Hot dark matter
Dark matter is a form of matter that neither emits nor absorbs light. Within physics, this behavior is characterized by dark matter not interacting with electromagnetic radiation, hence making it dark and rendering it undetectable via conventional instruments in physics. Data from galaxy rotation curves indicate that approximately 80% of the mass of a galaxy cannot be seen, forcing researchers to innovate ways that indirectly detect it through dark matter's effects on gravitational fluctuations. There exists no consensus in the theoretical physics community as to whether dark matter is divisible into various 'types', but there exists evidence for differentiating dark matter into "hot" (HDM) and "cold" (CDM) types–some even suggesting a middle-ground of "warm" dark matter (WDM). The terminology is not meant to invoke any association with temperature, but instead refer to the size of the purported dark matter particles (WIMPs). In turn, the size of the particles determines the velocities at which they travel at in an inverse relationship: HDM travels faster than CDM because the HDM particles are theorized to be of lower mass.
Role in Galaxy Formation
In terms of its application, the distribution of HDM could also help explain how clusters and superclusters of galaxies formed after the Big Bang. Theorists claim that there exist two classes of dark matter: 1) those that "congregate around individual members of a cluster of visible galaxies" and 2) those that encompass "the clusters as a whole." Because CDM possesses a lower velocity, it could be the source of "smaller, galaxy-sized lumps," as shown in the image. HDM, then, should correspond to the formation of larger mass aggregates that surround whole galaxy clusters. However, data from the cosmic microwave background radiation, as measured by the COBE satellite, is highly uniform, and such high-velocity HDM particles cannot form clumps as small as galaxies beginning from such a smooth initial state, highlighting a discrepancy in what dark matter theory and the actual data are saying. Theoretically, in order to explain relatively small-scale structures in the observable Universe, it is necessary to invoke CDM or WDM. In other words, HDM being the sole substance in explaining cosmic galaxy formation is no longer viable, placing HDM under the larger umbrella of mixed dark matter (MDM) theory.
An example of a hot dark matter particle is the neutrino. Neutrinos have very small masses, and do not take part in two of the four fundamental forces, the electromagnetic interaction and the strong interaction. They theoretically interact by the weak interaction, and gravity, but due to the feeble strength of these forces, they are difficult to detect. A number of projects, such as the Super-Kamiokande neutrino observatory, in Gifu, Japan are currently studying these neutrinos.
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