Based on the number of galaxies visible in the Hubble Deep Field (Figure 15.1 in the textbook in your textbook), the estimated number of galaxies in our observable universe is about ______.
What is main-sequence fitting?
a method for determining the distance to a star cluster by assuming that its main sequence should line up with the main sequence on a standard H-R diagram
What two observable properties of a Cepheid variable are directly related to one another?
the period between its peaks of brightness and its luminosity
Given that white dwarf supernovae are such good standard candles, why don't we use them to measure the distance to all galaxies?
They are rare events, so we have observed them in only a tiny fraction of all galaxies.
When we use an analogy that represents the expanding universe with the surface of an expanding balloon, what does the inside of the balloon represent?
The inside of the balloon does not represent any part of our universe.
If we say that a galaxy has a lookback time of 1 billion years, we mean that _________.
its light traveled through space for 1 billion years to reach us
Although the entire universe may be much larger than our observable universe, we can see only within our observable universe. The "boundary" of our observable universe is called _________.
the cosmological horizon
Telescopes designed to study the earliest stages in galactic lives should be optimized for observations in ______.
Which of the following is an important starting assumption in models of galaxy formation?
Some regions in the universe start out denser than others.
Current understanding holds that a galaxy's type (spiral, elliptical, or irregular) ______.
may either be the result of conditions in the protogalactic cloud that formed it or the result of later interactions with other galaxies
Collisions between galaxies typically unfold over a period of _________
hundreds of millions of years
Why are collisions between galaxies more likely than collisions between stars within a galaxy?
Relative to their sizes, galaxies are closer together than stars.
Which of the following features is not a feature of central dominant galaxies?
They are often spiral galaxies
Why should galaxy collisions have been more common in the past than they are today?
Galaxies were closer together in the past because the universe was smaller.
The distinguishing feature of a starburst galaxy is _________.
a rate of star formation that may be 100 or more times greater than that in the Milky Way
According to current understanding, what is a quasar?
an active galactic nucleus that is particularly bright
Which of the following phenomena is probably not related to the presence of a supermassive black hole?
the presence of globular clusters in the halos of galaxies
The mass of a supermassive black hole thought to power a typical bright active galactic nucleus is roughly ______.
1 billion solar masses
According to the theory that active galactic nuclei are powered by supermassive black holes, the high luminosity of an active galactic nucleus primarily consists of ______.
light emitted by hot gas in an accretion disk that swirls around the black hole
the collective activity of many supernova events in relatively small volume of a galaxy can create ______
the energy for all active galactic nuclei is thought to comes from in-fall of matter into _____________
super massive black holes
_______ are often characterized by a sources of immense energy located hundreds of thousands of light years away on either side of their centers
the type of galaxy known as a ___________ was mpre common in the universe 10 billion years ago
Which of the following best summarizes what we mean by dark matter?
Matter that we have identified from its gravitational effects but that we cannot see in any wavelength of light
Which of the following best summarizes what we mean by dark energy?
It is a name given to whatever might be responsible for causing the expansion of the universe to accelerate with time.
The text states that luminous matter in the Milky Way seems to be much like the tip of an iceberg. This refers to the idea that _________.
the unseen dark matter represents much more mass and extends much further from the galactic center than the luminous matter in the Milky Way
What is a rotation curve?
A graph showing how orbital velocity depends on distance from the center for a spiral galaxy
What do we mean when we say that the rotation curve for a spiral galaxy is "flat"?
Gas clouds orbiting far from the galactic center have approximately the same orbital speed as gas clouds located further inward.
What is the main way that we determine the mass distribution of a spiral galaxy?
Constructing its rotation curve, which we make by looking for Doppler shifts in the 21-cm emission from atomic hydrogen in its disk.
Although we know less about dark matter in elliptical galaxies than we do in spiral galaxies, what does current evidence suggest?
Elliptical galaxies probably contain a similar proportion of their mass in the form of dark matter as do spiral galaxies
Which of the following is NOT one of the three main strategies used to measure the masses of galaxy clusters?
Measuring the temperatures of stars in the halos of the galaxies
When we say that a cluster of galaxies is acting as a gravitational lens, what do we mean?
It bends or distorts the light coming from galaxies located behind it.
Which of the following statements best summarizes what studies of the masses of individual galaxies and of clusters of galaxies have told us about dark matter?
Dark matter is the dominant form of mass in both clusters and in individual galaxies
What is the distinguishing characteristic of what we call ordinary or baryonic matter?
It consists of atoms or ions with nuclei made from protons and neutrons
Some dark matter may consist of what astronomers call MACHOs (massive compact halo objects). Which of the following would NOT be considered a type of MACHO if it were found in the halo of the galaxy?
What do we mean when we say that particles such as neutrinos or WIMPs are weakly interacting?
They interact with other matter via the weak force but do not interact with the electromagnetic force and hence cannot emit light.
Which of the following best sums up current scientific thinking about the nature of dark matter?
We have been able to rule out the possibility that most dark matter is made of a known form of matter, meaning that it must be made of some as-yet-undiscovered type of particle.
What do astronomers mean by the large-scale structure of the universe?
The overall arrangement of galaxies, clusters of galaxies, and superclusters in the universe
What do we mean by the critical density of the universe?
The average density the universe would need in order for its gravity to ultimately win out over the current rate of expansion (if it weren't accelerating)
Evidence from distant supernovae (as well as from studies of the cosmic microwave background) suggests that the universe is doing something that goes against what nearly all astronomers expected just a decade ago. What is this phenomenon, which is variously attributed to things called dark energy, the cosmological constant, or quintessence?
The expansion of the universe seems to be getting even faster.
What exactly is dark energy?
We really don't know, but since it's not a form of matter, we use "dark energy" as the all-purpose explanation for the observed acceleration of the expansion of the universe.
Hubble's law expresses a relationship between __________.
e distance of a galaxy and the speed at which it is moving away from us
We can always determine the recession velocity of a galaxy (at least in principle) from its redshift. But before we can use Hubble's law, we must first calibrate it by __________.
measuring the distances to many distant galaxies with a standard candle technique
Suppose that you measure a galaxy's redshift, and from the redshift you determine that its recession velocity is 30,000 () kilometers per second. According to Hubble's law, approximately how far away is the galaxy?
1.4 billion light-years
Based on what you have learned, which of the following best describes the meaning of Hubble's constant ( )?
It describes the expansion rate of the universe, with higher values meaning more rapid expansion.
Why does the rotation curve for the solar system show speeds that become slower with increasing distance from the Sun?
Because the Sun contains most of the mass of the solar system
an object the size of earth located in the halo of our galaxy would be an example of the form of dark matter known as _____
________ are defined as subatomic particles that have more mass than neutrions but do not interact with light
the _______ of spiral galaxies provide strong evidence for the existence of dark matter
the ________ of spiral galaxies provide strong evidence for the existence of dark matter
models show that the _________ of the universe is better explained when we include the effects of dark mater along with the effects of luminous matter
large scale structures
Radar, the first link in the cosmic distance chain, is used to establish the baseline distance necessary for the second link, parallax. What baseline distance must we know before we can measure parallax
the Earth-Sun distance
Which cosmic distance measurement techniques are considered standard candle techniques?
white dwarf supernovae (distant standards)
Suppose that Cepheids did not exist and there were no other standard candle technique that worked at the same distances. Which statement would be true?
We would not be able to measure the distances of distant galaxies
Why do we use Hubble's law to estimate the distances of most distant galaxies, rather than using white dwarf supernovae in all cases?
We have not observed white dwarf supernovae in most galaxies.
Which technique is the most useful for measuring the distance to a galaxy located 10 million light-years away?
Based on our current understanding of physics, we can understand the conditions that prevailed in the early universe as far back in time as about _________.
one ten-billionth of a second after the Big Bang
What happens when a particle of matter meets its corresponding antiparticle of antimatter?
The combined mass of the two particles is completely transformed into energy (photons).
What is the significance of the Planck time?
Before it, conditions were so extreme that our current understanding of physics is insufficient to predict what might have occurred.
The four fundamental forces that operate in the universe today are _________.
strong force, weak force, electromagnetic force, gravity
A "GUT" (grand unified theory) refers to theories that _________.
unify the strong force with the electromagnetic and weak forces
What do we mean by inflation?
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
Which of the following statements correctly summarizes the events in the early universe according to the Big Bang theory?
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.
The Big Bang theory is supported by two major lines of evidence that alternative models have not successfully explained. What are they?
(1) the theory predicts the existence of and the specific characteristics of the observed cosmic microwave background; (2) the theory correctly predicts the observed overall chemical composition of the universe.
Measuring the amount of deuterium in the universe allows us to set a limit on _________.
the density of ordinary (baryonic) matter the universe
The idea of dark matter arose to explain gravitational effects observed in galaxies and clusters of galaxies. However, studies of the early universe (especially of the cosmic microwave background and of chemical abundances) also tell us something about dark matter. What do they tell us?
They add further support to the idea that dark matter really exists and is made of non-ordinary (nonbaryonic) matter, such as WIMPs.
Which of the following observations cannot be explained by the Big Bang theory unless we assume that an episode of inflation occurred?
the fact that the temperature of the cosmic microwave background is almost the same everywhere
The idea of inflation makes one clear prediction that, until the discovery of an accelerating expansion, seemed to contradict the available observations. What is this prediction?
The universe should be geometrically "flat" (in the four dimensions of spacetime).
Olbers's paradox is an apparently simple question, but its resolution suggests that the universe is finite in age. What is the question?
Why is the sky dark at night?
Which of the following statements cannot be tested by science today?
Prior to the Planck time, our universe sprouted from another universe.
predicts that the strong, weak, and electromagnetic forces should become indistinguishable at high temperatures
Grand Unified Theory
a dramatic expansion of the universe thought to have occured when the universe was only a tiny fraction of a second old
observations of the ______ provide a way to test our theory of the big bang
Cosmic Microwave Background
When a particle of ordinary matter meets it precise opposite particle of antimatter, the result is ______ with complete conversion of mass into energy.