From UC Santa Cruz: Women in STEM- “First-year Subaru Hyper Suprime-Cam survey results yield cosmological constraints” Alexie Leauthaud

UC Santa Cruz

From UC Santa Cruz

September 25, 2018
Tim Stephens

NAOJ Subaru Hyper Suprime-Cam

NAOJ/Subaru Telescope at Mauna Kea Hawaii, USA,4,207 m (13,802 ft) above sea level

Using the powerful Japanese Subaru telescope, the Hyper Suprime-Cam (HSC) survey collaboration team has made and analyzed the deepest wide-field map of the three-dimensional distribution of matter in the universe.

The results place new constraints on the nature of the mysterious dark energy that is accelerating the expansion of the universe, said Alexie Leauthaud, assistant professor of astronomy and astrophysics at UC Santa Cruz and a member of the international collaboration.

Alexie Leauthaud (Photo by T. Stephens)

The HSC survey, led by the astronomical communities of Japan and Taiwan and Princeton University, is an unprecedented effort to measure gravitational lensing, in which the gravity of a galaxy in the foreground of an image bends the light from a more distant galaxy as it passes by. Gravitational lensing can reveal dark matter, which accounts for about 80 percent of the mass in the universe but cannot be observed directly.

Leauthaud has been involved in the survey from the beginning, joining the project when she was at the Kavli Institute for the Physics and Mathematics of the Universe (IPMU) at the University of Tokyo, before she joined the faculty at UC Santa Cruz. She helped to write the proposal that secured 300 nights for the survey on the Subaru telescope and has provided expertise with weak lensing measurements and performed critical data quality checks.

Left panel: The 3-dimensional dark matter map of the universe inferred from one of the six HSC observation areas is shown in the background with various shades of blue (brighter areas have more dark matter). The map was inferred from the distortions of shapes of galaxies in the HSC data which are indicated by white sticks. The stick lengths represent the amount of distortion and the angle of the stick corresponds to the direction of the distortion. Right panel: The measurements are enabled by the light from distant galaxies that travels through the Universe and gets deflected by matter at different epochs in the Universe, before reaching the Subaru telescope. (All images credit: C. Hikage et al.)


The cosmological constraints on the fractional contribution of matter to the energy budget of the universe (the rest of it corresponds to dark energy), and the clumpiness of the matter distribution today as inferred from the analysis of the 3d dark matter map. The results of the clumpiness of the matter distribution from HSC observations of the distant universe using weak gravitational lensing are consistent with results from other similar observations (Dark Energy Survey and the Kilo Degree Survey) of more nearby universe. The results from the cosmic microwave background observations during the universe’s infancy obtained by the Planck satellite are shown in blue.

A team of scientists led by Chiaki Hikage at the Kavli IPMU has now used the gravitational distortion of images of about 10 million galaxies to make a precise measurement of the lumpiness of matter in the universe. By combining this measurement with observations of the cosmic microwave background by the European Space Agency’s Planck satellite and other cosmological experiments, the team has been able to further constrain dark energy.

“Taken together in the context of other weak lensing data sets, the hints of a deviation from Planck are very intriguing,” Leauthaud said. “It is exciting to see what we have been able to achieve this early on in our survey. With our full data set, we will have the power to say whether or not there is a tension with Planck. That is incredibly exciting to me because we may be on the verge of learning something very interesting about the physics of dark energy.”

Though quite weak, the gravitational lensing effect results in small but measurable distortions in the images of distant galaxies. Like a pointillist painting, the distorted images of millions of galaxies located further and further away paint a three-dimensional picture of the distribution of matter in the universe. The research team has characterized the precise amount of fluctuations in the matter distribution and their change over billions of years, from the universe’s adolescence to its adulthood.

Precise measurements

Cosmological constraints on the dark energy equation of state: blue contours alone from HSC, red contours correspond to constraints after combining with cosmological results from the Planck CMB satellite and other contemporary cosmological measurements.

The study required precise measurements of galaxy shapes. Since the weak lensing effect is quite small, the HSC team had to control various problems affecting the measurement of shapes, such as distortions due to the atmosphere and the instrument itself. The team overcame these difficulties by using detailed painstaking image simulations of the HSC survey based on Hubble Space Telescope images.

When carrying out precise measurements of very small effects, it is known that people have a tendency to decide that their analysis is complete if their results confirm earlier results. The HSC team performed a so-called blind analysis of their data in order to avoid such “confirmation bias.” They carried out many tests of their catalogs for more than a year without ever seeing the actual values of cosmological parameters from their analysis or comparing with results from other experiments.

The HSC weak lensing measurement is used to determine the lumpiness of matter in the universe, quantified by a parameter called S8. Larger S8, for example, would mean more structure such as galaxies in the universe. With the high-precision HSC data, the team determined S8 with a precision of 3.6 percent, which is similar to the precision with which it was measured by the weak lensing analysis from the Dark Energy Survey (DES). The DES surveyed​ ​14 times more area on the sky than HSC, but focused on the more nearby universe. With a deeper survey that images even fainter galaxies, the HSC team was able to map out a sharper dark matter distribution than was previously possible and achieved a similar precision measurement with a smaller area. This showcases the strength and complementarity of HSC among ongoing projects worldwide.

Consistent picture

When compared to the fluctuations expected from those seen in the universe’s infancy by the Planck satellite, the HSC measurements offer a consistent picture of the cosmological model. The universe today is dominated by dark matter and dark energy, and that dark energy behaves like Einstein’s cosmological constant—the simplest model.

However, taken together the results from weak lensing surveys prefer a slightly smaller value of fluctuations than that predicted by the Planck satellite. This could just be a statistical fluctuation due to the limited amount of data, or it might be a signature of the breakdown of the standard model of the universe based on general relativity and the cosmological constant. The new HSC results come from a mere one tenth of the planned survey. When completed, the survey has the potential to deepen scientists’ understanding of the standard cosmological model by shedding light on the behavior of dark energy.

The research paper has now been submitted to the journal ​Publications of the Astronomical Society of Japan​ and will undergo rigorous peer review by the scientific community.

The HSC survey began in the spring of 2014 using the National Astronomical Observatory of Japan’s Subaru telescope located at the summit of Mt. Maunakea on the Big Island of Hawaii. The telescope features a large collecting area corresponding to a diameter of 8.2 meters, a wide angle camera that fits an area equal to about 9 moons in a single shot, and superb image quality, making it well suited to conduct a wide yet deep imaging survey of the sky. The survey has covered about 140 square degrees of sky (the area of 3,000 full moons) over 90 nights on the telescope​.

The research will be uploaded to the preprint server and will be submitted to the Publication of the Astronomical Society of Japan.

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UCSC Lick Observatory, Mt Hamilton, in San Jose, California, Altitude 1,283 m (4,209 ft)


UCO Lick Shane Telescope
UCO Lick Shane Telescope interior
Shane Telescope at UCO Lick Observatory, UCSC

Lick Automated Planet Finder telescope, Mount Hamilton, CA, USA

Lick Automated Planet Finder telescope, Mount Hamilton, CA, USA

UC Santa Cruz campus
The University of California, Santa Cruz, opened in 1965 and grew, one college at a time, to its current (2008-09) enrollment of more than 16,000 students. Undergraduates pursue more than 60 majors supervised by divisional deans of humanities, physical & biological sciences, social sciences, and arts. Graduate students work toward graduate certificates, master’s degrees, or doctoral degrees in more than 30 academic fields under the supervision of the divisional and graduate deans. The dean of the Jack Baskin School of Engineering oversees the campus’s undergraduate and graduate engineering programs.

UCSC is the home base for the Lick Observatory.

Lick Observatory's Great Lick 91-centimeter (36-inch) telescope housed in the South (large) Dome of main building
Lick Observatory’s Great Lick 91-centimeter (36-inch) telescope housed in the South (large) Dome of main building

Search for extraterrestrial intelligence expands at Lick Observatory
New instrument scans the sky for pulses of infrared light
March 23, 2015
By Hilary Lebow
The NIROSETI instrument saw first light on the Nickel 1-meter Telescope at Lick Observatory on March 15, 2015. (Photo by Laurie Hatch) UCSC Lick Nickel telescope

Astronomers are expanding the search for extraterrestrial intelligence into a new realm with detectors tuned to infrared light at UC’s Lick Observatory. A new instrument, called NIROSETI, will soon scour the sky for messages from other worlds.

“Infrared light would be an excellent means of interstellar communication,” said Shelley Wright, an assistant professor of physics at UC San Diego who led the development of the new instrument while at the University of Toronto’s Dunlap Institute for Astronomy & Astrophysics.

Wright worked on an earlier SETI project at Lick Observatory as a UC Santa Cruz undergraduate, when she built an optical instrument designed by UC Berkeley researchers. The infrared project takes advantage of new technology not available for that first optical search.

Infrared light would be a good way for extraterrestrials to get our attention here on Earth, since pulses from a powerful infrared laser could outshine a star, if only for a billionth of a second. Interstellar gas and dust is almost transparent to near infrared, so these signals can be seen from great distances. It also takes less energy to send information using infrared signals than with visible light.

Frank Drake, professor emeritus of astronomy and astrophysics at UC Santa Cruz and director emeritus of the SETI Institute, said there are several additional advantages to a search in the infrared realm.

“The signals are so strong that we only need a small telescope to receive them. Smaller telescopes can offer more observational time, and that is good because we need to search many stars for a chance of success,” said Drake.

The only downside is that extraterrestrials would need to be transmitting their signals in our direction, Drake said, though he sees this as a positive side to that limitation. “If we get a signal from someone who’s aiming for us, it could mean there’s altruism in the universe. I like that idea. If they want to be friendly, that’s who we will find.”

Scientists have searched the skies for radio signals for more than 50 years and expanded their search into the optical realm more than a decade ago. The idea of searching in the infrared is not a new one, but instruments capable of capturing pulses of infrared light only recently became available.

“We had to wait,” Wright said. “I spent eight years waiting and watching as new technology emerged.”

Now that technology has caught up, the search will extend to stars thousands of light years away, rather than just hundreds. NIROSETI, or Near-Infrared Optical Search for Extraterrestrial Intelligence, could also uncover new information about the physical universe.

“This is the first time Earthlings have looked at the universe at infrared wavelengths with nanosecond time scales,” said Dan Werthimer, UC Berkeley SETI Project Director. “The instrument could discover new astrophysical phenomena, or perhaps answer the question of whether we are alone.”

NIROSETI will also gather more information than previous optical detectors by recording levels of light over time so that patterns can be analyzed for potential signs of other civilizations.

“Searching for intelligent life in the universe is both thrilling and somewhat unorthodox,” said Claire Max, director of UC Observatories and professor of astronomy and astrophysics at UC Santa Cruz. “Lick Observatory has already been the site of several previous SETI searches, so this is a very exciting addition to the current research taking place.”

NIROSETI will be fully operational by early summer and will scan the skies several times a week on the Nickel 1-meter telescope at Lick Observatory, located on Mt. Hamilton east of San Jose.

The NIROSETI team also includes Geoffrey Marcy and Andrew Siemion from UC Berkeley; Patrick Dorval, a Dunlap undergraduate, and Elliot Meyer, a Dunlap graduate student; and Richard Treffers of Starman Systems. Funding for the project comes from the generous support of Bill and Susan Bloomfield.