The effects of the Universe’s large-scale structure on the light from distant galaxies
Everything we know — galaxies, stars, planets, our families, friends, and even pets — makes up just 5% of the Universe. The remaining 95% is made up of mysterious components that scientists call dark energy (68%) and dark matter (27%). What are they, and how do they influence the structure and evolution of the Universe? Researchers like Andrés Alejandro Plazas Malagón, Rubin Operations Scientist at SLAC National Laboratory and Community Scientist and Calibration Scientist at Rubin Observatory, hope to tackle these questions with the upcoming Legacy Survey of Space and Time (LSST), conducted with Rubin Observatory in Chile. With its immense sky coverage and ability to detect faint objects, Rubin’s LSST will provide scientists with the enormous dataset needed to unravel these and other mysteries of the Universe.
In the 1970s American astronomer Vera C. Rubin
provided the most convincing evidence at that time for the existence of
an unseen ‘dark’ matter in the Universe. Dark matter is called dark
because that’s pretty much all we know about it, other than its
gravitational influence on stars and gas in galaxies: it’s a substance
in the Universe that has mass but doesn’t give off or reflect light.
This invisible material makes up about 80% of all matter, and its
properties affect how the Universe evolves — how galaxies form and grow
as well as how they clump together to form long filaments that make up
the structure scientists call the cosmic web.
But shaping the large-scale structure of the Universe is a game of
cosmic tug-of-war between dark matter and an elusive force known as dark
energy. “You can think of dark matter as trying to build the cosmic
structures, while dark energy is actually trying to dilute them and
push them apart,” says Plazas Malagón. Most scientists think that
dark energy drives the accelerating expansion of the Universe, and that
its behavior is described by a quantity known as the cosmological constant.
This explanation is commonly agreed on because it aligns with the
evidence we’ve collected so far. But, while the cosmological constant is
currently a fundamental piece of the equations that describe the
Universe, researchers are still trying to pin down its exact value — and
whether it’s the explanation for dark energy at all.
Calculating the cosmological constant, and thus placing firm
constraints on the equations describing the Universe, is an effort at
the forefront of cosmology. And Rubin will enable the most precise
measurements of it yet by opening new possibilities for using a subtle
effect called weak gravitational lensing to explore the complex interplay between dark matter and dark energy.
Cosmologists use weak lensing to infer the ‘clumpiness’ of matter by observing how its gravity bends light. But unlike strong gravitational lensing, which often produces giant, beautiful arcs
around clusters of galaxies, weak lensing produces effects that are
less dramatic: tiny distortions of the light from distant galaxies.
While weak lensing can occur at the outskirts of a strong lensing
system, it also exists everywhere across the Universe as light from
background galaxies makes its way through the filaments of galaxies
connecting galaxy clusters and superclusters known as the cosmic web. “If
strong lensing is like looking through the bottom of a wine glass, weak
lensing is like looking through a large, very subtly warped window,” says Theo Schutt, a PhD candidate at Stanford University who collaborates with Plazas Malagón.
These weak distortions of distant galaxies are too slight to be
measured by observing just a single galaxy — even with data from
thousands of galaxies, scientists can’t tell whether individual
galaxies’ observed shapes are their true shapes or whether they’ve been
distorted by weak lensing. To really understand the big picture, they
need a big dataset to calculate the collective distortion across the
entire observable sky. Rubin Observatory, with its ability to observe
huge patches of the sky while also being able to see very faint and
distant galaxies, will be the first observatory in history to provide
data on not just millions, but billions of galaxies and their shapes.
Rubin Observatory will be a leading resource for cutting-edge
astronomy and astrophysics when it comes online in 2025. Using an
8.4-meter telescope equipped with the largest digital camera in the world,
it will scan the entire southern hemisphere sky every few nights for
ten years, providing the most comprehensive view of the Universe we’ve
ever seen. Rubin Observatory is jointly funded by the National Science
Foundation (NSF) and the US Department of Energy (DOE). Rubin is a Program of NSF’s NOIRLab, which, along with SLAC National Accelerator Laboratory, will operate Rubin.
Current surveys, such as the Dark Energy Survey, the Hyper Suprime Cam survey, and the Kilo-Degree Survey,
are already untangling some of the mysteries of dark matter and dark
energy. But there is a trade-off, explains Plazas Malagón, between a
wide survey and a deep survey, limited by factors such as camera size
and resolving power. The LSST Camera
at Rubin Observatory has the unmatched wide-field view and high
resolution that give astronomers and astrophysicists the best of both
worlds. In fact, Rubin Observatory was designed from the start to help scientists map dark matter in the Universe using gravitational lensing of billions of distant galaxies.
“With Rubin, we’re going to have everything,” said Plazas Malagón.
“We’re going to measure the properties of vastly more galaxies than
what we have now, which is going to give us the statistical power to use
weak lensing to both map the distribution of dark matter and study how
dark energy evolves with time.”
It’s also possible that Rubin will produce new evidence to support
alternative explanations for the phenomena we observe in our Universe
beyond the most common theories of dark matter and dark energy.
“Dark energy is a concept that fits with the accepted theory of gravity within Einstein’s general theory of relativity,” said Plazas Malagón, “but Rubin and the LSST will also allow us to explore alternatives to that, which is incredibly exciting as well.”
More information
The National Science Foundation (NSF) is an independent federal agency created by Congress in 1950 to promote the progress of science. NSF supports basic research and people to create knowledge that transforms the future.
SLAC is operated by Stanford University for the US Department of Energy’s Office of Science. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time.
NSF’s NOIRLab (National Optical-Infrared Astronomy Research Laboratory), the US center for ground-based optical-infrared astronomy, operates the international Gemini Observatory (a facility of NSF, NRC–Canada, ANID–Chile, MCTIC–Brazil, MINCyT–Argentina, and KASI–Republic of Korea), Kitt Peak National Observatory (KPNO), Cerro Tololo Inter-American Observatory (CTIO), the Community Science and Data Center (CSDC), and Vera C. Rubin Observatory (in cooperation with DOE’s SLAC National Accelerator Laboratory). It is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with NSF and is headquartered in Tucson, Arizona. The astronomical community is honored to have the opportunity to conduct astronomical research on Iolkam Du’ag (Kitt Peak) in Arizona, on Maunakea in Hawai‘i, and on Cerro Tololo and Cerro Pachón in Chile. We recognize and acknowledge the very significant cultural role and reverence that these sites have to the Tohono O’odham Nation, to the Native Hawaiian community, and to the local communities in Chile, respectively.
Links
Contacts
Andrés Alejandro Plazas Malagón
Rubin Operations Scientist, SLAC National Accelerator Laboratory
Email: plazas@slac.stanford.edu Kristen Metzger
Communications Manager for Education and Public Outreach, Rubin Observatory
Email: kristen.metzger@noirlab.edu
Bob Blum
Director for Operations, Vera C. Rubin Observatory, NSF’s NOIRLab
Tel: +1 520-318-8233
Email: bob.blum@noirlab.edu
Željko Ivezić
Director of Rubin Construction
Tel: +1-206-403-6132
Email: ivezic@uw.edu
Josie Fenske
Communications NSF’s NOIRLab
Email: fenske.josie@noirlab.edu
Manuel Gnida
Media Relations Manager, SLAC National Accelerator Laboratory
Tel: +1 650-926-2632 (office)
Cell: +1 415-308-7832 (cell)
Email: mgnida@slac.stanford.edu
