Why is there a discrepancy between the distance and time taken for light from a galaxy with a redshift of z = 0.4 to reach Earth?
The discrepancy between the distance and the time taken for light from a galaxy to reach Earth with a redshift of z = 0.4 can be explained by the expansion of the universe.
Expansion of the Universe
The expansion of the universe is a fundamental concept in cosmology. It suggests that the universe is constantly growing and galaxies are moving away from each other. This expansion affects the light emitted from distant galaxies in various ways.
Redshift: As the universe expands, the space between galaxies also expands, causing the light emitted from distant galaxies to be stretched out or redshifted. The degree of redshift is directly proportional to the distance between the observer and the source of the light. Therefore, a greater redshift corresponds to a greater distance.
Speed of Galaxies: The redshift of z = 0.4 indicates that the galaxy was moving away from us at a speed of 40% the speed of light when the light was emitted. This speed, combined with the expansion of the universe, results in the galaxy being much farther away than it was when the light was emitted.
Effect on Distance: As the light traveled through space, the universe continued to expand, causing the distance between us and the galaxy to increase. This means that even though the light has only been traveling for 4.4 billion years, the galaxy is actually now at a distance of 5.2 billion light years from Earth.
Therefore, the discrepancy between the distance and the time taken for light to reach Earth from the galaxy with a redshift of z = 0.4 is due to the expansion of the universe over the course of the light's journey.