Question 1

Which of the following changes would make the raisin bread analogy more similar to the actual expansion of the universe?

Accepted Answer

The universe is considered to be infinite or unbounded in extent, so making the raisin loaf infinitely big would better represent the actual expansion of the universe.

Question 2

What does the resolution of Olbers's paradox say about why it gets dark at night?

Accepted Answer

The resolution of Olbers's paradox, which questions why the night sky is dark if the universe is infinite and filled with stars, is that the universe is not infinite in age. This means that not all light has had enough time to reach us, resulting in a dark night sky.

Question 3

About how many times larger are the wavelengths of CMB photons today compared to when they were first emitted?

Accepted Answer

The wavelengths of Cosmic Microwave Background (CMB) photons have stretched by about a factor of 1000 since they were first emitted, due to the expansion of the universe. This stretching corresponds to the redshift of the photons from the early universe to the present.

Question 4

What is the approximate redshift of the CMB?

Accepted Answer

The Cosmic Microwave Background (CMB) has a redshift of approximately 1000, indicating the universe has expanded by a factor of about 1000 since the emission of the CMB.

Question 5

Why is the cosmic microwave background (CMB) so cold, if the early universe was so hot?

Accepted Answer

The cosmic microwave background (CMB) is so cold because the expansion of the universe has stretched out the wavelengths of the photons, shifting them towards the red end of the spectrum. This redshift effectively cools their temperature, leading to the CMB's current low temperature despite the universe's initially hot state.

Question 6

What is meant by the term "observable universe"?

Accepted Answer

The observable universe refers to the region of the universe from which light has had time to reach us since the beginning of the cosmological expansion, meaning its look-back time is smaller than the age of the universe.

Question 7

What happens when a proton and an antiproton collide?

Accepted Answer

When a proton and an antiproton collide, they undergo annihilation, which means they are converted into pure energy in the form of gamma rays. This occurs because the proton and antiproton have opposite charges and when they come into contact, they undergo a mutual destruction, releasing all the energy that was binding them together.

Question 8

If the Hubble constant were twice as large and the expansion rate was constant, what would be the approximate age of the universe?

Accepted Answer

The age of the universe is inversely proportional to the Hubble constant. If the Hubble constant were twice as large, the universe would appear to be expanding twice as fast, implying it would reach its current state in half the time. Given the current estimate of the universe's age is about 14 billion years, doubling the Hubble constant would suggest an age of about 7 billion years.

Question 9

What happened when the average energy of gamma rays in the early universe was no longer high enough to create protons and neutrons?

Accepted Answer

When the average energy of gamma rays in the early universe decreased to the point where it was no longer sufficient to create protons and neutrons, most of the existing protons combined with antiprotons. This process is part of what is known as baryogenesis, where the slight asymmetry in the laws of physics led to a universe dominated by matter (protons) rather than being annihilated by equal amounts of matter and antimatter (antiprotons).

Question 10

What is the point of the raisin bread analogy for the expanding universe?

Accepted Answer

The raisin bread analogy is used to help people understand the expansion of the universe. In this analogy, the raisins represent galaxies and the bread represents the universe. As the bread dough rises, all the raisins move away from each other, just like galaxies in the universe seem to be moving away from each other. This demonstrates the point that all observers see other galaxies expanding away from them.

Question 11

Where did the cosmic microwave background (CMB) come from?

Accepted Answer

The cosmic microwave background (CMB) came from photons that were released when electrons and nuclei combined for the first time, which happened about 380,000 years after the Big Bang during the recombination era. This light has traveled through the universe and has been redshifted (stretched out) by the expansion of the universe, resulting in it being seen today as the cosmic microwave background radiation.

Question 12

Approximately how long after the big bang did all four fundamental forces become distinct?

Accepted Answer

The four fundamental forces became distinct much less than one second after the Big Bang, during the very early stages of the universe's expansion and cooling.

Question 13

When people hear the phrase "big bang," they often think of an explosion. What word would be a better description than "explosion"?

Accepted Answer

The "big bang" theory describes the rapid expansion of the universe from a highly hot and dense state, making "expansion" a more accurate term than "explosion."

Question 14

Galaxy A is twice as far away as Galaxy B (both are very distant) and they happen to appear beside each other in the sky. What would observers in Galaxy A see?

Accepted Answer

Observers in Galaxy A would see the Milky Way Galaxy receding at twice the rate as Galaxy B because the distance to Galaxy A is twice that of Galaxy B, implying that due to the expansion of the universe, the Milky Way would appear to be moving away faster from Galaxy A than Galaxy B is.

Question 15

Which of the following statements best describes Olbers's paradox?

Accepted Answer

Olbers's paradox is the argument that if the universe were infinite, static, and filled uniformly with stars, then every line of sight in the night sky should end on a star, making the night sky bright instead of dark. This paradox highlights the contradiction between the darkness of the night sky and the assumption of an infinite and eternal universe filled with stars.

Question 16

What is the Hubble time?

Accepted Answer

The Hubble time is an estimate of the age of the universe assuming a constant rate of expansion. It is derived from the Hubble constant, which measures the rate at which the universe is expanding.

Question 17

Why is there a limit to the size of the observable universe?

Accepted Answer

The limit to the size of the observable universe is due to the universe having a finite age, meaning light from distant objects has had only a limited amount of time to reach us since the beginning of the universe.

Question 18

During the first moments of the big bang when elements could be created, nuclear fusion reactions made few heavy elements. Why not?

Accepted Answer

The absence of stable nuclei with masses of 5 or 8 hydrogen masses created a bottleneck in the process of nucleosynthesis during the early moments of the Big Bang, preventing the formation of heavier elements beyond helium and a small amount of lithium.

Question 19

If we lived in a static, unchanging universe, which of the following would you expect to see?

Accepted Answer

In a static, unchanging universe, galaxy properties would remain constant with distance, as there would be no expansion or evolution affecting their appearance.

Question 20

What property of an antiproton is opposite to that of a proton?

Accepted Answer

The electric charge of an antiproton is opposite to that of a proton. Protons have a positive electric charge while antiprotons have a negative electric charge.

Quiz 11: Cosmology in the 21st Century