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Chapter 23: Problem 20

Neutrinos are an example of a. hot dark matter. b. cold dark matter. c. charged particles. d. both a and c

Short Answer

Expert verified
a. hot dark matter.

Step by step solution

01

Understanding Neutrino Properties

Neutrinos are fundamental particles that are very light and interact only via the weak nuclear force and gravity. They do not carry electric charge and are nearly massless.
02

Classifying Dark Matter

Dark matter is typically classified into two categories based on its temperature when it decoupled from other particles: hot dark matter (HDM) and cold dark matter (CDM). Hot dark matter consists of particles that were moving relativistically (nearly at the speed of light) when they decoupled, while cold dark matter particles were moving much slower.
03

Identifying Hot Dark Matter

Given that neutrinos are very light particles and move at relativistic speeds, they fall into the category of hot dark matter.
04

Checking for Electric Charge

Neutrinos are neutral particles, meaning they do not carry any electric charge. This immediately rules out options that suggest they are charged particles.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Hot Dark Matter
Hot dark matter (HDM) refers to dark matter particles that were moving at relativistic speeds, or near the speed of light, when they decoupled from the rest of the matter in the universe. Relativistic speeds are those that are a significant fraction of the speed of light. Because of this, HDM particles tend to spread out more and have a smoothing effect on the structure of the universe.

An excellent example of hot dark matter is the neutrino.
  • They are extremely light particles.
  • They move very fast, almost at the speed of light.
  • They interact only through the weak nuclear force and gravity.
Neutrinos were crucial in the early universe, but their speedy nature means they cannot account for the more clumpy structures we see in galaxies today. Instead, HDM plays a role in larger-scale structures like galaxy clusters.
Cold Dark Matter
Cold dark matter (CDM) is another classification of dark matter. Unlike HDM, CDM consists of particles that were moving much more slowly when they decoupled from other matter.

CDM particles move almost non-relativistically and have significant effects on galaxy formation. Because they move so slowly, they clump together more easily, which helps in the formation of smaller structures, such as individual galaxies, and eventually stars and planets.

Examples of cold dark matter candidates include:
  • Weakly Interacting Massive Particles (WIMPs)
  • Axions
Understanding the nature of CDM helps scientists figure out how galaxies and other cosmic structures formed and evolved over billions of years.
Electric Charge
Electric charge is a fundamental property of particles that causes them to experience a force when placed in an electromagnetic field. Particles with charge can be either positive or negative and interact via the electromagnetic force.

Key things to know about electric charge include:
  • Like charges repel each other, and opposite charges attract.
  • Protons carry a positive charge, while electrons carry a negative charge.
  • Neutrinos, however, are neutral particles, meaning they have no electric charge.
Because neutrinos do not carry an electric charge, they do not interact electromagnetically. This property is part of what makes them so difficult to detect and why they do not fit into the category of charged particles.

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Most popular questions from this chapter

Currently, the Milky Way and Andromeda galaxies are separated by about 2.3 million light-years and are moving toward each other at about 110 kilometers per second \((\mathrm{km} / \mathrm{s})\) Estimate how long it might take for the two to collide. Why do you think your answer may or may not be a good estimate of how long it will take these galaxies to fully merge?

Galaxy formation is similar to star formation because both a. are the result of gravitational instabilities. b. are dominated by the influence of dark matter. c. end with the release of energy through fusion. d. result in the formation of a disk.

T/F: Hot dark matter cannot account for the formation of today's galaxies.

Which observation reveals the most about how the universe will end? a. The universe is expanding. b. The universe is accelerating. c. The universe contains large amounts of dark matter. d. The universe contains large amounts of normal matter.

What is the primary difference between galaxy groups and galaxy clusters? a. the volume they occupy b. the number of galaxies c. the total mass of the galaxies d. all of the above
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