For many years, most physicists supported one of two cosmological theories: the steady-state universe, and the Big Bang. The theory of the steady-state universe states that the universe has always existed exactly as we observe it at present, whereas the Big Bang theory postulates that the universe was conceived from a singularity in space-time that has expanded into the current universe. The validity of either theory was not tested until 1929, when Edwin Hubble famously discovered what is now known as Hubble’s Law. Hubble’s experiment is now a famous benchmark in modern physics. Hubble, using the Mount Wilson Observatory, observed a class of stars known as Cephied variables, luminous stars that blink and flicker with a rate that depends on their distance from the observer. Using this relation and years of observing, Hubble calculated the distance to many of these variable stars. Milton Humason, a fellow astronomer, helped Hubble to calculate the stars’ relative velocities to Earth. When Hubble combined the two data sets he found an interesting relationship: all the stars appeared to be moving away from us! In fact, the speed at which they were moving increased with an increasing distance from Earth. Hubble realized, from this small set of data, that the earth was a part of the expanding universe. As the universe expands outward in all directions, any observer from a fixed vantage point will look out and see everything running away from them. The further away any two points are, the more the expansion affects them, and the faster they appear to be moving away from each other. Hubble’s result was the first experimental proof that we do not live in a steady-state universe, but rather a dynamic and expanding one.

Which of the following best expresses the main idea of the passage?

(A) Edwin Hubble discovered Hubble’s Law, a benchmark in modern physics.
(B) Hubble discovered that the universe is expanding, disproving the theory of the steady-state universe.
(C) Before 1929, most physicists supported one of two theories of the universe.
(D) All objects in space are receding from each other because of the expansion of the universe.
(E) Modern physics would not have progressed without Hubble’s discovery of the expanding universe.



Which of the following is not mentioned in the passage regarding Hubble’s experiment in which he deduced Hubble’s Law?

(A) It used years of data on Cepheid variable stars.
(B) Hubble accumulated data using the Mount Wilson Observatory and help from a fellow astronomer.
(C) Hubble found that all the observed stars appeared to be moving away from Earth.
(D) Hubble deduced the distance to Cepheid variable stars based on the rate at which they blinked and flickered.
(E) Hubble deduced the velocity of Earth to find the stars’ absolute velocities.



Consider each of the answer choices separately and indicate all that apply.

Which of the following can be properly inferred from the passage?

• The steady-state universe theory does not allow for an expanding universe.
• The closer any two points in the universe are, the less expansion effects them, and the slower they appear to be moving apart.
• After Hubble’s discovery of the expanding universe, the Big Bang was the only cosmological theory that could be valid.