Comparing Viewpoints & Hypotheses Practice Test

PASSAGE III

PHYSICS: This passage presents three hypotheses regarding the discrepancy between the observed mass of galaxies and their rotational speeds.

Introduction

In the 1970s, astronomer Vera Rubin observed that stars at the edges of spiral galaxies move just as fast as stars near the center. According to Newtonian physics, stars further from the center should move slower due to the decrease in gravitational pull. This observation implies that galaxies contain far more mass than what is visible in stars and gas. This missing mass is referred to as "Dark Matter." Three scientists propose different explanations for its nature.

Scientist 1

The missing mass consists of Weakly Interacting Massive Particles (WIMPs). These are subatomic particles that have mass but do not interact with electromagnetic radiation (light), making them invisible. WIMPs interact only through gravity and the weak nuclear force. They were produced in the early universe and now form a vast, spherical "halo" that surrounds every galaxy. Computer simulations of the universe’s formation match observational data only when this "cold dark matter" is included. If WIMPs exist, they should eventually be detectable by sensitive underground experiments designed to catch rare collisions between WIMPs and atomic nuclei.

Scientist 2

The missing mass is not some exotic new particle; it is simply normal ("baryonic") matter that is too dim to see. These objects are called Massive Compact Halo Objects (MACHOs). They include black holes, neutron stars, brown dwarfs (failed stars), and rogue planets. Because they emit little to no light, they have escaped detection. The gravitational pull of these objects accounts for the high rotational speeds of galaxies. Evidence for MACHOs comes from "microlensing" events, where the gravity of a massive, invisible object bends the light of a distant star, causing it to brighten temporarily.

Scientist 3

There is no missing mass. The discrepancy is caused by a flaw in our understanding of gravity itself. This theory is known as Modified Newtonian Dynamics (MOND). Newtonian laws work well on the scale of our solar system, where accelerations are high. However, on the galactic scale, where accelerations are incredibly low ($< 10^{-10} m/s^2$), gravity behaves differently. Below this threshold, gravitational force decays more slowly with distance ($1/r$ instead of $1/r^2$). This stronger effective gravity eliminates the need for invisible halos or new particles. The rotation curves are exactly what the laws of physics predict when corrected for this scale.

PASSAGE III

PHYSICS: This passage presents three hypotheses regarding the discrepancy between the observed mass of galaxies and their rotational speeds.

Introduction

In the 1970s, astronomer Vera Rubin observed that stars at the edges of spiral galaxies move just as fast as stars near the center. According to Newtonian physics, stars further from the center should move slower due to the decrease in gravitational pull. This observation implies that galaxies contain far more mass than what is visible in stars and gas. This missing mass is referred to as "Dark Matter." Three scientists propose different explanations for its nature.

Scientist 1

The missing mass consists of Weakly Interacting Massive Particles (WIMPs). These are subatomic particles that have mass but do not interact with electromagnetic radiation (light), making them invisible. WIMPs interact only through gravity and the weak nuclear force. They were produced in the early universe and now form a vast, spherical "halo" that surrounds every galaxy. Computer simulations of the universe’s formation match observational data only when this "cold dark matter" is included. If WIMPs exist, they should eventually be detectable by sensitive underground experiments designed to catch rare collisions between WIMPs and atomic nuclei.

Scientist 2

The missing mass is not some exotic new particle; it is simply normal ("baryonic") matter that is too dim to see. These objects are called Massive Compact Halo Objects (MACHOs). They include black holes, neutron stars, brown dwarfs (failed stars), and rogue planets. Because they emit little to no light, they have escaped detection. The gravitational pull of these objects accounts for the high rotational speeds of galaxies. Evidence for MACHOs comes from "microlensing" events, where the gravity of a massive, invisible object bends the light of a distant star, causing it to brighten temporarily.

Scientist 3

There is no missing mass. The discrepancy is caused by a flaw in our understanding of gravity itself. This theory is known as Modified Newtonian Dynamics (MOND). Newtonian laws work well on the scale of our solar system, where accelerations are high. However, on the galactic scale, where accelerations are incredibly low ($< 10^{-10} m/s^2$), gravity behaves differently. Below this threshold, gravitational force decays more slowly with distance ($1/r$ instead of $1/r^2$). This stronger effective gravity eliminates the need for invisible halos or new particles. The rotation curves are exactly what the laws of physics predict when corrected for this scale.