Our
universe is filled with galaxies, in all directions as far as our instruments
can see. Some researchers estimate that there are as many as two trillion galaxies in the observable universe. At first glance, these
galaxies might appear to be randomly scattered across space, but they’re not.
Careful mapping has shown that they are distributed across the surfaces of
giant cosmic “bubbles” up to several hundred million light-years across. Inside
these bubbles, few galaxies are found, so those regions are called cosmic
voids. NASA’s Nancy Grace Roman Space Telescope will allow us to measure these
voids with new precision, which can tell us about the history of the universe’s
expansion.
This narrated video sequence illustrates how NASA's
Nancy Grace Roman Space Telescope will be able to observe cosmic voids in the
universe. These highly detailed measurements will help constrain cosmological
models.
Credit: Video: NASA, STScI; Visualization: Frank
Summers (STScI); Script Writer: Frank Summers (STScI); Narration: Frank Summers
(STScI); Audio: Danielle Kirshenblat (STScI); Science: Giulia Degni (Roma Tre
University), Alice Pisani (CPPM), Giovanni Verza (Center for Computational
Astrophysics/Flatiron Inst.)
“Roman’s ability to observe wide areas of the sky to great depths, spotting
an abundance of faint and distant galaxies, will revolutionize the study of
cosmic voids,” said Giovanni Verza of the Flatiron Institute and New York
University, lead author on a paper published in The Astrophysical Journal.
Cosmic Recipe
The cosmos is made of three key
components: normal matter, dark matter, and dark energy. The gravity of normal
and dark matter tries to slow the expansion of the universe, while dark energy
opposes gravity to speed up the universe’s expansion. The nature of both dark
matter and dark energy are currently unknown. Scientists are trying to
understand them by studying their effects on things we can observe, such as the
distribution of galaxies across space.
“Since they’re relatively empty of
matter, voids are regions of space that are dominated by dark energy. By
studying voids, we should be able to put powerful constraints on the nature of
dark energy,” said co-author Alice Pisani of CNRS (the French National Centre
for Scientific Research) in France and Princeton University in New Jersey.
To determine how Roman might study
voids, the researchers considered one potential design of the Roman
High-Latitude Wide-Area Survey, one of three core community surveys that Roman will conduct. The High-Latitude Wide-Area Survey will look away from the plane of our galaxy
(hence the term high latitude in galactic coordinates). The team found that
this survey should be able to detect and measure tens of thousands of cosmic
voids, some as small as just 20 million light-years across. Such large numbers
of voids will allow scientists to use statistical methods to determine how
their observed shapes are influenced by the key components of the universe.
To determine the actual, 3D shapes
of the voids, astronomers will use two types of data from Roman — the positions
of galaxies in the sky and their cosmological redshift, the latter of which is determined using spectroscopic data. To convert redshift to a physical distance, astronomers make assumptions
about the components of the universe, including the strength of dark energy and
how it might have evolved over time.
Pisani compared it to trying to
infer a cake recipe (i.e., the universe’s makeup) from the final dessert served
to you. “You try to put in the right ingredients — the right amount of matter,
the right amount of dark energy — and then you check whether your cake looks as
it should. If it doesn’t, that means you put in the wrong ingredients.”
In this case, the appearance of the
“cake” is the shape found by statistically stacking all of the voids detected
by Roman on top of each other. On average, voids are expected to have a
spherical shape because there is no “preferred” location or direction in the
universe (i.e., the universe is both homogeneous and isotropic on large
scales). This means that, if the stacking is done correctly, the resulting
shape will be perfectly round (or spherically symmetric). If not, then you have
to adjust your cosmic recipe.
Power of Roman
The researchers emphasized that to
study cosmic voids in large numbers, an observatory must be able to probe a
large volume of the universe, because the voids themselves can be tens or
hundreds of millions of light-years across. The spectroscopic data necessary to
study voids will come from a portion of the Roman High-Latitude Wide-Area
Survey that will cover on the order of 2,400 square degrees of the sky, or
12,000 full moons. It will also be able to see fainter and more distant
objects, yielding a greater density of galaxies than complementary missions
like ESA’s (European Space Agency’s) Euclid.
“Voids are defined by the fact that
they contain so few galaxies. So to detect voids, you have to be able to
observe galaxies that are quite sparse and faint. With Roman, we can better
look at the galaxies that populate voids, which ultimately will give us greater
understanding of the cosmological parameters like dark energy that are
sculpting voids,” said co-author Giulia Degni of Roma Tre University and INFN
(the National Institute of Nuclear Physics) in Rome.
The Nancy Grace Roman Space
Telescope is managed at NASA’s Goddard Space Flight Center in Greenbelt,
Maryland, with participation by NASA’s Jet Propulsion Laboratory in Southern
California; Caltech/IPAC in Pasadena, California; the Space Telescope Science
Institute in Baltimore; and a science team comprising scientists from various
research institutions. The primary industrial partners are BAE Systems, Inc. in
Boulder, Colorado; L3Harris Technologies in Melbourne, Florida; and Teledyne
Scientific & Imaging in Thousand Oaks, California.
By Christine Pulliam
Space Telescope Science Institute, Baltimore, Md.
Source: NASA’s Roman Telescope Will Observe Thousands of Newfound Cosmic Voids - NASA
