Introduction: Measuring movement of celestial bodies
In 1929, Hubble discovered that the more distant a galaxy, the faster it is moving away from us. This fact proves that our universe started with the Big Bang and is continuously expanding. In the 1960s, Rubin et al. also discovered that the rotation speed of gases of our Galactic System does not drop even if they are located at the far outer area where stars no longer exist. This means that most of the galactic mass is occupied by unseen matter, not stars or gases. In other words, this is a discovery of "dark matter." Meanwhile, in 1995, Mayor et al., by measuring precisely the movement of star 51 Pegasi, discovered that something orbited it; specifically, they discovered an "extrasolar planet." There is one thing common to these three discoveries: they all used the Doppler Effect to measure movement of celestial bodies more precisely than ever before.
The Doppler Effect is the phenomenon where light emitted from a moving object at speed is measured as a different wavelength from its original wavelength. This is the same principle as when the tonal pitch of an ambulance sounds differently during its approach and its departure. We can measure velocity based on changes in wavelength of emission or absorption line. When above results were initially discovered, errors were large and many people doubted the results. In consideration, however, these are foresight, a historical discoveries. The measurement of celestial bodies' movement by the Doppler Effect has been a fundamental of astronomy.
Using the same principle, we investigated plasma motion in a galaxy cluster. One day we may discover something new (dark energy?) based on this measurement method. In this article, I introduce our "discovery."
Collision of galaxy cluster's plasma
In the universe, stars gather to form a galaxy, and galaxies form a "galaxy cluster," the largest structures in the universe. X-ray radiation was detected in galaxy clusters, and in the 1960s, high-temperature plasmas in addition to visible stars were also discovered in them.
Plasma denotes an ionization state where atoms separate into electrons and ions. On Earth, plasmas exist only in very special hot-temperature places. In the universe, however, most ordinary substances exist in the form of plasma. For example, inside the Sun, ionized gases in galaxies, and the disks around black holes are all plasmas.
X-ray observation of galaxy clusters revealed that plasma mass exceeds the total mass of stars, and plasma is the primary element of ordinary substances (i.e., not dark matter) in the universe. Plasma is very high-temperature ranging from 10 million to 100 million Kelvin. It is not easy to heat intergalactic matter up to such high-temperatures. To understand ow the plasma has been heated is a key to elucidate the structural formation of the universe. Most astronomers believe that plasmas have been heated through the process that small structures collide and merge repeatedly to grow to large structures. At the scale of a galaxy cluster, this means that the gravitational energy of dark matter occupying most of the mass is converted to thermal energy through the plasma's kinetic energy. In past observations, the temperature distribution of plasmas has been well explored. The galaxy clusters' plasma motion that determines temperature distribution, however, has only been suggested based on minute changes in the shape of the plasma caused by shock waves and other processes, through X-ray image analysis.