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Gravitational lensing helps see the invisible dark matter

A team of astrophysicists and cosmologists at various international institutes, including Dr. Surhud More from the Inter-University Center for Astronomy and Astrophysics (IUCAA), has characterized the structure of dark matter in the Universe using images taken from the Subaru telescope in Hawai’i. Scientists used the gravitational lensing effect to map the dark matter in the Universe, and how it is distributed around galaxies, in multiple different tomographic slices of distances away from us, similar to a medical CT scan. They measured a value for the “clumpiness” of the Universe’s matter, known to cosmologists as S8, of 0.76, which agrees with values that other gravitational lensing surveys have found in looking at the relatively recent Universe — but does not agree with the value of 0.83 predicted by the Cosmic Microwave Background (CMB), which dates back to the origin of the Universe, when it was about 380,000 years old.

Figuer 1
Figure 1: Scientists have measured the clumpiness of the matter in the Universe by using the Subaru Hyper Suprime-Cam survey and compared it to the value predicted by the observations of the Cosmic Microwave Background in the standard cosmological model. The measurements are smaller than expected, and thus can inform our understanding of dark matter and dark energy.

Scientific background:

The growth of fluctuations in the Universe is a result of a competition between dark matter, which causes fluctuations to grow, and dark energy, the substance that causes the accelerated expansion of the Universe and moves everything further apart (see Fig. 1). By mapping out the dark matter distribution in the Universe and its distribution around galaxies, the team was able to show that the clumpiness of matter in the Universe is smaller than that expected from observations of the CMB, if the standard cosmological model is correct.

Figuer 2
Figure 2: One amongst the many images taken by the Subaru HSC survey. The survey is the deepest survey amongst the three worldwide surveys trying to characterize the clumpiness of the Universe. This allows the survey to image galaxies that are much farther away and also get a sharper view of the dark matter around galaxies.

There are three gravitational lensing experiments currently being conducted by scientists worldwide; the deepest amongst which is the Hyper Suprime-Cam (HSC) Survey carried out by this team (see Fig. 2). All of these surveys, independently, agree with each other and find less clumpiness in the dark matter distribution in the Universe today, than expected from the CMB. Individually, the significance of these differences found by each of the surveys does not yet rise to the gold standard 5𝜎 threshold. However, taken together, it is quite intriguing that all three surveys consistently find less clumpiness than expected. This raises the possibility that there’s some as-yet unrecognized error in these measurements or that of the CMB. Once this is conclusively ruled out, such a difference could indicate that the standard cosmological model is incomplete in some interesting ways.

According to Einstein's general relativity, light bends when it passes near a massive object, in a phenomenon known as gravitational lensing. Lensing causes the shapes of galaxies to be distorted by a tiny amount as the light from such galaxies travels through the Universe. Scientists had to measure these tiny changes imprinted on the shapes of galaxies. “Teasing out the gravitational lensing signal meticulously from images of millions of galaxies was quite a challenging task. Our team had to toil for many years in order to measure and model the signal”, said Dr. Surhud More, Associate Professor at IUCAA, who chairs the weak lensing working group of the HSC survey along with Dr. Hironao Miyatake, Associate Professor at Nagoya University. “The simplicity of the standard cosmological model lies in the small number of parameters required to describe it”, said Dr. More. The density of dark matter in the Universe and the amount of clumpiness of matter, are quite important to understand how structure in the Universe evolves with time. “By mapping out dark matter in the Universe in a tomographic manner, that is in 3D, we are able to carry out a stringent test of the cosmological model”, he said.

Dark energy and dark matter make up 95% of our Universe today, but we understand very little about what they actually are and how they’ve evolved over the history of the universe. Cosmologists are eager to test the current model by constraining the few parameters that describe the standard model, such as by observing the fluctuations in the CMB, modeling the expansion history of the universe, or measuring the clumpiness of the universe in the relatively recent past.

Blind analysis of data

When analyzing data of such high quality, it is very important that scientists do not bias themselves one way or the other and don’t stop testing their analyses as soon as the results match their preconceived ideas about what the results should be. Therefore, the team devised a careful procedure to blind the data as well as the results before carrying out a battery of tests to ensure that the choices they make in the analysis are not driven by such biases. The team worked on four different independent but complementary ways to analyze the data. After spending roughly a year on the analysis, the team got together on a teleconference call to unblind the result of the analyses. A number of early career scientists in Japan and the US had put a lot of their effort into designing and conducting these analyses.

The unblinding of the results was quite a tense moment, as it was not known whether all the four techniques would agree on their inferences. “It was both a moment of excitement and relief to see all the analyses yielding consistent results. The blinding procedure increases our confidence in the robustness of our results”, said Dr. More.

A huge survey with the world's largest telescope camera

The observations used one of the most powerful astronomical cameras in the world, the Hyper Suprime-Cam (HSC) mounted on the 8.2 m diameter Subaru Telescope on the summit of Maunakea in Hawai’i, a big island in the middle of the Pacific Ocean. The Hyper Suprime-Cam survey collaboration is led by astronomers from the scientific community of Japan, Taiwan and Princeton University. The survey used by the research team covers approximately 420 square degrees of the sky, about the equivalent of 2000 full moons. It is not a single contiguous chunk of sky, but split among six different pieces, each about the size of a person's outstretched fist. The 25 million galaxies the researchers surveyed are so distant that instead of seeing these galaxies as they are today, the HSC recorded how they were billions of years ago.

Each of these galaxies glow with the light from tens of billions of suns, but because they are so far away, they are extremely faint, as much as 25 million times fainter than the faintest stars we can see with the naked eye.

The five papers:

The results of the analyses performed by the team are summarized in 5 new different research articles prepared by the HSC team. These results will now undergo a rigorous peer review process from the scientific community. The team will present these results to the scientific community on April 3, 2023 at 20:30 IST.
  1. More, S., Sugiyama, S. et al. 2023, Hyper Suprime-Cam Year 3 Results: Measurements of the clustering of SDSS-BOSS galaxies, galaxy-galaxy lensing and cosmic shear
  2. Miyatake, H., Sugiyama, S. et al. 2023, Hyper Suprime-Cam Year 3 Results: Cosmology from galaxy clustering and weak lensing with HSC and SDSS using the emulator based halo model
  3. Sugiyama, S. et al. 2023, Hyper Suprime-Cam Year 3 Results: Cosmology from galaxy clustering and weak lensing with HSC and SDSS using the minimal bias model
  4. Dalal, R. et al 2023, Hyper Suprime-Cam Year 3 Results: Cosmology from cosmic shear power spectra
  5. Li, X. et al. 2023, Hyper Suprime-Cam Year 3 Results: Cosmology from cosmic shear two-point correlation functions
The website summarizing the main results and these research publications will be available at:
https://hsc-release.mtk.nao.ac.jp/doc/index.php/wly3/ (Available on April 3, 2030 IST)

Research contacts:

Surhud More Dr. Surhud More
Associate Professor

E-mail: surhud_at_iucaa.in
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