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Chapter 23: Problem 18

Imagine that there are galaxies in the universe composed mostly of dark matter, with relatively few stars or other luminous normal matter. If this were true, how might you learn of the existence of such galaxies?

Short Answer

Expert verified
Detect dark matter-dominated galaxies via gravitational lensing, galaxy rotation curves, studying the CMB, and mass discrepancies.

Step by step solution

01

- Understand the Concept of Dark Matter

Dark matter does not emit, absorb, or reflect light, making it invisible to traditional telescopes. Its presence is inferred from its gravitational effects on visible matter, radiation, and the large-scale structure of the universe.
02

- Look for Gravitational Lensing

Gravitational lensing occurs when a massive object, like a galaxy primarily composed of dark matter, bends the light from objects behind it. Detecting distortions in light from background galaxies can indicate the presence of a dark matter-dominated galaxy.
03

- Study Galaxy Rotation Curves

Analyze the rotation curves of galaxies. If a galaxy is composed mostly of dark matter, stars at various distances from the center will rotate at similar speeds. This constant rotational velocity suggests the presence of dark matter.
04

- Observe the Cosmic Microwave Background (CMB)

Study the Cosmic Microwave Background radiation for fluctuations. The distribution and presence of dark matter influence how matter clumps together in the early universe, which is observable through the CMB.
05

- Look for Relational Inconsistencies

Compare the observed mass of galaxies with the mass inferred from their gravitational effects. A significant discrepancy between these masses may suggest a large component of dark matter.

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Key Concepts

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

gravitational lensing
Gravitational lensing is a phenomenon where massive objects like galaxies warp the space around them, bending the light from objects behind them. This effect is like a cosmic magnifying glass.
When light from a distant galaxy passes through a region with a large mass, such as a dark matter-dominated galaxy, it gets bent and distorted.
By studying these distortions, astronomers can detect the presence of otherwise invisible dark matter.

Key points:
  • Gravitational lensing helps detect dark matter by observing light distortions.
  • Light bending reveals unseen massive structures.
  • Even if a galaxy doesn't emit light, its mass can still warp space-time.
Detecting and analyzing these lens effects can provide evidence for galaxies mostly composed of dark matter.
The amount and pattern of lensing can help estimate the mass and distribution of dark matter.
galaxy rotation curves
Galaxy rotation curves are crucial for understanding dark matter.
Normally, we expect stars farther from a galaxy's center to rotate more slowly, like planets in our solar system.
However, observations show that stars in the outer parts of many galaxies rotate at similar speeds to those near the center.

This suggests something additional is influencing the galaxy’s rotation. The consistent rotational velocity implies a hidden mass component – dark matter.

Key points:
  • Stars at different distances from the center should rotate at different speeds.
  • The observed constant speed hints at hidden mass.
  • Dark matter explains the unexpected rotation curves.
Such analysis supports the idea that most of the mass in these galaxies is not visible but rather dark matter.
cosmic microwave background
The Cosmic Microwave Background (CMB) is the afterglow of the Big Bang, providing a snapshot of the early universe.
Studying the CMB helps us understand the distribution of matter and energy.

Fluctuations in the CMB give insights into the presence of dark matter.
Dark matter played a crucial role in the early universe, affecting how matter clumped together.

Key points:
  • CMB is the relic radiation from the Big Bang.
  • It shows slight temperature variations.
  • These variations help track the influence of dark matter.
By analyzing the CMB, scientists can infer how dark matter influenced the formation and evolution of structures in the universe.

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Most popular questions from this chapter

Reionization of the neutral gas in the universe occurred because of the a. decay of dark matter particles. b. emission of neutrinos by the first stars that formed. c. release of jets of charged particles from supermassive black holes. d. radiation from the first stars, supernovae, and black holes that formed.

Which is not a characteristic difference between cold and hot dark matter? a. temperature b. composition c. the way they clump under the influence of gravity d. mass

What are some of the observational signs that dark matter exists? Explain why this evidence challenged earlier theories and forced astronomers to change their minds about the existence of matter they could not see.

As clumps containing cold dark matter and normal matter collapse, they heat up. When a clump collapses to about half its maximum size, the increased thermal motion of particles tends to inhibit further collapse. Whereas normal matter can overcome this effect and continue to collapse, dark matter cannot. Explain the reason for this difference.

If 300 million neutrinos fill each cubic meter of space, and if neutrinos account for only 5 percent of the mass density (including dark matter) of the universe, estimate the mass of a neutrino.
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