggers a runaway reaction that detonates the thief once it reaches a specific point where it has gained so much mass that it becomes unstable. Astronomers have also found evidence supporting another scenario, involving two white dwarfs that spiral toward each other until they merge. If their combined mass is high enough that it leads to instability, they, too, may produce a type Ia supernova.
These explosions peak at a similar, known intrinsic brightness, making type Ia supernovae so-called standard candles – objects or events that emit a specific amount of light, allowing scientists to find their distance with a straightforward formula. Because of this, astronomers can determine how far away the supernovae are by simply measuring how bright they appear.
Astronomers will also use Roman to study the light of these supernovae to find out how quickly they appear to be moving away from us. By comparing how fast they’re receding at different distances, scientists will trace cosmic expansion over time. This will help us understand whether and how dark energy has changed throughout the history of the universe.
Previous type Ia supernova surveys have concentrated on the relatively nearby universe, largely due to instrument limitations. Roman’s infrared vision, gigantic field of view, and exquisite sensitivity will dramatically extend the search, pulling the cosmic curtains far enough aside to allow astronomers to spot thousands of distant type Ia supernovae.
Roman will study dark energy’s influence in detail over more than half of the universe’s history, when it was between about four and 12 billion years old. Exploring this relatively unprobed region will help scientists add crucial pieces to the dark energy puzzle.
Roman will use type Ia supernovae to help unravel the mysteries of dark energy.
Credit: NASA's Goddard Space Flight Center
The animation above shows how Roman will measure dark energy with exploding stars called type Ia supernovae. These blasts are caused by the total destruction of a white dwarf star and each one emits similar amounts of light. But the farther away they are, the fainter the explosions look. By measuring how bright type Ia supernovae appear to be, we have a way to measure their distances. It was by comparing supernovae redshifts to their apparent brightness that astronomers discovered dark energy. These studies showed that explosions at greater redshifts were dimmer than they should be in any model where the expansion of the universe was not speeding up.