Cosmology
Together, Roman and Webb will unlock enormous stretches of the universe’s history. Astronomers will use observations of different cosmic eras to piece together how the universe transformed over billions of years to its present state.
Dark Energy
Scientists have discovered that the universe’s expansion is speeding up, but no one knows why. A mysterious pressure dubbed “dark energy” has been theorized as a possible explanation. Exploring the nature of dark energy is one of Roman’s primary goals, and Webb will offer clues, too.
Roman will combine the powers of imaging and spectroscopy to unveil more than a billion galaxies. Imaging will reveal the locations, shapes, sizes, and colors of objects like distant galaxies, and spectroscopy will measure the intensity of light from those objects at different wavelengths, allowing astronomers to determine how far away they are.
Doing both across the same enormous swath of the universe will yield enormous, deep 3D images that will help astronomers discern between the leading theories that attempt to explain why the expansion of the universe is accelerating. Webb has powerful spectrographs too, but its smaller view renders it impractical to survey enough sky to measure large-scale galaxy clustering, which carries the imprint of dark matter and dark energy.
Roman will also trace cosmic expansion using a special kind of exploding star called a type Ia supernova. These explosions, which happen roughly once every 500 years in the Milky Way, peak at a similar, known intrinsic brightness. That allows astronomers to determine how far away the supernovae are by simply measuring how bright they appear. Astronomers can 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 across billions of years. This will help us understand whether and how dark energy has changed throughout the history of the universe, and could clear up mismatched measurements of the Hubble constant — the universe’s current expansion rate.
Roman’s gigantic view will cast such a wide net that astronomers will see thousands of type Ia supernovae. Webb can study these explosions more closely to help refine the way they’re used to determine cosmic distances.
Dark Matter
Roman and Webb will also add pieces to the dark matter puzzle—another key component of the universe that we don’t understand well. This invisible material is detectable only through its gravitational effects on normal matter. Scientists are trying to determine what exactly dark matter is made of so they can detect it directly, but our current understanding has so many gaps, it’s difficult to know just what we’re looking for.
In its very first science image, Webb found dark matter hidden among distorted galaxies. Anything with mass warps the fabric of space-time — the greater the mass, the stronger the warp. Light that passes nearby follows the curved path around the object. For things as large as galaxies and galaxy clusters, this effect—called gravitational lensing — can warp light so strongly that distant galaxies are smeared into arcs and streaks in images. Astronomers can determine how massive an intervening object is by seeing how much it distorts light from more distant sources.
Roman will be sensitive enough to use a more subtle version of the same effect (called weak lensing) to see how clumps of dark matter warp the appearance of distant galaxies. By observing lensing effects on this small scale over a gigantic area, Roman will map how dark matter is distributed and explore its structure. This will help astronomers fill in more of the gaps in our understanding of dark matter. Their findings could even lead to adjustments to our current cosmological model of the universe.