Chapter 17
Today, most scientists accept the Big Bang theory because its predictions agree so well with observations. But a scientific theory can always be revised or discarded if future observations do not agree with its predictions. Consider the following hypothetical future observations. Which one(s) would be inconsistent with the Big Bang theory? Check all that apply.
-Astronomers discover distant protogalactic clouds with a helium abundance below 20%. -Careful studies of quasar spectra show that, 12 billion years ago, the temperature of the cosmic microwave background is actually 2.71 K rather than 2.73 K. -Over the next 10 years, the temperature of the cosmic microwave background falls to 1 K.
Which of the following represent observations or measurements that have confirmed key predictions of the Big Bang model? Select all that apply.
-Characteristics of the cosmic microwave background. -The abundance of helium in the universe.
Complete each of the following sentences about the early universe. Drag words from the left to complete the sentences on the right.
-Most of the helium in the universe formed during the ERA OF NUCLEOSYNTHESIS. -The GUT ERA refers to a time period in which the strong, weak, and electromagnetic forces were all unified. -The PLANCK ERA refers to a time period in which all four forces are thought to have been unified. -The COSMIC MICROWAVE BACKGROUND is our name for the photons we see that were first released when the universe was about 380,000 years old. -The BIG BANG is the name we give to the moment when the expansion of the universe began. -A very short period of extremely rapid expansion, called INFLATION, is thought to have occurred when the universe was about 10^-38 second old.
What will the temperature of the cosmic microwave background be when the average distances between galaxies are twice as large as they are today? (Hint: The peak wavelength of photons in the background will then also be twice as large as it is today.)
1.4 K
Using the inverse square law for light, determine the apparent brightness our Sun would have if it were at a distance of 15 billion light-years. Express your answer using two significant figures.
1.5 x 10^-27
Using the inverse square law for light, determine the apparent brightness of the Sun in our sky. Express your answer using two significant figures.
1350 W/m^2
From your answers to parts A and B, estimate how many stars like the Sun would need to exist at a distance of 15 billion light years for their total apparent brightness to equal that of our Sun. Express your answer using two significant figures.
9.0 x 10^29
The current temperature of the universe as a whole is
A few K
What is the earliest time in the universe that we can directly observe?
A few hundred thousand years after the Big Bang.
What was the approximate temperature of the universe at an age of 5 minutes?
About 10^9 K
The Big Bang theory is closely linked to Hubble's discovery that the universe is expanding, which seems to imply that there was a time in the past when the expansion first began. Nevertheless, the Big Bang theory did not gain widespread acceptance among scientists until the 1960s. Why wasn't expansion alone enough to convince scientists that the Big Bang really happened?
Although expansion seems to imply a Big Bang, no other specific predictions of the Big Bang theory were tested and confirmed until the 1960s.
Although we can divide the history of the universe into many distinct "eras", sometimes it's useful just to have a broad perspective on what events occurred in the very early universe and what events occurred later. Listed following are events that occurred either before or after the first five minutes in the history of the universe. Match these events to the appropriate time period.
Before the First 5 Minutes: -Inflation occurred -Strong force and electroweak force first became distinct -Antielectrons (positrons) as common as electrons -Temperature fell to 10^15 K After the First 5 Minutes: -Stars formed -Galaxies formed -Photons of cosmic microwave background released -Carbon nuclei formed by fusion
Shown following are the same eras shown in Part A. Rank the eras from left to right based on the temperature of the universe during each one, from coolest to hottest.
Coolest -> Hottest -Era of Galaxies -Era of Atoms -Era of Nuclei -Era of Nucleosynthesis -Particle Era -Electroweak Era -GUT Era -Planck Era
Shown following are several times in the history of the universe. Rank these times from left to right based on the average temperature of the universe at each time, from coolest to hottest.
Coolest -> Hottest -Today -1.5 billion years after the Big Bang -100 million years after the Big Bang -1 million years after the Big Bang -500,000 years after the Big Bang
Shown following are the names of several eras that are used to describe the history of the universe. Rank the eras from left to right in the order in which they occurred, from first to last.
First -> Last -Planck Era -GUT Era -Electroweak Era -Particle Era -Era of Nucleosynthesis -Era of Nuclei -Era of Atoms -Era of Galaxies
How much cooler is the universe now (at an age of 4 x10^17s) than it was at an age of 1 second?
Its current temperature is one ten-billionth (10^-10) the temperature at an age of 1 second.
During the history of the universe, what important event occurred about 380,000 years after the Big Bang?
Light began to travel freely through the universe.
Which of the following offers at least some evidence that inflation really occurred in the early universe?
Measurements of brightness variations in the cosmic microwave background.
During the history of the universe, what important event occurred about 0.001 seconds after the Big Bang?
Most matter was annihilated by antimatter.
Which of the following does not provide strong evidence for the Big Bang theory?
Observations of the amount of hydrogen in the universe
Which of these pieces of evidence supports the idea that inflation really happened?
Observations of the cosmic microwave background that indicate a flat geometry for the universe.
Shown following are several times in the history of the universe. Imagine that you were able to watch a single photon that has been part of the cosmic microwave background since it first became present in the universe. Rank these times from left to right based on the wavelength this photon would have at each time, from shortest to longest.
Shortest -> Longest -500,000 years after the Big Bang -1 million years after the Big Bang -100 million years after the Big Bang -1.5 billion years after the Big Bang -Today
Shown following are several times in the history of the universe. Rank these times from left to right based on the peak wavelength in the spectrum of the cosmic microwave background, from shortest to longest.
Shortest -> Longest -500,000 years after the Big Bang -1 million years after the Big Bang -100 million years after the Big Bang -1.5 billion years after the Big Bang -Today
Shown following are the same eras shown in Part A and B. Rank the eras from left to right based on the size of the observable universe during each one, from smallest to largest.
Smallest -> Largest -Planck Era -GUT Era -Electroweak Era -Particle Era -Era of Nucleosynthesis -Era of Nuclei -Era of Atoms -Era of Galaxies
Which of the following important events occurred earliest in the history of the universe?
Space-time rapidly expanded during a brief period of inflation.
Which of the following best explains why the night sky is dark?
The observable universe is not infinite.
Which of the following does inflation help to explain?
The uniformity of the cosmic microwave background
In basic terms, what does the graph show?
The universe is getting cooler with time.
When the universe was 380,000 years old, its thermal radiation spectrum consisted mostly of
Visible and infrared photons
Compare your answer to part C with the estimate of the total number of stars in our observable universe. Use your answer to explain why the night sky is much darker than the daytime sky. How much larger would the total number of stars need to be for "night" to be as bright as day?
We estimated the number of stars in the observable universe to be around 10^(22), so we would need many times more stars than we have in the observable universe to make the night sky as bright as day. This explains why the night sky is dark and why even a finite number of stars would make the sky as bright as the Sun (and life as we know it impossible).
Essentially all the hydrogen nuclei that will ever exist in our universe were created ___________.
by the time the universe was about 3 minutes old
When a proton and an antiproton collide, they
convert into two photons.
Compared to when the cosmic microwave background was first released, the radiation of the cosmic microwave background today is ____________.
fainter and has most of its photons at longer wavelengths
From Part C, you know that in basic terms, the graph shows that the temperature of the universe decreases with time. In more specific terms, this graph shows that the temperature of the universe ____________.
fell very rapidly when the universe was young, but is changing very gradually today.
The charge of an antiproton is
negative
