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Chapter 16: Problem 32

Choose the best answer to each of the following. Explain your reasoning with one or more complete sentences. Based on the observational evidence, is it possible that dark matter doesn't really exist? (a) No, there is too much evidence to think that it could be in error. (b) Yes, but only if there is something wrong with our current understanding of how gravity should work on large scales. (c) Yes, but only if all the observations themselves are in error.

Short Answer

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(b) Yes, but only if there is something wrong with our current understanding of gravity.

Step by step solution

01

Examine Option (a)

Option (a) suggests that it is not possible for dark matter to not exist because there is too much evidence to think it could be in error. Observational evidence for dark matter comes from various sources such as galaxy rotation curves and gravitational lensing, which show mass that cannot be accounted for by visible matter alone. Hence, dark matter is a critical component of current cosmological models.
02

Examine Option (b)

Option (b) posits that it is possible dark matter doesn't exist if there is something wrong with our current understanding of gravity on large scales. This reflects theories such as Modified Newtonian Dynamics (MOND) which propose modifications to gravity that could account for the observations without needing dark matter.
03

Examine Option (c)

Option (c) suggests that dark matter might not exist if all the observations themselves are in error. This is less plausible because the observational evidence is broad, from different types of measurements and observations that consistently point to dark matter's presence.
04

Compare Options and Decide

Analyzing the options, option (b) is the most plausible if we allow for the possibility that our current gravitational models might be incomplete or incorrect on large scales. While dark matter explanations are widely accepted, alternative theories of gravity offer a coherent argument.

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Key Concepts

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

Galaxy Rotation Curves
Galaxy rotation curves are one of the most compelling pieces of evidence in favor of dark matter. When astronomers measure how fast stars orbit the center of a galaxy, they find something unexpected. The speed of stars remains nearly constant even far from the center where you'd expect it to drop. According to Newton's law of gravity, the rotation speed should decrease as one moves away from the mass concentrated at the center.

However, the actual observations defy these expectations:
  • Stars on the outer edges of galaxies rotate at much higher speeds than can be explained by the gravity from visible matter.
  • This discrepancy suggests the presence of an unseen mass known as dark matter, which influences the motion of stars.
This phenomenon demonstrates that visible matter alone isn't enough to explain galactic dynamics. Without dark matter, galaxies would fly apart due to their high rotation speeds.
Gravitational Lensing
Gravitational lensing is another powerful tool providing evidence for dark matter. It occurs when a massive object, like a galaxy cluster, bends light from objects behind it due to its gravity. This bending can magnify and distort the background objects, much like a lens:
  • The amount of lensing observed often indicates more mass than what's visible in galaxies or galaxy clusters.
  • The calculations of this bending require a much larger mass than accounted for by visible material alone.
Dark matter is the most straightforward explanation for this extra mass:

It doesn't emit, absorb, or reflect light, making it invisible but influential through gravity.
Gravitational lensing provides a "map" of this invisible matter, enhancing our understanding of mass distribution in the universe.
Modified Newtonian Dynamics (MOND)
Modified Newtonian Dynamics, or MOND, is an alternative theory proposing that our current understanding of gravity might need adjustments to explain galactic observations without dark matter. MOND suggests that Newton's laws are not entirely accurate at very low acceleration levels, typical of outer regions of galaxies:
  • It introduces a new constant to adjust for these low acceleration environments, potentially explaining flat galaxy rotation curves without requiring dark matter.
  • While MOND can sometimes mimic the effects attributed to dark matter in certain contexts, it doesn't explain all cosmological phenomena.
The MOND framework challenges the conventional dark matter theory, providing invaluable insights into the gravitational laws at cosmic scales.

Despite its success in explaining some observations, it remains controversial because it doesn't naturally fit within the broader framework of modern physics.

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Most popular questions from this chapter

Decide whether the statement makes sense (or is clearly true) or does not make sense (or is clearly false). Explain clearly; not all of these have definitive answers, so your explanation is more important than your chosen answer. The distance to a white dwarf supernova with a particular redshift is larger in an accelerating universe than in a universe with no acceleration.

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 a high concentration of dark energy

Choose the best answer to each of the following. Explain your reasoning with one or more complete sentences. The major evidence for the idea that the expansion of the universe is accelerating comes from observations of (a) white dwarf supernovae. (b) the orbital speeds of stars within galaxies. (c) the evolution of quasars.

Choose the best answer to each of the following. Explain your reasoning with one or more complete sentences. Dark energy has been hypothesized to exist in order to explain (a) observations suggesting that the expansion of the universe is accelerating. (b) the high orbital speeds of stars far from the center of our galaxy.(c) the giant voids between large-scale structures in the universe.

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
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