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ASTR 2030 Final
| Question | Answer |
|---|---|
| Black hole | Region of space enclosed by an event horizon - No matter, radiation, or information can flow outward |
| Dark stars | Imagine stars so compact they "trapped" light - First concept of black holes |
| Special relativity | 1) Speed of light is the same in all reference frames 2) Laws of physics are the same in all reference frames |
| Relativity of simultaneity | Whether two events are simultaneous, depends on where you are relative to the two events |
| Twin paradox | If one twin travels to space at near the speed of light, they will return younger than their twin that remained on Earth, not a symmetric effect |
| Drive-thru paradox | a long object moving at a relativistic speed appears to fit inside a shorter space due to length contraction, even thought it wouldn’t fit if it were stationary |
| Equivalence principle | inertial mass and gravitational mass are the same thing. Gravitational force is proportional to inertial mass, and the proportionality is independent of the kind of matter |
| Gravitational lensing | a phenomenon causing the warping of space and time, causing light to bend and magnify as it passes by |
| Gravitational redshift | when light appears redder because it loses energy as it escapes a strong gravitational field |
| Gravitational time dilation | time passes slower closest to a strong gravitational field Balance of pressure and gravity |
| Main sequence | Stars that are fusing hydrogen into helium in their cores |
| Neutron star | >10 Msun main sequence stars end up in Type 2 supernovae |
| Electron degeneracy pressure | a force that arises when electrons are squeezed very close together in a small space - white dwarfs are supported by electron degeneracy |
| Pulsars | spinning neutron stars that emit beams of radio wavelength light |
| Galaxy | Massive, gravitationally bound system of stars, gas, and dust |
| Stellar mass black hole | 3-100 Msun found throughout the galaxy |
| Supermassive black hole | 10^6 to 10^9 Msun found in a galaxy’s center |
| AGN | Produced by disk accretion onto a central supermassive black hole |
| Quasars | Found to be at large redshifts, so they must be very limunous - Luminous due to accretion of gas onto supermassive black hole |
| Masers | Microwave Amplification by Stimulated Emission of Radiation - Measures most accurate masses of supermassive black holes |
| AGN feedback | links the energy released by the AGN to the surrounding gaseous medium |
| Binary pulsar | Indirect evidence for gravitational waves |
| LIGO | First direct detection of gravitational waves (merging stellar mass black holes) |
| Pulsar timing arrays | First detection of gravitational wave background from merging supermassive black holes |
| LISA | Designed to detect gravitational waves from individual mergin supermassive black holes |
| GRBs | Distributed throughout the sky and must have a cosmological origin - short (0.1s) and long (10s) |
| Hawking's area theorem | Event horizon must stay the same size or increase |
| Hawking radiation | Due to the quantum uncertainty principle, black holes cna evaporate |
| Information paradox | Lost information when it goes into a black hole, but that violates quantum reversibility. Fix is that Hawking radiation isn't random and somehow returns the information to the Universe. |
| Singularity theorem | Every trapped surface must contain a singularity |
| What will you experience if you fall into a black hole? | 1) gravitational redshift and time dilation 2) spaghettification due to tidal forces 3) cannot escape once inside the event horizon 4) see singularity at the inner horizon |
| White dwarfs | <10 Msun main sequence stars become white dwarfs, possibility of Type 1a supernovae |
| AGN | Small region at the center of some galaxies that is far brighter than can be explained by the stellar population alone |
| What are intermediate mass black holes (IMBHs) and how do we think they could form? | Intermediate mass black holes are black holes with masses 10^3 − 10^4 Msun. No established formation mechanism, but they may: form from accretion onto stellar mass bhs, be supermassive bh “seeds" that didn't grow, or form from the collapse of a very mass |
| Where would we want to look for IMBHs? | We could look for them in: binary systems, centers of star clusters, halos of massive galaxies, or centers of dwarf galaxies |
| What do we think ultra luminous X-ray sources are? | They might be IMBHs, but they could also be regular X-ray binaries that are “beamed” towards us making them seem brighter. |
| Why do we not see black holes with mass less than 2 MSun forming in the present day universe? | They cannot form with currently possible astrophysical processes. Objects with lower mass are stable as neutron stars, white dwarfs, planets, etc. and will not turn into black holes |
| How do we think low mass black holes might have formed in the early universe? | The early universe was very dense and was “lumpy”, and some of these “lumpy” regions could have collapsed gravitationally and formed primordial black holes. |
| How could we detect primordial black holes? | 10^11 − 10^12 kg: possibly detect from gamma-ray flashes from their evaporation 10^−3 − 10^2 MSun: gravitational lensing 10^5 MSun: possibly gravitational effects |
| Could we create a black hole at a particle accelerator? Explain. | No. To create the smallest black hole possible, it would take about 10^15 times more energy than possible at the LHC. But, if gravity was spread out over extra dimensions, then the energy required in our 3D might be low enough that it is possible |
| How does the event horizon radius of a black hole depend on its angular momentum? | As the angular momentum of a black hole increases, its event horizon radius decreases. |
| If we inject mass into a black hole, is it possible to reduce the event horizon radius of a black hole? | No, as the gained mass will increase the event horizon radius |
| How is the area of a black hole’s event horizon related to its entropy, and what does this mean for black hole accretion? | The area of a black hole’s event horizon is proportional to its entropy. This means that for any matter accreted, the event horizon surface area (and therefore the entropy) must increase. |
| What other properties, in addition to entropy, did Hawking attribute to the black hole event horizon? | Black hole event horizons have temperature and they radiate energy like any hot object |
| describe the process of Hawking radiation | Near a black holes event horizon, a particle-antiparticle pair forms. The negative-energy particle falls in, and the positive-energy one escapes. The black hole loses mass equivalent to the escaping particles energy to conserve energy, reducing its size. |
| Compare the temperature and evaporation time of a larger and a smaller black hole. How do both relate to the black hole’s mass? | A larger black hole is cooler and evaporates over a longer time. A smaller black hole is hotter and evaporates more quickly. Temperature scales as 1/M and evaporation time scales as M^3. |
| What are the four fundamental forces and how does the one we talk about most in this class compare to the other three? | Electromagnetism, strong nuclear force, weak nuclear force, gravity. Gravity is by far the weakest of the four forces (about 10^40 times weaker than electromagnetism) |
| Explain what the hierarchy problem is and a speculative solution to this problem. | The hierarchy problem is that the Planck mass is vastly larger than particle masses. One idea suggests gravity is spread across extra dimensions, making it appear weaker in our 3D space. However, there's no experimental evidence for this yet. |
| What is quantum gravity and why do we need a theory of it? | Quantum gravity is gravity at the quantum scale (particle size and smaller). We need this in order to understand what happens inside a black hole or what happens if two particles collide with enough energy to create a black hole. |
| Why is quantum gravity so different and much more difficult than quantum theories for the other three forces? | Gravity relies on distance to determine if events can causally interact via light signals. In quantum gravity, spacetime may be fuzzy at small scales, making distances uncertain and causality hard to define. |
| Explain how string theory could help solve the problem with quantum gravity above. | Maybe particles are not pointlike particles but extended strings, and these strings operate in higher dimensions that could explain quantum gravity interactions. |
| How do we expect orbital velocities around a massive object to behave as the distance from the massive object increases? What do we notice about orbital velocities in galaxies? What does this imply? | The orbital velocity should decrease as distance increases. In galaxies however, we observe that orbital velocities plateau instead. This means that there is spread out mass that we cannot see in the galaxy, and we call this dark matter |
| Why do we call dark matter “dark”? | We call it dark because, as far as we can see, dark matter does not emit or absorb much light anywhere in the electromagnetic spectrum |
| Compare the amount of dark matter to the amount of visible matter in the Universe | There is 5x more dark matter than luminous matter in the Universe. |
| Where does dark matter exist in a galaxy? | It exists in a dark matter halo that is spherical and much larger than the visible matter portion of the galaxy. |
| What could dark matter be? | MACHOs (e.g., brown dwarfs, white dwarfs, black holes) account for ~20% of the Milky Way's dark matter via gravitational lensing. WIMPs, subatomic particles that are massive and not prone to interactions (similar to neutrinos), are another possibility an |
| What type of light does a WIMP create when it annihilates with its anti-particle, and how are we trying to detect WIMPs? | Experiments aim to detect WIMPs through their interactions with atomic nuclei. When WIMPs annihilate, they may emit gamma rays. The Fermi telescope observes the Galactic Center in gamma rays to search for this evidence. |
| What is an alternate explanation for galaxy orbital velocities that doesn’t invoke dark matter? | Maybe general relativity is not the correct theory of gravity on large spatial scales. Modified Newtonian Dynamics (MOND) could possibly explain the flat rotation curves of galaxies, but MOND is not consistent with other astronomical observations |
| Name a specific observation that we discussed in class that cannot be explained by this alternate explanation for galaxy orbital velocities that doesn't invoke dark matter. | The Bullet Cluster shows two colliding galaxy clusters where gas slowed due to gravitational and electromagnetic interactions, but dark matter slowed less, interacting only gravitationally. This cannot be explained by MOND. |
| What does an observer see when they watch someone fall into a black hole | The observer sees the person become redder (gravitational redshift) and their time slow (gravitational time dilation), appearing to fall more slowly. The person never crosses the event horizon and eventually becomes too redshifted to be visible. |
| What mechanism causes spaghettification? | As objects fall toward a black hole, tidal forces stretch them radially and squeeze them laterally. Gravity is stronger at the bottom (closer to the black hole) than at the top, causing this effect, aptly named spaghettification. |
| How does the “magnitude” of spaghettification at the event horizon depend on the size of the black hole? | The smaller the black hole, the greater the tidal forces at the event horizon |
| What is the singularity of a black hole, do all black holes have one, and can we ever see light coming from one? | a point where spacetime curvature and mass/energy density become infinite. All black holes are theorized to have a singularity. We can never see one as they are always behind an event horizon, EXCEPT maybe for a moment right as a black hole evaporates |
| What is the inner horizon of a black hole, and what could you possibly see if you went beyond it? | The inner horizon is the horizon within which you could see the singularity. If you went beyond the inner horizon, you could see the singularity and you may also see a white hole or wormhole if they exist. |
| How would you die (and you definitely would) if you entered a black hole? | If it was a smaller black hole, you would die from tidal forces early on. If it was a supermassive black hole, you could get all the way to the inner horizon before you are vaporized. |
| Name some upcoming observations that will help us learn more about black holes. | Pulsar timing arrays will soon be able to detect gravitational waves from an individual system of binary supermassive black holes, and LISA will enable us to detect gravitational waves from supermassive black hole binaries in the very early universe. |
Created by:
sella_matthews
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