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Chapter 11: Problem 23

What is meant by cosmological expansion? Express your answer in terms of a Hubble graph and the red shift of distant starlight.

Short Answer

Expert verified
Cosmological expansion is the observed phenomenon of the universe expanding, causing galaxies to move away from each other. This can be expressed through a Hubble graph, which plots the relationship between the recessional velocities of galaxies and their distances from the observer. Redshift occurs when light from distant sources is stretched, causing the wavelength to increase and shift towards the red end of the spectrum. As the redshift increases, the recessional velocity and distance of the distant galaxy also increase. The Hubble graph illustrates this relationship, demonstrating that the farther away a galaxy is, the faster it is moving away from us due to cosmological expansion.

Step by step solution

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1. Hubble Graph

Hubble graph is a plot representing the relationship between the recessional velocities of galaxies and their distances from the observer. The graph represents the expansion of the universe by demonstrating that galaxies farther away from us are moving away faster. The slope of this graph determines the Hubble constant (H0), which indicates the current rate of expansion. Note: The Hubble constant is given by \(H_0 = \frac{v}{d}\), where \(v\) is the recessional velocity, and \(d\) is the distance from the observer.
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2. Redshift

Redshift occurs when light from a distant source, such as a star or galaxy, is stretched as it travels through the expanding universe. This stretching causes the wavelength of the light to increase, shifting it towards the red end of the spectrum. The redshift of a galaxy can be used to determine its distance and velocity, both of which are related to cosmological expansion.
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3. Relation between Cosmological Expansion, Hubble Graph, and Redshift

The cosmological expansion is the overall increase in distance between galaxies due to the expansion of the universe. Observations of the redshift of distant starlight provide evidence of this expansion. As the redshift increases, the recessional velocity of the distant galaxy increases as well. The Hubble graph represents this relationship between redshift and distance, illustrating that the farther away a galaxy is, the faster it is moving away from us due to the cosmological expansion. In essence, cosmological expansion is expressed through the Hubble graph by relating the redshift observed in starlight from distant sources to the measured distance of those sources from Earth.

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