Figure 1: Conceptual image of this research. Distribution of invisible "dark matter" was investigated by combining the cosmic microwave background (CMB) and the HSC-SSP images. Credit: Reiko Matsushita (Nagoya University)
How do we see something that happened so long ago? Because of the finite
speed of light, we see distant galaxies not as they are today, but
rather as they were billions of years in the past. But even more
challenging, how do we see something like dark matter, that by its
nature does not emit light? Consider a very distant source galaxy, even
further away than the galaxy whose dark matter one wants to investigate.
The gravitational pull of the foreground galaxy, including its dark
matter, distorts the surrounding space and time, as predicted by
Einstein’s theory of General Relativity. As the light from the source
galaxy travels through this distortion, it bends, changing the apparent
shape of the galaxy in the sky. The greater the amount of dark matter,
the greater the distortion. Thus, scientists can measure the amount of
dark matter around the foreground galaxy (the "lens" galaxy) from the
distortion.
However, at this point, scientists encounter a problem. The
galaxies in the deepest reaches of the Universe are incredibly faint. As
a result, the further away from Earth you look, the less effective this
technique becomes. The lensing distortion is subtle and difficult to
detect in most cases, so one needs many background galaxies to detect
the signal. Most previous studies remained stuck at the same limits.
Unable to detect enough distant source galaxies to measure the
distortion, they could only analyze dark matter from no further back
than 8-10 billion years ago. These limitations left open the question
about the distribution of dark matter between this time and 13.7 billion
years ago, around the beginnings of our Universe.
To overcome
these challenges and observe dark matter in the furthest reaches of the
Universe, a research team led by Nagoya University’s Hironao Miyatake,
in collaboration with the University of Tokyo, National Astronomical
Observatory of Japan, and Princeton University, used a different source
of background light, the microwaves released from the Big Bang itself.
How
did they do this? First, using data from the observations of the Hyper
Suprime-Cam Subaru Strategic Program (HSC-SSP), the team identified 1.5
million lens galaxies using visible light, selected to be seen 12
billion years ago. Next, to overcome the lack of galaxy light even
further away, they employed microwaves from the cosmic microwave
background (CMB), the radiation residue from the Big Bang. Using
microwaves observed by the European Space Agency’s Planck satellite, the
team measured how the dark matter around the lens galaxies distorted
the microwaves.
"Look at dark matter around distant galaxies?" asks Professor
Masami Ouchi of the National Astronomical Observatory of Japan and the
University of Tokyo, who made many of the observations. "It was a crazy
idea. No one realized we could do this. But after I gave a talk about a
large distant galaxy sample, Hironao came to me and said it may be
possible to look at dark matter around these galaxies with the CMB."
"Most
researchers use source galaxies to measure dark matter distribution
from the present to eight billion years ago", adds Assistant Professor
Yuichi Harikane of the Institute for Cosmic Ray Research, University of
Tokyo. "However, we could look further back into the past because we
used the more distant CMB to measure dark matter. For the first time, we
were measuring dark matter from almost the earliest moments of the
Universe."
After a preliminary analysis, the researchers soon
realized they had a large enough sample to detect the distribution of
dark matter. Combining the large distant galaxy sample and the lensing
distortions in the CMB, they detected dark matter even further back in
time, from 12 billion years ago. This is only 1.7 billion years after
the beginning of the Universe, and thus these galaxies are seen soon
after they first formed.
"I was happy that we opened a new window
into that era," Miyatake says. "12 billion years ago, things were very
different. You see more galaxies that are in the process of formation
than at the present; the first galaxy clusters are starting to form as
well." Galaxy clusters consist of 100-1000 galaxies bound by gravity
with large amounts of dark matter.
"This result gives a very
consistent picture of galaxies and their evolution, as well as the dark
matter in and around galaxies, and how this picture evolves with time,"
says Neta Bahcall, Eugene Higgins Professor of Astronomy, professor of
astrophysical sciences, and director of undergraduate studies at
Princeton University.
One of the most exciting of the
researchers’ findings was related to the clumpiness of the dark matter.
According to the standard theory of cosmology, the Lambda-CDM (Cold Dark
Matter) model, subtle fluctuations in the CMB form pools of densely
packed matter by attracting surrounding matter through gravity. This
creates inhomogeneous clumps that form stars and galaxies in these dense
regions. The group’s findings suggest that their clumpiness measurement
was lower than predicted by the Lambda-CDM model. Miyatake is excited
about the possibilities. "Our finding is still uncertain", he says. "But
if it is true, it would suggest that the entire model is flawed as you
go further back in time. This is exciting, because it could suggest – if
the result holds after the uncertainties are reduced – an improvement
of the model that may give insight into the nature of dark matter
itself."
This study used data available from existing
telescopes, including Planck and the Subaru Telescope. The group has
only reviewed a third of the HSC-SSP data. The next step will be to
analyze the entire data set, which should allow for a more precise
measurement of the dark matter distribution. In the future, the team
expects to use an advanced data set like the Vera C. Rubin Observatory's
Legacy Survey of Space and Time (LSST) to explore more of the earliest
parts of space. "LSST will allow us to observe half the sky," Harikane
says. "I don’t see any reason we couldn’t see the dark matter
distribution 13 billion years ago next."
These results appeared as Miyatake et al. "First
Identification of a CMB Lensing Signal Produced by 1.5 Million Galaxies
at z∼4: Constraints on Matter Density Fluctuations at High Redshift" in Physical Review Letters on August 1, 2022.
Relevant Links
