ANSWERS for QUIZ 1:
1) From our location in the Milky Way, as we look at the center we are
looking through the disk. There is dust in the Galaxy and that dust is
efficient at scattering blue and optical light, due to the size of the
dust particles. Thus, some of the background light is scattered
creating a dark band.
2) The orbit of Uranus was not aligned that well with the expectations
for how gravity worked. From these deviations from expectations, they
were able to predict that another body was influencing the orbit. When
astronomers pointed their telescopes at that location, they discivered
Neptune.
3) The LIGO experiment is actually measuring the distance very
accurately between two detectors.
4) First, Newton's gravity law assumes "action at a distance" which
implies that any change in mass is instantly transmitted throughout
the universe. Thus, it would have information travel at faster than
the speed of light. Second, Newton's gravity implies that massless
particles would not be affected by another body, and we have measured
that photons (which are massless) can have their path bent.
5) A solar flare happens in the Sun and charged partcles (e.g.,
protons and electrons) stream towards the earth. They get caught up in
the magnetic fields of the Earth and stream along them. As they
stream, they collide with atoms in the atmosphere, and excite these
atoms. The atoms then fall back into a lower energy state, and then
emit light.
6) A graviton is the carrier of the gravitational wave.
7) Instead of a force pulling two objects together, Einstein assumed
that a massive body warps spacetime. This warping then causes a nearby
body to fall in towards it.
8) The pulsars are moving through spacetime, causing it to move up and
down which emits gravity waves. The energy of the gravity wave
emission comes from the orbital energy of the pulsars. So over time,
the pulsars are getting closer together.
9) For which star has a more massive planet it is ambiguous. Star A
has a planet that is closer in than in Star B.
10) Looks like the plot on the course webapge.
ANSWERS for QUIZ 2:
1) Due to its limited size LIGO cannot measure black holes that are
too much more massive than these. The problem is that with a very long
gravity wavelength, LIGO would only see a small fraction of it and
hence not be sensitive. For smaller black holes, they are definitely
in the LIGO data stream, but these first mergers were large enough
that they had a huge signal compared to the smaller ones.
2) the wavelength is shifted towards the blue; the speed is unchanged
3) Entropy is a measure of disorder; you get an accurate measure of
entropy by counting available states that a systems can be in. An
unstretched rubber band has higher entropy than a stretched one.
4) The two black holes had no material around them when they
merged. And since the black holes warp spacetime in on itself,
material can never be ejected from inside the black holes. Thus, as
they merge, no light can escape.
5) When a system (like a galaxy) forms stars, it will form stars over
a variety of masses, small mass to high mass. The high mass stars burn
their fuel much quicker, which causes them to both be very bright and
live a short amount of time. Also, more massive stars produce more
blue light since they have more energetic collisions between
particles. Thus, when a system has recent star formation, i.e. young,
it appears blue, and then eventually fades to red as it gets old.
6) For the direction of the rotation, we look at the blue stars
compared to the red stars. The side of the arm that has the blue stars
defines the direction of rotation. The argument is that the spiral arm
is a density wave and this wave builds up density on the side in which
it is moving. This density build up forms stars, and thus we see them
as blue (since they are recently formed).
7) GAIA is using parallax to get the distances. As it orbits about the
Sun, it gets a different perspective of the nearby stars by taking
very accurate images. There apparent motion, called parallax, then
determines how far away they are.
8) The point where spacetime is closed in on itself; the point where
the escape velocity is equal to the speed of light; the event horizon;
the Schwarzschild radius as determined from the mass
9) Consider a rocket accelerating upwards and having a person at the
top of the rocket measure the timed pulse from a person at the bottom
of the rocket. Since they are accelerating as the pulses are traveling
towards them, the pulses take longer to reach them. Thus, the person
at the top concludes that the person at the bottom is in a slower time
frame. Time runs slower in an accelerated field. Since acceleration is
equal to gravity, then time runs slower near a massive body.
10) The 40 solar mass black hole merger is the one on the webpage. The
more massive black hole merger would have a longer period and
amplitude. The shape change would be similar (period increases).
ANSWERS for QUIZ 3:
1) velocity and distance of an orbiting body
2) The earth's atmosphere both blocks some light (mainly optical is
used since it gets through) and it causes the light to be blurred
3) redshift and chemical composition
4) We observe the light coming from Jupiter when Europa is in front of
it. As the edge of Europa, some of the light is fainter. We interpret
this dimming of light as being due to water geyers coming off of
Europa and reflecting and refracting some of the light.
5) We measure their redshift and that tells us they are moving very
fast away from us. Since the universe is expanding, the redshift tells
us they are very far away. Since they are already bright and very far
away, they are intrinsically extremely bright. The onlw known energy
soure to generate this amount of light is accretion onto a black hole.
6) gravity first causes the cloud to contract; as it contracts, due to
conservation of angular momentum, the body begins to spin up; as it
spins up and contract, due to particle interactions the system
flattens into a disk.
7) We use proper motions of the stars as the orbit. We take an image
of the galaxy centers at various times over decades and watch the
stars move. The velocity and radius then determines the mass.
8) M_BH = 450/3 = 150 M_sun
9) Hawking radiation, which is a process that due to the Heisenberg
Uncertainty Principles creates particle pairs just outside the event
horizon; one of the pairs falls into the black hole and the other
radiated away causing the black hole to appear to create light.
10) Solid body rotates perpendicular to the axis of
rotation. Particles in a gravitationally-bound structure orbit about
the center.
ANSWERS for QUIZ 4:
1) dark matter and a modification of gravity
2) we use the age of the universe and the speed of light
3) black holes, neutron stars, white dwarf stars, planets, rocks,
anything dark (including WIMPs, I guess).
4) rotation curves in galaxies, movement of galaxies in a cluster,
x-ray gas in a cluster, gravitational lensing
5) In each, we use an observation to infer the total mass. Rotation
curves use velocity of material in a galaxy, movement of galaxies use
the velocities and distances to infer mass, x-ray gas just uses the
brightest of the gas to infer total mass, and gravitational lensing
uses the strength of the arcs. Then in each case we use the amount of
light we see to infer the observed mass. In each case, the total mass
is more than the observed, which implies dark matter
6) As material falls towards a black hole, it gets squeezed to a very
high density since the black hole is so small. In fact, the density
gets so high that it simply cannot all make it into the black
hole. Most of it escapes by getting funneled into a low density
region. Thus, it gets squirted out of the area around the black hole,
making a jet.
7) Normal and dark matter where initially distributed the same in a
galaxy. Normal matter then interacted with itself, lost velocity, and
fell into the center of a galaxy, whereas the dark matter remained at
large radii.
8) We sometimes find black holes not at the center of a galaxy, so
they might be many of them at large radii. Also, the mass we find,
including the measurement from gravitational waves, is consistent with
expectations for having many black holes at large radii in a
galaxy. This unseen mass could then account for the dark matter.
9) dark energy, dark matter, protons, photons, sriracha
10) axes are radii and velocity. Plot should cross at origin and then
flatten on either side.
ANSWERS to QUIZ 5
1. V=H0xD. V is velocity measured in km/s. D is distance, generally measured
in Mpc. H0 is Hubble's constant at around 7 km/s/Mpc
2. We measure how bright an object is. We then determine how bright an
object is intrinsically (how bright is actually is). From these two
measurements, the ratio then is related to the distance. We must
however use a calibration to determine how bright an object actually
is.
3. Their mutual gravitational attraction caused them to come together
despite the expansion of space between them. They were close enough
initially that gravity dominated.
4. It decreses.
5. If dark energy were larger, then it would have caused the universe
to expand more, and it would have been hard to form galaxies and very
large structure. If it were much smaller, matter would have dominated
more and it might have closed in on itself. In both cases, we would
probably not exist.
6. dark energy, matter, radiation, and shape
7. We need to measure a velocity and a distance.
8. As the average distance of objects in the universe increase (e.g.,
universe expands), the effect of gravity is weaker since the material
is further separated. However, since there is more space, this would
also cause dark energy to increase it's effect.
9. Yes, No (or not thought to be according to our models).
10. UT football Team, microwave photons, hydrogen, dark matter, dark energy.
ANSWERS to QUIZ 6
1. Higgs particle
2. point, line, plane, volume (cube, sphere, etc.), adding time,
imaging all timelines
3. the universe is expanding, the early SN's have less energy (less
mass), there is more dust in the past, the early SN are asymmetric
4. The idea is that they are all a specific mass. Once they reach this
exact mass, the star explodes in a SN. This mass is converted to
energy which is seen at light. Thus, because they are the same mass,
they are the same intrinsic brightness.
5. velocity and distance
6. For the Cepheid, we need to measure a velocity and we do that by
getting a spectrum. We then get the distance assuming they are
standard candles. We measure their period by taking images of how the
light varies over time. The period is then related to the intrinsic
brightness. We can then measure how bright they appear to us. With the
intrinsic and observed, we get the distance. Then distance and
velocity give expansion rate.
7. a candle, flashlight with low batteries, variable star, flickering
neon sign, computer screen
8. air, heat, dark matter, dark energy...
9. Einstein used the assumptions that the universe was small (size of
MW), old, and static. From his equations, he showed that it should have
collapsed by now. Since it didn't, he invented a force that countered
gravity exactly. This is what he called the cosmological constant.
10. He dismissed the idea of the cosmological constant when Hubble
made observations showing that the Universe is not static, that is was
expanding and large. If he had taken the next step, he could have
checked for stability in his model of the cosmological constant. He
would have seen that there were two solutions: one that the universe
has collapsed, and the other that it is expanding. Since it didn't
collapse, he would have concluded that the universe is expanding. This
was his blunder.
ANSWERS to QUIZ 7
1. flat
2. CMB and Big Bang Neucleosynthesis
3. 1.36 K
4. 100 times closer
5. The universe expands to an infinite size, thereby stretching out
the photons to having essentially zero energy.
6. The horizon problem is that opposite sides of the universe are
effectively at the same temperature, yet they are separated by too
large a distance for information to have traveled between them, given
the age of the universe.
7. Why haven't advanced alien races visited Earth given that the
universe is so large and there are so many chances for life.
8. BBNS assumes that we can predict the relative abundances of the
elements by using our understanding of interactions at higher energy
and density. We basically have a giant fusion reaction in the early
universe. Thus, BBNS makes a solid prediction for what elements should
exist. We then measure those abundances in very pristine systems. The
theoretical expectations match the observations very well. Thus, the
assumption that the universe starts out small and hot is validated.
9. That would imply a higher density of normal matter since that would
cause there to be more collisions, and hence better ability to convert
H to He.
10. It looks like a parabola with present near the minimum. High in
past and in future. Axes are time and expansion rate.