Dark Matter Spatial Fluctuations in the Universe Detected on Small Scale

Monday - November 13 2023, 16:32 UTC - 1 year ago

A research team led by Professor Kaiki Taro Inoue at Kindai University has discovered fluctuations in dark matter distribution on scales smaller than massive galaxies using the world’s most powerful radio interferometer - the Atacama Large Millimeter/submillimeter Array (ALMA). This result is consistent with cold dark matter theory, and shows that dark matter is distributed not only within galaxies, but also in the intergalactic space. The research is an important step to understanding the true nature of dark matter.

Groundbreaking observations reveal dark matter fluctuations below the scale of galaxies, affirming cold dark matter theories and providing new insights into the Universe’s composition.

A research team led by Professor Kaiki Taro Inoue at Kindai University (Osaka, Japan) has discovered fluctuations in dark matter distribution in the Universe on scales smaller than massive galaxies using the world’s most powerful radio interferometer, the Atacama Large Millimeter/submillimeter Array (ALMA), located in the Republic of Chile. This is the first time that the spatial fluctuations of dark matter in the far Universe has been detected on a scale of 30,000 light-years. This result shows that cold dark matter[1] is favored even on scales smaller than massive galaxies, and is an important step toward understanding the true nature of dark matter. The article will be published in The Astrophysical Journal.

Dark matter, the invisible material that makes up a large fraction of the mass of the Universe, is thought to have played an important role in the formation of structures such as stars and galaxies.[2] Since dark matter is not uniformly distributed in space but is distributed in clumps, its gravity can slightly change the path of light (including radio waves) coming from distant light sources. Observations of this effect (gravitational lensing) have shown that dark matter is associated with relatively massive galaxies and clusters of galaxies, but how it is distributed on smaller scales has not been known.

The research team decided to use ALMA to observe an object at a distance of 11 billion light-years from the Earth. The object is a lensed quasar,[3] MG J0414+0534[4] (hereinafter referred to as "this quasar"). This quasar appears to have a quadruple image due to the gravitational lensing effect of the foreground galaxy. However, the positions and shapes of these apparent images deviate from those calculated solely from the gravitational lensing effect of the foreground galaxy, indicating that the gravitational lensing effect of the distribution of dark matter on scales smaller than massive galaxies is at work.

It was found that there are spatial fluctuations in the density of dark matter even at the scale of about 30,000 light-years, which is far below the cosmological scale (several tens of billions of light-years). This result is consistent with the theoretical prediction of cold dark matter, which predicts that dark matter clumps reside not only within galaxies (pale yellow color in Figure 2), but also in the intergalactic space (orange in Figure 2).

The gravitational lensing effects due to the clumps of dark matter found in this study are so small that it is extremely difficult to detect them alone. However, thanks to the gravitational lensing effect caused by the foreground galaxy and the high resolution of ALMA, we were able to detect the effects for the first time. Thus, this research is an important step to verify the theory of dark matter and to elucidate its true nature.

This research was presented in a paper "ALMA Measurement of 10 kpc-scale Lensing Power Spectra toward the Lensed Quasar MG J0414+053", written by Professor Taro Inoue and his colleagues, and published in The Astrophysical Journal.