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Chapter 17: Problem 19

Which statement(s) about redshift and look-back time is/are true? Choose all that apply. a. The greater the redshift, the greater the look-back time. b. The smaller the value of \(z\), the shorter the look-back time. c. Objects with greater redshift appear younger than they actually are today. d. Look-back time is limited by the speed of light and the age of the Universe. e. The greater the redshift, the closer the look-back time approaches the Big Bang.

Short Answer

Expert verified
All statements (a, b, c, d, e) are true.

Step by step solution

01

Understanding Redshift and Look-back Time

Redshift is a measure of how much the wavelength of light has been stretched due to the expansion of the Universe. Look-back time is the time difference between when light was emitted by a distant object and when we observe it. As the universe expands, objects that are further away from us have higher redshift, meaning the light takes longer to reach us.
02

Evaluating Statement (a)

Statement (a) states that "The greater the redshift, the greater the look-back time." This is true because a higher redshift indicates that the light from the object has been traveling for a longer time to reach us, resulting in a greater look-back time.
03

Evaluating Statement (b)

Statement (b) claims "The smaller the value of \(z\), the shorter the look-back time." This is true since a smaller redshift implies that the object is closer to us and the light took less time to reach us, resulting in a shorter look-back time.
04

Evaluating Statement (c)

Statement (c) asserts "Objects with greater redshift appear younger than they actually are today." This statement is true because we see the light from these objects as it was emitted a long time ago (due to the time light takes to travel), thus showing the object as it was in the past, not as it is today.
05

Evaluating Statement (d)

Statement (d) states "Look-back time is limited by the speed of light and the age of the Universe." This is true because the speed of light determines how fast information can travel across the universe, and the look-back time cannot exceed the age of the Universe.
06

Evaluating Statement (e)

Statement (e) suggests "The greater the redshift, the closer the look-back time approaches the Big Bang." This is true because higher redshifts correspond to times closer to the beginning of the Universe when the Big Bang occurred. As the redshift increases, the look-back time approaches the moment of the Big Bang.

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Key Concepts

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

Look-back time
Look-back time refers to the concept in astronomy that describes the period between when light was initially emitted by a distant celestial object and the moment we observe it here on Earth. This is an intriguing part of studying the universe because it means that when we gaze at far-off galaxies, we are essentially looking backward in time. The farther an object is, the longer its light has taken to reach us, providing a glimpse into the universe's past. To put it simply:
  • Light emitted from a distant galaxy takes time to travel across space.
  • The moment it reaches our telescopes, we're seeing that galaxy as it was when the light began its journey.
This allows astronomers to observe the early stages of stars and galaxies. Understanding look-back time helps us unveil the historical structure and evolution of the universe.
Expansion of the Universe
The universe has been expanding since the Big Bang, and this expansion is a key concept in astronomy. As the universe expands, the distances between galaxies increase over time. This expansion can be illustrated through the phenomenon of redshift, which occurs as light stretches, causing its wavelength to increase, while its inherent frequency decreases. This effect shifts the light towards the red end of the spectrum, hence the name "redshift." The expansion of the universe tells us a lot about its past and future. For instance:
  • It shows that the universe was once condensed to a single point in the Big Bang.
  • It suggests that the universe will continue to expand, possibly indefinitely, depending on dark energy dynamics.
Understanding this concept helps astronomers predict cosmological events and the general fate of the cosmos.
Speed of light
In the vast distances of space, the speed of light serves as a cosmic speed limit. At approximately 299,792 kilometers per second, light travels incredibly fast, yet still takes significant time to traverse the universe's vastness. This speed is crucial in shaping our understanding of astronomy.
  • The speed of light defines how we measure astronomical distances, often in "light years," representing the distance light travels in one year.
  • It also dictates how quickly we can receive information about distant cosmic events or objects.
However, the speed of light poses a natural limit on our observations. We can only see as far back as light has traveled since the universe's birth. Ultimately, the speed of light governs our ability to look backward in time, setting the maximum "look-back time" based on the age of the universe.
Big Bang
The Big Bang theory is the prevailing explanation for the universe's origin, describing how it expanded from an extremely hot and dense initial point approximately 13.8 billion years ago. This moment marked the inception of both space and time. As the universe expanded and cooled, subatomic particles formed atoms, eventually giving rise to stars, galaxies, and larger cosmic structures.
  • The Big Bang signifies the start of the observable universe.
  • It offers insight into cosmic background radiation, a faint glow left over from this formation period.
By studying the Big Bang and its aftermath, astronomers aim to understand the universe's composition, age, and the fundamental forces at play. This theory is central to cosmology and underpins many aspects of modern astronomical research.

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Most popular questions from this chapter

What is required for the formation of an Einstein ring?

What does a redshift of \(z=0\) mean? a. The object is almost at the edge of the Universe. b. The object is nearly 14 billion years old. c. The object is not moving. d. The object shows no redshift. e. The object is obscured from our view by the plane of the Milky Way.

Which of the following provide(s) evidence of dark matter? Choose all that apply. a. the Solar System b. galaxies c. galaxy clusters d. superclusters e. black holes

What galaxy types are found in the Local Group, and how are they arranged?

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