Chapter 23
Conceptual Quiz
Part A
How do we determine the conditions that existed in the very early universe?
Hint A.1
Study Section 23.1
ANSWER:

By looking all the way to the cosmological horizon, we can see the actual conditions that prevailed all the way back to the first instant of the Big Bang. We can only guess at the conditions, since we have no way to calculate or observe what they were. The conditions in the very early universe must have been much like those found in stars today, so we learn about them by studying stars. From the current expansion rate we can work backward to estimate temperature and densities at various times in the early universe.
Part B
Why can't current theories describe what happened during the Planck era?
Hint B.1
Study Section 23.1
ANSWER:

We do not yet have a theory that links quantum mechanics and general relativity. We do not know how hot or dense the universe was during that time. We do not understand the properties of antimatter. The Planck era was the time before the Big Bang, and we cannot describe what happened before that instant.
Part C
When we say that the electroweak and strong forces "freeze out" at 10--35 seconds after the Big Bang, we mean that ______.
Hint C.1
Study Section 23.1
ANSWER:

these forces are only important at temperatures below the freezing point of water --- a temperature that the universe reached at an age of about 10--35 seconds. prior to this time, the electroweak and strong forces maintained a single identity, but they possessed separate identities following this time. following this time, neither the strong nor electroweak forces are ever important in the universe again. "freezing out" was a term coined by particle physicists who think that the Big Bang theory is really cool.
Part D
According to the Big Bang theory, how many forces -- and which ones --- operated in the universe during the GUT era?
Hint D.1
Study Section 23.1
ANSWER:

2: gravity and a single force that later became the strong, weak, and electromagnetic forces. 2: the strong force and the electroweak force 3: gravity, the strong force, and the electroweak force 1 force that represented the unification of all four forces that operate today.
Part E
Laboratory experiments conducted with particle accelerators confirm predictions made by the theory that unifies ______.
Hint E.1
Study Section 23.1
ANSWER:

the strong, weak, and electromagnetic forces into the GUT force. the unification of all four forces into a single "superforce." the strong and weak forces into the combined nuclear force. the electromagnetic and weak forces into the electroweak force.
Part F
What was the significance of the end of the era of nucleosynthesis, when the universe was about 3 minutes old?
Hint F.1
Study Section 23.1
ANSWER:

The basic chemical composition of the universe had been determined. It marks the time at which the first stars formed. The proportions of dark matter and luminous matter had been determined. It marks the time at which the expansion of the universe had settled down to its current rate.
Part G
According to the Big Bang theory, why do we live in a universe that is made of matter rather than antimatter?
Hint G.1
Study Section 23.1
ANSWER:

GUT theories predict that under the conditions that prevailed in the early universe, the normal laws of physics would have been suspended so that only matter particles were created, and no particles of antimatter. The fact that we live in a universe made of matter is not surprising, because antimatter has never been shown to exist for real. Einstein's famous equation E = mc 2 tells us that energy can turn into matter, but does not tell us that it can turn into antimatter. During the first 0.001 second after the Big Bang, particles and antiparticles were made in almost but not perfectly equal numbers. Everything annihilated except the very slight excess of matter particles.
Part H
Which of the following is NOT a characteristic of the cosmic microwave background?
Hint H.1
Study Section 23.2
ANSWER:

It has a perfect thermal radiation spectrum, Its temperature is the same everywhere, except for small variations at the level of 1 part in 100,000. Its temperature is a little less than 3 Kelvin (3 degrees above absolute zero). It contains prominent spectral lines of hydrogen, the primary chemical ingredient of the universe.
Part I
In principle, if we could see all the way to the cosmological horizon we could see the Big Bang taking place. However, our view is blocked for times prior to about 380,000 years after the Big Bang. Why?
Hint I.1
Study Section 23.2, and review Chapter 20 if you've forgotten what we mean by the cosmological horizon.
ANSWER:

Before that time, the universe was too crowded with stars. Before that time, the gas in the universe was dense and ionized and thus did not allow light to travel freely. Before that time, the universe was dark so there was no light to illuminate anything. 380,000 years after the Big Bang marks the time when stars were first born, and thus began to shine the light by which we can see the universe.
Part J
If astronomers had discovered that the cosmic microwave background was precisely the same everywhere, rather than having very slight variations in temperature, then we would have no way to account for:
Hint J.1
Study Section 23.2
ANSWER:

the fact that our universe is expanding. the relationship between the strong and the weak force. how galaxies came to exist. the existence of helium in the universe.
Part K
In stars, helium can sometimes be fused into carbon and heavier elements (in their final stages of life). Why didn't the same fusion processes produce carbon and heavier elements in the early universe?
Hint K.1
Study Section 23.2
ANSWER:

Temperatures in the early universe were never above the roughly 100 million Kelvin required for helium fusion. No one knows --- this is one of the major mysteries in astronomy. Helium fusion occurred, but the carbon nuclei that were made were later destroyed by the intense radiation in the early universe. By the time stable helium nuclei had formed, the temperature and density has already dropped too low for helium fusion to occur.
Part L
How does the idea of inflation account for the existence of the "seeds" of density from which galaxies and other large structures formed?
Hint L.1
Study Section 23.3
ANSWER:

Inflation predicts that gravity would have been very strong and thereby made mass get concentrated into seeds. Inflation would have caused random, microscopic quantum fluctuations to grow so large in size that they became the seeds of structure. Inflation tells us that the universe should have a "flat" overall geometry, and this led to the flat disks of galaxies. Inflation predicts that temperatures and densities should have become nearly equal throughout the universe.
Part M
Which of the following is NOT consistent with recent observations of the cosmic microwave background by the WMAP satellite?
Hint M.1
Study Section 23.3
ANSWER:

The matter density (both luminous and dark matter combined) in the universe is only about one-fourth of the critical density. The majority of the energy in the universe is NOT in the form of mass-energy. The universe is at least 20 billion years old The universe is geometrically "flat" (in the four dimensions of spacetime).
Part N
Based on the results from the WMAP satellite, the overall composition of the energy of the universe is ______.
Hint N.1
Study Section 23.3
ANSWER:

4% ordinary (baryonic) matter, 23% non-baryonic dark matter, 73% dark energy 1% ordinary (baryonic) matter, 99% non-baryonic dark matter 100% ordinary (baryonic) matter 15% ordinary (baryonic) matter, 85% non-baryonic dark matter
Part O
Which adjective does NOT describe a known feature of the early universe?
Hint O.1
Study Section 23.1
ANSWER:

Small. Filled with intense radiation. Dense. Hot.
Part P
The Big Bang theory seems to explain how elements were formed during the first few minutes after the Big Bang. Which hypothetical observation below (these are NOT real observations) would call our current theory into question?
Hint P.1
Study Section 23.2
ANSWER:

The discovery of a planet that with no helium in its atmosphere. The discovery of a galaxy with 27% helium rather than the 25% that theory tells us was produced in the Big Bang. The discovery of a galaxy with a helium abundance of only 10% by mass. The discovery of a star-like object made entirely of carbon and oxygen.
Part Q
If observations of the cosmic microwave background (such as those from the WMAP satellite) had shown that the universe was much smoother than, say, 1 part in 100,000, what theory would have been most at risk?
Hint Q.1
Study Section 23.2
ANSWER:

Galaxy formation from tiny "seeds" made up of WIMPs. Darwin's theory of evolution. General Relativity. The hot Big Bang.
Part R
If the density of normal (baryonic) matter in the universe were actually 10 times higher we now think it is, we would expect to see _____.
Hint R.1
Study Section 23.2
ANSWER:

A more rapid acceleration of the expansion of the universe. Less deuterium than we do. A higher abundance of heavy elements. More deuterium than we do.