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Inoue Lab. Kindai Univeristy
Department of Physics

Research

Probing the largest-scale structures in the universe

Supervoid as the origin of the Cold Spot:

In 2004, Vielva et al. found a strange signal in the temperature in the sky. They found a strange patch spanning around 10 degree in the sky showing a decrement of 70 micro Kelvin, which is called the "Cold Spot" later on.  The mean amplitude of fluctuation at this scale is around 20 micro Kelvin: the Cold Spot is anomalously cold. In theory, the chance of having such a cold region is less than 2 per cent.  In 2006, I and Joseph Silk proposed a theory that can explain the origin of the Cold Spot by assuming a large void with a radius of  several hundred mega parsec,  around 1 bilion light year, where the density of matter is smaller than average. Such a region can be called  a "supervoid".  If the light coming from an ancient universe  goes through a large underdense region, it "feels" the change of gravitational potential of the local region. If the gravitational force by the local region is strong enough, the "supervoid" grows. In this case, the height of the "mountain" of the gravitational potential  becomes large and the energy of the light is increased. On the other hand, if the gravitational force is not so strong, the energy of light can be decreased. For instance, if the repulsive force by the dark energy is strong enough,  gravitational potential of the local force can decay. In this case, we would observe a cold spot in the line of sight to the supervoid since the energy of the light is decreased.  This mechanism is called the "integrated Sachs-Wolfe effect". If the origin of the Cold Spot is indeed a hypothetical supervoid, it implies an existence of either dark energy or modified gravity.  In 2015, Szapudi et al. found a supervoid with a radius of  three hundred mega parsec at 1 giga parsec away. The claimed mass deficit is ~14 percent. The probability of having such a large underdense region is less than 1 per cent. The supervoid yields a temperature decrement of about 20 micro Kelvin at the center,  which is enough to deviate the signal that comes from the ancient universe.  If the size and mass scale of the supervoid are confirmed, the discovery strongly supports our theory in which we have predicted such a supervoid. More elaborate analysis could probe the origin of cosmic acceleration that is bailed in mystery.

Temperature in the sky (measured by WMAP) and the position of the Cold Spot (red ellipse).