Dark Matter

• A. The redshift of distant quasars
• B. Fritz Zwicky's measurements of the velocities of galaxies in the Coma Cluster ✓
• C. The cosmic microwave background temperature
• D. The discovery of Type Ia supernovae

Why: In the 1930s, Fritz Zwicky measured galaxy velocities in the Coma Cluster and found they were moving far too fast to be held together by the visible mass alone, implying unseen mass was present.

• A. Stars at the outer edge orbit more slowly than stars near the center
• B. Stars at the outer edge orbit at roughly the same speed as those nearer the center, implying extra unseen mass ✓
• C. The galaxy is shrinking over time
• D. The galaxy contains no gas

Why: Based on visible matter alone, orbital speeds should drop with distance from the center. Observed flat rotation curves mean there is substantial mass distributed in a halo extending beyond the visible disk, which is attributed to dark matter.

• A. The absorption of light by interstellar dust
• B. The bending of light from a background object by the gravity of a massive foreground object ✓
• C. The slowing of clocks near a black hole
• D. The emission of radio waves by pulsars

Why: General relativity predicts that mass curves spacetime, bending the path of light. The amount of bending reveals the total mass of the lensing object, including dark matter, regardless of whether it emits light.

• A. The precise pattern of temperature fluctuations, which matches a universe with about 5 times more dark matter than ordinary matter ✓
• B. The brightness of the Sun over time
• C. The speed of sound in air
• D. The number of planets in the Solar System

Why: The acoustic peaks in the CMB power spectrum depend on the densities of ordinary and dark matter. The observed pattern requires dark matter to make up roughly 85% of all matter, with ordinary matter forming only a small fraction.

• A. It contains only dark matter and no stars
• B. During a galaxy cluster collision, the gas (seen in X-rays) was slowed down, while the mass (mapped by lensing) passed through, separating from the gas ✓
• C. It proves that dark matter emits light
• D. It shows that gravity does not exist

Why: Most of the mass measured by gravitational lensing is located where the galaxies are, not where the hot X-ray gas is. This separation suggests the dominant mass is collisionless, as dark matter is expected to be, rather than being explained by modified gravity alone.

• A. The chemical composition of stars across the galaxy
• B. How orbital speed of stars and gas varies with distance from the galactic center ✓
• C. The brightness of the galaxy as seen from Earth over time
• D. The temperature distribution of gas clouds in the galactic disk

Why: A rotation curve plots the orbital velocity of stars and gas against their distance from the galactic center, revealing how mass is distributed within the galaxy.

• A. They should increase steadily without limit
• B. They should remain constant at all distances
• C. They should decrease, falling off with distance ✓
• D. They should oscillate periodically

Why: If most of a galaxy's mass is concentrated in the bright central region, the enclosed mass stops growing outward, so orbital speeds should drop off at larger radii, much like planets farther from the Sun orbit more slowly.

• A. Orbital speeds dropped sharply beyond the visible disk
• B. Orbital speeds stayed roughly flat at large distances instead of declining ✓
• C. Galaxies rotated in the opposite direction to their spiral arms
• D. Rotation speeds were identical for all galaxies regardless of size

Why: Rubin's measurements showed that stars far from the center orbit at nearly the same speed as those nearer in, a flat curve that contradicts the expected decline from visible mass alone.

• A. Stars are moving faster than expected because of magnetic fields
• B. Galaxies contain a halo of unseen dark matter providing extra gravitational mass ✓
• C. Newton's law of gravity is wrong at all scales
• D. Galaxies are expanding so rapidly that their stars drift outward

Why: Adding an extended halo of dark matter, which does not emit light, increases the enclosed mass at large radii and keeps orbital speeds high and flat, matching observations.

• A. It directly photographs dark matter particles
• B. It shows gravitational effects that visible matter alone cannot account for ✓
• C. It proves that dark matter is made of black holes
• D. It measures the exact chemical makeup of dark matter

Why: Rotation curves reveal the total gravitating mass through its effect on motion, and that total substantially exceeds the mass we can see, indicating the presence of unseen matter.

• A. The absorption of light by dense dust clouds in space
• B. The bending of light paths by the gravity of massive objects ✓
• C. The slowing of light as it passes through a galaxy's halo
• D. The reflection of light off the surfaces of black holes

Why: Gravitational lensing occurs because massive objects curve spacetime, which bends the paths of light rays passing near them, as predicted by general relativity.

• A. The redshift of distant galaxies
• B. The apparent shift in positions of stars near the Sun ✓
• C. The orbit of Mercury around the Sun
• D. The cosmic microwave background radiation

Why: Arthur Eddington's 1919 eclipse expedition measured starlight passing near the Sun appearing displaced from its expected position, supporting Einstein's prediction of light bending.

• A. A ring-shaped planet orbiting a distant star
• B. A ring of dust surrounding a black hole's event horizon
• C. A circular image of a background galaxy produced by perfect alignment with a foreground mass ✓
• D. A ring of dark matter surrounding a spiral galaxy

Why: When a background source, a foreground lens, and the observer are almost perfectly aligned, the lensed light forms a ring around the foreground mass, called an Einstein ring.

• A. Dark matter emits strong radio signals that lensing amplifies
• B. Lensing reveals the total mass distribution, including mass that does not emit light ✓
• C. Dark matter absorbs light, creating shadows that lensing detects
• D. Lensing only occurs around dark matter particles

Why: Because lensing depends on gravitational mass rather than light emission, it can map dark matter, which neither emits nor absorbs light but still bends it.

• A. The lens ✓
• B. The filter
• C. The mirror
• D. The prism

Why: The intervening massive object acts like an optical lens, which is why it is called the lens, while the distant object being magnified is the source.

• A. Radiation emitted by the first stars after they formed
• B. Leftover thermal radiation from the early universe, released when atoms first formed ✓
• C. Microwave signals produced by distant quasars
• D. Radio waves emitted by dark matter particles as they decay

Why: The CMB is the relic radiation from about 380,000 years after the Big Bang, when the universe cooled enough for electrons and protons to combine into neutral atoms and light could travel freely.

• A. About 2.7 Kelvin ✓
• B. About 270 Kelvin
• C. About 27 Kelvin
• D. About 0.27 Kelvin

Why: Due to cosmic expansion, the radiation has been stretched to microwave wavelengths and cooled to roughly 2.7 K, just above absolute zero.

• A. It shows the positions of galaxies as they looked billions of years ago
• B. It was emitted at a single moment when the universe became transparent to light ✓
• C. It records every supernova that occurred in the early universe
• D. It captures the light from the very first black holes

Why: The CMB was released across the whole sky at roughly the same time, at the epoch of recombination, so it represents a picture of the universe at that moment.

• A. That the CMB was coming from a single point in the sky
• B. That the CMB has a nearly perfect blackbody spectrum and tiny temperature fluctuations ✓
• C. That the CMB is polarized in a circular pattern
• D. That the CMB temperature is higher than 100 Kelvin

Why: COBE measured a near-perfect blackbody spectrum and detected small anisotropies of about one part in 100,000, which seeded later structure formation.

• A. They show that dark matter does not exist
• B. They reveal the density variations that dark matter helped amplify into galaxies ✓
• C. They prove dark matter is made of photons
• D. They measure the exact mass of a dark matter particle

Why: The fluctuations reflect early density variations; dark matter's gravity amplified these into the large-scale structure we see today, and the CMB pattern constrains how much dark matter exists.

• A. Photon
• B. Weakly Interacting Massive Particle (WIMP) ✓
• C. Electron
• D. Gluon

Why: WIMPs are hypothetical particles that interact via gravity and the weak nuclear force but not electromagnetically, which makes them a natural fit for dark matter that is invisible to light. They have long been a leading candidate in particle physics models.

• A. It is located in regions of space with no stars
• B. It does not emit, absorb, or reflect light ✓
• C. It is made of black holes
• D. It has a negative mass

Why: Dark matter does not emit, absorb, or reflect electromagnetic radiation, so it cannot be seen directly with telescopes. Its presence is inferred from its gravitational effects on visible matter and light.

• A. The orbital speeds of stars within galaxies ✓
• B. The color of distant nebulae
• C. The temperature of the Sun's core
• D. The brightness of supernovae in our galaxy

Why: Stars at the outer edges of galaxies orbit faster than expected based on the visible mass alone. This discrepancy, noted in galaxy rotation curves, suggested the presence of additional unseen mass, which is attributed to dark matter.

• A. A large, dense star that emits no light
• B. A very light hypothetical particle proposed to solve a problem in quantum chromodynamics ✓
• C. A type of black hole formed in the early universe
• D. A neutron star made of dark matter

Why: Axions are hypothetical, extremely light particles first proposed to resolve the strong CP problem in quantum chromodynamics. They are also considered a viable dark matter candidate because they could be produced in large numbers in the early universe.

• A. Sterile neutrinos
• B. Primordial black holes
• C. Visible red dwarf stars ✓
• D. Axions

Why: Visible red dwarf stars emit light and are part of ordinary baryonic matter, so they cannot account for dark matter, which by definition does not emit or absorb light. Sterile neutrinos, primordial black holes, and axions are all actively studied dark matter candidates.

• A. To produce dark matter particles in high-energy collisions
• B. To observe dark matter particles scattering off atomic nuclei in a detector ✓
• C. To measure the gravitational lensing of distant galaxies
• D. To detect neutrinos emitted from the Sun's core

Why: Direct detection experiments look for rare interactions in which a dark matter particle from the galactic halo scatters off a nucleus in a sensitive detector, producing a measurable recoil signal.

• A. Underground rock provides a stronger magnetic field for particle tracking
• B. The overburden shields the detector from cosmic rays that would create background signals ✓
• C. Underground temperatures are needed to keep detector materials superconducting
• D. Underground sites are closer to the galactic center where dark matter density is highest

Why: Cosmic rays and their secondary particles can mimic dark matter signals. Placing detectors deep underground, under kilometers of rock, greatly reduces this background.

• A. A burst of visible light from nuclear fusion
• B. A small energy deposit in the form of nuclear recoil, such as heat, ionization, or scintillation ✓
• C. A change in the detector's electrical resistance caused by magnetic flux
• D. A high-energy gamma ray emitted during annihilation

Why: When a WIMP strikes a nucleus, the nucleus recoils and deposits a small amount of energy that can be read out as heat (phonons), ionization, or scintillation light, depending on the detector technology.

• A. Heavy nuclei are easier to cool to cryogenic temperatures
• B. Heavy nuclei give a larger coherent scattering cross-section for WIMPs, increasing the expected event rate ✓
• C. Heavy nuclei are radioactive and emit signals that are easy to identify
• D. Heavy nuclei absorb neutrons, which removes background noise

Why: For spin-independent WIMP-nucleon interactions, the cross-section scales roughly with the square of the nucleus mass number, so heavier targets like xenon offer much higher sensitivity.

• A. Dark matter particles are too fast to be slowed by any detector material
• B. Radioactivity in detector materials and environmental radiation can produce similar energy deposits ✓
• C. Dark matter interacts only with light, so optical detectors are blocked by dust
• D. The detectors must be kept at room temperature, which causes thermal noise

Why: Natural radioactivity from detector components and surrounding rock, as well as neutrons, produce electron and nuclear recoils that can mimic WIMP signals, so experiments must carefully screen materials and discriminate event types.

• A. Galaxies emit no light at all
• B. Stars in galaxies orbit faster than visible matter alone can explain ✓
• C. Telescopes have detected dark matter particles directly
• D. Black holes are constantly emitting dark matter

Why: Measurements of galaxy rotation curves show that outer stars orbit at speeds that would require much more mass than the visible matter provides, suggesting an invisible mass component.

• A. The redshift of distant quasars
• B. The cosmic microwave background temperature
• C. The gravitational lensing of background galaxies beyond the cluster ✓
• D. The color of cluster stars

Why: Gravitational lensing reveals that clusters bend light far more strongly than their visible matter could, indicating a large amount of unseen mass.

• A. About 5%
• B. About 27% ✓
• C. About 68%
• D. About 95%

Why: Cosmological measurements, such as those from the Planck satellite, indicate that dark matter makes up about 27% of the universe's mass-energy, while ordinary matter is only about 5%.

• A. It repels ordinary matter, spreading galaxies apart
• B. Its gravity pulls together into clumps that attract ordinary gas, forming galaxies ✓
• C. It converts directly into stars and planets
• D. It prevents any gravitational collapse in the early universe

Why: Dark matter, which does not interact electromagnetically, began clumping early under gravity. Its dense halos then drew in ordinary gas, which cooled and formed the galaxies we observe today.

• A. It absorbs all light that strikes it
• B. It does not emit, absorb, or reflect electromagnetic radiation significantly ✓
• C. It exists only in regions with no stars
• D. It is located deep inside black holes

Why: Dark matter does not interact strongly with light, so it neither emits nor absorbs electromagnetic radiation in any significant way, making it invisible to telescopes that rely on light.