ACT Science Practice Test 114

Directions: Each passage is followed by several questions. After reading a passage, choose the best answer to each question and fill in the corresponding oval on your answer document. You may refer to the passages as often as necessary.

You are NOT permitted to use a calculator on this test.


In recent decades, astronomers deduced that there is approximately five times more material in clusters of galaxies than expected based on visible galaxies and hot gas. Most of the material in these galaxies is, in fact, invisible. Since galaxies are the largest structures in the universe held together by gravity, some scientists concluded that most of the matter in the entire universe is invisible. They called this invisible material dark matter. Two scientists offer theories on whether dark matter exists.

Scientist 1

Recent studies by researchers at Northeastern University and the University of Victoria may suggest that dark matter- a substance previously considered viable in light of Newton's theories of gravity-does not actually exist. Dark matter is not readily observable because it does not directly refract light or energy. Its existence could only be deduced because of the perceived gravitational effect that it has on surrounding matter, (i.e., warped galaxies).

This new research is based upon Einstein's theory of general relativity. Although Newtonian physics may provide for the cohesive nature of solar systems, when applied to galaxies the 'numbers' do not add up. Because there is not enough visible matter for the various gravitational equations to balance, dark matter was theorized to make up this deficit. Without a source for the rest of the missing matter, there was previously nothing in Newtonian physics to explain the movement or shape of galaxies.

In terms of general relativity, a galaxy, seen collectively, has its own gravity and essentially drives its own rotation at a constant rate. Aaron Romanowsky of Harvard University and several colleagues point to the existence of several elliptical-shaped galaxies surrounded by very little dark matter as evidence that dark matter is not, in fact, the cause of the warped galaxies. The results of their studies cast doubt on some of the conventional theories of galaxy formation and manipulation.

This theory does not explain everything, such as how large clusters of galaxies are able to bind to one another, but it does allow for already proven equations to explain the motion of galaxies without dark matter.

Scientist 2

Without dark matter, there are many cosmological phenomena that are difficult to explain. Some scientists believe that the interaction between dark matter and other smaller, nearby galaxies is causing the Milky Way galaxy to take on a warped, elliptical profile. This interaction involves two smaller galaxies (called Magellanic Clouds) near the Milky Way, moving through an enormous amount of dark matter, which in effect enhances the gravitational pull that the two Magellanic Clouds could exert on the Milky Way and other surrounding bodies. Computer models from the University of California at Berkeley seem to support this theory. Without the existence of the dark matter, the Magellanic Clouds would not have sufficient mass to have such a strong effect on the bend of the Milky Way galaxy.

The strongest evidence for the validity of this hypothesis rests in Newtonian physics and the hypothesis that anything with mass will exert a gravitational pull. However, there is nothing readily observable in the vicinity of the Milky Way with sufficient mass that could cause such a high level of distortion via gravitational pull.

In addition, theoretical arguments for the existence of dark matter can be made by looking at the cosmic microwave background in the universe. This "leftover" light radiation, emitted only a few hundred thousand years after the formation of the universe, provides information about conditions in the universe on a very large scale. Measurements of cosmic microwave radiation imply the existence of dark matter, although even dark matter cannot solve all of the mysteries of the universe.

1. Which of the following statements is most consistent with Scientist 1's viewpoint?

A. The application of the theory of general relativity to observed phenomena requires the inclusion of dark matter.
B. Einstein invented dark matter to cover up deficiencies in his theory of relativity.
C. Newton's theories are completely dependent upon the proven existence of dark matter in the universe.
D. New research shows that dark matter is not required to explain astronomers' observations.

2. According to the passage, a similarity between the two viewpoints is that both scientists believe that:

F. dark matter has little to no effect on galaxy shape.
G. there are still many unexplained cosmological phenomena.
H. cosmic microwave radiation suggests the presence of dark matter.
J. dark matter can be easily observed in the universe.

3. Which of the following best summarizes Scientist 2's position?

A. The existence of dark matter is a scientific fraud perpetrated by astronomers and physicists.
B. The existence of dark matter is probable based on currently available evidence.
C. Dark matter is misnamed because it is visible using modern instruments.
D. Dark matter is no longer a necessary part of the general theory of relativity.

4. With which of the following statements would both Scientist 1 and Scientist 2 most likely agree?

F. Astronomical observations of the known universe are of no value when it comes to explaining the shape of the Milky Way.
G. Warped galaxies are a convenient fiction created by astronomers and physicists.
H. Newtonian physics can account for the existence of warped galaxies without resorting to dark matter as part of the explanation.
J. Warped galaxies such as the Milky Way present an astronomical puzzle that is worth investigating.

5. Scientist 2's position would be most weakened by which of the following observations?

A. The Magellanic Clouds actually move more quickly than previously thought.
B. The Magellanic Clouds are actually much more massive than previously thought.
C. The Milky Way is warped more than previously thought.
D. U.C. Berkeley computer models are much more accurate than previously thought.

6. According to the passage, the main point of the disagreement between Scientist 1 and Scientist 2 is:

F. the existence of dark matter in the universe.
G. the source of dark matter in the universe.
H. the likelihood that Einstein was aware of Newton's theories.
J. the existence of the Magellanic Clouds near the Milky Way.

7. Scientist 1's position would be most weakened by:

A. the revelation that Einstein's general theory of relativity is significantly flawed.
B. the appearance of several newly-discovered warped galaxies similar to the Milky Way.
C. the discovery that previous estimates of the mass of galaxies were too high.
D. proof that Einstein was aware of Newton's theories at the time he postulated his general theory of relativity.