Using images obtained by the Hyper Suprime-Cam instrument on Japan’s Subaru Telescope , experts from a team led by the National Astronomical Observatory of Japan (NAOJ) and the University of Tokyo have produced a larger and sharper dark matter map than all that had been done so far. The distribution of dark matter in the universe is estimated using the weak gravitational lensing technique. The astronomers located the positions and lens signals of the dark matter halos and found hints that the number of halos might not match what the simplest cosmological model suggests. This could confirm that the expansion of the Universe is accelerating.
In the 1930s, Edwin Hubble discovered that the universe is expanding , which came as a surprise to most astronomers, who thought that it had remained the same and stable throughout eternity. But it was necessary to find a formula that related matter and the geometry of space-time to mathematically express this expansion of the Universe. Coincidentally, Einstein had already developed that formula . Modern cosmology is based on Einstein’s theory of gravity.
The expansion was thought to slow down over time because the elements contained in the universe attract each other. But in the late 1990s, the expansion was found to have accelerated from about 8 billion years ago . This was another surprise that earned the astronomers who found the expansion a Nobel Prize in 2011. To explain the acceleration, you have to consider that there must be something in the universe that repels space. The simplest solution is to put the cosmological constant back into Einstein’s equation .
The cosmological constant was originally introduced by the German physicist to explain a static universe, but was discarded after the expansion was discovered. The standard cosmological model (called LCDM) incorporates the cosmological constant. LCDM supports many observations, but we still don’t know what is causing the acceleration. This is one of the biggest questions in modern cosmology. Now, the team of Japanese astrophysicists is analyzing large-scale images using Hyper Suprime-Cam (HSC) to investigate the mystery of the accelerated universe. The key is to examine the history of the expansion of the Universe. In the primitive cosmos, matter was distributed almost evenly but not completely.
There were slight fluctuations in density that can now be observed through temperature variations of the cosmic microwave background . These slight fluctuations of matter evolved during cosmic time due to the mutual gravitational pull of matter, and eventually the large-scale structure of the current universe became visible. It is known that the speed of growth of the structure depends largely on how the universe is expanding. For example, if the expansion rate is high, it is difficult for matter to contract. This means that the expansion can be tested inversely by observing the growth rate. But the growth rate cannot be calculated well if we only look at visible matter (stars and galaxies). This is because we now know that almost 80% of matter is an invisible substance called dark matter.
Japanese astrophysicists used the weak gravity lens technique . The images of distant galaxies are slightly distorted by the gravitational field generated by the distribution of dark matter in the foreground . Systematic distortion analysis allows us to reconstruct the dark matter distribution in the foreground. This technique is difficult to carry out observations of the cosmos, because the distortion of each galaxy is generally very subtle. Precise measurements of faint and seemingly small galaxies are required. This motivated the team to develop Hyper Suprime-Cam Hyper Suprime-Cam from March 2014.
Currently 60% of the work has been completed . So far they have presented the dark matter map based on image data taken in April 2016, which is 11% of what will be the final planned map. But so far there has never been a dark matter map so sharp that it covers such a wide area. Image 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 faint background galaxies (called a photometric redshift). At the same time, the effectiveness of the lens is greatest when it is placed directly between the distant galaxy and the observer. Using the photometric information from the redshift, the galaxies are grouped into redshift containers. Using this sample of clustered galaxies, the dark matter distribution is reconstructed using tomographic methods and thus the 3-D distribution (in the image above) can be obtained.
The 30 square degree data is used to reconstruct the redshift range between 0.1 (about 1.3 G light years) and 1.0 (about 8 G light years). At the 1.0 redshift, the spanned angle corresponds to 1.0 G x 0.25 G light years. This extensive 3-D dark matter map is also fairly new. It is the first time that the increase in the number of dark matter halos over time can be observed observationally. These results were published in Publications of the Astronomical Society of Japan .