Unprecedentedly wide and sharp dark matter map

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.

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Credit: NAOJ/University of Tokyo

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.

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.

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Credit: NAOJ

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.

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(Credit: NAOJ)

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.

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Credit: University of Tokyo/NAOJ

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.

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Two-dimensional dark matter map estimated by weak lensing technique. The dark matter is concentrated in dense clumps.

 

Hubble tracks down a galaxy cluster's dark matter.