AST201 exam Flashcards
1
Q
Based on Newton’s law of gravity which planet would orbit the fastest?
A
mercury: the closest planet
2
Q
What did Vera Rubin observe about the speed of galaxies?
A
The further an object is from the center of the galaxy, the faster it orbits.
3
Q
What is the ratio to dark-light matter
A
roughly 1:20
4
Q
What are the present day ratios of content in the universe?
A
70% Dark Energy, 5% Ordinary Matter, 25% Dark Matter
5
Q
Why do galaxies in clusters move faster?
A
Dark Matter
6
Q
Is motion absolute?
A
No, all motion is relative
7
Q
Is the speed of light variant?
A
No, all observers measure the speed of light to be the same
8
Q
What does special relativity study?
A
Relativity for objects in non accelerating frames.
9
Q
What are the postulates of special relativity?
A
- Laws of physics are the same in all reference frames that move at constant speeds.
- The speed of light is the same.
10
Q
What is the main consequence of special relativity?
A
Observers do not agree on the time between two events.
11
Q
What is time dilation?
A
The difference between the times observed by two observers
12
Q
What does the famous saying “Moving clocks run slower” mean?
A
The moving frame of reference will say that less time has passed.
13
Q
What does the invariance of the speed of light bound?
A
Space and time, to create Space-time
14
Q
When does time dilation become a major problem?
A
When you approach the speed of light
15
Q
If Fritz is on Earth and Vera travels in a rocketship at 0.6c, what would each twin say about the other’s clocks?
A
Both twins would say the other is moving and hence the other twins clock runs slower.
16
Q
What is the Lorentz factor?
A
The factor by which time differs in two moving frames of references.
17
Q
Which twin is younger? The moving twin or the still one?
A
The moving one
18
Q
What breaks the twin paradox?
A
The frame of reference changing.
19
Q
What is Newton’s law of gravity?
A
Gravity is an accelerating force. It is the product of the Gravitational constant times the masses divides by the square of the distance.
20
Q
Newton’s first law
A
An object in motion (or rest) remains in motion (or rest) unless acted upon by an external force
21
Q
Newton’s Second law
A
Force = mass * acceleration
22
Q
Newton’s Third law
A
Each action has a reaction equal in magnitude and opposite in direction.
23
Q
What are the four fundamental forces of nature?
A
Strong Nuclear, ElectroMagnetic, gravity and weak nuclear
24
Q
Arrange the four fundamental forces in ascending order in terms of strength.
A
gravity < weak nuclear < electromagnetic < strong nuclear
25
What fundamental forces have infinte range?
gravity and electromagnetic
26
What seperate gravity from the other fundamental forces?
it does not have a carrier particle.
27
What does the strong nuclear force do?
hold atomic nuclei tgth
28
what does the electromagnetic force do?
light friction and stuff
29
what does the weak nuclear force do?
involved in radioactive decay
30
What is the carrier particle for the strong nuclear force?
gluons
31
What is the carrier particle for Electromagnetism?
photons
32
What is the carrier particle for Weak nuclear force?
W and Z bosons
33
What is the equivalence principle?
we cant distinguish gravity from acceleration.
34
What is Einstein's theory of gravity?
Gravity is curved space time and objects try to follow the straightest path.
35
What is the difference between Einstein's law of gravity and newtons?
In einstein's, gravity is curved spacetime whereas in newton's gravity is a force.
36
What does Einstein's gravity model explain that newtons doesnt?
1. precession of planets.
2. gravitational lensing
37
Where does time run slower?
Time runs slower in places of strongly curved spacetime i
38
What is an escape speed?
The speed required for a projectile to break free from an object's gravity
39
What is the formula for escape speed?
v = sqrt(2GM/r)
M is the mass of the object and r is the distance from the center of mass.
40
What is a blackhole?
A region of space where the matter is so dense, that the escape vel. exceeds the speed of light
41
What is the event horizon?
The mathematical outer boundary of a black hole.
It is the distance from the center where escape speed = c.
42
What is a singularity?
Where density goes to infinity
43
What is the schwartzchild radius?
The distance between the singularity and event horizon.
44
What does the schwarzschild radius depend on?
mass of the black hole
= (2G/c^2) *m
45
If Star A becomes a blackhole, is the gravity of the blackhole stronger or weaker than the gravity of the star?
It is the same.
46
What is a microscopic blackhole?
an object produced by energetic particle collisions.
47
What types of blackhole do we have no evidence for?
microscopic
48
What are the types of blackholes?
microscopic, stellar mass black hole, intermediate mass, supermassive
49
What is a stellar mass black hole?
Form from the death of a star through a supernova
50
What is an intermediate mass black hole?
Form from the collision of smaller objects
51
What is a supermassive black hole?
Usually in the middle of galaxies but we don't know how they form.
52
What is the evidence for intermediate blackholes?
gravitational waves
53
Why would you not survive falling into a blackhole?
tidal forces (difference in gravity) cause spaghettification
54
Are smaller black holes stronger than bigger black holes?
Yes, smaller black holes are more "dense."
55
What do tidal forces caused?
Tidal disruption events are caused by the tidal forces that are further caused by things falling into blackholes.
56
How can we detect tidal diruption events?
X-ray telescopes
57
What would a clock appear to do as it falls into a blackhole?
The clock would tick slower and slower until it stopped when it reached the event horizon.
58
What is the earliest evidence for a stellar mass black hole?
X-ray binaries
59
What causes an accretion disk?
When two stars in a binary star system get too close, they'll steal matter from another.
60
What was the first widely accepted black hole?
Cygnus X1
61
What are the evidence for blackholes
1. Stars orbiting nothing
2. X-ray
3. Gravitational waves
62
What is the age and size of our sun?
avg.
63
What is the distance between the sun and the earth
8 light minutes aka 1 AU
64
As frequency increases, the colours become more
purple
65
As wavelength increases, the colours become more
red
66
What object absorbs all wavelengths of light?
blackbody?
67
what happens when you heat a blackbody?
it's colour becomes cooler and it becomes more bright
68
What is electron emission?
When electrons jump from a higher shell to a lower
69
What is electron absorption?
When electrons jump to higher shells.
70
What determines the colour of light emitted or absorbed?
the change in energy
71
What colour does the sun's blackbody spectra peak at?
Green
72
Where are the absorption lines in the suns spectra
at green and red
73
How can we figure out what a star is made up of?
It's spectra
74
What happens when you perceive equal amounts of every wavelength?
you see white
75
What colour is hot?
Blue!!
76
What colour is cold
red!!
77
what colour will you never see a star being?
green
78
What determines the wavelength of the peak ofa blackbody?
the temperature
79
Which principle is used when a star generates energy?
the mass-energy principle (it converts its mass into energy.
80
When we stick nuclear particles together. what is that process called?
Nuclear fusion
81
What powers the sun?
the fusion of four protons into helium.
82
Explain the proton-proton chain
4 hydrogen atoms becomes 1 helium atom, 2 antielectrons, 2 nuetrinos and 2 photons.
83
How do we know that the proton-proton chain powers the sun?
Because we detect the nuetrino particles always in pairs of two?
84
what is the luminosity of our sun?
4 x 10^26 watts or 1 L
85
What is luminosity (absolute magnitude) ?
How bright a light source acc is
86
What is apparent brightness (apparent magnitude)?
How bright a light source may appear
87
what do we need to convert between the luminosity and apparent brightness of a star?
its distance
88
How do we measure the distance of nearby stars
parallax (closing one eye, then the other, you need two to percieve depth)
89
using parallax, the angle for a further object will be ___, than an object at a closer
smaller (distance = 1/ parallax)
90
What is the inverse square law?
apparent brightness is proportional to Luminosity/distance^2
91
What is the key piece of information a star's spectral classification will give you?
the general number of absorption lines and temperature
92
From Hottest to coldest, name all the sequence types?
O B A F G K M
93
what is the balmer series?
absorption and emission lines begging and ending at n = 2 for hydrogen
94
Will you see Balmer lines in a cool star?
no because most electrons are in ground state and cant absorb light to make the balmer lines
95
Will you see Balmer lines in a hotish star?
Yes, because most electrons are at n=2
96
Will you see Balmer lines in a very hot star?
No, because most electrons are at higher shells
97
Why do cooler stars have more emission lines?
they can house more molecules
98
What colour are the balmer lines in?
red
99
As temperature increases, the absorptions lines will get stronger in what order of elements/molecules?
molecules, neutral metals, ionized metals, hydrogen, natural helium and ionized helium
100
As temperature increases, the absorptions lines will get stronger in what order of elements/molecules?
molecules, neutral metals, ionized metals, hydrogen, natural helium and ionized helium
101
What is a HR diagram measuring?
Luminosity (low to high) vs Surface temperature (high to low)
102
What is the main sequence?
General diagonal line on an HR diagram
103
what are stars on the main sequence doing?
Fusing hydrogen aka staying alive
104
What is the type of scale for luminosity on an HR diagram?
logarithmic
105
What is the stefan-boltzmann equation
Luminosity = Surface area * constant * Temp^4 (for spherical objects SA is 4pi r^2?)
106
What do most astronomers label most main sequence stars?
dwarf stars
107
What are M-class main sequence stars called?
red dwarfs
108
What are G-class main sequence stars called?
yellow dwarfs
109
Amongst main sequence stars all M stars are....
small and red
110
Amongst main sequence stars all O stars are....
large and blue
111
As we move up the main sequence what do u see?
higher mass, lower lifespan
112
Can stars go up or down the HR Sequence?
no :(
113
Are dead stars like white dwarfs above or below the main sequence?
below
114
Are dying stars above ro below the main sequence?
above?
115
Most stars are ___?
red dwarfs
116
Most visible stars are?
Supergiants or giants
117
What are the five luminosity classes?
Supergiants, bright giants, giants, subgiants and main sequence.
118
What does a luminosity class tell you about a star?
Its evolutionary state
119
Why do massive stars have a shorter life time on the main sequence?
Because they "burn"through hydrogen much quicker
120
What is the relationship between lifetime, mass and luminosity
lifetime ~= mass/luminosity
121
If a star cluster has all stars present, then it must be very young or old?
young
122
What is the age of a cluster determined by?
lifespan of the shortest lived star on the main sequence?
123
What is the main sequence turn off age?
It is the age of the cluster
124
What are large pink areas in a galaxy indicative of?
Ionized Hydrogen. Typically H II regions.
125
What are large dark areas in a galaxy indicative of?
molecular Hydrogen (H2), stars form here
126
What are the engines of star formation?
Molecular clouds
127
Explain how stars form.
1. Collapse – A dense region in a molecular cloud collapses under gravity.
2. Heating – Gravitational energy turns into heat, forming a hot, dense core.
3. Disk Formation – Rotation causes the collapsing cloud to flatten into a disk.
4. Temperature Rises – Radiation becomes dominant, raising the surface temperature.
5. Fusion Begins – Core gets hot enough for hydrogen fusion; energy balances gravity → star is born.
128
What is the pre-main-sequence
phase of the protostar?
1. Collapse – A dense region in a molecular cloud collapses under gravity.
2. Heating – Gravitational energy turns into heat, forming a hot, dense core.
129
How to protons overcome the natural electromagnetic repulsion?
They collide at very high speeds (Very very hot)
130
In a 1 M star, what is the minimum core temperature for protons to start fusion?
10 million K.
131
Once a proton-proton chain is established in the core of the protostar, what happens next?
The energy creates pressure to counterbalance gravity and it becomes a zero age main sequence star.
132
What is hydrostatic equilibrium?
outward pressure of fusion = inward pull of gravity
133
What type of star is in hydrostatic equilibrium?
A main sequence star.
134
When do stars start to die?
When the stars run out of fuel in their cores. This makes inwards gravity > outwards pressure
135
The death of a star depends on what?
Its mass
136
What are high mass stars?
Mass > 8 solar masses aka all O and some B stars
137
What are low mass stars?
Mass < 8 solar masses aka A-M stars
138
How does a low mass star die?
1. Hydrogen Depletion:
Core hydrogen is exhausted → fusion stops → pressure drops → gravity causes the core to contract and heat up.
2. Shell Burning & Expansion: Hydrogen fuses in a shell around the inert helium core → extra energy pushes outer layers outward → star enters subgiant, then red giant phase. The outer layers expand and cool, but the core keeps contracting and heating.
3. Helium Flash:Once the degenerate helium core hits ~100 million K, helium fusion ignites explosively (helium flash) → core expands, lifting degeneracy → hydrogen shell burning slows → star settles into helium core fusion + hydrogen shell fusion on the horizontal branch.
4. Helium Exhaustion & AGB: Helium in the core fuses into carbon and oxygen, but the core can't fuse those. Now, with a C/O inert core, helium and hydrogen fusion continue in shells → outer layers expand again → asymptotic giant branch (AGB).
5. Death:
The star becomes unstable, sheds its outer layers as a planetary nebula, and leaves behind a white dwarf — a hot, dense, non-fusing C/O core. It’s "dead" because no new energy is produced.
139
What is the chemical composition of a star on the main sequence?
10% helium, 90% hydrogen in volume
140
What is degeneracy pressure?
The pressure exerted by electrons refusing to get closer due to EMF.
141
Why does a low mass star not collapse under its gravity and produce a black hole?
The inert degeneracy pressure will balance out the gravity?
142
What is a white dwarf?
The exposed C/O core of a star after death
143
What are the expelled contents of a low mass star after death?
Usually a planetary nebula
144
Whats the main difference between low mass stars and high mass stars when it comes to their death?
high mass stars get hot enough to fuse helium while still on the main sequence
145
Are masses of nuclear particles constant?
No, they depend on the nucleus they're in.
146
Why is the mass of nuclear particles not constant?
Separating two objects that are attracted, requires energy, that energy gets added to the mass of the object
147
What reactions can reduce the mass per nuclear particle?
Only those that liberate energy. For lighter elements this is fusion and for heavier, it is fission.
148
What is the bottom most element on a mass per nuclear fusion vs atomic mass graph?
Iron.
149
What is the significance of Iron having the lowest mass per nuclear particle?
Neither fusion or fission can liberate energy.
Once a star has an iron core it can no longer resist gravity.
150
What does a massive star become after leaving the main sequence?
A supergiant
151
Why doesn't electron degeneracy stop the collapse of a massive star?
Its simply not strong enough to counteract the gravity.
152
How is the collapse of a massive star?
Very violent and sudden
153
In some massive stars, as the core collapse what is it converted into?
neutrons: proton + electron = neutron + neutrino
154
How does a massive star die?
1: Neutronization: all the mass is converted into neutrons very quickly
2. Implosion: since its suddenly unsupported against gravity, it implodes almost as quick as the speed of light.
3. Contraction: The core contracts to a ball of neutrons. Degeneracy pressure kicks in as neutrons do not wish to go closer.
4. Explosion: The Neutrinos try to push out of the dense core but they're stuck, this pressure caused an explosion causing a supernova
155
What is the type of supernova that is a result of a star dying?
Core collapse or Type II
156
What is a type la remnant?
When a white dwarf in a binary star gets too close to the other star, it can steal mass and exceed the chandrasekhar limit. This immediately causes fusion and a supernova.
157
What is the Chandrasekhar limit?
The maximum mass of a white dwarf star is 1.4 M.
158
What does the final state of a dead star depend on?
It's initial mass and the sequence of events that happened to it (damn fat shaming and trauma :( )
159
Why is harder to predict the end state of more massive stars?
They lose a lot of their mass randomly due to stellar winds
160
For a neutron star to form what must the mass after a supernova be?
1.5 - 3 M
161
During a Type II supernova, if the ball of neutrons does not collapse to form a blackhole, what does it become?
A neutron star.
162
If the net external force on an object is zero, the angular momentum must remain?
constant
163
To maintain the conservation of angular momentum, when a star collapses, what does it do?
It spins even faster.
164
A pulsar is a type of what star?
Nuetron star
165
What is a pulsar?
A pulsar has a strong magnetic field, that sweep across the sky as the star rotates rapidly. It was detected through radiowaves.
166
The collison of binary neutron stars result in?
strong gravitational waves and a kilonova
167
Does everyone see a pulsar as a pulsar?
No, you only see it as a pulsar if you're in the direction of the poles while it rotates
168
Most low mass stars end up as ___ after their deaths?
white dwarfs
169
Most high mass stars end up as ___ after their deaths?
black holes, neutron stars or supernovas
170
What is our home galaxy?
The milky way
171
what type of galaxy is the milky way?
A spiral galaxy ~ a barred spiral
172
What is the total diameter of the luminous part of the milky way galaxy?
100, 000 light years
173
What are the three main parts of the milky way/ any spiral galaxy?
Disk, halo and bulge.
174
What is the galactic disk?
Thin layer (1000 ly) that consists of most of the dust and gas in the milky way. Star formation sites, appears blue
175
Why do blue stars appear close to the spiral arm and red and yellow dont?
Blue stars die before one complete orbit so they have no chance to move. Lower mass stars complete many orbits and have time to populate the whole disk.
176
What is a starburst galaxy?
Galaxy with rapid star formation
177
What is a starburst galaxy triggered by?
Gravitational interactions between galaxies
178
What does the bulge of the galaxy contain?
older redder stars and often a suppermassive blackhole (ours does). Very little amounts of gas and stars
179
What is in the galactic halo?
Mostly globular clusters of stars and majority of dark matter. Very little amounts of gas and stars
180
How is the movement of stars in different parts of the galaxy?
Stars in the disk orbit in a circle, with little variance.
Stars in the halo and bulge orbit randomly.
181
What did scientist pre rubin expect the rotation curve to look like?
an asymptote towards infinity as x approached 0 and infinity. reciprocal fn.
182
Why did scientist pre rubin expect the rotation curve to look like an reciprocal fn?
They assumed all mass was centered in the galaxy, and applied newtons law.
F= ma
a = v^2/r
F = mv^2/r
v = sqrtGM/r
183
What assumption explains Vera rubin's findings of the rotation curves
mass is distributed smoothly within and beyond the galaxy.
184
What does Vera Rubin's rotation curve imply
There is more mass than we can find
185
How much of the Milky way is dark matter?
90%
186
What is gravitational lensing?
when galaxies appear to curve due to spacetime.
187
What is another proof of dark matter?
There is far too much gravitational lensing is far to be explained simply by just the mass of objects.
188
What did the Bullet Cluster show us?
That Dark matter doesn't interact with itself except through gravity.
189
If Dark matter interacted with itself what would that have implied?
That it would have used EMF or another force.
190
What is our best theory as to what dark matter is?
A WIMPs. a particle that only interacts through weak forces like gravity.
191
What are the three main types of galaxies?
Spiral, elliptical and irregular.
192
What is the general shape of an elliptical galaxy?
Round or oval shapes with
little structure
193
What types of stars do elliptical galaxies contain? Is there a lot of gas?
Older red stars and lil gas
194
What is the scale for elliptical gaalaxies.
E0 (spherical) to E7 (elongated)
195
What is the galaxy that is a bit like elliptical and a bit like spiral?
SO also known as Lenticular galaxies.
196
What are some characteristics of Lenticular Galaxies
They have no arms but a bulge and disk. Older star population. Can have a bar.
197
What are Irregular galaxies like?
Random shape and structure. Often due to gravitational interactions. Contain all types of gas and dust. Young stars
198
Who will we collide with
Andromeda in 4 billion years
199
How many galaxies does a galaxy group hold?
100 or fewer like the local group
200
How many galaxies do Galaxy clusters have?
from hundreds to thousands.
201
What are super clusters?
Massive
structures
containing
multiple galaxy
clusters
202
What is the cosmic web?
Galaxies, galaxy groups and clusters and
superclusters are arranged in twisting, threadlike structures called the cosmic web
203
What technique can measure distances over the range of cosmic distance?
None
204
What is the cosmic distance ladder?
The cosmic distance ladder is an
interlocking set of techniques for measuring
distances out to billions of light years
205
What are standard Candles?
A standard candle is an object whose luminosity can be determined without knowing its distance
206
What is an example of a standard candle?
the main sequence, type la remanants
207
What is main sequence fitting?
when there are two main sequences, we know the difference between them is caused by distance.
208
If the main sequence if hyades is 7.5 times brighter than pleiades, then it is?
pleides is sqrt7.5 times farther.
209
What is a variable star?
A star whos brightness varies with time
210
What is a cepheid variable star?
A star whos luminosity changes as the star pulses
211
What is the Cepheid period luminosity relationship?
The star's period is proportional to it's luminosity.
212
How are typa la remanants, standard candles?
They all burst at the Chandrashekar limit. so they all have the same luminosity.
213
What were the two discoveries made because of cepheid and type la distance measuring?
1. The universe had a beginning.
2. Most of the universe is dark energy
214
How did we know that the milky way wasnt the entire galaxy?
Hubble measured variable stars in andromeda using cepheid variable stars to prove it wasnt a part of the milky way.
215
What is the steady state model?
universe is eternal and essentially unchanging
216
What is Olbers paradox?
if the universe is infinite in size and has always existed, then
every line of sight should ultimately end at the surface of a star.
The sky should be blindingly bright. Yet it is mostly dark.
217
The further away a galaxy is, the more its spectral lines are ___ to longer wavelengths.
redshifted.
218
What is Hubbles Law?
redshift = Hubbles constant * distance form Milky way
219
What did astronomers intially believe the redshifts to be?
Doppler shifts. If a source is moving away from you it is shifted red. (longer wavelength)
220
Are galaxies flying through space?
No, the space between each galaxy is expanding.
221
Why does the raisin bread analogy not apply to closer galaxies?
because gravity is the much stronger force there.
222
Is the expansion of the universe centered?
No it's symmetric and everyone is in the middle.
223
What is the main thing that hubbles law tells us?
That the universe had a beginning. The Big bang.
224
How do we get that the universe had a beginning fromn hubbles law?
Well if the universe was expanding and all the area between galaxies were getting bigger, then in the past they must have overlapped at a time. This is the singularity.
225
Why was the early universe opaque, and what made it become transparent?
At first, the universe was hot and dense—full of ionized plasma where free electrons scattered light, making it opaque.
As it expanded and cooled to around 3000 K, electrons combined with protons to form neutral atoms (recombination).
With no free electrons left to block light, photons could finally travel freely—so the universe became
transparent.
This released the Cosmic Microwave Background (CMB).
226
What does the big bang model explain that the steady state model doesnt?
WHy the sky is almost uniformly bright in microwaves. The big bang model says that it redshifted into the microwave part of spectra.
227
What provides strong evidence for the Big bang?
Cosmic microwave background.
228
When does Hubble's law deviate from a straight line?
At large distances, this tells us that the expansion is accelerating.
229
Is there any independent evidence for the big bang?
Yes, Big bang nucleosynthesis.
230
Explain Big bang nucleosynthesis
Laws of nuclear physics predict the exact combinations of elements formed.
231
What did Einstein write into his equations for general relativity?
A fudge factor, the erm lambda, so he could preserve the steady state model. We need this factor but not for the reasons he thought.
232
What does einstein's constant account for?
The accelerating expansion of the universe. We call it Dark energy.
233
What does Hubble's law tell us about the end of the universe?
heat death
234
What is the farthest thing we can see?
The cosmic microwave background
235
What is our observable universe contained to?
objects whose light has had time to reach us.
236
What has our observable universe been stretched to?
46.5 billion light years
237
How would you measure the curvature of the earth?
Pick three points and check what the sum of angles adds up to 180.
238
What is a flat universe?
A triangle is 180 degrees
239
What is a positively curved universe?
A closed object with triangular angles exceeding 180 degrees, (Sphere)
240
What is a negativly curved universe?
An open object with triangular angles less 180 degrees (pringle)
241
How do we know the universe is flat?
Measure angles using the cosmic microwave background
242
What do scientists mean by a flat universe?
no intrinsic curvature in space. Parallel lines remain parallel forever. Infinite size.
243
What does the geometry of the universe have to
do with its ultimate fate?
The geometry is determined by how much “stuff”
(matter and energy) there is in the universe
244
What is the density equation for the universe.
Total density = density of all matter + density of dark energy.
When Density = 1: flat
density < 1: open
density > 1 : closed
245
When there is excess matter in the universe, how does the world end?
The Big crunch
246
When there is balance of matter and no dark energy in the universe, how does the world end?
we get a flat universe, that expands forever at a decelerating rate.
247
When there is less matter and no dark energy in the universe, how does the world end?
We get an Open universe that expands forever.
248
What is our universe actually like.
Our actual universe is FLAT (Ω = 1) but there is enough dark energy to cause the expansion to accelerate indefinitely.
249
What does the conventional Big Bang model predict ab the death of the universe?
Star formation will slow. Night skies will grow dark. Galaxies will be so far apart we wont be able to detect anything. The cosmic background will be very red shifted. Everything will be cold. This is the heat death of our universe.
250
What are the four main problems with the Big Bang theory?
1. The flatness problem
2. The horizon problem
3. The Hubble Tension
4. Why there is something rather than nothing
251
What is the Flatness problem?
If there are an infinte number of possible ways for the universe to be closed or open, but only one way for it to be flat, why is it flat?
252
What is an observable universe?
a spherical region centered on any given observer whose radius is equal to the maximum distance light could have travelled since the big bang
253
What is the horizon problem?
Two opposing points of a straight line in the observable universe, had enough time to send information to the middle but not enough time to convey information to each other. So how did they come to be the same temperature and contact?
254
If two completely opposite sides of the sky
have never communicated with one another, how
did they come to be the exact same
temperature?
Inflation dragged them apart that speeds much
higher than the speed of light, making it appear that they were never able to
communicate.
255
If these two completely opposite sides of the sky
have never communicated with one another, what does both of them having the same properties imply?
They were once in physical contact
256
What is inflation?
When the universe was born, it expanded much more rapidly then it does now.
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What solves the flatness problem?
Inflation, strong curvature in the beginning would be virtually impossible to measure present day.
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What is Hubble Tension?
disagreement among different measurements of the Hubble constant. Scientists think that dark energy varies intime. (very recent result)
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Why do many people prefer universes that end with a Big crunch?
Because it seems like the universe sprung from nothing. If it crunches, then it can repeat infintly
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What is Eternal Inflation?
In this model, pockets of existing universes undergo periods of inflation that spawn what appear to their occupants to be new universes
