Astronomy Chapter 17 Practice Questions – Flashcards
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Why do scientists assume that the early universe was much hotter and denser than the universe of today?
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The fact that the universe is expanding implies that objects were closer together in the past, and compressing material makes it hotter and denser.
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What happens when a particle of matter meets its corresponding antiparticle of antimatter?
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The combined mass of the two particles is completely transformed into energy (photons).
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Experiments allow physicists today to reproduce (on very small scales) energy and temperature conditions thought to have prevailed in the early universe as far back in time as about _________.
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one ten-billionth (10-10) of a second after the Big Bang
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The four fundamental forces that operate in the universe today are _________.
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strong force, weak force, electromagnetic force, gravity
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What is the significance of the Planck time?
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Before it, conditions were so extreme that our current understanding of physics is insufficient to predict what might have occurred.
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A "GUT" (grand unified theory) refers to theories that _________.
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unify the strong force with the electromagnetic and weak forces
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In terms of the "eras" that scientists use to describe different times in the history of the universe, we live in the __________.
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era of galaxies
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What do we mean by inflation?
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a sudden and extremely rapid expansion of the universe that occurred in a tiny fraction of a second during the universe's first second of existence
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Which of the following statements correctly summarizes the events in the early universe according to the Big Bang theory?
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The universe began with the forces unified. During the first fraction of a second, the forces separated and there was a brief but important episode of inflation. Subatomic particles of both matter and antimatter then began to appear from the energy present in the universe. Most of the particles annihilated to make photons, but some became protons, neutrons, electrons, and neutrinos. The protons and neutrons underwent some fusion during the first three minutes, thereby determining the basic chemical composition of the universe.
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Which statement about the cosmic microwave background is not true?
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It is the result of a mixture of radiation from many independent sources, such as stars and galaxies.
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What is the approximate temperature of the universe (as a whole) today?
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3K
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The Big Bang theory is widely accepted today because it successfully predicts and explains several key observed features of the universe. Two of its important predictions that have been verified are:
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(1) the existence and specific characteristics of the observed cosmic microwave background; (2) the observed overall chemical composition of the universe.
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Which of the following observations cannot be explained by the Big Bang theory unless we assume that an episode of inflation occurred?
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the fact that the temperature of the cosmic microwave background is almost the same everywhere
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The critical density of the universe is the _______.
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the total density of matter and energy needed to give the universe a "flat" geometry (in spacetime).
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Models of the Big Bang that include inflation predict that the overall geometry of the universe should be "flat" (in spacetime). This prediction _________.
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agrees with data found by studying the cosmic microwave background
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Which of the following statements cannot be tested by science today?
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Prior to the Planck time, our universe sprouted from another universe.
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Olbers's paradox is an apparently simple question, but its resolution suggests that the universe is finite in age. What is the question?
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Why is the sky dark at night?
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How do we determine the conditions that existed in the very early universe?
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We work backward from current conditions to calculate what temperatures and densities must have been when the observable universe was much smaller in size.
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Why can't current theories describe what happened during the Planck era?
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We do not yet have a theory that links quantum mechanics and general relativity.
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Which of the following statements best explains what we mean when we say that the electroweak and strong forces "froze out" at 10-38 second after the Big Bang?
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These two forces first became distinct at this time.
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According to the Big Bang theory, how many forces - and which ones - operated in the universe during the GUT era?
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2 forces: gravity and a single force that later became the strong, weak, and electromagnetic forces
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Laboratory experiments conducted with particle accelerators confirm predictions made by the theory that unifies _________.
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the electromagnetic and weak forces into the electroweak force
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What was the significance of the end of the era of nucleosynthesis, when the universe was about 5 minutes old?
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The basic chemical composition of the universe had been determined.
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According to the Big Bang theory, why do we live in a universe that is made of almost entirely of matter rather than antimatter?
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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.
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Which of the following is not an observed characteristic of the cosmic microwave background?
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It contains prominent spectral lines of hydrogen, the primary chemical ingredient of the universe.
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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?
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Before that time, the gas in the universe was dense and ionized and therefore did not allow light to travel freely.
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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 _________.
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how galaxies came to exist
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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?
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By the time stable helium nuclei had formed, the temperature and density had already dropped too low for helium fusion to occur.
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How does the idea of inflation account for the existence of the "seeds" of density from which galaxies and other large structures formed?
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Inflation would have caused random, microscopic quantum fluctuations to grow so large in size that they became the seeds of structure.
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Suppose that inflation did not occur. In that case, the fact that the cosmic microwave background has the same temperature in opposite directions of the sky would be considered ___________.
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surprising, because those locations would never have been close enough to have any light or matter exchanged between them
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Which analogy best explains why inflation predicts that the overall geometry of the observable universe should appear to be flat?
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Earth is so big that it the part explored by an ant appears flat.
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Suppose that the universe were infinite in both extent and age. In that case, we would expect the night sky to be ___________.
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uniformly bright
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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?
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The discovery of a galaxy with a helium abundance of only 10% by mass.
