According to the Big Bang theory, which proposes that the universe is roughly 13.7 billion years old, all matter and energy were at one time compressed into a single microscopic point. This point then exploded outward in all directions in a rapid expansion. The expansion has continued to the present day, which has allowed matter to cool to a state at which stable atomic components can form. The Big Bang theory proposes that our universe is finite in age, and since nothing can travel faster than the speed of light, there exists a cosmological horizon, which is the maximum distance light or energy could have travelled since the occurrence of the Big Bang. Since the universe is still expanding, however, regions of space that are visible from our vantage point are not within each other's cosmological horizons. For example, if galaxy A is 10 billion light years away from us, and galaxy B is 10 billion light years away from us in the opposite direction, there is a total distance of 20 billion light years between them. The universe has only existed long enough for light, energy, or information to travel 13.7 billion light years between them; thus, it is not possible for any contact to have been made between the two galaxies. Yet, even these vastly separated regions of space have been observed to be extremely homogeneous—they have remarkably similar features and properties despite being so far away from each other. The question, therefore, is what caused this apparent homogeneity observed in the universe. If matter rapidly expanded outward, why does the universe have such a uniform appearance in every direction? If the Big Bang theory is correct, some explanation for this horizon problem is needed.

Scientist 1

In the current state of the universe there exist regions that lie beyond the cosmological horizons of others, and therefore cannot possibly be influenced by them. This was not always the case. At a point in time mere microseconds after the Big Bang, all of the matter in the universe experienced a period of exponential expansion, known as inflation, before the rate of expansion fell to a more stable level. This inflation led to all regions of the universe having homogeneous features, even though they are not capable of affecting one another in any way in their current state.

Scientist 2

Although there is ample evidence that a Big Bang occurred, the horizon problem, as well as the flatness problem, suggest that the Big Bang is not the full story of the inception of the universe. The horizon problem can be solved if, instead of viewing the Big Bang as the "beginning of everything," we stipulate that the expansion seen after the Big Bang was already occurring for some time before the Big Bang occurred. This marks the Big Bang as a sort of "causal horizon," which disallows us from directly observing evidence from any period beforehand. If we assume the universe is cyclic, the homogeneity of the universe is explained as the result of a continuous cycle of expansion and compression, which would naturally lead to a universe having uniform features.

From the context of the passage, "light year" most nearly means __________.

Earth’s moon rotates like a satellite around Earth. It is the fifth largest moon in the Solar System and is best seen at night. The Earth’s moon is about 384,400 km from Earth and has an orbital period of twenty-seven days. Most scientists agree that the Moon formed about 4.5 billion years ago; however, there are several conflicting theories on the moon’s origin. Below two scientists discuss what they believe to be true.

Scientist 1

The Fission Theory states that the Moon and Earth were once the same formation. A part of the formation separated from Earth and became the moon. The formation that broke off to form the moon most likely came from the Pacific Ocean Basin. The rock densities of the moon are similar to the rock densities of the Earth’s mantle. This is because the part that broke off from the Earth to form the moon broke off from the outer part of the Earth’s mantle. The theory that the Moon and Earth formed separately is highly unlikely. For this theory to be true, Earth’s gravitational field would have had to pull the moon into orbit. This is unlikely because it would have required a very particular set up. Most objects that come into the Earth’s gravitational field have elliptical orbits. If the Moon was pulled into orbit with the Earth, it would have a comet-like elliptical orbit—which it does not.

Scientist 2

The Impactor Theory states that a small planet collided with the Earth just after the solar system was formed. This caused large amounts of materials from the outer shell of both planets to break off. This debris started orbiting the Earth and forming one collective body of material. That collective piece is what we now call the moon. The lunar rocks studied are burnt, implying they were heated at one time. This would make sense because when the small planet and Earth collide, the material became heated due to impact. In addition, the Moon does not have a magnetic field like Earth, but some of the rocks on the surface of the Moon hint the Moon could have had some sort of magnetic qualities at one time. This is because the Moon was partially made up of Earth’s outer rocks.

If research concluded that the Moon's composition was the same as the Earth's composition, which viewpoint would this support?

According to the Big Bang theory, which proposes that the universe is roughly 13.7 billion years old, all matter and energy were at one time compressed into a single microscopic point. This point then exploded outward in all directions in a rapid expansion. The expansion has continued to the present day, which has allowed matter to cool to a state at which stable atomic components can form. The Big Bang theory proposes that our universe is finite in age, and since nothing can travel faster than the speed of light, there exists a cosmological horizon, which is the maximum distance light or energy could have travelled since the occurrence of the Big Bang. Since the universe is still expanding, however, regions of space that are visible from our vantage point are not within each other's cosmological horizons. For example, if galaxy A is 10 billion light years away from us, and galaxy B is 10 billion light years away from us in the opposite direction, there is a total distance of 20 billion light years between them. The universe has only existed long enough for light, energy, or information to travel 13.7 billion light years between them; thus, it is not possible for any contact to have been made between the two galaxies. Yet, even these vastly separated regions of space have been observed to be extremely homogeneous—they have remarkably similar features and properties despite being so far away from each other. The question, therefore, is what caused this apparent homogeneity observed in the universe. If matter rapidly expanded outward, why does the universe have such a uniform appearance in every direction? If the Big Bang theory is correct, some explanation for this horizon problem is needed.

Scientist 1

In the current state of the universe there exist regions that lie beyond the cosmological horizons of others, and therefore cannot possibly be influenced by them. This was not always the case. At a point in time mere microseconds after the Big Bang, all of the matter in the universe experienced a period of exponential expansion, known as inflation, before the rate of expansion fell to a more stable level. This inflation led to all regions of the universe having homogeneous features, even though they are not capable of affecting one another in any way in their current state.

Scientist 2

Although there is ample evidence that a Big Bang occurred, the horizon problem, as well as the flatness problem, suggest that the Big Bang is not the full story of the inception of the universe. The horizon problem can be solved if, instead of viewing the Big Bang as the "beginning of everything," we stipulate that the expansion seen after the Big Bang was already occurring for some time before the Big Bang occurred. This marks the Big Bang as a sort of "causal horizon," which disallows us from directly observing evidence from any period beforehand. If we assume the universe is cyclic, the homogeneity of the universe is explained as the result of a continuous cycle of expansion and compression, which would naturally lead to a universe having uniform features.

From the context of the passage, "light year" most nearly means __________.

Earth’s moon rotates like a satellite around Earth. It is the fifth largest moon in the Solar System and is best seen at night. The Earth’s moon is about 384,400 km from Earth and has an orbital period of twenty-seven days. Most scientists agree that the Moon formed about 4.5 billion years ago; however, there are several conflicting theories on the moon’s origin. Below two scientists discuss what they believe to be true.

Scientist 1

The Fission Theory states that the Moon and Earth were once the same formation. A part of the formation separated from Earth and became the moon. The formation that broke off to form the moon most likely came from the Pacific Ocean Basin. The rock densities of the moon are similar to the rock densities of the Earth’s mantle. This is because the part that broke off from the Earth to form the moon broke off from the outer part of the Earth’s mantle. The theory that the Moon and Earth formed separately is highly unlikely. For this theory to be true, Earth’s gravitational field would have had to pull the moon into orbit. This is unlikely because it would have required a very particular set up. Most objects that come into the Earth’s gravitational field have elliptical orbits. If the Moon was pulled into orbit with the Earth, it would have a comet-like elliptical orbit—which it does not.

Scientist 2

The Impactor Theory states that a small planet collided with the Earth just after the solar system was formed. This caused large amounts of materials from the outer shell of both planets to break off. This debris started orbiting the Earth and forming one collective body of material. That collective piece is what we now call the moon. The lunar rocks studied are burnt, implying they were heated at one time. This would make sense because when the small planet and Earth collide, the material became heated due to impact. In addition, the Moon does not have a magnetic field like Earth, but some of the rocks on the surface of the Moon hint the Moon could have had some sort of magnetic qualities at one time. This is because the Moon was partially made up of Earth’s outer rocks.

If research concluded that the Moon's composition was the same as the Earth's composition, which viewpoint would this support?

Earth’s moon rotates like a satellite around Earth. It is the fifth largest moon in the Solar System and is best seen at night. The Earth’s moon is about 384,400 km from Earth and has an orbital period of twenty-seven days. Most scientists agree that the Moon formed about 4.5 billion years ago; however, there are several conflicting theories on the moon’s origin. Below two scientists discuss what they believe to be true.

Scientist 1

The Fission Theory states that the Moon and Earth were once the same formation. A part of the formation separated from Earth and became the moon. The formation that broke off to form the moon most likely came from the Pacific Ocean Basin. The rock densities of the moon are similar to the rock densities of the Earth’s mantle. This is because the part that broke off from the Earth to form the moon broke off from the outer part of the Earth’s mantle. The theory that the Moon and Earth formed separately is highly unlikely. For this theory to be true, Earth’s gravitational field would have had to pull the moon into orbit. This is unlikely because it would have required a very particular set up. Most objects that come into the Earth’s gravitational field have elliptical orbits. If the Moon was pulled into orbit with the Earth, it would have a comet-like elliptical orbit—which it does not.

Scientist 2

The Impactor Theory states that a small planet collided with the Earth just after the solar system was formed. This caused large amounts of materials from the outer shell of both planets to break off. This debris started orbiting the Earth and forming one collective body of material. That collective piece is what we now call the moon. The lunar rocks studied are burnt, implying they were heated at one time. This would make sense because when the small planet and Earth collide, the material became heated due to impact. In addition, the Moon does not have a magnetic field like Earth, but some of the rocks on the surface of the Moon hint the Moon could have had some sort of magnetic qualities at one time. This is because the Moon was partially made up of Earth’s outer rocks.

Which of the following best states the basis for the belief of Scientist 1?

Earth’s moon rotates like a satellite around Earth. It is the fifth largest moon in the Solar System and is best seen at night. The Earth’s moon is about 384,400 km from Earth and has an orbital period of twenty-seven days. Most scientists agree that the Moon formed about 4.5 billion years ago; however, there are several conflicting theories on the moon’s origin. Below two scientists discuss what they believe to be true.

Scientist 1

The Fission Theory states that the Moon and Earth were once the same formation. A part of the formation separated from Earth and became the moon. The formation that broke off to form the moon most likely came from the Pacific Ocean Basin. The rock densities of the moon are similar to the rock densities of the Earth’s mantle. This is because the part that broke off from the Earth to form the moon broke off from the outer part of the Earth’s mantle. The theory that the Moon and Earth formed separately is highly unlikely. For this theory to be true, Earth’s gravitational field would have had to pull the moon into orbit. This is unlikely because it would have required a very particular set up. Most objects that come into the Earth’s gravitational field have elliptical orbits. If the Moon was pulled into orbit with the Earth, it would have a comet-like elliptical orbit—which it does not.

Scientist 2

The Impactor Theory states that a small planet collided with the Earth just after the solar system was formed. This caused large amounts of materials from the outer shell of both planets to break off. This debris started orbiting the Earth and forming one collective body of material. That collective piece is what we now call the moon. The lunar rocks studied are burnt, implying they were heated at one time. This would make sense because when the small planet and Earth collide, the material became heated due to impact. In addition, the Moon does not have a magnetic field like Earth, but some of the rocks on the surface of the Moon hint the Moon could have had some sort of magnetic qualities at one time. This is because the Moon was partially made up of Earth’s outer rocks.

Which of the following best states the basis for the belief of Scientist 1?

A rover on Mars tests the soil on the planet in Gale Crater. An instrument on the rover dectects steep spikes in methane levels in the soil within 60 days. Additionally, satellite measurements from the area detect unusual plumes of methane from this specific area, which scientists suggest may have once contained an ancient freshwater lake. Two different scientists discuss whether this evidence is conclusive of the existence of life on the planet.

Scientist 1

This is the first evidence that organic life exists on Mars. On Earth, 95% of methane gas is produced by microbial organisms, which points towards the presence of similar biological processes on Mars. The rover has also found evidence of water bound to soil in the crater, and pictures showing carvings in the sides of the crater walls suggest the existence of once-flowing water. Because this water could have supported life in the crater, this methane originated from a bacterial source and shows there is life on the planet.

Scientist 2

There is no evidence of life on Mars. These methane spikes came from organic matter left behind from recent meteor impacts as they degraded in the sun's rays. Although the crater was once filled with water, now all water is bound to minerals in the soil and it not accessible. Methane has also been measured on planets such as Saturn, Uranus, and Neptune, which have conditions too hostile to support life. There is not even enough evidence to suggest that the original water in the crater could have supported life.

According to Scientist 1, which of the following must be present if there is life on Mars?

A rover on Mars tests the soil on the planet in Gale Crater. An instrument on the rover dectects steep spikes in methane levels in the soil within 60 days. Additionally, satellite measurements from the area detect unusual plumes of methane from this specific area, which scientists suggest may have once contained an ancient freshwater lake. Two different scientists discuss whether this evidence is conclusive of the existence of life on the planet.

Scientist 1

This is the first evidence that organic life exists on Mars. On Earth, 95% of methane gas is produced by microbial organisms, which points towards the presence of similar biological processes on Mars. The rover has also found evidence of water bound to soil in the crater, and pictures showing carvings in the sides of the crater walls suggest the existence of once-flowing water. Because this water could have supported life in the crater, this methane originated from a bacterial source and shows there is life on the planet.

Scientist 2

There is no evidence of life on Mars. These methane spikes came from organic matter left behind from recent meteor impacts as they degraded in the sun's rays. Although the crater was once filled with water, now all water is bound to minerals in the soil and it not accessible. Methane has also been measured on planets such as Saturn, Uranus, and Neptune, which have conditions too hostile to support life. There is not even enough evidence to suggest that the original water in the crater could have supported life.

Which of the following describes the phenomena to which the scientists attribute methane on Mars?

Two scientists are interesting in studying the orbits of Jupiter's moons around Jupiter. Based on their own observations, the two scientists express their conclusions below:

Scientist 1:The moons of Jupiter vary greatly in size and mass and yet all have very different orbital radii (distances from Jupiter) which show no correlation with each moon's mass. This is likely attributed to the fact that while they have different masses, they are very small in comparison to the mass of Jupiter, so we can say that, relative to Jupiter's mass, the masses of Jupiter's moons are essentially equivalent. Jupiter's moons also vary greatly in the speed at which they orbit Jupiter. Unlike mass, this does correlate with orbital radius. A greater speed corresponds to a smaller orbital radius.

Scientist 2:Orbital radius is a quantity that does not depend on the orbiting object's mass. Instead, it depends on the mass of the central object (Jupiter, in this case) and the speed of the orbiting object. If, for example, Europa (one of Jupiter's moons) were orbiting a planet of half the mass of Jupiter at the same speed, it would have half the orbital radius. On the other hand, if Europa were orbiting Jupiter and Europa suddenly doubled its rotational speed, its orbital radius would decrease by a factor of four.

If Jupiter's mass were to suddenly decrease significantly, with which statement would Scientist 1 most likely agree and Scientist 2 most likely disagree?

Two scientists are interesting in studying the orbits of Jupiter's moons around Jupiter. Based on their own observations, the two scientists express their conclusions below:

Scientist 1:The moons of Jupiter vary greatly in size and mass and yet all have very different orbital radii (distances from Jupiter) which show no correlation with each moon's mass. This is likely attributed to the fact that while they have different masses, they are very small in comparison to the mass of Jupiter, so we can say that, relative to Jupiter's mass, the masses of Jupiter's moons are essentially equivalent. Jupiter's moons also vary greatly in the speed at which they orbit Jupiter. Unlike mass, this does correlate with orbital radius. A greater speed corresponds to a smaller orbital radius.

Scientist 2:Orbital radius is a quantity that does not depend on the orbiting object's mass. Instead, it depends on the mass of the central object (Jupiter, in this case) and the speed of the orbiting object. If, for example, Europa (one of Jupiter's moons) were orbiting a planet of half the mass of Jupiter at the same speed, it would have half the orbital radius. On the other hand, if Europa were orbiting Jupiter and Europa suddenly doubled its rotational speed, its orbital radius would decrease by a factor of four.

Upon which of the following statements are both Scientist 1 and Scientist 2 most likely to agree?