A galaxy is approximately \(1.2 × 10^{26}\) m from the Earth.
(a) Scientists observe light from the distant galaxy. The wavelength of the observed light is longer than the wavelength of the light emitted from the galaxy. State the name of this effect.
(b) (i) State the current estimate for the Hubble constant \(H_0\).
(ii) Calculate the speed at which the galaxy is moving away from the Earth.
(iii) Scientists have measured the speeds at which distant galaxies are moving away from the Earth and their distances from the Earth. These measurements suggest that all the Universe was once at a single point. Explain why.
Correct Answer: redshift
Detailed solution: When a light source moves away from an observer, the wavelength of the light is stretched, shifting it towards the red end of the spectrum; this phenomenon is known as redshift.
Correct Answer: \((H_0 =)\) \(2.2 \times 10^{–18}\) per second
Detailed solution: The current widely accepted estimate for the Hubble constant, which describes the rate of expansion of the universe, is approximately \(2.2 \times 10^{-18}\) per second.
Correct Answer: \(2.6 \times 10^8\) m / s
Detailed solution: Using Hubble’s law \(v = H_0 d\), multiply the distance \(d = 1.2 \times 10^{26}\text{ m}\) by the Hubble constant \(H_0 = 2.2 \times 10^{-18}\text{ s}^{-1}\). The calculation yields \(2.64 \times 10^8\text{ m/s}\), which rounds to \(2.6 \times 10^8\text{ m/s}\).
Correct Answer: any two from:
• more distant galaxies are moving away faster
• universe is expanding
• (if galaxies are moving away from each other now, then) in the past galaxies were closer together
Detailed solution: The observation that galaxies are receding at speeds proportional to their distance implies the universe is expanding. Extrapolating this expansion backwards in time leads to the conclusion that all matter originated from a single, dense point.
