What is the difference between hot dark matter and cold dark matter? What difference does it make to cosmology?

Question

Leah Abbate · Accepted Answer

Hot Dark Matter : Dark Matter moving at relativistic speeds.
Cold Dark Matter: Dark Matter moving at non-relativistic speeds.
They affect the way structures form in the Universe.

Particles are categorized into the following groups based on the difference between their rest mass energy and kinetic energy: Non-relativistic: #E_k \lt\lt E_0#, Relativistic : #E_k \approx E_0#, Super-relativistic: #E_k \gt\gt E_0#; HDM & CDM: Dark matter particles are referred to as Hot Dark Matter (HDM) if they are relativistic or super-relativistic, and as Cold Dark Matter (CDM) otherwise. The current consensus is that Dark Matter is mostly cold because, if it were hot, structure formation theories predict that all structures smaller than massive galaxies would have been destroyed by a process called Free Streaming. Since we have structures smaller than galaxies, we conclude that Dark Matter cannot be predominantly hot, though there may be a small amount of hot dark matter mixed among predominantly cold matter. This is because Dark Matter has not been detected yet.

Nova Adams · Answer

undefined Hot dark matter consists of fast-moving particles like neutrinos, while cold dark matter consists of slower-moving particles like WIMPs. The difference affects the structure formation in the early universe; cold dark matter leads to large-scale structures, while hot dark matter inhibits small-scale structures. This distinction influences the evolution of cosmic structures and has implications for cosmological models and observations.