Question 1

If observations had shown that the cosmic microwave background was perfectly smooth (rather than having very slight variations in temperature),then we would have no way to account for

Accepted Answer

The slight variations in the cosmic microwave background radiation are believed to have led to the formation of galaxies as well as other large-scale structures in the universe through gravitational collapse. If the cosmic microwave background had been perfectly smooth, there would be no way to explain how galaxies (and therefore, large-scale structure) came to exist.

Question 2

Based on the results from Planck,the overall composition of the universe is

Accepted Answer

The results from Planck suggest that the universe is composed of 4% ordinary matter (baryonic matter), 23% nonbaryonic dark matter, and 73% dark energy. This is known as the standard model of cosmology and has been confirmed by a variety of observations, including the cosmic microwave background radiation and the large scale structure of the universe.

Question 3

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?

Accepted Answer

In the early universe, the temperature and density were much higher than in stars, but as the universe expanded and cooled, the density and temperature dropped too low for helium fusion to occur. By the time stable helium nuclei had formed, the conditions necessary for further fusion had already passed.

B and C are not correct because there was not enough radiation yet in the early universe to destroy the carbon nuclei that were formed, and temperatures were high enough for helium fusion to occur, at least initially. D is not correct because we do have some understanding of why this didn't happen in the early universe.

Question 4

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?

Accepted Answer

The Big Bang theory predicts that about 25% of the universe's mass should be helium, as it was produced in the first few minutes after the Big Bang. A galaxy with only 10% helium by mass would significantly deviate from this prediction, suggesting that our understanding of the early universe and element formation might be incorrect.

Question 5

Which of the following is not consistent with recent observations of the cosmic microwave background by Planckt?

Accepted Answer

Recent observations by Planckt suggest that the universe is around 13.8 billion years old, which contradicts option D that states the universe is at least 20 billion years old. Therefore, option D is not consistent with recent observations.

Question 6

According to the Big Bang theory,why do we live in a universe that is made of almost entirely of matter rather than antimatter?

Accepted Answer

According to the Big Bang theory, particles and antiparticles were created in equal numbers during the first 0.001 seconds after the Big Bang. However, due to a slight excess of matter particles, everything annihilated except for the matter particles. This is known as baryogenesis and is the most widely accepted explanation for why we live in a universe made mostly of matter.

Question 7

Which of the following is not an observed characteristic of the cosmic microwave background?

Accepted Answer

The cosmic microwave background does not contain prominent spectral lines of hydrogen, as it is a thermal radiation spectrum that is characteristic of a blackbody radiation.

Question 8

Which adjective does not necessarily describe a known feature of the early universe? (Be sure to consider the universe as a whole,not just the observable universe. )

Accepted Answer

While we know the early universe was dense, hot, and filled with intense radiation, we do not necessarily know that it was small. In fact, some theories suggest that the early universe underwent a period of rapid expansion known as inflation, which would have caused it to become much larger.

Question 9

How does the idea of inflation account for the existence of the "seeds" of density from which galaxies and other large structures formed?

Accepted Answer

Inflation would have caused random, microscopic quantum fluctuations to grow so large in size that they became the seeds of structure. These fluctuations would have led to the formation of tiny variations in density, which were the starting point for the formation of galaxies and other large structures.

Question 10

What was the significance of the end of the era of nucleosynthesis,when the universe was about 5 minutes old?

Accepted Answer

The end of the era of nucleosynthesis marked the time when the basic chemical composition of the universe had been determined. During this era, light elements such as hydrogen and helium were formed through nuclear fusion, which set the stage for the formation of heavier elements later on. The exact abundances of these elements are determined by the physical conditions during this era, and they are consistent with observations of the universe today.

Question 11

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?

Accepted Answer

Before about 380,000 years after the Big Bang, the universe was filled with a dense, ionized gas that scattered and absorbed any light that tried to travel through it. This means that the earliest light that was emitted after the Big Bang was effectively "trapped" by this gas and couldn't travel freely through space until it had dissipated enough to allow light to pass through. This event, called recombination, occurred around 380,000 years after the Big Bang, which is why we can't see anything prior to that time.

Quiz 22: The Birth of the Universe

If observations had shown that the cosmic microwave background was perfectly smooth (rather than having very slight variations in temperature) ,then we would have no way to account for