Which is not a characteristic difference between cold and hot dark matter? a. temperature b. composition c. the way they clump under the influence of gravity d. mass

Cold dark matter (CDM) is a type of dark matter that moves at speeds much slower than the speed of light. This slow movement allows it to clump together under the influence of gravity, forming structures like galaxies and galaxy clusters. CDM is thought to be composed of particles that interact very weakly with electromagnetic forces, meaning they do not emit, absorb, or reflect light, making them invisible and detectable only through their gravitational effects. The candidates for these particles include Weakly Interacting Massive Particles (WIMPs) and axions.

Hot dark matter (HDM) differs from cold dark matter primarily in its velocity. HDM particles move at relativistic speeds, close to the speed of light, which prevents them from clumping together easily. This means they have a tendency to remain spread out in the universe. Neutrinos are the most likely candidates for hot dark matter, as they are very light particles that travel quickly and have only weak interactions with other matter.

Gravitational clumping refers to the way dark matter attracts and pulls together to form larger structures under the force of gravity. Cold dark matter, due to its slow movement, clumps very effectively, leading to the formation of galaxies, galaxy clusters, and other large-scale structures in the universe. On the other hand, hot dark matter, because of its high speed, resists clumping and tends to remain more evenly distributed. This difference in clumping behavior is crucial for understanding the evolution of the universe's structure.

The composition of dark matter is still a subject of much research and debate. Cold dark matter is believed to be made up of heavy particles like WIMPs, which are massive and move slowly. Hot dark matter, however, consists of much lighter particles such as neutrinos. These compositional differences influence the behavior of these particles. Specifically, the mass and speed of these particles dictate whether they clump under gravity or remain spread out.

One key difference between cold and hot dark matter is the mass of the particles that make them up. Cold dark matter particles are generally much more massive than hot dark matter particles. For example, WIMPs (a candidate for CDM) are much heavier than neutrinos (a candidate for HDM). This mass difference significantly impacts how these particles behave. Heavy particles move more slowly and clump together under gravity, while lighter particles move quickly and do not clump as easily. Despite these differences, mass itself is not always singled out as a primary distinguishing feature, because the behavior of dark matter (temperature, clumping, composition) tends to encompass these attributes.