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Chapter 22: Problem 27

Choose the best answer to each of the following. Explain your reasoning with one or more complete sentences Dark matter is inferred to exist because (a) we see lots of dark patches in the sky. (b) it explains how the expansion of the universe can be accelerating. (c) we can observe its gravitational influence on visible matter.

Short Answer

Expert verified
The answer is (c) because dark matter is inferred from its gravitational influence on visible matter.

Step by step solution

01

Understanding Dark Matter

To solve this problem, we need to understand what dark matter is. Dark matter is a form of matter that does not emit, absorb, or reflect light, making it invisible and undetectable by electromagnetic instruments. However, its presence is inferred from the gravitational effects it has on visible matter.
02

Evaluate Option (a)

Option (a) suggests dark matter is inferred from dark patches in the sky. These dark patches are usually areas with little to no stars or galaxies visible in optical wavelengths, not related to the presence of dark matter. Thus, dark patches do not provide evidence for dark matter.
03

Evaluate Option (b)

Option (b) suggests that dark matter explains the accelerated expansion of the universe. However, the accelerated expansion is explained by dark energy, not dark matter. Dark energy and dark matter are different concepts, so this option is incorrect.
04

Evaluate Option (c)

Option (c) points out that dark matter is inferred from its gravitational influence on visible matter. This is the correct explanation. We observe that galaxies rotate at speeds that cannot be accounted for by the mass of visible matter alone. The additional gravitational influence needed to explain these speeds is attributed to dark matter.

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Key Concepts

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

Gravitational Influence
Gravitational influence refers to the effect that gravity has on objects in the universe. In the context of dark matter, we cannot see it directly because it does not emit or interact with light. However, we know it exists because of the gravitational effects it has on visible matter.
For example:
  • Objects orbiting within galaxies move faster than they should based on the visible mass alone.
  • Clusters of galaxies show more gravitational pull than expected from their stars and gas.
These observations suggest that there is more mass present than we can see, leading to the conclusion that dark matter exerts gravitational force and plays a crucial role in cosmic structures.
Visible Matter
Visible matter includes everything we can see in the universe, such as stars, planets, and gas clouds, which emit or reflect light. Unlike dark matter, visible matter interacts with light, allowing us to observe it using telescopes and other instruments.
Despite being the "main characters" we usually see:
  • Visible matter only makes up a small portion of the total mass of the universe.
  • The interaction with light gives us clues about the presence of other unseen forces, like dark matter.
The visible components of galaxies still leave a lot unexplained about the dynamics we observe, pushing scientists to seek answers beyond what is visible.
Galaxy Rotation
The rotation of galaxies presents a mysterious case in astrophysics. When astronomers observe the speed at which stars revolve around the center of galaxies, they notice something unusual. The rotational speeds are too high for the amount of visible matter present.
Here's what's perplexing:
  • In a typical spinning object, you'd expect speeds to decrease with increasing distance from the center. However, in galaxies, outer stars are moving at surprising speeds.
  • This rotation curve suggests an unseen mass is contributing to the gravitational forces, known as dark matter.
This phenomenon led scientists to propose the existence of dark matter to explain the "missing mass" that accounts for these higher-than-expected speeds.
Electromagnetic Instruments
Electromagnetic instruments are devices that detect and measure electromagnetic radiation. This includes light visible to the human eye and other types of waves like radio, infrared, and X-rays. Telescopes equipped with these technologies allow us to observe stars, galaxies, and other astronomical bodies.
Why dark matter remains elusive:
  • Dark matter neither emits nor reflects electromagnetic radiation, making it invisible to these instruments.
  • The lack of interaction with electromagnetic radiation means dark matter can only be inferred indirectly through gravitational influences.
These tools are crucial for observing the universe, despite their limitations in detecting dark matter, they help us study the visible universe and infer the presence of dark matter indirectly.

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Most popular questions from this chapter

Be sure to show all calculations clearly and state your final answers in complete sentences. How Many MACHOs? Imagine a galaxy whose stars are all identical to the Sun but that has an overall mass-to-light ratio of \(30 M_{\mathrm{Sun}} / L_{\mathrm{Sun}}\) a. What is the ratio of dark matter to luminous matter in this galaxy? b. Suppose all the dark matter consists of MACHOs similar to Jupiter, each with a mass of \(0.001 M_{\text {Sun. }}\) How many of these MACHOs must the galaxy contain for each ordinary star? Explain.

Be sure to show all calculations clearly and state your final answers in complete sentences. Supergiant \(M / L\). What is the mass-to-light ratio of a \(30 M\) sun supergiant star with a luminosity of \(300,000 L_{\text {Sun }}\) ?

Choose the best answer to each of the following. Explain your reasoning with one or more complete sentences Which region of the early universe was most likely to become a galaxy? (a) a region whose matter density was lower than average (b) a region whose matter density was higher than average (c) a region with an unusual concentration of dark energy

Assuming that the accelerating expansion of the universe is real, what does it imply for the fate of the universe? What does current evidence suggest for the fate of the universe if the acceleration is not real? Explain.

Decide whether the statement makes sense (or is clearly true) or does not make sense (or is clearly false). Explain clearly; not all these have definitive answers, so your explanation is more important than your chosen answer. If the universe has more dark matter than we think, then it is also younger than we think.
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