Question 1

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

Accepted Answer

Helium, along with other light elements, was primarily formed during the first three minutes of the universe in a process known as Big Bang nucleosynthesis.

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 transition from a radiation-dominated universe to a matter-dominated universe occurred approximately 30,000 years after the Big Bang. This transition is significant because it marks the point at which the density of matter became greater than the density of radiation, leading to the formation of structures in the universe.

Question 3

When the universe was about 30,000 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

The formation of atoms became possible when the average photon energy decreased to a level where it could no longer ionize hydrogen, allowing electrons to combine with protons to form neutral hydrogen atoms.

Question 4

As astronomers look at more distant regions of space, they see the regions as they existed at earlier times, but their 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 "wall" refers to the surface of last scattering, which occurred when the universe cooled enough for electrons and protons to combine and form neutral hydrogen atoms. This event made the universe transparent to radiation, allowing light to travel freely through space.

Question 5

The symmetry breaking that led to the existence of matter is MOST likely associated with which one of the fundamental forces?

Accepted Answer

The symmetry breaking that led to the existence of matter over antimatter in the universe is most closely associated with the weak nuclear force. This force is responsible for processes such as beta decay and plays a key role in the Standard Model of particle physics, particularly in explaining the observed asymmetry between matter and antimatter.

Question 6

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

Accepted Answer

The cosmic microwave background radiation photons had their last interaction with matter during the era of recombination, approximately 380,000 years after the Big Bang, when the universe cooled enough for electrons to combine with protons, forming neutral hydrogen atoms and allowing photons to travel freely.

Question 7

If astronomers 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 they have found?

Accepted Answer

The cosmic microwave background (CMB) radiation is the afterglow of the Big Bang, and as the universe expands, this radiation cools down. 4.5 billion years ago, the universe was younger and hotter, so the peak intensity of the CMB would have been at a shorter wavelength than it is now.

Question 8

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 the start of the Big Bang?

Accepted Answer

As the universe expanded, it cooled, leading to a decrease in the energy of gamma rays. This reduced energy was insufficient to create particle-antiparticle pairs.

Question 9

What is the period of quark confinement?

Accepted Answer

During the period of quark confinement, the universe had cooled down enough for quarks to combine into hadrons (like protons and neutrons) without being separated by high-energy photons. This allowed for the stable formation of matter as we know it.

Question 10

What order of events do cosmologists predict in the early universe, from earlier to later?

Accepted Answer

The correct order of events in the early universe is: Big Bang, Planck time, inflation, formation of protons and neutrons.

Question 11

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

During the early universe, for every billion antiprotons, there were slightly more than a billion protons. This slight excess of protons over antiprotons is what led to the matter-dominated universe we observe today, as matter and antimatter annihilate each other when they come into contact. This imbalance is crucial for the existence of the universe as we know it.

Question 12

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 antiparticles created in the early universe annihilated with their particle counterparts, producing photons. This process significantly reduced the number of particles and antiparticles, leaving a slight excess of particles, which went on to form the matter we observe in the universe today.

Question 13

When a gamma-ray photon undergoes pair production, what is one possibility for the final products?

Accepted Answer

Pair production is a process where a gamma-ray photon, under the right conditions, can transform into a particle and its antiparticle pair. In this case, the correct pair is an electron and a positron (the antielectron). This process conserves both charge and energy.

Question 14

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

Accepted Answer

During the first millionth of a second of the universe, it was too hot and dense for quarks to bind together into protons and neutrons, meaning protons (hydrogen nuclei) could not exist as they do in cooler, less dense conditions.

Question 15

Which of these four periods of time describes this universe at present?

Accepted Answer

The universe is currently in a period where quarks are confined within protons and neutrons, and cannot travel freely, which is consistent with the period of confinement.

Question 16

The fact that matter exists today is evidence that matter particles outnumber antimatter particles in the universe. This imbalance was caused by

Accepted Answer

Symmetry breaking is the phenomenon that explains the imbalance between matter and antimatter particles in the universe. It suggests that during the early moments after the Big Bang, physical laws did not apply uniformly, leading to a slight excess of matter over antimatter. This excess allowed matter to dominate as the universe evolved.

Question 17

How was MOST of the helium in the universe created?

Accepted Answer

Most of the helium in the universe was created by nuclear reactions during the first 3 minutes following the Big Bang, in a process known as Big Bang nucleosynthesis. This period allowed for the formation of most of the helium, along with hydrogen and small amounts of other light elements.

Question 18

When two high-energy gamma-ray photons collide, the gamma-ray photons could

Accepted Answer

When two high-energy gamma-ray photons collide, they can indeed disappear and create a particle-antiparticle pair, such as an electron and its antiparticle, the positron. This process is a manifestation of the principle of energy-mass equivalence, as described by Einstein's equation E=mc^2, where the energy of the gamma-ray photons is converted into the mass of the particle-antiparticle pair.

Question 19

What experimental evidence do physicists have for the direct transformation of energy into matter?

Accepted Answer

The conversion of gamma-ray photons into electron-positron pairs is a direct demonstration of the transformation of energy into matter, as described by the equation E=mc^2. This process, known as pair production, is a clear experimental evidence supporting the theory that energy and matter can be converted into each other.

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 19: Cosmology