chapter 18 assignment 1

Arrange the steps in the history of the Universe, starting from the earliest (top). Instructions

1) inflation 2) protons and neutrons 3) electrons 4) helium forms 5) the microwave background 6) stars form

Match the average temperatures of the Universe with what was happening in it at the time it cooled to that value. Instructions

10^25K -Inflation ends1 0^9 K- Helium is created through nucleosynthesis. 3000 K- electrons recombine with hydrogen and emit light that eventually forms the CMB 100 K- Stars and galaxies begin to form 2.725K- Humans live on Earth.

Based on the current estimate of the Hubble constant being about 70 km/s per Mpc, it's been about ______ years since the Big Bang.

14 billion

The graph shows Hubble's law. If you measure a recession velocity of 15,000 kmseckmsec, you can conclude that the distance to the galaxy is ____.

200 Mpc

The cosmic microwave background radiation was emitted when the Universe was much like the surface layers of a red giant star, an ionized plasma with a temperature of 3000 K. Today, as a result of the expansion of the Universe, these photons have an effective temperature of 3 K. From this, we can infer that the Universe today is about _____ times larger than at the last scattering epoch.

1000

The most abundant photons in the cosmic microwave background radiation have a wavelength of about 1 millimeter. This light was stretched by the expansion of the universe as it traveled for 14 billion years. If the photons were just a bit longer than red light, say 1000 nanometers when emitted, then the universe was about ______ times ______ at recombination when they were emitted. (Select two answers.)

smaller 1000

In the balloon analogy for the expansion of the Universe (image), it looks like the center of the expansion is at A. However, you can tell there is no center to the expansion on the balloon's surface because ____.

buttons A and C moved as far from B as B and C from A

Astronomers interpret galaxy recession velocities as evidence that the Universe is expanding. The "expansion of the Universe" means that is expanding.

space

Astronomers estimate that the oldest galaxies are no more than about 13 billions years old, based on the ages of the oldest

stars

Cosmology is the study of the ______.

structure and evolution of the universe

There is no actual center or preferred direction to the universe. Therefore, everything looks the same from every location. Astronomers describe this idea as the princicple.

cosmological

In some directions, the cosmic microwave background (CMB) appears slightly hotter than average. These regions are where ____.

dark matter clumped and attracted normal matter the density of matter in the Universe was slightly higher clusters of galaxies would later form

For two photons to form an electron and position pair, the combined energy of the photons must be ____.

equal to or greater than 2 × me × c2

In a later generation star such as the Sun, most of the helium present in its atmosphere was created ______.

in the early universe right after the Big Bang.

Currently, it is believed galaxies initially formed because ____.

of the gravitational attraction of dark matter on itself and regular matter

If we observe a distant galaxy as appearing to be receding from us at 6300 km/s, aliens in the distant galaxy would observe us ______.

receding from them. appearing to move at 6300 km/s.

The era was a critical time in the early Universe when the expansion finally stretched ambient radiation to wavelengths too long to ionize hydrogen, and atoms captured electrons to become neutral gas.

recombination

We think that the universe is approximately homogeneous because we observe ____.

that the cosmic microwave background doesn't depend on direction about the same number of galaxies in each direction that galaxies seem to be made of the same elements all throughout the universe

The Hubble constant can be used to find ____.

the age of the universe, assuming it has expanded at a constant rate

The Big Bang model is the only theory that successfully explains the presence of ____.

the cosmic microwave background

Evidence demonstrates that Earth is not at the center of the Universe, nor is the Sun, nor even the Milky Way. In fact, there may not even be a unique center or any preferred location. This is a statement of ____.

the cosmological principle

The light travel time distance is ________.

the distance that light would have traveled from source to us, if the Universe were not expanding

George Gamow and other astronomers thought that elements should have been produced by the early Universe because ____.

the high temperature and density of the Universe would have been like that of the interiors of stars

The cosmic horizon is ____.

the largest distance from which a signal can reach us if it has traveled for the whole age of the Universe

The best explanation why some galaxies appear to be receding at more than the speed of light is that ______.

the space itself between the Milky Way and these galaxies is expanding, stretching the light rays passing through

Under very dense and hot conditions, it is slightly more likely that quarks are created than anti-quarks. As a result, ______.

there is more than just light in the universe today. the universe has lots of protons and neutrons and few antiprotons and antineutrons.

As we look at galaxies that are closer to the cosmic horizon, ____. (Select all that apply.)

they are younger than nearer galaxies they are farther away from us than nearer galaxies they are farther from us now than when the light we see was emitted

Match the objects and their approximate ages. Instructions

- 14 billion years : The Universe. - 4.6 billion years : The Sun. - 13 billion years : The Milky Way, The oldest stars.

Match the dates with events in the study of cosmology.

- 1920s : The discovery of Hubble's law leads to theories of a Big Bang. - 1940s : Gamow, Alpher, and Hermann develop a theory of a cosmic background from last scattering. - 1960s : Penzias and Wilson detect radio emission from the CMB.- 1978 : Penzias and Wilson win the Nobel Prize.

Select all of the reasons that the sky is not bright all the time (Olbers' paradox).

- There have not always been galaxies. - Cosmological redshift reduces the energy of light from distant galaxies. - There is a cosmic horizon beyond which we can't see.

The age of an expanding Universe with a Hubble constant, H, in kilometers per second per megaparsec can be calculated from t = 1H1H. This works because ____. (Select all that apply.)

- distance cancels out of t = d/V when you use Hubble's law to find V. - kilometers and megaparsecs are both units of distance.

Match the astronomer to how they might have described the Universe. Instructions

-Aristotle : Earth is at the center of spheres holding planets, and beyond them, a sphere holding stars. - Galileo : Stars extend in all directions, but there are more in some directions (the Milky Way) than others. - Harlow Shapley (1920) : The Milky Way is an island of stars, with some smaller islands around it. - Carl Sagan (Twentieth century) : Billions of billions of galaxies like the Milky Way, distributed in clusters evenly spread as far as we can see.

Arrange the events in the order that they occurred. Instructions

1) einstein 2) hubble 3) gamow 4)penzias and wilsion discoverd 5) penzais and wilson won

Which of the following theories states that the Universe was born in a hot, dense state and expanded rapidly?

Big Bang theory

Hubble's law is V = H × d. Therefore, a galaxy moving away from us at 30,000 kmseckmsec must be three times as far away from us as one moving at kmseckmsec. (Enter a whole number.)

Blank 1: 10,000 or 10000

The Universe began with the , an explosion from a tiny, hot, dense state.

Blank 1: Big Blank 2: Bang

is the study of the structure and evolution of the Universe.

Cosmology

Select the best description of the expansion of the Universe.

Everything is moving away from everything else.

If an astronomer refers to the light travel time distance of a galaxy as being 7 billion light years, that means ______.

It took the light from the galaxy 7 billion years to reach us. The galaxy would be 7 billion light years distant from us right now if the universe had never been expanding or contracting.

True or false: On their way to us, the photons detected as the cosmic microwave background today have been absorbed and emitted many times since the last scattering epoch.

False

True or false: Since the development of more advanced telescopes in the 1800s, astronomers have been aware the Universe is composed of countless galaxies.

False

From Earth, we see galaxy A receding at 700 km/sec. What would an observer in galaxy A see?

Galaxy Y receding at 2100 km/sec and galaxy B receding at 700 km/sec.

Select all the true statements about the formation of elements in the early Universe. (Select all that apply.)

It continued until about 24% of the Universe was helium. It continued until the Universe cooled to temperatures below that of a stellar core. It happened throughout the Universe.

How do scientists believe the Universe began?

It expanded from a tiny, dense size in a violent burst.

Select all of the conversions that can occur according to the laws of physics.

Positrons and electrons annihilate and become two photons. Two photons combine to make a particle-antiparticle pair.

Main-sequence stars form 4He, and so did the Big Bang. Select the correct statements about the history of 4He in the Universe.

The Big Bang processed 24% of the mass of the Universe into 4He. Stars have processed 3% of the mass in the Universe into 4He.

Why is the cosmic microwave background (CMB) such a perfect blackbody?

The Universe was a very uniform plasma at recombination.

Olbers' paradox, that every direction on the sky should intersect a star somewhere, can be resolved by the Universe having an edge in time or space. However, if the Universe began with the very hot Big Bang, why do we not simply see intense radiation from the Big Bang in every direction?

The expansion of the Universe has reduced the energy of photons from the beginning of the Universe.

Which statement describes Olbers' paradox?

The night sky should be bright, but it is not.

Wien's law relates the temperature of a body to the peak wavelength emitted by that body. If the temperature of the Universe has dropped by a factor of 1000, how has the wavelength of peak emission changed?

The wavelength has increased by a factor of 1000.

In the early Universe, matter and antimatter were created from energy and annihilated to form photons. Why did any matter survive, and why is our galaxy normal matter, not antimatter?

There is an asymmetry in the laws of physics that makes it slightly easier for antiquarks to decay than quarks.

Select all the premises that can be used to argue that the night sky should be bright (Olbers' paradox). Since the night sky is not bright, these are assumptions that must be investigated!

There is an infinite number of stars. The Universe has existed forever.

Select all the statements that describe the Universe at recombination.

Very uniform in density Temperature about 3000 K, the same as a red star Much smaller than today

The cosmic microwave background radiation today has an effective temperature of 3 K and a wavelength of around a millimeter. As a result of the expansion of the Universe, it originally had ____. (Select two.)

a shorter wavelength a higher temperature

The figure shows the positions of galaxies (white dots) over the entire sky. (The blue haze represents the Milky Way's disk along our view.) Over regions that are the size of the yellow circle, the number of galaxies in different directions is ______.

approximately constant in every direction.

The age of the oldest galaxies is believed to be ____.

at least 13 billion years, since that the age of the oldest detected stars