Gravitational lensing probes ‘clumpiness’ of dark matter

Pune: A team of astrophysicists and cosmologists from international institutes, including Surhud More from Pune’s Inter-University Centre for Astronomy and Astrophysics (IUCAA), has characterised 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 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 (measurement of cosmological parameters), 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.

The study

“The growth of density fluctuations in the Universe is a result of 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. 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 CMB observations, if the standard cosmological model is correct,” said More.

The results of the analysis performed by the team are summarised in five new research articles prepared by the Hyper Suprime-Cam (HSC) Survey team. The team presented the results to the scientific community for rigorous peer review on April 3, 2023.

The experiments

According to the research team, there are three gravitational lensing experiments — HSC Survey, Dark Energy Survey and Kilo Degree Survey — currently being conducted by scientists worldwide; the deepest amongst which is the HSC survey carried out by them. All 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. The significance of 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-unrecognised 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.

The model

“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 More, associate professor at IUCAA, who chairs the weak lensing working group of the HSC survey along with Hironao Miyatake, associate professor at Nagoya University in Japan.

“The simplicity of the standard cosmological model lies in the small number of parameters required to describe it,” said 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.

The team worked on four different independent but complementary ways to analyse 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 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 More.

The instrument

The observations used one of the most powerful astronomical cameras in the world, the Hyper Suprime-Cam (HSC) mounted on the 8.2 m diametre Subaru Telescope on the summit of Maunakea in Hawai’i, a big island in the middle of the Pacific Ocean. The survey used by the research team covers approximately 420 square degrees of the sky, about the equivalent of 2,000 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,” said More.