Cosmic Speed Limits: Hot vs Cold Dark Matter Quiz

The terms hot and cold refer to the speed of the particles shortly after the Big Bang. Hot dark matter consists of particles moving at relativistic speeds (near the speed of light), while cold dark matter consists of slower, heavier particles. This velocity determines how easily the matter can clump together to form structures.

In a cold dark matter scenario, small density fluctuations collapse first because the particles are moving slowly enough to be trapped by gravity. These small clumps then merge over billions of years to form larger galaxies and clusters. This "bottom-up" process matches most astronomical observations of the modern universe.

Free-streaming occurs when particles, like those in hot dark matter, move at such high velocities that they "stream" out of small regions of high density. This prevents small structures like individual galaxies from forming early on, as the particles fly away before gravity can pull them together into a cohesive unit.

Hot dark matter particles are relativistic, meaning they travel extremely fast. Because they move so quickly, they can only be caught by very large gravitational wells. This leads to a "top-down" model where massive superclusters form first and later fragment into smaller galaxies.

The cosmic microwave background provides a "snapshot" of the early universe's density. The specific patterns and sizes of the temperature ripples found in this light align perfectly with a universe dominated by slow-moving, cold dark matter. A universe filled with hot dark matter would have much smoother fluctuations.

Neutrinos are subatomic particles that were produced in massive quantities during the Big Bang. Because they have nearly zero mass, they travel at almost the speed of light. While they are a form of non-baryonic matter, they are classified as "hot" and cannot account for all the dark matter in the universe.

In a top-down model, associated with hot dark matter, large-scale structures like "pancakes" or filaments form first. These massive structures would eventually break apart to form smaller components like galaxies. However, observations show that distant, early galaxies already existed, which contradicts the pure hot dark matter theory.

Scientists look for particles that have mass but do not interact with light. WIMPs and Axions are leading theoretical candidates for CDM. Unlike protons, which are baryonic, these particles are non-baryonic and move slowly enough to allow for the hierarchical growth of galaxies and clusters.

Cold dark matter allows for hierarchical structure formation. This means that small "halos" or pockets of dark matter form first. As the universe ages, these small units are pulled together by gravity to create larger and more complex systems, eventually resulting in the massive galaxy clusters seen today.

Velocity is the deciding factor in how "smooth" or "clumpy" the universe is. Cold, slow particles can be captured by even small amounts of gravity, creating a very detailed web with small galaxies. Hot, fast particles "erase" small-scale details, resulting in a much coarser and less detailed structure.

If the universe formed "top-down," we would expect to see only massive structures in the early universe. However, deep-space telescopes have observed small, mature galaxies existing only a few hundred million years after the Big Bang. This suggests that small structures formed first, supporting the cold dark matter model.

The Big Bang theory relies on the composition and distribution of matter to explain cosmic history. The velocity of dark matter particles dictates the density of the early universe, which in turn influenced the production of hydrogen and helium and the eventual locations of all baryonic matter.

In the CDM model, dark matter forms "gravitational wells" first. Because the dark matter is slow, these wells stay put. Normal baryonic gas is then attracted to these wells. Once the gas is concentrated enough, it cools and collapses to ignite the first stars.

In a hot dark matter universe, the fast-moving particles would prevent small-scale gravitational collapse. We would see a universe where large clusters exist, but the individual small galaxies that make them up would have taken much longer to appear, which does not match our current observations.

The velocity of dark matter particles is the "tuning knob" for the universe's structure. By adjusting the speed in simulations, scientists can see how the universe changes from hot (smooth) to cold (clumpy). Current data confirms that a "cold" velocity is required to produce the universe as we see it today.

Warm dark matter is a theoretical middle ground. It suggests particles that move faster than CDM but slower than HDM. This model is studied to see if it can better explain the number of small "satellite" galaxies orbiting larger ones, which sometimes differs from pure CDM predictions.

Gravitational lensing allows astronomers to "see" the shape and density of invisible dark matter. By mapping how light bends around galaxy clusters, scientists can determine if the matter is concentrated in small, dense clumps (Cold) or spread out in large, smooth clouds (Hot), further confirming the CDM model.

The origin of the universe is supported by the observed expansion (redshift), the leftover heat from the Big Bang (CMB), and the specific ratios of hydrogen and helium. The behavior of dark matter during this expansion is what allowed these elements to eventually organize into the cosmic web.

After the Big Bang, the universe was filled with intense light. Because baryonic matter is made of charged particles (protons/electrons), it was constantly pushed around by this light. Non-baryonic cold dark matter does not "feel" light, so it could start clumping long before the gas could.

A dark matter halo is a dense region of non-baryonic matter that acts as the gravitational anchor for a galaxy. Whether these halos started as small units (Cold Dark Matter) or huge sheets (Hot Dark Matter) is the central question in understanding how the cosmic web evolved.