A galaxy is observed receding from Earth with a speed of 2800 km/s. If the Hubble constant is 22 km/s per million light-years, estimate how many years ago the light that we presently see from the galaxy actually left the galaxy. (c = 3.00 × 10⁸ m/s, 1 ly = 9.461 × 10¹⁵ m, 1 y = 3.156 × 10⁷ s) A) 1.3 × 10⁸ y B) 4.2 × 10⁸ y C) 4.2 × 10⁷ y D) 1.4 × 10⁷ y E) 4.2 × 10⁶ y

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Quizplus · Accepted Answer

The Hubble constant is defined as the ratio of the recession velocity of a galaxy to its distance from Earth. In this case, the recession velocity is 2800 km/s and the Hubble constant is 22 km/s per million light-years. So, the distance to the galaxy is:$$distance = \frac{recession velocity}{Hubble constant} = \frac{2800 km/s}{22 km/s per million light-years} = 127.3 million light-years$$The light that we presently see from the galaxy actually left the galaxy 127.3 million years ago.

A Galaxy Is Observed Receding from Earth with a Speed