Figure 1: 2 dimensional dark matter map estimated by
weak lensing technique. The dark matter is concentrated in dense clumps.
We can identify massive dark matter halos (indicated by oranges
circles). The area shown in this figure is approximately 30 square
degrees (a total of 160 square degrees were observed this time). The
distribution map without the orange circles is available here. (Credit: NAOJ/University of Tokyo)
A research team of multiple institutes, including the National
Astronomical Observatory of Japan and University of Tokyo, released an
unprecedentedly wide and sharp dark matter map based on the newly
obtained imaging data by Hyper Suprime-Cam on the Subaru Telescope. The
dark matter distribution is estimated by the weak gravitational lensing
technique (Figure 1, Movie).
The team located the positions and lensing signals of the dark matter
halos and found indications that the number of halos could be
inconsistent with what the simplest cosmological model suggests. This
could be a new clue to understanding why the expansion of the Universe
is accelerating.
Mystery of the accelerated Universe
In the 1930's, Edwin Hubble and his colleagues discovered the
expansion of the Universe. This was a big surprise to most of the people
who believed that the Universe stayed the same throughout eternity. A
formula relating matter and the geometry of space-time was required in
order to express the expansion of the Universe mathematically.
Coincidentally, Einstein had already developed just such a formula.
Modern cosmology is based on Einstein's theory for gravity.
It had been thought that the expansion is decelerating over time (blue and red lines in Figure 2)
because the contents of the Universe (matter) attract each other. But
in the late 1990's, it was found that the expansion has been
accelerating since about 8 Giga years ago. This was another big surprise
which earned the astronomers who found the expansion a Nobel Prize in
2011. To explain the acceleration, we have to consider something new in
the Universe which repels the space.
The simplest resolution is to put the cosmological constant back into
Einstein's equation. The cosmological constant was originally
introduced by Einstein to realize a static universe, but was abandoned
after the discovery of the expansion of the Universe. The standard
cosmological model (called LCDM) incorporates the cosmological constant.
The expansion history using LCDM is shown by the green line in Figure 2.
LCDM is supported by many observations, but the question of what causes
the acceleration still remains. This is one of the biggest problems in
modern cosmology.
Figure 2: Expansion history of the Universe. The blue
line shows what was believed to be likely in the early days of
cosmology. Later this cosmological model fell out of favor because it
predicts a higher growth rate and more structures, inconsistent with
the observed galaxy distribution. Thus a much lighter Universe model was
proposed which is shown by the red line. This light model also solved
the so called "age problem," the existence of globular clusters older
than the age of the Universe predicted by the blue track. But both the
blue and red lines conflict with the inflation cosmology. Later when the
acceleration of the Universe was discovered, LCDM represented by the
green track, was adopted as the most likely model. Thanks to the
addition of the cosmological constant, LCDM becomes consistent with the
inflation model. (Credit: NAOJ)
Wide and deep imaging survey using Hyper Suprime-Cam
The team is leading a large scale imaging survey using Hyper
Suprime-Cam (HSC) to probe the mystery of the accelerating Universe. The
key here is to examine the expansion history of the Universe very
carefully.
In the early Universe, matter was distributed almost but not quite
uniformly. There were slight fluctuations in the density which can now
be observed through the temperature fluctuations of the cosmic microwave
background. These slight matter fluctuations evolved over cosmic time
because of the mutual gravitational attraction of matter, and eventually
the large scale structure of the present day Universe become visible.
It is known that the growth rate of the structure strongly depends on
how the Universe expands. For example, if the expansion rate is high, it
is hard for matter to contract and the growth rate is suppressed. This
means that the expansion history can be probed inversely through the
observation of the growth rate.
It is important to note that growth rate cannot be probed well if we
only observe visible matter (stars and galaxies). This is because we now
know that nearly 80 % of the matter is an invisible substance called
dark matter. The team adopted the 'weak gravitation lensing technique.'
The images of distant galaxies are slightly distorted by the
gravitational field generated by the foreground dark matter
distribution. Analysis of the systematic distortion enables us to
reconstruct the foreground dark matter distribution.
This technique is observationally very demanding because the
distortion of each galaxy is generally very subtle. Precise shape
measurements of faint and apparently small galaxies are required. This
motivated the team to develop Hyper Suprime-Cam. They have been carrying
out a wide field imaging survey using Hyper Suprime-Cam since March
2014. At this writing in February 2018, 60 % of the survey has been
completed.
Figure 3: Hyper Suprime-Cam image of a location with a
highly significant dark matter halo detected through the weak
gravitational lensing technique. This halo is so massive that some of
the background (blue) galaxies are stretched tangentially around the
center of the halo. This is called strong lensing. (Credit: NAOJ)
Unprecedentedly wide and sharp dark matter map
In this release, the team presents the dark matter map based on the imaging data taken by April 2016 (Figure 1).
This is only 11 % of the planned final map, but it is already
unprecedentedly wide. There has never been such a sharp dark matter map
covering such a wide area.
Imaging observations are made through five different color filters.
By combining these color data, it is possible to make a crude estimate
of the distances to the faint background galaxies (called photometric
redshift). At the same time, the lensing efficiency becomes most
prominent when the lens is located directly between the distant galaxy
and the observer. Using the photometric redshift information, galaxies
are grouped into redshift bins. Using this grouped galaxy sample, dark
matter distribution is reconstructed using tomographic methods and thus
the 3D distribution can be obtained. Figure 4
shows one such example. Data for 30 square degrees are used to
reconstruct the redshift range between 0.1 (~1.3 G light-years) and 1.0
(~8 G light-years). At the redshift of 1.0, the angular span
corresponds to 1.0 G x 0.25 G light-years. This 3D dark matter mass map
is also quite new. This is the first time the increase in the number of
dark matter halos over time can be seen observationally.
Figure 4: An example of 3D distribution of dark matter
reconstructed via tomographic methods using the weak lensing technique
combined with the redshift estimates of the background galaxies. All of
the 3D maps are available here. (Credit: University of Tokyo/NAOJ)
What the dark matter halo count suggests and future prospects
The team counted the number of dark matter halos whose lensing signal
is above a certain threshold. This is one of the simplest measurements
of the growth rate. The histogram (black line) in Figure 5
shows the observed lensing signal strength versus the number of
observed halos whereas the model prediction is shown by the solid red
line. The model is based on the standard LCDM model using the
observation of cosmic microwave background as the seed of the
fluctuations. The figure suggests that the number count of the dark
matter halos is less than what is expected from LCDM. This could
indicate there is a flaw in LCDM and that we might have to consider an
alternative rather than the simple cosmological constant (Note 1).
Figure 5: Number of dark matter halos versus their
lensing signal strength (black histogram) and number count expected from
LCDM and the most recent CMB observation by the Planck satellite.
(Credit: NAOJ/University of Tokyo)
The statistical significance is, however, still limited as the large error bars (vertical line on the histogram in Figure 5)
suggest. There has been no conclusive evidence to reject LCDM, but many
astronomers are interested in testing LCDM because discrepancies can be
a useful probe to unlock the mystery of the accelerating Universe.
Further observation and analysis are needed to confirm the discrepancy
with higher significance. There are some other probes of the growth rate
and such analysis are also underway (e.g. angular correlation of galaxy
shapes) in the team to check the validity of standard LCDM.
These results were published on January 1, 2018 in the HSC special
issue of the Publications of the Astronomical Society of Japan (Miyazaki
et al. 2018, "A large sample of shear-selected clusters from the Hyper
Suprime-Cam Subaru Strategic Program S16A Wide field mass maps", PASJ,
70, S27; Oguri et al. 2018 "Two- and three-dimensional wide-field weak
lensing mass maps from the Hyper Suprime-Cam Subaru Strategic Program
S16A data", PASJ, 70, S26). The projects are supported by Grants-In-Aid
by MEXT and JSPS JP15H05892, JP15H05887, JP15H05893, JP15K21733,
JP26800093, JP15K17600, JP16H01089 as well as JST's CREST JPMJCR1414.
Note 1: Empty space is known to have energy caused by quantum
effects and this is one candidate for the source of the cosmological
constant. However, the energy density of the cosmological constant is
many orders of magnitude weaker than what would be predicted based on
this "vacuum energy" and it is hard to reconcile the discrepancy.
Astronomers started to consider the existence of some other physical
mechanism to explain the energy density, that concept is now called dark
energy. The energy density can change over time in this generalization.
If the dark energy was stronger in the past, the acceleration would
have been more significant and suppressed the growth rate. This would
result in fewer dark matter halos.
Source: Subaru Telescope
