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231 Cards in this Set

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Why wasn't the Milky way discovered until the 1920s
1.) Its big
2.) We are inside it (like if you were in the forest)
3.) Dust – obscures the disk of the Galaxy (most distant stars are the hardest to see)
- TYPICAL or MEDIAN distance to the 6000 brightest stars in teh Milky way
1000ly
WHAT is the Milky Way?
- Integrated light from many stars in the disk of our galaxy …
- You are not seeing the light from individual stars a bunch of stars in the same direction
- The band strongly suggests that we live in a FLATTENED system of stars…no real obvious center
What is DUST???
Small specks of solid matter

- Typical size – 0.4 microns (um) range (.1 – 0.4um)
- Composition: silicates(sand and glass) and carbon compounds (graphite or soot)
- Where from? Evolved stars (in planetary nebula phase – EJECT dust particles…)
Why does dust block light?
Interactions between photons and dust…
1.) Absorption – photon is absorbed and converted to something else
2.) Scattering – photon’s direction is changed…
Collectively called Extinction –
Absorption and scattering are sensitive to what in photons?
wavelength - Interactions are most effective if the DIAMETER of the dust particle is bigger or equal to the wavelength of light…
- The average diameter of the dust particles are about 0.4 um which is the wavelength of blue light…
- Red wavelengths are longer and less absorption and scattering SO…
- We want wavelengths LONGER than visible light so we look at the universe with infrared..in ORDER to see it…
Why do we know that we are not in the center of the Milky Way?
- IF there were a spherical distribution of stars, we would see the same amount of stars in all directions
- If we were we would see a MUCH higher density of bright stars…
HOW DO GALAXIES FORM?
From Star clusters
GLOBULAR
- 10^ 5-10^6 stars
- In the HALO mostly though a few in the Disk and Bulge
- Few heavy elements (Population II)
- OLDER >10^10 years
OPEN cluster
- 10^2 – 10^3 stars
- Only in DISK of the galaxy
- Heavy elements in the Stars (Population I Stars) – heavier than helium
- Younger < 109 years
How do we know that all the stars in a globular cluster are the same age?
- With the GC there is a MAIN SEQUENCE TURN-OFF
- When you see that – you know they are ALL THE SAME AGE ….
no MS turn off in regular HR diagrams
FUNDAMENTAL differences in galaxies
Mass, spin, gas content, ages of stars
MORE superficial differences in galaxies –
spiral structure and bars…
what kind of galaxy is the MW?
An Sb galaxy
What is the problem with Hubble's classification system?
- Based on visual appearance not on objective properties (may not even be fundamental properties) –
- Not based on quantified measurements…
- Classification or sub classification of elliptical misleading because based on apparent shape not true shape
- Bars are not all or nothing…. There is a continuum of bar strengths –
- Bars are not really a fundamental property …
- It suggests elliptical evolve into spirals - but it is not clear whether that is true (Not true)
- IT IS TRUE that SPIRALS turn into elliptical ( but that is not the only way to make elliptical)
which is more important distinguishing feature of a galaxy
the amount of the stars in the disk
(or the bulge - as this is related to mass)
Ellipticals (E)
- Smooth and featureless
- some are round and some are flatter - appearance is not always the actual shape
- most are like a squashed football
- NO STAR formation
- LITTLE DUST
- COLD GAS
- They are DEAD – no raw material for star formation
SPIRALS (Sa, Sb, Sc)
- Disk and Bulge
- BULGE:significantly extended in all three directions…centrally concentrated – more so than the disk - like an E galaxy – not in every way, but in many respects
- DISK – thin, less centrally concentrated than the bulge… in spirals the DISK contain GAS and DUST and ongoing STAR formation…
- Sa – bigger bulge relative to disk
- Sb – about the same
- Sc – bigger disk relative to bulge
- Take the gas and turn it into stars - STILL forming and evolving… )
LENTICULAR (S0)
- Contains a disk and a bulge like a spiral
- UNLIKE spirals – DISK has no GAS, DUST or ONGOING star formation
- REMOVED all the gas and dust… from a Spiral galaxy… OR leave it alone long enough and it will turn into stars…
BARS in galaxies
not that fundamental to make it a distinct branch of the spirals.
MAIN difference between disk and bulge
1.) Gas content – disks contain lots of Cold Gas – BULGES do not
2.) Ages of the stars – DISKS – young stars and ongoing star formation; elipticals and bulges DON’T
- The MW galaxy would have had MORE GAS – 3 billion years ago
3.) SHAPE – related to something
4.) Motion of the stars - they travel in nearly circular obits close to the disk plane…
Hubble's system leaves out which galaxies:
most Of the galaxies in the UNIVERSE –
1.) SMALL galaxies
2.) Interacting and disturbed galaxies
3.) LOW surface brightness galaxies…
4.) MOST MASSIVE galaxies – (cD galaxyes – supergiants in the center of clusters)
Small galaxies are important why?
- Dominant by NUMBER but not mass
- PROBABLY the building blocks for bigger galaxies…
- disk systems which lack symmetry (and beauty) – do not have arms because of LOW MASS
- Dwarf galaxy companions to M31
Motions of stars
Most stars in a disk travel in nearly circular orbits close to the disk plane.
What is the problem with Hubble's classification system?
- Based on visual appearance not on objective properties (may not even be fundamental properties) –
- Not based on quantified measurements…
- Classification or sub classification of elliptical misleading because based on apparent shape not true shape
- Bars are not all or nothing…. There is a continuum of bar strengths –
- Bars are not really a fundamental property …
- It suggests elliptical evolve into spirals - but it is not clear whether that is true (Not true)
- IT IS TRUE that SPIRALS turn into elliptical ( but that is not the only way to make elliptical)
which is more important distinguishing feature of a galaxy
the amount of the stars in the disk
(or the bulge - as this is related to mass)
Ellipticals (E)
- Smooth and featureless
- some are round and some are flatter - appearance is not always the actual shape
- most are like a squashed football
- NO STAR formation
- LITTLE DUST
- COLD GAS
- They are DEAD – no raw material for star formation
SPIRALS (Sa, Sb, Sc)
- Disk and Bulge
- BULGE:significantly extended in all three directions…centrally concentrated – more so than the disk - like an E galaxy – not in every way, but in many respects
- DISK – thin, less centrally concentrated than the bulge… in spirals the DISK contain GAS and DUST and ongoing STAR formation…
- Sa – bigger bulge relative to disk
- Sb – about the same
- Sc – bigger disk relative to bulge
- Take the gas and turn it into stars - STILL forming and evolving… )
LENTICULAR (S0)
- Contains a disk and a bulge like a spiral
- UNLIKE spirals – DISK has no GAS, DUST or ONGOING star formation
- REMOVED all the gas and dust… from a Spiral galaxy… OR leave it alone long enough and it will turn into stars…
BARS in galaxies
not that fundamental to make it a distinct branch of the spirals.
MAIN difference between disk and bulge
1.) Gas content – disks contain lots of Cold Gas – BULGES do not
2.) Ages of the stars – DISKS – young stars and ongoing star formation; elipticals and bulges DON’T
- The MW galaxy would have had MORE GAS – 3 billion years ago
3.) SHAPE – related to something
4.) Motion of the stars - they travel in nearly circular obits close to the disk plane…
Hubble's system leaves out which galaxies:
most Of the galaxies in the UNIVERSE –
1.) SMALL galaxies
2.) Interacting and disturbed galaxies
3.) LOW surface brightness galaxies…
4.) MOST MASSIVE galaxies – (cD galaxyes – supergiants in the center of clusters)
Small galaxies are important why?
- Dominant by NUMBER but not mass
- PROBABLY the building blocks for bigger galaxies…
- disk systems which lack symmetry (and beauty) – do not have arms because of LOW MASS
- Dwarf galaxy companions to M31
Motions of stars in galaxies
Most stars in a disk travel in nearly circular orbits close to the disk plane.
What is the problem with Hubble's classification system?
- Based on visual appearance not on objective properties (may not even be fundamental properties) –
- Not based on quantified measurements…
- Classification or sub classification of elliptical misleading because based on apparent shape not true shape
- Bars are not all or nothing…. There is a continuum of bar strengths –
- Bars are not really a fundamental property …
- It suggests elliptical evolve into spirals - but it is not clear whether that is true (Not true)
- IT IS TRUE that SPIRALS turn into elliptical ( but that is not the only way to make elliptical)
which is more important distinguishing feature of a galaxy
the amount of the stars in the disk
(or the bulge - as this is related to mass)
Ellipticals (E)
- Smooth and featureless
- some are round and some are flatter - appearance is not always the actual shape
- most are like a squashed football
- NO STAR formation
- LITTLE DUST
- COLD GAS
- They are DEAD – no raw material for star formation
SPIRALS (Sa, Sb, Sc)
- Disk and Bulge
- BULGE:significantly extended in all three directions…centrally concentrated – more so than the disk - like an E galaxy – not in every way, but in many respects
- DISK – thin, less centrally concentrated than the bulge… in spirals the DISK contain GAS and DUST and ongoing STAR formation…
- Sa – bigger bulge relative to disk
- Sb – about the same
- Sc – bigger disk relative to bulge
- Take the gas and turn it into stars - STILL forming and evolving… )
LENTICULAR (S0)
- Contains a disk and a bulge like a spiral
- UNLIKE spirals – DISK has no GAS, DUST or ONGOING star formation
- REMOVED all the gas and dust… from a Spiral galaxy… OR leave it alone long enough and it will turn into stars…
BARS in galaxies
not that fundamental to make it a distinct branch of the spirals.
MAIN difference between disk and bulge
1.) Gas content – disks contain lots of Cold Gas – BULGES do not
2.) Ages of the stars – DISKS – young stars and ongoing star formation; elipticals and bulges DON’T
- The MW galaxy would have had MORE GAS – 3 billion years ago
3.) SHAPE – related to something
4.) Motion of the stars - they travel in nearly circular obits close to the disk plane…
Hubble's system leaves out which galaxies:
most Of the galaxies in the UNIVERSE –
1.) SMALL galaxies
2.) Interacting and disturbed galaxies
3.) LOW surface brightness galaxies…
4.) MOST MASSIVE galaxies – (cD galaxyes – supergiants in the center of clusters)
Small galaxies are important why?
- Dominant by NUMBER but not mass
- PROBABLY the building blocks for bigger galaxies…
- disk systems which lack symmetry (and beauty) – do not have arms because of LOW MASS
- Dwarf galaxy companions to M31
Motions of stars in most galaxies
Most stars in a disk travel in nearly circular orbits close to the disk plane.
Different in ellipticals
How are orbits in Elliptaical galaxies different?
in an elliptical, stars look like a swarm of bees! They orbit
around the center, but in this case the stars are orbiting in different
directions. Orbits are not confined to a plane.
Spiral arms-
spiral shaped regions of enhanced density & enhanced star
formation. The amount of extra stars in an arm is modest compared to overall
density, and stars move in and out of these arms
What causes spiral arms?
? Caused by a wave or disturbance in the disk.
? Arms are not material arms (stuff does not stay in them) but a wave feature
through which stars and gas pass
2 reasons for excess light in the spiral arms
1.) stars slow down in spiral arms, causing a traffic jam of all stars.
2.) Gas clouds are compressed and shocked on the arm. Some massive stars explode
as supernovae on the backside of the arm. Star formation is thus triggered on
the arms.
Most luminous young stars are massive O stars, which blow up
directly after formation on the arms. They explode as Type II supernovae before they move far from the arm.
What is gas?
Gas – individual atoms and molecules
WHERE does Gas Come from?
a.) Primordial – created in the big bang – existed before stars and galaxies – protostar is a cloud of gas
b.) Recycled through stars – stars are always losing mass even on the main sequence… shooting out ––
WHy is it important that we be able to DETECT ATOMIC HYDROGEN in the UNIVERSE with a SPECTRAL LINE OF H1?
- HI – most Abundant element in the Universe – 74% by mass of the matter in the Universe is in the form of H…
- Helps us detect Doppler shift
Spontaneous spin flip transition –
a.) an HI has the proton and the electron spin aligned – they spin parallel to one another..
- If they are parallel it will not stay like this forever – it will spontaneously switch to another state.
b.) They switch to ANTI-PARALLEL conditions
- Lower E state - EMITS a photon
Why is the spin flip transition important in H1 line detection?
because it is the most popular element – and photons released travel far uninterrupted…. We can measure the amount of H in the galaxy
Where is the mass in spirals?
Mgas/Mtotal = 1%
Mdust/Mtotal = 0.01%
Mstars/Mtotal = 9%
Mdarkmatter/Mtotal = 90%
So ten times more in stars as gas
KEY POINTS about Doppler Shift
- Apparent change in wavelength λ (and frequency ν) of a wave due to relative motion between the source and the observer
- Motion along the LINE OF SIGHT toward the observer (a negative velocity – v<0) causes a blueshift (λ<λ1) (wavelength less than original)
- Motion along the LINE OF SIGHT away from the observer causes a redshift (λ>λ1) (wavelength more than original)
Three things which cause the wavelength - λ - of light to shift
1.) The Doppler shift
2.) The cosmological redshift
3.) Gravitational redshift…
Doppler shift – equations
z = Δλ/λ = v/c - as v approaches c
HOW do we measure the SPEED of the galaxy ?
- YES – MEASURE the DOPPLER shift – from one side of the galaxy to the other…
Measuring the Rotational Velocities of Spiral Galaxies Using Doppler Shifts of HI 21 cm LINE
RADIO telescopes – map flux vs wavelength (assuming galaxy at rest with respect to earth) Center - λ = 21.12 cm
Moving AWAY λ= 21.13cm
MOVING TOWARD λ =21.11cm
Both sides of the galaxy are moving at velocity V(rot)
SHIFT is equal to
z=λ – λ0/λ0
IF DOPPLER SHIFT then
Velocity (v rot) = cz
Newtons 2nd Law of Motion:
F=ma
Newton’s Law of Gravity
F=GMm/r2
Velocity of Rotation
Mv(rot)/r =GMm/r2
and if only force is gravity then
SO (m) cancels and (r) cancels and we get
v (rot) = Gm/r
IMPORTANT – why is the equation sensitive to the MASS INSIDE the galaxy and NOT OUTSIDE the orbit?
symmetric mass distribution cancels
- For a spherical distribution of matter – the force of S1 negates S2
- Mass on either side of the galaxy the small amount on your side cancels out the LARGE On the other
how do we detect DARK MATTER in galaxies?
The observed rotation curves are nearly flat and have a much higher amplitude than predicted - the rotation of the galaxy is far to fast at the edge of light - so there must be more mass farther away.
Newtons 2nd Law of Motion:
F=ma
Newton’s Law of Gravity
F=GMm/r2
Velocity of Rotation
Mv(rot)/r =GMm/r2
and if only force is gravity then
SO (m) cancels and (r) cancels and we get
v (rot) = Gm/r
IMPORTANT – why is the equation sensitive to the MASS INSIDE the galaxy and NOT OUTSIDE the orbit?
symmetric mass distribution cancels
- For a spherical distribution of matter – the force of S1 negates S2
- Mass on either side of the galaxy the small amount on your side cancels out the LARGE On the other
how do we detect DARK MATTER in galaxies?
- According to Newtons and Einstein’s laws of GRAVITY – there is not enough mass in Known stars and gas to account for the HIGH rotational velocity of known stars and galaxies..
Two explanations for the high rotational velocities
1.) There is some undetected and unknown form of mass contributes most of the mass to galaxies (called Dark Matter) – more people here
2.) Maybe the LAW of Gravity needs modification (for either BIG DISTANCES or SMALL ACCELERATIONS) – less people here
REVIEW – go back to look at the rotational velocities of stars

Other Possibilities Discounted
NO measurement ERROR – cause they have done it carefully
NOT black holes – shouldn’t modify the laws of gravity – cause black holes are HUGE masses
SPEEDS of initial stuff… in Big BANG – laws of gravity only if the galaxy is in equilibrium (so maybe the galaxy is NOT in equilibrium) – Most do not think so – cause we can tell we are in equilibrium…
WHAT IS DARK MATTER – 2 possibilities
1.) Faint lumps of “normal” (baryonic) matter
- (e.g.brown dwarfs, black holes, and prunes etc – do not emit light) – MACHOS – Massive compact Halo Objects -
- NOT likely – though hard to find not impossible to find – use gravitational lensing

2.) MOST LIKELY – some exotic subatomic particle – WIMPS – weakly interacting massive particle. i.e. Neutrinos – sun emits them every second – pass through body every second but DO NOT INTERACT…
Groups of Galaxies
Group – 2-3mly
- 10 -50 galaxies –
- Fairly irregular shape
- mostly spirals and dwarfs
- NO central concentrations - center of mass yes – but no object there
Clusters of Galaxies
Clusters – 10 mly
- About 1000 galaxies
- Shape – roughly spherical
- High central concentration of mass
- In most clusters there is a Big galaxy at the center of mass – Elliptical or CD galalxies (biggest in the Universe)
- Contain = lots of elliptical and lenticular galaxies - why? The environment of the cluster helps determine what kinds of galaxies are found there
Superclusters of Galaxies
Superclusters – 100mly
- Filamentary
- Where they intersect – we find clusters and groups
- Voids – there is something there – low low density GAS

Zooming out to superclusters a 1billion ly
WHAT is the effect of environment on galaxy?
Messy galaxies – interacting and disturbed galaxies are ones that are at a very important evolutionary stage – that something is changing the galaxy
What will happen to the Sun when the MW collides with the M31
- Its ORBIT will change
What will happen to the galaxies when the MW collides with the M31
- They make a disorganized system of stars
- A starbust ( spontaneous making of stars)
- An elliptical galaxy is formed
- The galaxies merge
Why don’t all galaxies fall into one another
Because the universe is expanding
What happens to the stars when two galaxies collide?
Stars:
- Orbits get scrambled and the stars tend to make a big bulge or an elliptical
- Disks get destroyed
- Collisions of stars are extremely rare – because stars are so small compared to the space between them
- GALAXIES are nearly empty with regard to STARS - typical distance between stars as compared to the mass of stars – is so LARGE
- TOTAL gravity field form each galalxy DEFLECTS the orbits of stars int eh other – which Changes the distribution of stars and the SHAPE of the galaxies (cause orbits are different)
What happens to the gas when two galaxies collide?
Gas:
- Volume of the galaxy is all gas
- GAS DOES collide – gas is shocked and heated to high temps
- SQUEEZE THE GAS - Some compressed and triggers a large burst of star formation
- When they collide – they LOOSE ENERGY – so gas can settle to form a new disk …
- If you have stars from in that gas disk – get a stellar disk …
GRAVITATIONAL Lensing
- GRAVITY bends light – predicted by Einstein
- A way to predict Dark Matter
- Theory of gen relativity
- You can get multiple images – you will see the galaxy in multiple wrong places..
- ANYTHING with mass can be gravitational lenses - clusters of galaxies are especially good at doing this!
- Amount of bending depends on the amount of MASS – so this is another good way to estimate the mass of a cluster
- SAME ANSWER – 90% of mass has to be dark
What is a QUASAR
Point sources of light
- Light from a SMALL region - so NOT GALAXIES – because all light coming from a tiny PIECE of the galaxy – less than 1ly in size

How do we know that they are so small?
RAPID TIME VARIABILITY
- Small angle and parallax are not able to be used because the quasar is So small
How do we know that Quasars are small?
Light is VARIABLE IN TIME:
- the brightness varies –
- Time scales are a YEAR or LESS – tells us that it must be less than a lightyear in size

Explanation - A blob emitting light – a sudden burst – all parts flash on and off at the same time – on earth observing –
- Even if the blob lights up all at once – you will NOT SEE the flash continually – cause the light from the front gets to your eye before the light from the back of the blob

The Size of the blob must be less than or equal to the speed of light times the time of burst:
Clicker question: What is the Energy source of AGN?
Gravity
AGN
– supermassive black holes in the centers of galaxies… between a million and a billion suns: 106 – 109 mass of the sun
How can black hole emit lots of E?
It cannot – but stuff falling into it can emit lots of E just before it goes in…
What are the jets we see in some AGNs?
- STUFF SHOT OUT IN JETS: small fraction of the mass which is heading into the Black Hole – not at the black hole boundary yet – but in vicinity – might somehow – get accelerated to a very high speed and be ejected from the nuclear region before it reaches the black hole.
Radio GALAXY
really the Jets of an AGN seen head on
- Gas swirling around the center the DISK – is so HOT – that it emits light at that level…
The Unified Model of AGN:
DISK
- Accretion disk – produces thermal or BBody radiation
- optical emissions (tens of thousands of degrees), x-ray emission (millions of degrees)
- Disk size is light years to light days
JETS
- Non- thermal radiation
- See them at optical and more commonly at RADIO wavelengths
- Move at CLOSE to the Speed of light – v=c
- Jets connect out to lobes –
RADIO LOBES –
- Size is a million ly
- Beyond the stars in the galaxies
QUASARS
- Highest luminosities
- Bright in optical
- In interacting or merging galaxies –
Seyfert Galaxies
- Like Quasars - fainter
- But found in Spiral galaxies
RADIO GALAXIES
- Faint at optical wavelengths
- Not much accretion disk
- Mostly in Elliptical galaxies
BLAZARS –
- RADIO GALAXY where the jet is viewed end –on
APPARENT SUPERLUMINAL MOTION of AGNs
- Optical ILLUSION – happens when a jet is shot right at you close to the speed of light
What is clear evidence that an AGN contains a black hole
Fast orbital motion indicating a lot of mass in a small space
- Not relativistic jets
- Not non- stellar spectrum
What is a Black hole?
Any object whose escape speed is greater than the speed of light.
Escaping gravity is dependent on what?
Velocity (speed)
Conservation of E
E before(sum of KE and PE before) = E after ( KE and PE after)
Escape velocity equation
So v (esc) = square root of 2GM/R
Orbital velocity equation
So v (orbit) = square root of GM/R
Escape from earth
– 7miles/sec
Orbit earth
– 5 miles/sec
Why do you feel weightless in orbit/space?
Because you are free falling, but missing earth....
Orbit
A free fall that misses the object
Describe the radius of a black hole
R = 2GM/c^2
(its escape velocity basically)
Schwartzchild's radius
Any object can become a black hole - just very small one
Black hole for a human (80kg)
10 ^-25 m
Black hole for the sun
3km
Black hole for the galaxy center (10^6 solar masses
3 times 10^6 km
If the sun became a black hole – what would happen to the earth?
– The orbit will shrink but the orbit will stay the same (same gravitational attraction)
Two areas where you need Einstein and not Newton:
1.) Speeds (v) that approach (c) speed of light: SPECIAL THEORY OF RELATIVITY
2.) Strong Gravitational Fields: near a black hole or Neutron star or White Dwarf… OR for Universe on LARGE scales - COSMOLOGY: GENERAL RELATIVITY
First principle of Special Relativity
First Principle: The laws of physics are the same for all observers, as long as they are moving at constant velocity (not accelerating – if accelerating – turn to General theory)
Second theory of special relativity
Second Principle: Regardless of your speed or direction of motion, you will always measure the speed of light to be the same. (Took the MIchaelson-Moorely experiment and said okay lets take this as a principle – see how it affects nature and space and time)
Consequence – overturning of the common sense views of space and time. Cannot be thought of as separate entities, unrelated to one another – they are intrinsically linked.
Consequence – space and length contraction: moving sticks are shorter); a stick has a length: L when at rest:
But
When moving at a velocity (V) – it is shorter …
Length contraction:
moving things are shorter
Time Dilation
Moving clocks tick slower
You will appear to age more slowly to your friend if you are moving near c
Your friend will appear to age more slowly if your friend is moving near c
Relativistic Mass increase
As velocity approaches c
Then the Mass Approaches INFINITY
AN INFINITE force would be required to get a mass moving at the speed of light .
thus
Particles of Matter cannot GO The SPEED OF LIGHT
What is more important - space/time or EVENTS for observers?
DISAGREE
- On size of train and tunnel ( space )
- When the gates close( and whether they close simultaneously – ( time)

AGREE
- EVENTS and their OUTCOMES
- Events: something that occurs in space at a point in time
- Front gate misses front of the train
- Back gate misses the back of the train
Special relativity is only relevant
for things moving at CONSTANT VELOCITY (no acceleration or deceleration)
TWIN PARADOX ( time)
One takes a trip at relativistic speeds and comes back
The other stays
The twin who takes the trip at relativistic speeds returns younger. WHY??
ACCELERATION - she accelerated at the end and at the beginning…
EQUIVALENCE PRINCIPLE (General Relativity)
No way to locally distinguish between gravity and acceleration
Relativistic Mass increase
As velocity approaches c
Then the Mass Approaches INFINITY
AN INFINITE force would be required to get a mass moving at the speed of light .
thus
Particles of Matter cannot GO The SPEED OF LIGHT
What is more important - space/time or EVENTS for observers?
DISAGREE
- On size of train and tunnel ( space )
- When the gates close( and whether they close simultaneously – ( time)

AGREE
- EVENTS and their OUTCOMES
- Events: something that occurs in space at a point in time
- Front gate misses front of the train
- Back gate misses the back of the train
Special relativity is only relevant
for things moving at CONSTANT VELOCITY (no acceleration or deceleration)
TWIN PARADOX ( time)
One takes a trip at relativistic speeds and comes back
The other stays
The twin who takes the trip at relativistic speeds returns younger. WHY??
ACCELERATION - she accelerated at the end and at the beginning…
EQUIVALENCE PRINCIPLE (General Relativity)
No way to locally distinguish between gravity and acceleration
APPLE in the elevator
ON EARTH: in a gravitational field –
IN Space – Accelerating up (relative to stars) -
APPLE MOVES THE SAME…
Basic Ideas of General RELATIVITY
- Mass of the object alters properties of Space and time around it
- Gravity causes space and time to slow down
- DOES NOT TALK ABOUT A FORCE Of GRAVITY – eliminates this idea
- Gravity is CURVED space-time….
TESTS of General Relativity
1.) Bending of light
- Principle of Equivalence implies that gravity can bend light since acceleration can
2.) The Deflection of Starlight by a mass
- Gravitational lensing – galaxy lenses – a quasar 4 times….
3.) The Orbit of Mercury … the Advance of the Perihelion of the elipse
- The orientation of the eclipse – proceed over time … and Newton predicts that procession should happen
- Einstein too – But at different rate
- Einstein predicted faster ( or slower) – Einstein was right and Newton wrong
4.) Gravitional Time delay – Time passes more slowly in a gravitational field
- Two clocks and measure the passage of time at diff heights in the gravitational field…
5.) Viking space craft – radio signals to earth
- Time delay around the sun – where space is curved and gravitation is strong…
Relativistic Mass increase
As velocity approaches c
Then the Mass Approaches INFINITY
AN INFINITE force would be required to get a mass moving at the speed of light .
thus
Particles of Matter cannot GO The SPEED OF LIGHT
What is more important - space/time or EVENTS for observers?
DISAGREE
- On size of train and tunnel ( space )
- When the gates close( and whether they close simultaneously – ( time)

AGREE
- EVENTS and their OUTCOMES
- Events: something that occurs in space at a point in time
- Front gate misses front of the train
- Back gate misses the back of the train
Special relativity is only relevant
for things moving at CONSTANT VELOCITY (no acceleration or deceleration)
TWIN PARADOX ( time)
One takes a trip at relativistic speeds and comes back
The other stays
The twin who takes the trip at relativistic speeds returns younger. WHY??
ACCELERATION - she accelerated at the end and at the beginning…
EQUIVALENCE PRINCIPLE (General Relativity)
No way to distinguish between gravity and acceleration on large scales
APPLE in the elevator
ON EARTH: in a gravitational field –
IN Space – Accelerating up (relative to stars) -
APPLE MOVES THE SAME…
LOCALLY (in a box) you think you can distinguish between gravity and acceleration
Two parts of a black hole
Two Parts:
1.) The Singularity
- All the mass of the Black Hole is crushed to very small volume perhaps infinite density at the singularity
2.) The Event Horizon
- Horizon beyond which nothing can escape: No matter, no light
- We have no way of learning ANYTHING about what goes on inside the event horizon - no light escapes
- No events, physical processes
Three properties of BHoles
Three Properties
a.) Mass
b.) Electric Charge – actually neutral… but theoretically charged – so if there is a big thing with a big charge, it will attract the opposite and neutralize
c.) Spin or Angular Momentum
Clicker Question: How do you avoid going in to a black hole?
Go into orbit
What determines the trajectory or orbit?
a.) How the sheet or space is curved by masses
b.) The Speed of the thing
DOESN”T depend on the masses of the things moving around OR orbiting…
CURVATURE of SPACE is equal to the STRENGTH of Gravity
- M=3 solar masses – of normal star, then neutron star (more dense) and BH (densest)….
- The strength of gravity is the same out at the top…. Only different very close to the mass
- Effects of Relativity are only important where the gravity walls are steep…. – or near VERY DENSE Objects…
TIDAL Forces
- Differential gravitational force
- Difference in gravitational force on different parts of the object.
- Force of gravity is different in different ways on parts of your body
- BECAUSE – different “R” for different parts of body
F=GMm/R2
As you move toward the BLACK HOLE
- There is a stronger Force of Gravity at the FEET than at the HEAD – STRETCHING
- Force of Gravity at Sides in different directions – SQUEEZING
Strength of the tidal forces just outside the Event Horizon –
depends on the MASS of the Black hole (size)
- FOR A solar mass BH – tidal force is Strong
- FOR a SUPER MASSIVE BH the tidal forces are WEAK
Time Dilation near a black hole
infinite - - Time between successive ticks is longer and longer
Stellar Mass Black holes – 3-30 M solar
- Form from the collapsed cores of massive stars
- There could be many million of these –most massive stars are rare…
- WE think there are millions of stellar mass BH –
- WE HAVE detected Ten of them
GALACTIC or Supermassive Black Holes M= 10^6-10^9 solar masses
- Form from mergers of stellar mass BHs
- The densities of stars and other stuff is deep and
- Collect more over time… one way that galaxies GROW…
Medium Mass Black Holes
- Probably exist … there must be stage though not detected
Mini-holes = M = less than one solar mass
- No good theories
- But also possible
COSMOLOGY definition
The study of the structure and evolution of the Universe as a whole
The EXPANSION of the UNIVERSE - how did we discover it?
Hubble measured increasing redshifts for galaxies that were farther away - plotted on a line makes the equation
v= H0d
v = recessional velocity
H 0= Hubble “constant” which is the slope of the line = 71 km per second per Mpc
d= distance
v= H0d
v = recessional velocity
H 0= Hubble “constant” which is the slope of the line = 71 km per second per Mpc
d= distance
explains the expanding universe
NOTE: two distances - simplify to 1 over seconds
– the AGE of the universe
This kind of relationship tells us the universe is expanding and the inverse of this tells us HOW LONG is has been expanding
Hubble constant related to the AGE of the UNIVERSE
SIGNIFICANCE:
1.) Linear relationship evidence for expansion of the universe
2.) Use it to get distances to galaxies: d=v/ H0
(BUt must know d or v first)
Clicker Question: For which galaxies would the linear Hubble relation be expected to hold?
ONLY for galaxies at intermediate distances…
Cosmological redshift
z(cos) =Δλ/λ;
- then interpret as a recession speed using the Doppler formula, even though it is not a Doppler shift:
- v (rec)=cz(cos)=c Δλ/λ : (FOR LOW z; z <0.1)


FOR HIGH z, z >0.1
z = square root of (c+v/c-v) -1
BUT we do not really need to know this ....
What is the difference between the Doppler shift and the cosmological redshift?
Doppler shift – it is caused by the objects motion THROUGH space
Cosmological redshift - it is caused by the EXPANSION of SPACE
What accounts for the large redshifts observed in the universe?
Photons are “stretched out” to longer wavelengths as space in universes expands
Proof of expansion
- The further away an object is the faster it recedes from us – AND the same in every direction
Cosmological redshift equations
Observed wavelength/Emitted Wavelength = λ2/ λ1 = 1 + z (cos) = d( t1)/d (t2)

z (cos) = (Observed wavelength/Emitted Wavelength) - 1
Expansion velocity between the objects
change in distance/Change in time
Clicker Question: the Expansion of the Universe takes place:
Only on scales of super clusters and voids and larger – galaxies Do NOT expand
What is it that keeps localized regions of space such as planetary systems et al from participation in expansion …
Their mutual self-gravity ….
what is the universe expanding into?
- It is not expanding into anything – expanding means the distances in BETWEEN the – galaxies – are increasing with time…
LINEAR HUBBLE LAW And the relation between the distance and the recession speed DOES not work well for
the NEAREST and MOST distant galaxies…
Why doesn't the linear Hublle Law work for Galaxies closer than about 20 Mpc – i.e. Andromeda moves towards us
- Gravitational Attraction is strong enough to overcome the Hubble law –
- Galaxies can have motion THROUGH space (Doppler shift) in addition to recession due to expansion of space ( cosmological redshift)
- Motions through space can be 100-2000 km/sec
- Cluster as a whole expanding away from us
- So the Doppler shift (motion through space) dominate measurements.
Why doesn't the Linear Hubble law work for Galaxies farther than zrec > 0.1; vrec>30,000 km/sec; d> 400 Mpc
- Relativistic effects make the relationship metween observed redshift and distance more complicated
- The Hubble constant is not CONSTANT… Not the same for galaxies with LARGE distances – why? The expansion of the Universe changes over time…
- Looking back in TIME – since expansion rate VARIES with TIME - so
- H0 – is the PRESENT value of the expansion rate of the universe
Einstein's COSMOLOGICAL PRINCIPLE:
EINSTEIN assumed that the UNIVERSE was Homogenous ( the same in all places) and ISOTOPIC (same in all directions) on LARGEST scales (>10^8 ly )
- He assumed it and we have now PROVEN it
COSMOLOGICAL Constant: Λ
Einstein;s mistaken fudge factor - the invented force that acts opposite gravity to prevent the universe from contracting in on itself IF IT IS STATIC
Recent Evidence that not only is the universe expanding but SPEEDING up over time:
- From high redshift supernovae and CMB
- ONE WOULD expect the expansion of the Universe to be SLOWING DOWN if there were only GRAVITY acting… evidence for DARK ENERGY
DARK ENERGY
DARK ENERGY: There is a mysterious form of pressure and energy which causes the universe's expansion to accelerate
Clicker question: what determines the fate of the universe?
.
The average density of ( all) matter and dark energy…
- Is there enough mass and gravity in the universe to slow expansion to a halt and cause the universe to RECOLLAPSE?
No:
Calculate total energy – in normal way with E=mc2
- If E < 0 (no escape, bound system and will collapse)
- If E> 0 (escape, expand forever)
Sum of E = KE +PE = 1/2mv2 – GMm/r
Where:
- M=density times the volume (ρv) which is rho = 4/3 ρ (matter) π^3
- v= H0 r (Hubble law)
- SUBSTITUTE
GET:
E= ½ m (Ho r)^2 – Gm (4ρcπr^3/3)/r
CRITICAL DENSITY
Critical density = ρ (crit)
if ρ (mass)= ρ (crit), THEN E=0
The density corresponding to this critical case ( E=0) is defined to be the Critical density of the universe
ρ (crit) = 3 Ho^2/8πG
cosmological mass density parameter
Ώ = ρ (mass)/ρ (crit) = 8πG/3 Ho^2
If Ώ is >1 – then the universe is bound
If Ώ is <1 - then the universe is unbound and will expand forever
Ho
71 km/s/Mpc
AGE OF THE UNIVERSE:
- Imagine 2 galaxies distance d apart. There are expanding away from each other at v
• Slope of line v = H_o D
o Assuming that they always have the same expansion speed (unlike in graph) then the age is d/v = d/H_o d = 1/ H_o
 So inverse of the hubble constant gives an estimate for the age of the universe! For H_o = 71 km/s/Mpc
• T = 14 Billion year (no gravity, no dark energy)
o So this is good, but not a great estimate
The real universe, with gravity and dark energy (actual case)
 First, gravity dominated, then DE kicked in and it bagan speeding up
 T could be more or less that 1/Ho, depends on the relative strengths of gravity and dark energy
 Our best estimate = 13.7 +/- .5 billions years
• This agrees with the ages of star
FOUR different pieces of evidence that the universe has evolved over time:
1.) Expansion of the Universe over time…
2.) The Night sky is Dark
3.) Most quasars (brightest AGNs) and most star formation at redshifts (z) between 2 -4; z corresponds to a time and a distance

4.) Cosmic Background radiation …
Residual glow from the bigbang… a relic of a hotter, denser universe (learned a tremendous amount from this)
Clicker Questions: How do we know the distance of objects with a z>1 (most DISTANT Objects)?
No standard candles so -
- Measure Cosmological redshift (z) plus H_o, plus Omega (m) and Omega (Λ)
The redshift is the only thing you really can measure….
What we observe: z (cos) : what can we know from this?
Distance was (z+1) smaller
Area was (z+1)^2 smaller
Density was (z+1) ^3 higher
The Cosmological parameters
1.) Everything about the expansion: current expansion rate (Ho) and how it has changed over time (gravity makes the expansion slow and dark E pushes it faster)
2.) So amount of MASS in the universe acting to decelerate the expansion - Ώm
3.) The Amount on Dark Energy - ΏΛ - (amount of Dark energy in order to close the universe)
when we look into space - what are we doing?
we look back into time
- High redshifts - we get messy galaxies ( quasars, blazars et al); cause we are looking back into time
So the rate of quasar observation PEAKS
at redshifts (z) of 2 and 3 ….times of 1.5 -3 billion years after the BIG BANG… when black holes were being FED AT THE MAX RATE….
Peak of star formation happened
at the same time as the peak in quasar activity…
Z = 2-4
Time = 1.5 -4 billion years
Olber’s Paradox
Night sky is DARK - if it is a Newtonian ( no expansion or relativity) – Infinite in space and infinitely old; AND uniformly filled with stars…The night sky would be BRIGHT
Why is the night sky dark?
Light from objects farther away than a certain distance has not yet reached us – the UNIVERSE is NOT infinite in time – it had some sort of beginning….
Resolution for OLBER’s PARADOX
- The finite age of the UNIVERSE
- For all objects within our cosmic light horizon there has been enough time for the light from them to reach us…
SUPPOSE there are objects that are outside our light horizon; but there has not been enough time for light to reach us from them – but we would not know it…
The COSMIC LIGHT HORIZON
is the extent of the observable universe… that observable universe…
- The distance to the cosmic light horizon gets bigger… the more time there is the more we SEE
- Now the horizon is at 13.7 billion light years…
How can galaxies exist beyond the CLH if it is 13.7 billion years ago and the universe is only that old….
- Because SPACE can expand faster than the speed of light…
- THAT Is how there can be stuff beyond our observable horizon – even though it is only 14 Byrs old
- SPECIAL relativity – NOTHING CAN MOVE THROUGH Space faster than c – but space can exand – but GEN relativity says – distant regions of space can expand away from each other at rates greater than the speed of light….
COSMIC MICROWAVE Background
- Compelling evidence for the universe was once much
- HOTTER
- DENSER
- MORE UNIFORM
compelling evidence for the BB
Discovered in the 1960s – by Penzias and Wilson with their telescope…
TWO KEY Properties of the CMB:
1.) Spectrum of a nearly perfect BB (the most nearly perfect blackbody of ALL TIME – Stars are like BB – but they have bumps and wiggles
- Never seen that before
- Temperature of 2.725 degrees K – perfect measurement, cause the BB so perfect.
- Spectrum peaks in the microwave part of the radio spectrum λ = 1.1 millim.
- Most Perfect BB ever measured…

2.) NEAR Isotropy of the Cosmic Microwave Background
- Intensity of the photons nearly the same in all directions
- So photons come from the universe as a whole and not from discrete bodies .
When did the Universe cool down to about 3 K –
- Just recently cooled down RECENTLY
- Only with the EXPANSION OF THE UNIVERSE
Before Recombination
- the universe is opaque to photons
- photons have enough energy to ionize H - and basically no atoms can form because they are immediately blasted apart by cosmic photons
- as the universe cools - the E of the Photons decreases
and SUDDENLY - the universe is transparent to photons
After Recombination
- the photons of the current CMB have cooled enough to not be able to interact with H atoms
- they stream freely through the universe (matter and Photons DECOUPLED), cool as they expand and are the CMB we see today
When did cosmic particles have enough energy to ionize H?
- But THEN – at z= 1000 – the peak of the spectrum was not E enough to ionize H; but the TAIL is…
- T= 3000 K
What redshift does recombination correspond to?
WEIN’s LAW
- λ = 0.29 /T K = 1.1 mm then T = 2.7 K
- so λmax = to be Ionizing hydrogen
- then the T mist be 1100 larger
- So the wavelength increased by the expansion of the universe by a factor of 1000
- So the T must also
So when λ is right then T = 3000K then the z = 1100
WHAT TIME does recombination correspond to?
- t = 380,000 yrs ABB
we figure out what z is; and then get (t) of recombination from z plus the cosmological parameters ( Ho, Ώ m, ΏΛ)
THINGS THE CMB tells us:
- Universe was much hotter and denser and more uniform billions of years ago – (likely started in BB)
- How “structure” (stars, planets, galaxies and clusters ) began to form (cosmic photons tell us this)
- We are not in any special Location in the universe
- Evidence for Dark Energy
- How space is CURVED
Where did the Big Bang Happen?
everywhere all at once
THOUGH the CMB is Isoptropic
There are two pretty important ANISOTROPIES
1.) Dipole Anisotropy
2.) Small Scale Fluctuations – when dipole signal and MW artifacts are subtracted from the CMB map
Dipole Anisotropy of the CMB
- The CMB spectrum is slightly blueshifted in the direction of LEO - and
- REDshifted in the direction of Aquarius …
- Three spectrum – the Average; the Red and the Blue shifted spectrum
- WHY – because the Earth’s motion relative to the CMB…. THIS IS A DOPPLER Shift…
Why are we moving with respect to space itself - or CMB?
- The pull of local sources of gravity….
How do we find our relative motion with respect to the CMB
Addition of Several different motions
- Earth around the Sun – 30 km/ Sec
- The Sun around the Center of the MW – 220 km/sec
- MW pulled toward the Virgo Cluster – 350 km/sec
- MW and VC – toward the “Great Attractor” 620 km/sec
ADD – 370 km/sec – (not all in the same direction) so TOWARDs LEO – which is in between
WHAT is the Great Attractor
is a large concentration of super clusters and GALAXIES in this region – in the PLANE of the MW – so hard to see it –
- LOOKED really hard and found evidence of superclusters in that region of the sky…
Small Scale Fluctuations in the CMB spectrum – when dipole signal and MW artifacts are subtracted from the CMB map
- Remove 99. 999% of the signal - Really a fluctuation of Temperature
- Correspond to T differences in the early universe
Three interesting things about the small scale fluctuations in the CMB:
1.) These are the seeds of structure in the Universe
- The origin of super clusters clusters, galaxies stars etc. – the regions that were really dense at this time ended up forming SUPERCLUSTERS etc.
2.) Evidence about the contents of the Universe –
- How much matter – how much DARK E
- Contain evidence of Dark E –
3.) Evidence of the Curvature of space
Why is matter not the same as it was in the early universe…?
Recombination – first time that matter left alone by photons – allowed matter to collapse under its own GRAVITY
Why was there no gravitational collapse in the era before recombination?
because of conservation of energy - E must be released ( usually in the form of photons) in order for matter to collapse on itself. In the early universe - these photons could not "escape" matter and so there could be no collapse until after recombination
We measure the Temperature (today and during decoupling (equations) how do we know that matter’s density changed?
WE know that the universe was dense – cause it was UNIFORM in T. - At the beginning matter and energy were TIGHTLY coupled…
- BUT after – when not OPAQUE anymore
-
Why do we think that there is Dark energy?
1.) The expansion of the universe is accelerating – measure on Supernovae
2.) Space is curved - how it is curved and how much stuff is in it
How do we know that the large scale dipole pattern is due to a Doppler shift and the small-scale fluctuations are temp differences?
- The Spectra of the spatial patterns on the sky
Why are we moving with respect to space itself - or CMB?
- The pull of local sources of gravity….
How do we find our relative motion with respect to the CMB
Addition of Several different motions
- Earth around the Sun – 30 km/ Sec
- The Sun around the Center of the MW – 220 km/sec
- MW pulled toward the Virgo Cluster – 350 km/sec
- MW and VC – toward the “Great Attractor” 620 km/sec
ADD – 370 km/sec – (not all in the same direction) so TOWARDs LEO – which is in between
WHAT is the Great Attractor
is a large concentration of super clusters and GALAXIES in this region – in the PLANE of the MW – so hard to see it –
- LOOKED really hard and found evidence of superclusters in that region of the sky…
Small Scale Fluctuations in the CMB spectrum – when dipole signal and MW artifacts are subtracted from the CMB map
- Remove 99. 999% of the signal - Really a fluctuation of Temperature
- Correspond to T differences in the early universe
Three interesting things about the small scale fluctuations in the CMB:
1.) These are the seeds of structure in the Universe
- The origin of super clusters clusters, galaxies stars etc. – the regions that were really dense at this time ended up forming SUPERCLUSTERS etc.
2.) Evidence about the contents of the Universe –
- How much matter – how much DARK E
- Contain evidence of Dark E –
3.) Evidence of the Curvature of space
Why is matter not the same as it was in the early universe…?
Recombination – first time that matter left alone by photons – allowed matter to collapse under its own GRAVITY
Why was there no gravitational collapse in the era before recombination?
because of conservation of energy - E must be released ( usually in the form of photons) in order for matter to collapse on itself. In the early universe - these photons could not "escape" matter and so there could be no collapse until after recombination
We measure the Temperature (today and during decoupling (equations) how do we know that matter’s density changed?
WE know that the universe was dense – cause it was UNIFORM in T. - At the beginning matter and energy were TIGHTLY coupled…
- BUT after – when not OPAQUE anymore
-
Why do we think that there is Dark energy?
1.) The expansion of the universe is accelerating – measure on Supernovae
2.) Space is curved - how it is curved and how much stuff is in it
How do we know that the large scale dipole pattern is due to a Doppler shift and the small-scale fluctuations are temp differences?
- The Spectra of the spatial patterns on the sky
Here is how it is differ:
1.) The dipole pattern the SAME perfect BB curve – but wavelengths are shifted – customary of Doppler shift – CURVES cross
2.) The smaller scale fluctuation – two separate curves peaking at different T – do not CROSS and still a black body spectrum
Curvature of Space – his definition
- IF space were positively curved ( spherical geometry); AND if you could travel infinitely fast; you could travel “straight” in any direction and end up where you started from
- BUT!!!! Since the fastest possible speed through space is the speed of light and the universe is expanding, this is not quite possible.
- Photons don’t have enough time to circumnavigate the universe because it is expanding (in some places away from you faster than the speed of light)
- What prevents this from happening? THE EXPANSION OF THE UNIVERSE
what is the furthest distance that a signal could have traveleld in the universe at the time when the CMB photons were produced?
Could have only travelled 380,000LY – cause that is the age of the universe at that time –
- The biggest Blob has to be no bigger than 380,000LY – the size of the largest cosmic affected regions
How do we know that space is curved?
So we know how big we think the blobs in the CMB really are… so what is the angular size
1.) If space is flat then the blobs should appear 1 degree
2.) If space is Hyperbolic – then blobs should be smaller
3.) If Positively curved then BIGGER…

What do we see?
Biggger blobs are one degree in size – nearly flat….
What is Dark Energy:
-
The mass energy needed to bridge the gap between the mass we know to exist and what is needed to make the universe flat…
What determines the curvature of space?
Contents of universe determine its curvature: total amount of matter , radiation, dark energy
The CMB fluctuations indicate that Ω=1 is NEARLY FLAT…. So if we really measured that the universe is
We think that
- Ωm = 0.27
- Ωrad = 0.00005

AND we know that
- Ω0 = 1 = Ωm + Ωrad+ ΩΔ
- So ΩΔ must equal 0.73
Other Evidence for Dark E
- increase in redshift of distant supernovae
-
Composition of the Universe;
Dark Energy: 73 percent
Dark Matter: 23 percent
Baryonic Matter: 4 percent
Stars can make lithium in nuclear reactions but most stars actually destroy lithium… WHY?
- It gets fused into other elements…
At t = 1-5 minutes after the BB
- T = 109 degrees – hot enough for nuclear reactions to occur – fusion
- If it ever was this hot then we expect nuclear reactions to have occurred
- formation of the light elements Deuterium (1), Helium (2), Lithium (3), Beryllium (4), Boron (5)
- there are too much of the light elements to be explained by making them in stars…
So what is different about stars and the early universe? How are conditions in the early universe different that the conditions in the core of a star?
- Same temperatures and densities – but existing for DIFFERENT lengths of times
- T=1-5 minutes vs. Stars existing for billions of years…
- The difference is the amount of time w/ the conditions suitable for fusion
How do we learn the Density of the universe a few minutes after the BB …
What we measure is the relative number of light elements which ; tells us the Density of baryonic normal matter at the time when the universe was hot enough to fuse;
So we know that the EARLY universe has 4 percent of the critical density in the form of atoms
Ratios of light elements means that the universe was 10^27 denser and 10^9 hotter
INFLATION:
a brief period of superfast expansion hypothesized to occur a fraction of a second after the BB
- The universe expanded by a factor of 10^ 50 in the time of 10 ^-32 seconds
The Horizon Problem:
a problem with the CMB – HOW CAN YOU HAVE very distant regions of space have very nearly identical temperatures?
- THEY ARE NOT WITHIN causal contact with each other…
- The largest causally connected regions are 380,000 LY – they are the largest regions that could possibly be in contact with each other…
Why do opposite sides of the universes have the same temperature even though they seem too far apart for light and heat to have traveled from one side to the other in the age of the universe?
- They used to be close together but then a superfast expansion carried them far apart.
- SO ONCE – everything that was in causal contact
The Flatness Problem
WHY DOES the universe now appear so flat? when densities suggest otherwise.....
- Can be understood if the universe expanded ENORMOUSLY at early times so much that the observable universe appears flat.
- the region of the supervast universe that fits within our CLH appears flat cause the whole is so big
EVIDENCE for the Big Bang
1.) Expansion of the universe
2.) CMB – evidence for the big Bang – that it was once a billion times denser
3.) Abundance of the light elements – the universe was once hot enough to undergo fusion everywhere – distinct stages in the evolution of the universe
TWO SCHOOLS OF THOUGHT on the BB
1.) Big bang theory describes HOW our universe is evolving, not how it began…. (Jim Peebles)
- Making the point that – the BB can refer to the evolution of the universe – but what happened after the beginning…
- This is what we have evidence of
2.) The standard cosmological model for the beginning of four things: SPACE, TIME, ENERGY and MATTER
- Not matter exploding into preexistent space
- But an explosion of space and time with energy and matter - all originating at the same process