Question 1

When the universe was about 2500 years old,the radiation density had decreased to the point where it was equal to the matter density.Yet it was only many years later that atoms formed.What happened to make possible the formation of atoms?

Accepted Answer

Atoms could not form until the average photon energy decreased to the point where it could no longer ionize hydrogen. This allowed electrons to combine with protons to form neutral hydrogen atoms, which eventually formed other elements.

Question 2

At about what time after the Big Bang did the universe pass through the transition from being radiation-dominated to being matter-dominated?

Accepted Answer

The universe transitioned from being radiation-dominated to matter-dominated approximately 30,000 years after the Big Bang.

Question 3

As we look at more distant regions of space,we see the regions as they existed at earlier times,but our farthest views are blocked by a "wall" beyond which the universe is opaque.What event occurred at the time marked by this wall?

Accepted Answer

The event that marked the "wall" beyond which the universe is opaque was the combination of electrons and protons to form neutral hydrogen atoms, which occurred about 380,000 years after the Big Bang. This is known as recombination, and it resulted in the release of cosmic microwave background radiation that we can still observe today. Beyond this point, the universe became transparent to light, allowing it to travel freely through space and allowing us to observe more distant regions of the cosmos.

Question 4

During the first one-ten-thousandth of a second (10-⁴ s)of the life of the universe,antiprotons were very common.For every billion antiprotons,how many protons were there?

Accepted Answer

The question states that there were "very common" antiprotons, but does not specify that they were created in equal numbers with protons. Therefore, it is likely that there were slightly more protons than antiprotons, which would result in the matter we see today. Option A is incorrect as it assumes equal creation of protons and antiprotons. Option C is incorrect as it states that the number is unknown, but this is not necessarily the case as we can make reasonable assumptions based on current knowledge. Option D is incorrect as it refers to dark matter, which is not mentioned in the question.

Question 5

How was most of the helium in the universe created?

Accepted Answer

Most of the helium in the universe was created during the first 3 minutes following the Big Bang, during a process known as Big Bang nucleosynthesis. This process involved the fusion of protons and neutrons to form helium-4 nuclei.

Question 6

Which statement CORRECTLY describes the universe for the entire first 500,000 years of its life?

Accepted Answer

The universe was filled with high-energy radiation and was opaque for the first 500,000 years, meaning that light could not travel through it. It was only after the universe cooled down enough that atoms could form and the radiation was able to travel freely, making the universe transparent. Option A is incorrect because quarks were confined inside protons and neutrons during this time. Option B is incorrect because the forces of nature did not unify until much later in the history of the universe. Option C is also incorrect because the universe was not filled with a sea of nuclear particles undergoing violent reactions, but rather a sea of high-energy particles, such as photons and leptons.

Question 7

Helium nuclei began permanent formation in the early universe,when the temperature dropped to the point where the gamma radiation no longer had enough energy to break nuclei apart as soon as they formed.Did this helium production cease? If so,why?

Accepted Answer

The production of helium in the early universe, a process known as Big Bang nucleosynthesis, ceased after a few minutes because the universe expanded and cooled to a point where the temperatures and densities were no longer sufficient for nuclear fusion to occur. This cooling prevented protons from overcoming their electromagnetic repulsion to fuse into helium nuclei.

Question 8

Why does the observable universe have an "edge"?

Accepted Answer

The observable universe has an "edge" because we cannot see any farther out into space than the distance that light has traveled over the lifetime of the universe. The observable universe is defined as the portion of the universe that is visible from Earth given the age of the universe and the speed of light, which limits how far light has had time to travel since the Big Bang. Therefore, any object beyond this distance is not observable to us.

Question 9

Symmetry breaking is associated with which one of the fundamental forces?

Accepted Answer

Symmetry breaking is a phenomenon where a system does not exhibit the same symmetries as its underlying laws. The weak nuclear force is associated with symmetry breaking because it is responsible for the breaking of the electroweak symmetry in particle physics. This symmetry breaking led to the Higgs mechanism and the generation of mass for elementary particles. Thus, the best choice is D) Weak nuclear force.

Question 10

During the first millionth of a second in the history of the universe,hydrogen atom could not exist.Why not?

Accepted Answer

The answer of During the first millionth of a second...

Question 11

The photons that enter our microwave detectors today (and that we interpret as the cosmic microwave background radiation)had their last interaction with matter:

Accepted Answer

The era of recombination occurred around 380,000 years after the Big Bang when the universe cooled down enough for electrons and protons to combine to form neutral hydrogen. This allowed photons to travel freely through space without interacting with matter. These photons eventually reached us today as the cosmic microwave background radiation, which is what our microwave detectors pick up.

Question 12

What significant event occurred about 500,000 years after the start of the Big Bang?

Accepted Answer

About 500,000 years after the start of the Big Bang, the temperature of the universe had cooled down enough for electrons and nuclei to combine into neutral atoms, which allowed light to travel freely through space, resulting in the formation of the cosmic microwave background radiation. This is a significant event in the timeline of the universe's history.

Question 13

If we could have turned a receiver toward the heavens in hopes of observing the cosmic microwave radiation 4.5 billion years ago,at the time the solar nebula was forming,what would we have found?

Accepted Answer

The cosmic microwave background (CMB) radiation was created about 13.8 billion years ago, shortly after the Big Bang. Over time, as the universe expanded, the CMB has redshifted, causing its peak wavelength to increase. 4.5 billion years ago, the universe was younger, and the CMB would have had a shorter peak wavelength than it does today.

Question 14

During what time interval was helium created in the Big Bang?

Accepted Answer

Helium was created during the first 3 minutes of the Big Bang, when the temperature and density were high enough for the fusion of protons and neutrons to form helium nuclei (alpha particles). This process ended when the temperature dropped below the threshold for further fusion reactions.

Question 15

Our view of the universe is limited because of what fundamental fact?

Accepted Answer

Light can only travel at a finite speed and has taken a finite amount of time to reach us from distant objects, so we cannot see objects beyond a certain distance because their light hasn't had enough time to reach us yet.

Question 16

Why did the formation of particle-antiparticle pairs (such as protons and antiprotons)from gamma rays come to an end in the first second after start of the Big Bang?

Accepted Answer

As the universe expanded, the temperature decreased, causing the energy of gamma rays to decrease as well. This meant that the gamma rays no longer had enough energy to create particle-antiparticle pairs. Therefore, the formation of particle-antiparticle pairs from gamma rays came to an end within the first second of the Big Bang.

Question 17

In the present theory of the Big Bang,what significant event occurred at about 500,000 years after the universe started expanding?

Accepted Answer

At about 500,000 years after the universe started expanding, the temperature of the universe had cooled enough for electrons to combine with nuclei and form neutral atoms, allowing photons to travel through the universe unimpeded. This event is known as recombination and resulted in the universe becoming transparent to photons of radiation. This radiation can still be seen today as the cosmic microwave background radiation.

Question 18

Within the first second after the Big Bang,the universe was filled with high-energy,gamma-ray photons.These photons created huge numbers of elementary particles and their antiparticles through pair production.What happened to these particles?

Accepted Answer

The particle and antiparticle pairs produced during the Big Bang annihilated each other to produce more gamma-ray photons. This process continued until the universe had expanded and cooled enough for particles and antiparticles to no longer be produced in large quantities.

Question 19

In the present universe,the "mass density" of radiation,using Einstein's equation E = mc² for the photons,is:

Accepted Answer

The mass density of radiation is much less than the mass density of matter, leading to a matter-dominated universe. This is supported by observations such as the cosmic microwave background radiation being much cooler than expected if the universe were still radiation-dominated. Additionally, the overall shape and distribution of large-scale structure in the universe also supports a matter-dominated universe.

Question 20

Which element(s)was/were created during the Big Bang?

Accepted Answer

During the Big Bang, the primary elements formed were hydrogen, helium, and lithium. These elements were produced in the first few minutes of the universe's existence through processes such as nucleosynthesis.

Quiz 14: Cosmology