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Chapter 20: Problem 26

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. We can't see galaxies beyond the cosmological horizon because their light is too dim.

Short Answer

Expert verified
The statement does not make sense; the limit is due to the universe's age, not light dimness.

Step by step solution

01

Understanding the Cosmological Horizon

The cosmological horizon is the limit beyond which we cannot observe any objects because their light hasn't had enough time to reach us since the universe's beginning. This limit is determined by the speed of light and the age of the universe.
02

Light Intensity and Distance

Light becomes dimmer as the distance from its source increases. This happens because light spreads out over a larger area and also because of cosmic redshift stretching the light wavelengths, which reduces the energy per photon.
03

Reasoning the Given Statement

The statement implies that the reason we can't see galaxies beyond the cosmological horizon is due to the dimness of their light. While dimming does occur due to distance, the primary reason we cannot see beyond this horizon is due to the finite speed of light and the age of the universe鈥攏ot simply dimness.

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Key Concepts

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

Speed of Light
The speed of light is a constant and fundamental property of the universe, crucial to various aspects of physics and astronomy. It defines the maximum speed at which information and matter can travel through space. In a vacuum, light travels at approximately 299,792 kilometers per second (or about 186,282 miles per second). This remarkable speed is considered universal and unreachable by any material object.
  • Light speed is integral in understanding cosmic distances, such as how long it takes for light from stars and galaxies to reach us.
  • The finite speed of light means we are always observing the universe as it was in the past, not as it is now.
Thus, the speed of light does not only set the ultimate speed limit but also significantly impacts how we perceive the universe's timeline and structure.
Age of the Universe
The age of the universe is estimated to be around 13.8 billion years. This age is crucial in determining the observable universe's size, as light from objects farther than 13.8 billion light-years has not yet had enough time to reach us. The universe's age helps astronomers understand the evolution of cosmic structures.
  • It provides a timeline for the formation of galaxies, stars, and planets.
  • Allows scientists to calculate the distances objects like the cosmic microwave background radiation have traveled.
Considering the universe's age, the cosmological horizon represents the edge of the observable universe, delineating the boundary up to which we can gather information.
Light Intensity
Light intensity diminishes with distance, which is governed by the inverse square law. This principle states that light's intensity decreases as it spreads out in space. The further light travels, the more it disperses, leading to reduced brightness. Intensity is also affected by cosmic redshift, which happens as light moves through the expanding universe.
  • Light is stretched into longer wavelengths, resulting in lower energy and diminished intensity.
  • This redshift not only causes light to appear dimmer but also shifts it toward the red end of the spectrum.
While distance does impact visibility, it is not the primary factor limiting our view at the cosmological horizon. Instead, it is a combination of distance and the universe's expansion.
Cosmic Redshift
Cosmic redshift is an essential concept in cosmology, explaining how light is stretched as the universe expands. This change in wavelength makes distant galaxies appear redder.
  • As the universe grows, space itself stretches, causing the light waves traveling through it to elongate.
  • The greater the redshift, the faster the object is moving away from us, indicating the universe's continuous expansion.
Cosmic redshift not only affects observations of distant galaxies but also provides insights into their velocities and distances. However, the view at the cosmological horizon is limited more by the finite age and light speed, rather than redshift-induced dimness.

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Most popular questions from this chapter

Extremely Distant Galaxies. The most distant galaxies observed to date have a redshift of approximately \(z=10 .\) How does the wavelength of light we observe from those galaxies compare with its original wavelength when it was emitted?

Choose the best answer to each of the following. Explain your reasoning with one or more complete sentences. Which of these galaxies do we see at its oldest age? (a) a galaxy in the Local Group (b) a galaxy observed at a distance of 5 billion light-years (c) a galaxy observed at a distance of 10 billion light-years.

What are the three major types of galaxies, and how do their appearances differ?

Be sure to show all calculations clearly and state your final answers in complete sentences. Cepheids in M100. Scientists using the Hubble Space Telescope have observed Cepheids in the galaxy M100. Here are the actual data for three Cepheids in \(\mathrm{M} 100\) : \(\bullet\)Cepheid 1: luminosity \(=3.9 \times 10^{30}\) watts brightness \(=9.3 \times 10^{-19} \mathrm{watt} / \mathrm{m}^{2}\) \(\bullet\) Cepheid 2: luminosity \(=1.2 \times 10^{30}\) watts brightness \(=3.8 \times 10^{-19} \mathrm{watt} / \mathrm{m}^{2}\) \(\bullet\) Cepheid 3: luminosity \(=2.5 \times 10^{30}\) watts brightness \(=8.7 \times 10^{-19}\) watt \(/ \mathrm{m}^{2}\) Compute the distance to M100 with data from each of the three Cepheids. Do all three distance computations agree? Based on your results, estimate the uncertainty in the distance you have found.

The peak luminosity of a white dwarf supernova is around \(10^{10} L_{\text {Sun }}\), and it remains above \(10^{8} L_{\text {Sun }}\) for about 150 days. In comparison, the luminosity of a bright Cepheid variable star is about \(10,000 L_{\text {Sun }}\) The Hubble Space Telescope is sensitive enough to make accurate measurements of apparent brightness for Cepheid variables at distances up to about 100 million light-years. Estimate the distance of a fading white dwarf supernova of luminosity \(10^{8} L_{\text {sun }}\) whose apparent brightness is comparable to that of a bright Cepheid variable star 100 million light-years from Earth. How does your distance estimate compare with the size of the observable universe?
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