Question 1

A universe is finite in time if its expansion will eventually stop, and finite in space if it does not extend indefinitely. What would a closed universe be?

Accepted Answer

A closed universe is one that is finite in both time and space, meaning it will eventually stop expanding and does not extend indefinitely in any direction.

Question 2

Which of the following is a characteristic of the first stars to form in the universe?

Accepted Answer

The first stars to form in the universe are known as Population III stars and are characterized by their extremely low metal content, as they formed from primordial gas consisting mostly of hydrogen and helium, with virtually no heavier elements.

Question 3

Which of the following has the events in the universe's history in chronological order?

Accepted Answer

C) proton-antiproton annihilation, electron-positron annihilation, recombination, reionization - This sequence correctly follows the chronological order of events in the universe's history. After the Big Bang, as the universe cooled, proton-antiproton annihilation occurred first, followed by electron-positron annihilation. Once the universe cooled further, electrons combined with nuclei to form neutral atoms in a process called recombination. Much later, the first stars and galaxies formed, emitting light that reionized the universe.

Question 4

What is the name for the period between when the CMB was emitted and when the first stars formed?

Accepted Answer

The period between when the Cosmic Microwave Background (CMB) was emitted and when the first stars formed is known as the dark age. This era is characterized by the absence of any luminous objects, hence the name.

Question 5

Approximately how long after the big bang did the first galaxies and stars form?

Accepted Answer

The first galaxies and stars are thought to have formed approximately a billion years after the Big Bang.

Question 6

How long after the big bang could gas begin to clump together due to gravity?

Accepted Answer

The correct answer is B because the temperature of the universe was too high for gas to clump together due to gravity until about 50,000 years after the Big Bang.

Question 7

The cosmic background radiation comes from a particular time after the origin of the universe. What was that time?

Accepted Answer

The cosmic background radiation originates from the time when electrons began to recombine with nuclei to form neutral atoms, about 380,000 years after the Big Bang. This process allowed photons to travel freely through space without being constantly scattered by free electrons, leading to the release of the cosmic microwave background radiation we observe today.

Question 8

As the universe cooled, it eventually reached a temperature of 3,000 K and protons were able to capture and hold electrons to form neutral hydrogen. What is this process called?

Accepted Answer

This process is called recombination, which refers to the epoch in the early universe when electrons and protons combined to form neutral hydrogen atoms, allowing the universe to become transparent to radiation for the first time.

Question 9

Why did electron-positron annihilation occur after proton-antiproton annihilation?

Accepted Answer

Electrons have less mass than protons, which means they require less energy to annihilate. The universe cooled down from the hot conditions of the Big Bang, and as it cooled, particles with higher masses like protons and antiprotons annihilated each other first. Electrons and positrons, being lighter, could survive longer until the universe cooled further to the point where they too could annihilate each other.

Question 10

What was the temperature of the universe when the recombination took place that produced the cosmic background radiation we observe today?

Accepted Answer

The temperature of the universe during the recombination era, which led to the formation of the cosmic microwave background radiation, was about 3,000 K. This event occurred roughly 380,000 years after the Big Bang, when the universe cooled enough for electrons to combine with protons and form neutral hydrogen atoms, allowing photons to travel freely.

Question 11

How does the geometry of a gravity-dominated universe relate to its density?

Accepted Answer

In a gravity-dominated universe, the geometry is determined by the critical density. If the density is equal to the critical density, the universe is flat. If the density is less than the critical density, the universe is open. If the density is greater than the critical density, the universe is closed. Therefore, if the universe is flat, the density must be equal to the critical density.

Question 12

What does a cosmological redshift tell you?

Accepted Answer

A cosmological redshift tells you how much the universe has expanded since the light was emitted.

Question 13

Which of the following applies if the universe is closed?

Accepted Answer

A closed universe implies that it has a positive curvature, leading to a high density that could eventually cause it to stop expanding and possibly contract. This concept does not inherently suggest that the universe has a center or edge, as those ideas are not supported by the general theory of relativity. The statement about expanding forever applies to an open or flat universe, not a closed one.

Question 14

What is the assumption of homogeneity?

Accepted Answer

The assumption of homogeneity is that the large-scale appearance of the universe is the same from any point. This means that if an observer were placed at any point in the universe and looked out, they would see a roughly uniform distribution of matter and energy on the large scale. This assumption is an important part of the cosmological principle, which underlies our current understanding of the universe.

Question 15

What characteristic of the universe governs whether the universe is open, closed, or flat?

Accepted Answer

The density of the universe determines whether it is open (low density), closed (high density), or flat (critical density).

Question 16

What properties does the cosmological principle require the universe to have?

Accepted Answer

The cosmological principle states that the universe is homogeneous (the same in all locations) and isotropic (looks the same in all directions).

Question 17

Under what conditions will space-time be flat?

Accepted Answer

Space-time will be flat if the average density of the universe is equal to the critical density. This condition means that the universe has just the right amount of matter and energy to make its geometry flat, according to the Friedmann-Lemaître-Robertson-Walker (FLRW) cosmological models.

Question 18

Suppose you found a gas cloud, the atoms of which had never been part of a star. What would you expect its composition to be?

Accepted Answer

The composition of a gas cloud that has never been part of a star would be primarily hydrogen and helium, reflecting the primordial nucleosynthesis after the Big Bang, which resulted in roughly 75% hydrogen and 25% helium by mass.

Question 19

Imagine three universes that have the same expansion rate and contain no dark energy: an open universe, a closed universe, and a flat universe. What can you say about the age of the open universe compared to the other two?

Accepted Answer

In the absence of dark energy, an open universe expands forever but at a decelerating rate, allowing more time to pass from the Big Bang to reach the same expansion rate as a flat or closed universe. Therefore, the open universe would be older.

Question 20

What is the assumption of isotropy?

Accepted Answer

The assumption of isotropy means that the universe appears the same in every direction when observed on a large scale, implying uniformity in its structure and appearance regardless of the direction in which one looks.

Quiz 11: Cosmology in the 21st Century