ASTR 101 final
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.
Which of the following statements correctly summarize key differences between the disk and the halo?
-Gas and dust are abundant in the disk but not in the halo. -Disk stars come in a broad range of masses and colors, while halo stars are mostly of low mass and red. -Clusters of young stars are found only in the disk. -Stars in the disk all orbit in the same direction and nearly the same plane, while halo stars have more randomly oriented orbits.
Which of the following accurately describe some aspect of gravitational waves?
-Gravitational waves carry energy away from their sources of emission. -The existence of gravitational waves is predicted by Einstein's general theory of relativity. -The first direct detection of gravitational waves came in 2015. -Gravitational waves are predicted to travel through space at the speed of light.
Which of the following observations were made by Vera Rubin and Kent Ford?
-Stars in the outskirts of the Andromeda galaxy orbit the galaxy's center at surprisingly high speeds. -Many spiral galaxies have hydrogen gas clouds orbiting their centers at surprisingly high speeds.
Which of the following statements about gravitational waves are true?
-The first direct detection of gravitational waves, announced in 2016, came from the LIGO observatory. -Two orbiting neutron stars or black holes will gradually spiral toward each other as a result of energy being carried away by gravitational waves. -The emission of gravitational waves from merging black holes is predicted by Einstein's general theory of relativity.
True (based on current science)
-a black hole is an object smaller than its own Schwarzschild radius -a black hole can form during a supernova explosion -material from a binary companion can form an X-ray-emitting accretion disk around a black hole -a black hole can have the mass of a star in a space less than a few kilometers across -two orbiting black holes can merge and emit gravitational waves
Not True
-a black hole will suck in any binary companion star -you would be squashed by gravity at the event horizon of any black hole -a 3-solar-mass black hole may be hidden between Jupiter and Saturn -black holes emit x-ray light from within their event horizons
Rank these objects based on their mass, from largest to smallest.
-a typical black hole -a typical neutron star -a one-solar-mass white dwarf -main-sequence star of spectral type M -jupiter -the moon
Which model(s) predict(s) that galaxies are getting farther apart NOW?
-accelerating -coasting -critical -recollapsing
Unknown
-black holes make up 1% of the mass of the Milky Way Galaxy -the singularity of a black hole has infinite density
Disk stars
-high-mass stars -the Sun -stars that all orbit in nearly the same plane -youngest stars
Listed following are several astronomical objects. Rank these objects based on their diameter, from largest to smallest.
-main sequence star of spectral type A -jupiter -a one-solar-mass white dwarf -the moon -a two-solar-mass neutron star -the event horizon of a two-solar-mass black hole
Halo stars
-stars whose orbits can be inclined at any angle -stars with the smallest abundance of heavy elements -globular clusters -oldest stars
According to current understanding of physics, which four of the following are the four fundamental forces in nature?
-strong force -electromagnetism -weak force -gravity
Rank these objects based on their density, from highest to lowest.
-the singularity of a black hole -a typical neutron star -a one-solar-mass white dwarf -a main-sequence star
Which of the four forces that exist today would be merged together at high energies in what scientists call the "GUT force"?
-weak force -strong force -electromagnetism
We have direct experimental evidence (from large particle accelerators) for the physical conditions in the universe back to about ________ after the Big Bang.
10-10 seconds
If you tried to fly into a ______________, you would be killed by tidal forces before you crossed the event horizon.
10-solar-mass black hole
Based on counting the number of galaxies in a small patch of the sky and multiplying by the number of such patches needed to cover the entire sky, the total number of galaxies in the observable universe is estimated to be approximately
100 billion
Based on current estimates of the value of Hubble's constant, about how old is the universe?
14 billion years old
According to the Big Bang theory, how many forces, and which ones, operated in the universe during the GUT era?
2 forces: gravity and a single force that later became the strong, weak, and electromagnetic forces
Approximately how long does it take the Sun to orbit the center of the Milky Way Galaxy?
200 million years
What is the temperature of the universe today?
3 K
What is the Schwarzschild radius of a 10 solar mass black hole?
30 km
Based on observations of the cosmic microwave background, the overall composition of the universe is approximately ________.
5% ordinary (baryonic) matter, 27% non-ordinary (nonbaryonic) dark matter, 68% dark energy
Which of the following best describes a black hole?
A place from which the escape velocity exceeds the speed of light.
Why might inflation have occurred at the end of the GUT era?
An enormous amount of energy was released when the strong and electroweak forces froze out from the GUT force.
Consider the statement from Part A reading "a 3-solar-mass black hole may be hidden between Jupiter and Saturn." How do we know this statement is not true?
An object of that mass would disrupt the orbits of the planets in our solar system.
What causes the radio pulses of a pulsar?
As the neutron star spins, beams of radio radiation sweep through space. If one of the beams crosses the Earth, we observe a pulse.
Which of the following does not accurately describe what we observe toward the Galactic center?
At optical wavelengths, we see a cluster of old, red stars.
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
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.
In principle, if we could see all the way back in time 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?
Before that time, the gas in the universe was dense and ionized and therefore did not allow light to travel freely.
Why can't we see past the cosmological horizon?
Beyond the cosmological horizon, we would be looking back to a time before the universe was born.
In 1924, Edwin Hubble proved that the Andromeda Galaxy lay far beyond the bounds of the Milky Way, thus putting to rest the idea that it might have been a cloud within our own galaxy. How was he able to prove this?
By observing individual Cepheid variable stars in Andromeda and applying the period-luminosity relation
Why did the Big Bang not produce heavier elements?
By the time helium could survive, the temperature had become too low for heavier elements to form.
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?
By the time stable helium nuclei had formed, the temperature and density had already dropped too low for helium fusion to occur.
What is the most accurate way to determine the distance to a nearby galaxy?
Cepheid variables
Some people wish that we lived in a recollapsing universe that would eventually stop expanding and start contracting. For this to be the case, which of the following would have to be true (based on current understanding)?
Dark energy does not exist and there is much more dark matter than we are aware of to date
Which of the following best sums up current scientific thinking about the nature of dark energy?
Dark energy probably exists, but we have little (if any) idea what it is.
Which of the following statements best summarizes current evidence concerning dark matter in individual galaxies and in clusters of galaxies?
Dark matter is the dominant form of mass in both clusters and in individual galaxies.
Which of the following statements about degeneracy pressure is not true?
Degeneracy pressure can arise only from interactions among electrons.
Notice that the "verdict" at the end of the Extraordinary Claims essay still leaves open the possibility that dark matter does not exist and that there is instead some problem with our theory of gravity. Which of the following hypothetical observations would give scientists substantially greater confidence in the existence of dark matter?
Direct detection of WIMPs in laboratories on Earth.
How does a black hole form from a massive star?
During a supernova, if the mass of the infalling core has enough gravity to overcome neutron degeneracy pressure, the core will collapse to a black hole.
According to the Big Bang theory, why do we live in a universe that is made of matter rather than antimatter?
During the rst 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.
Which of the following statements about electron degeneracy pressure and neutron degeneracy pressure is true?
Electron degeneracy pressure is the main source of pressure in white dwarfs, while neutron degeneracy pressure is the main source of pressure in neutron stars.
Who first noticed the surprisingly high speeds of galaxies within clusters?
Fritz Zwicky
Which of the following best describes how galaxies are distributed on large scales in the universe?
Galaxies appear to be distributed in chains and sheets that surround great voids.
Consider the statement from Part A reading "the singularity of a black hole has infinite density." Why is this statement in the "unknown" bin?
General relativity and quantum mechanics give different answers about the nature of singularity.
What are the two possible explanations for not seeing something glowing brightly along every line-of-sight in the sky? I. The universe is infinite in size and infinitely old, but dust absorbs starlight from far away stars. II. The universe has a finite number of stars. III. The universe is changing in some way as to prevent us from seeing an infinite number of stars
II and III
Which statement concerning black hole masses and Schwarzschild radii is not true?
In a binary system with a black hole, the Schwarzschild radius depends on the distance from the black hole to the companion star.
How does the idea of inflation account for the existence of the "seeds" of density from which galaxies and other large structures formed?
Inflation would have caused random, tiny quantum fluctuations in energy fields to grow into the density differences in the primordial gas from which galaxies formed.
Suppose there is a large amount of (weakly interacting) dark matter between us and a distant galaxy. How will this affect our view of the distant galaxy?
It does not affect our view; we can see right through the dark matter.
Why do we call dark matter "dark"?
It emits no radiation of any wavelength.
What do we mean by the singularity of a black hole?
It is the center of the black hole, a place of infinite density where the known laws of physics cannot describe the conditions.
Which of the following statements about the cosmic background radiation is not true?
It is the result of a mixture of radiation from many independent sources, such as stars and galaxies that formed within the first billion years of the Big Bang.
How does the interstellar medium affect our view of most of the galaxy?
It prevents us from seeing most of the visible and ultraviolet light from the galactic disk.
Consider a binary system of two neutron stars. How should the emission of gravitational waves affect this system?
It should cause the orbits of the two objects to decay with time.
If the electromagnetic force is stronger than gravity, then why is it overpowered by gravity on large scales?
Most objects are electrically neutral
Why can't the dark matter in galaxies be made of neutrinos?
Neutrinos travel at extremely high speeds and can escape a galaxy's gravitational pull
Why is the Big Bang theory the only theory of the universe you learn about in a typical astronomy class?
No competing theory has survived the test of evidence.
Does Hubble's law work well for galaxies in the Local Group? Why or why not?
No, because galaxies in the Local Group are gravitationally bound together.
On average, is the distance between galaxies increasing within clusters of galaxies?
No, because their gravity is strong enough to hold them together even while the universe as a whole expands
What direct evidence do we have that the weak and electromagnetic forces were once unified as a single force?
Particle accelerators on Earth can reach energies equivalent to the high temperatures of the Electroweak era and have produced particles predicted by the unified theory.
How does the theory of inflation explain the near-uniformity of the cosmic microwave background?
Prior to rapid inflation, all regions of space were close enough to bounce radiation back-and-forth and reach the same temperature.
Which of the following statements cannot be tested by science today?
Prior to the Planck time, our universe sprouted from another universe.
Open the Rotation Curve of a Spiral Galaxy animation and click at various distances from the galactic center to create a graph of the galaxy's rotation curve; be sure to start with points very close to the galactic center and continue to well beyond the visible part of the galaxy. Which of the following statements best describes the pattern of the graph?
Starting from the center, speeds at first rise rapidly, and then become nearly constant with increasing distance.
If all the "dark matter" in our universe were to be instantaneously removed, which of the following would not happen?
The Solar System would fly apart
How does an accretion disk around a neutron star differ from an accretion disk around a white dwarf?
The accretion disk around a neutron star is much hotter and emits higher-energy radiation.
Why do astronomers think the expansion rate of the universe is accelerating, rather than decelerating (as previously thought)?
The average distance between galaxies is greater than expected when looking at very distant galaxies.
What was the significance of the end of the era of nucleosynthesis, when the universe was about 5 minutes old?
The basic chemical composition of the universe had been determined
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)
Which statement below correctly describes the relationship between expansion rate and age for the universe?
The faster the rate of expansion, the younger the age of the universe.
Why isn't the space within our solar system or the Milky Way expanding according to Hubble's Law?
The gravity exerted by the solar system and the Milky Way is strong enough to hold them together against the expansion of the universe.
Which of the following is a consequence of Hubble's Law?
The more distant a galaxy is from us, the faster it moves away from us.
What evidence supports the existence of a very massive black hole at the center of our galaxy?
The motions of the gas and stars at the center indicate that it contains 4 million solar masses within a region no larger than our solar system.
What is it about the observations of Zwicky, Rubin, and Ford that suggests the existence of large quantities of dark matter?
The observed orbital speeds are too high to be explained by the gravity of the visible stars and gas clouds.
Based on observations, which of the following statements about stars in the Milky Way is generally true?
The older the star, the lower its abundance of heavy elements.
Why do we expect the cosmic background radiation to be almost, but not quite, the same in all directions?
The overall structure of the universe is very uniform, but the universe must have contained some regions of higher density in order for galaxies to form.
Why do we think tiny quantum ripples should have been present in the very early universe?
The principles of quantum mechanics require that the energy fields at any point in space be continually fluctuating.
How did star formation likely proceed in the protogalactic cloud that formed the Milky Way?
The stars that formed first could orbit the center of the galaxy in any direction at any inclination.
Why did the era of nuclei end when the universe was about 380,000 years old?
The universe had expanded and cooled enough for stable, neutral atoms to form.
Which of the following is not consistent with observations of the cosmic microwave background?
The universe is at least 20 billion years old.
Why do spiral galaxies appear blue in color?
Their light output is dominated by hot, massive blue stars.
Although most astronomers assume dark matter really exists, there is at least one other possible explanation for the phenomena attributed to dark matter. What is it?
There could be something wrong or incomplete with our understanding of how gravity operates on galaxy-size scales.
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.
What happened to all of the quarks that existed freely during the particle era?
They combined in groups to make protons, neutrons, and their antiparticles.
What do we mean when we say that particles such as neutrinos or WIMPs are weakly interacting?
They respond to the weak force but not to the electromagnetic force, which means they cannot emit light.
What makes white-dwarf supernovae very good standard candles for distance measurements?
They should all have approximately the same luminosity.
If WIMPs really exist and make up most of the dark matter in galaxies, which of the following is not one of their characteristics?
They travel at speeds close to the speed of light.
Which of the following statements best explains what we mean when we say that the strong force "froze out" at 10-38 second after the Big Bang?
This force first became distinct at this time
Suppose you drop a clock toward a black hole. As you look at the clock from a high orbit, what will you notice?
Time on the clock will run slower as it approaches the black hole, and light from the clock will be increasingly redshifted.
How many forces operated in the universe during the GUT era?
Two: gravity and the GUT force
Which of the following is the most likely candidate for what makes up the majority of dark matter?
WIMPs
Suppose we observe a Cepheid variable in a distant galaxy. The Cepheid brightens and dims with a regular period of about 10 days. What can we learn from this observation?
We can learn the distance to the galaxy.
Consider the statement from Part A reading "black holes make up 1% of the mass of the Milky Way Galaxy." Why is this statement in the "unknown" bin?
We cannot detect all black holes and therefore don't know the percentage of the galaxy's mass they make up.
How do we know that galaxy clusters contain a lot of mass in the form of hot gas that fills spaces between individual galaxies?
We detect this gas from the X-rays it produces.
Why can't current theories describe what happened during the Planck era?
We do not yet have a theory that links quantum mechanics and general relativity.
Which of the following statements best describes the current state of understanding regarding the apparent acceleration of the expansion of the universe?
We have moderately strong evidence that the acceleration is real, but essentially no idea what is causing it.
What evidence suggests that the Milky Way contains dark matter?
We observe clouds of atomic hydrogen far from the galactic center orbiting the galaxy at higher speeds than they would have if they felt only the gravitational attraction from objects that we can see.
Why are white dwarf supernovae more useful than massive star supernovae for measuring cosmic distances?
White dwarf supernovae all have roughly the same true peak luminosity, while massive supernovae come in a wide range of peak luminosities.
Which of the following is not a good reason why white-dwarf supernovae are good standard candles for distance measurements?
White-dwarf supernovae occur only among young and extremely bright stars.
Olbers' paradox is an apparently simple question, but its resolution suggests that the universe is finite in age. What is the simple question posed by Olbers' paradox?
Why is the sky dark at night?
How do X-ray measurements help us measure the amount of dark matter in galaxy clusters?
X-rays are emitted by hot gas, and the intracluster gas is heated by the gravitational effects of dark matter. More dark matter leads to greater heating, and hence stronger X-ray emission
Suppose you were unfortunate enough to fall into a black hole in a binary system where the black hole was accreting matter from its companion star. Which of the following is most likely to kill you first?
X-rays from the accretion disk
If you tried to visit a ______________, you would probably be killed by radiation well before you reached the black hole itself.
a black hole in an X-ray binary system
A standard candle is ________.
a light source of known luminosity
Which model of the universe gives the youngest age for its present size and expansion rate?
a re-collapsing universe
What do we mean by inflation in the context of the Big Bang theory?
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
What do we mean by inflation?
a sudden expansion of the universe driven by the energy released when the strong and electroweak forces froze out from the GUT force
This figure shows Hubble's law. If a galaxy is observed to be moving away from us at 30,000 km/s, how far away is it?
about 1400 million light-years
This figure shows how the luminosity of supernovae change over time. How long does it take a white dwarf supernova to decrease in luminosity by a factor of 100 from its peak?
about 200 days
The actual matter density of the universe, accounting for all of the luminous matter and all of the dark matter known to exist in galaxies and clusters, is what fraction of the critical density?
about 30 percent
The cosmic microwave background peaks at a wavelength of about 1 mm, and the universe has a temperature of about 3 K. If the microwave background peaked at a wavelength of 10 micrometers, what would its temperature be? (Hint: see Cosmic Calculations 5.1 and 17.1 in the textbook)
about 300 K
This figure shows the Cepheid period-luminosity relation. What is the approximate luminosity of a Cepheid star that varies in brightness on a 10 day cycle?
about 3000 times the luminosity of the sun
From an observational standpoint, what is a pulsar?
an object that emits flashes of light several times per second (or even faster), with near perfect regularity
What is the basic definition of a black hole?
an object with gravity so strong that not even light can escape
The distribution of the dark matter in a spiral galaxy is
approximately spherical and much larger than the galaxy's visible halo.
If we could view our galaxy from a distance of several hundred thousand light-years, it would appear ________.
as a flattened disk with a central bulge and spiral arms
The critical density of the universe is the ________.
average density the universe would need for gravity to someday halt the current expansion if dark energy did not exist
Suppose that Hubble's constant were 20 kilometers per second per million light-years. How fast would we expect a galaxy 100 million light-years away to be moving? (Assume the motion is due only to Hubble's law.)
away from us at 2,000 km/s
Why is the sky dark at night?
because the universe is not infinitely old, so the distance we can see is limited
The Planck era refers to the time period
before the Planck time.
Now, let's turn our attention to rotation curves for orbiting masses which are not rigidly held together like the points on a merry-go-round. The Rotation Curve of the Solar System animation shows the orbits of Mercury, Venus, Earth, and Mars. Click on each of the orbits, and watch the corresponding points appear on the graph below it. If a planet (or asteroid) orbited the Sun at three times the distance of Mars, its orbital speed would be __________.
between one-third and two-thirds the orbital speed of Mars
LIGO detects gravitational waves because the lengths of its arms change as gravitational waves pass by. About how much are these lengths expected to change when LIGO detects gravitational waves from the merger of two neutron stars or two black holes?
by an amount smaller than the diameter of a proton
How do we know the total mass of the Milky Way Galaxy that is contained within the Sun's orbital path?
by applying Newton's version of Kepler's third law (or the equivalent orbital velocity law) to the Sun's orbit around the center of the Galaxy
Given such small length changes (as noted in Part D), what can give scientists confidence that they have really detected a gravitational wave signal?
detecting the same changes at more than one location
The Rotation Curve for a Merry-Go-Round animation shows a rotating merry-go-round. Click several points on the merry-go-round, and watch the corresponding points appear on the graph below it. This graph is what we call a rotation curve, because it plots the speed at which any point rotates around the center (the orbital speed) against the distance of that point from the center. Based on the graph, you can conclude that doubling the distance of a point from the center of the merry-go-round would __________.
double its orbital speed
After a massive-star supernova, what is left behind?
either a neutron star or a black hole
Which of the following unifications as been directly tested and confirmed through experiments in particle accelerators?
electroweak unification
The most massive galaxies in the universe are
elliptical
Which of the following types of galaxies appear reddest in color?
elliptical
The most basic difference between elliptical galaxies and spiral galaxies is that ________.
elliptical galaxies lack anything resembling the disk of a spiral galaxy
The boundary from within which light cannot escape from a black hole is called the black hole's __________.
event horizon
What makes up the interstellar medium?
gas and dust
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 ________.
how galaxies came to exist
What kinds of atomic nuclei formed during the nucleosynthesis era?
hydrogen, helium and trace amounts of lithium, beryllium, and boron
The Sun's location in the Milky Way Galaxy is ________.
in the galactic disk, slightly over halfway out from the center
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
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
A 10 solar mass main sequence star will produce which of the following remnants?
neutron star
What is the primary form of evidence that has led astronomers to conclude that the expansion of the universe is accelerating?
observations of white dwarf supernovae
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
With current technology, we expect to be able to detect (directly) gravitational waves from a binary system of two neutron stars or two black holes __________.
only from the instant when the two objects merge into one
Considering the graph of expansion models, which model(s) predict(s) that galaxies will eventually get closer together?
recollapsing
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.)
small
Where is most of the dark matter in our galaxy thought to exist?
spread throughout the halo of the galaxy
What is the most accurate way to determine the distance to a nearby star?
stellar parallax
Degenearcy pressure arises when ________.
subatomic particles are packed as tightly as the laws of quantum mechanics allow
From Part D, you know that the galaxy's rotation curve is nearly flat, even well beyond the point at which we see any stars in the galaxy. According to the law of gravity as we understand it today, the only explanation for these this flatness is that __________.
substantial amounts of mass must reside at great distances from the galactic center
Ignoring any radiation, you could in principle survive the journey across the event horizon of a
supermassive black hole
Which of the following observatories is most likely to discover a black hole in a binary system?
the Chandra X-Ray Observatory
What caused the rapid spin of a neutron star that we see as a pulsar?
the conservation of angular momentum during the collapse of the original star's core
What are two key observational facts that led to widespread acceptance of the Big Bang model?
the cosmic background radiation and the helium content of the universe
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
Based on current evidence, a supercluster is most likely to have formed in regions of space where ________.
the density of matter was slightly higher than average when the universe was very young
Which of the following observations (if made) would disprove an important prediction of the Big Bang theory?
the discovery of a galaxy with 10% helium abundance
From what cosmic epoch do the photons in the cosmic background radiation originate?
the end of the era of nuclei
What is postulated to have caused a sudden inflation of the early universe?
the energy released from the "freezing out" of the strong force from the GUT force
Cosmological redshift is the result of ________.
the expansion of the universe
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
In a photo like the Hubble eXtreme Deep Field (the figure in your textbook), we see galaxies in many different stages of their lives. In general, which galaxies are seen in the earliest (youngest) stages of their lives?
the galaxies that are farthest away
What is the main idea of the Big Bang theory?
the idea that all matter and energy in the universe began in an unimaginably dense state, and then space itself began expanding in all directions
To understand why the galaxy rotation curve in Part D is flat, we must first understand how gravity determines the orbital speeds of stars around the galactic center. Consider any star on a circular orbit around the center of the galaxy. The speed of the star's orbit depends on the star's distance from the galactic center and __________.
the mass of the galaxy that resides within the star's orbit
Gravitational waves were first detected directly in 2015. According to models, the source of these gravitational waves was __________.
the merger of two black holes
Hubble's "constant" is not constant throughout
time
The first gravitational waves were detected in 2015 by the LIGO observatories in Washington and Louisiana. What event was thought to cause these gravitational waves?
two black holes merging
What are the Magellanic Clouds?
two small galaxies that orbit the Milky Way Galaxy
What two quantities did Edwin Hubble compare for a sample of galaxies to discover the expansion of the universe?
velocity and distance
Most stars in the Milky Way's halo are ________.
very old
What is the most accurate way to determine the distance to a very distant galaxy?
white dwarf supernova