Chapter20

Question 1

Why do we use different units of measurement for cosmology?

Answer 1

Bevause the distances are so far using it in standard form is pointless, and hard to compare

Question 2

So what are the three distances?

Answer 2

1) the AU
- average distance from earth tk the sun

2) light hear
- distance covered by speed of light in one earth year

3) the PARSEC

Question 3

Before defining the parsec, we must define the angles arcsecond and arc minutes!

Answer 3

In 1° there is 60 arc minutes
In 1 arc minute there is 60 arc seconds
Thus in 1\3600 degrees = 1 arc second

We use these as the angles measured are sooo small

Question 4

So whats the definition of a parsec?

Answer 4

A parsec is the distance at which an angle of 1 arcsecond subtends a distance ( radius) 1 AU

This forms a triangle , with oppsite being AU, adjacent being parsec and angle being 1 arc second

Question 5

As arcsecond is so small, what expression can we make

Therefore what can we say for the angles needed to subtend 1 AU at DIFFERENT parsecs?

Answer 5

Use small angle approximation

Therefore tan 1m arcsecond = roughly 1 AU/ 1 Parsec

So if we double distance away (from sun), and stil, want to subtended 1 AU, the only way this works is if the angle becomes 1/2 too (1/2 = 1/2)

Therefore theta is inversely proprtinsl to distance in parsecs! As an approximation

Question 6

So what’s the formula relating theta and parsec distance ti subtend a radius of 1 au , and how

Answer 6

Theta inversely prop to parsec distance

Due to small angle approx. Derived from definition of parsec …

Question 7

What is the stellar parallax technique used ti find?

What is stellar parallax

Answer 7

The distances of relatively close stars to earth , less than 100 pc

It is the APPARENT shift in positon kf a star against a FAR AWAY BACKDROP when observed fro, different places in the earths orbit

Question 8

Explain how the stellar parallax works

Answer 8

So when we are in postion A, looking at a star we see it in a specific place relative to the backdrop

Come back to this on its OPPSITE point of orbit, we saw the star shifted to a new place relative to its back drop

In reality the star hadn’t moved at all, it’s just an illusion due ti the ANGLE WE SEE IT AT

This angle is the PARALLAX ANGLE

As the distance is always 1 AU from the sun from both OPPSITE points of orbit, if thr angle is in arc seconds then we know the distance to the sun measured in par secs is = 1 / angle !

So if we know thr paralled angle, we can roughly work out how far away

Question 9

Why is the stellar parallax only used for stars less than 100pc away? Why not any more

Answer 9

Because the father awya you go, the less the stellar parallax is , and thus the harder it is to calculate the parallax angle

In fact it becomes impossible with current level of resolution of equipmemt on earth so far , thus we can’t

Question 10

Stellar parallax summary

Answer 10

Used to find the distance of a nearby star to thr sun using the illusion of the apparent shift in postion of the star relative to a fixed background drop

Take two points of oppsite orbit on the earth and see the parallax angle. As the distance being subtended by this angle is 1 AU, we can use d = 1/p to find how far it is

But must be in parsecs and arcsecond, don’t lack

Question 11

Parallax angle definition

Answer 11

This is the angle which subtends a raidus 1 au ar a certain distance away in parsecs from the sun

Question 12

What is the Doppler effect

Answer 12

The apparent shifts in frequency / wavelength due to the relative motion between the object and the observer

Where the change in freuwney is prop to the velocity of thr object

Question 13

Explain Doppler shift

Answer 13

As something moves further away, due to the wavelengths being more spread out from each other, the apparent wavkefnth observed at the observer is much bigger than what it was before, thus frewuency less too

As something moves closer to you, the wbakengghd catch up with each other, making the apparent wbskenght less and thus the freuwneyc increase

This effect is observed for any wave, so light as well as sound

Question 14

In the context of planets?

Answer 14

If a planet is moving awya from us, the frequencies will be less, as a result all th spectral lines for absorption will be shifted to the red side = red shift

If comes to us then blue shift

As most of them are red shift, we can conclude in general u inverse is expanding

Question 15

If you found the shifted frequency of one, how to find the shifted wl bery esislyl

Answer 15

Use speed of light

Question 16

When doing Doppler shift for sound, or ultrasound or whatever what is the equation changed

Answer 16

Need to use speed of thing, so sound, ultrasound etc

That’d why when I run fast as I can I don’t become red, I do but very slightly because of speed of light fsctor

But when sound moved away we do hear it become low frequency because the speed of the object is comparable to the speed of sound .

Question 17

IMPORTSNT POINT

Why within a spectral line, will the indicual lines not be the EXACT same shift?
What is the same ?

Answer 17

As the equation shows the change in wavelength is prop to wavlength and speed

If speed is the same fro each wavelength, then the change is only proprtinsl to its CURRENT WAVELENGTH

Therefore the absolute shifts are different but percentage is the same, with ratio v/c constsnt!
Thus if something red shifts, the red ones will red shift THE MOST and the blue ones thr lesdt

But in this case the PERCENAGGE SHIFT is the same for each one, as it must be

This is why you can’t straight compare the distances first, need to reverse the shift and then compare

Question 18

If further away stars have a more red shift, what does this mean fully

Answer 18

This means Fully that rhe stars are travelling faster (not to do with further away) as change in wavelength is proptinal to speed

As a result, if the further away stars are travelling faster than closer ones

• this means universe expanding at an ACCELERATING RATE

Question 19

What were the conclusions bubble made

Answer 19

1) that light coming from most GALAXIES (nit stars because too close ) confirmed were red shifted
2) that the further away thr galaxy was, the MORE red shifted (and thus faster speed ) it was travelling at

Question 20

What was thus HUBBLES law

Answer 20

After plotting receding speeds of many galaxies against distance away from earth , the trend seemed to show linear

Therefore v is prop to d

So the receding speed of a galaxy was roughly propritnsk to the distance away from earth

Question 21

What is HUBBLES constsnt , why is the value not well defined

Answer 21

Plotting on groan showed the trend was DEFINITELY LINEAR, however the gradient was hard as the uncertainty quite big

The HUBBLES constsnt is just the gradient

Question 22

What units do some astronomers like

Answer 22

They like km per MPC so might have ti convert, to give you s-1 especially when trying to find the age of thr universe

Question 23

What does the fact further away galaxies are more red shifted mean

Answer 23

That they are travlling at a higher speed away, thus the universe is expanding at an ACCELERATIMG RATE,

Here it’s the space between universe that expands

Question 24

What is the cosmological principle

Basically means the same everywhere

Answer 24

• the laws of physics are valid in all parts of the universe , any model on earth will work with sake physics over all of space time
• isotropic ( means that there is no edge or centre to the universe , and looks the same in all directions by any observer )
• homogenous (means that the density of the universe is uniform, matter is distributed evenly across universe and we see the same structures everywhere )

Again
- isotopic means to every observer in all directions the universe looks the same, no centre or edge
- homogenous means the density of the universe is uniform, mater is distributed even,y across u inverse and same structured found everywhere
- laws of physics work everywhere

Question 25

Why having these assumptions can mean we are very off some prediction?

Answer 25

Can never know for a fact if the lhysics are the same in every part of the universe, so whatever science says might not be the

Question 26

Why does bubbles law breakdown for galaxies close and very very far from us?

Answer 26

1) galaxies close to us could be attracted to us by gravitation pull so these actually show a blue shift, and hence don’t follow HUBBLES law. Only very immediate gs,sides are like this but k really we are expanding 2) galaxies very far have accelerated too much so have escaped the proprtinsl trend, it’s possible the trend changes too

Question 27

If Doppler shift spectral line and this shows a blue and red shift happened at the same time what does this show

Answer 27

It’d a BINARY STAR SYSTREM One star is coming closer to us and thus blue shift, other star will be moving away so red shift So light will show both at same time this can only be explained by binary star Remember binary star system the periods of roatiaon no matter mass is the same, and the centripetal force aimed at centre of mass but bravtwiknsk at centre of each planet

Question 28

Big Bang What are the two pieces of evidence that is used to prove the Big Bang?

Answer 28

1) HUBBLES law 2) CMBR with the TEMPERTAURE of the universe assuming it’s a black body emitter

Question 29

How does HUBBLES law suggest Big Bang to be true

Answer 29

HUBBLES law suggests universe is expanding from each other at an accelrwtion way, such that if we turned back time thr univerde would be concentrated into one point, which is what the big bangs suggests

Question 30

What is the big bangs theory

Answer 30

It’d thst the universe was a small dense and hot point which expanded and over time became what we are today

Question 31

Why couldn’t HUBBLES law be conclusive evidence for Big Bang alone?

Answer 31

Other theories could explain HUBBLES law too, meaning Big Bang wasn’t necessarily true So another piece of evidence was needed

Question 32

Why did Big Bang predict CMBR ?

Answer 32

Big Bang stated that when universe was young unjverde was saturated with high energy gamma photons . Expansion of space meant these gamma photons over time was stretched in wbakenght to become microwaves, which we find today - this means they would’ve been stretched through visible light too and now at microwaves - the stretch itself is both the stretch and a sort of red shift

Question 33

However what else backs that CMBR makes sense

Answer 33

If model the universe as a black body emitter, we can measure the background TEMPERTAURE and correspond thst to a peak wavelength using wiens constsnt This gives you order microwave radiation , which checks out with CMBR

Question 34

How does CMBR thus prove big biang

Answer 34

As only Big Bang could prove CMBR , which is backed up by TEMPERTAURE, and that as the universe cooled down the wbakenggh would’ve increased, this means Big Bang was true The only theory that could prove both HUBBLES law and CMBR

Question 35

How to estimate the age of thr universe What assumption

Answer 35

If we assume that the expansion of the universe happened at a CONSTSNT SPEED ( which It didn’t because it’s accelerating) (this makes it a very crude assumption , but atleast its 14 billion years ) Then time = d/ V And this = 1 /H0 in s-1 Which gives 14 billion years

Question 36

Big Bang theory summarised

Answer 36

Everything was fine treated in an infinty hot and dense singularity , which after the bug bangs space time expanded and what came after is what we are today

Question 37

Some key points of evolution about the Big Bang 1s , 380k

Answer 37

1) after 1s all the matter created in the universe stopped 2) 380k years the first atoms are made as the universe is cool enough . At this point it’s cold enough for CMBR to be present 3) 14 billion hears the humans eveokve

Question 38

How is thr mass made

Answer 38

First the fundamental lerticles, leptons And quarks are made and they Gain mass Then hadrons are made Then nuclei, then nuclei capture electrins and become atoms

Question 39

What is dark energy and what is it an explanation for?

Answer 39

Dark energy is an explanation for the accelerating universe, says that the accelrwtion needs some source of energy, which CANT BE DETECTED, and thus must be dark energy then

Question 40

Features of dark energy!

Answer 40

Dark energy permeates throughout all of space, and is the reason why universe addelrwtedn Acts as the fuel

Question 41

In a general galaxy, where most of thr mass is concentrated in the centre, what would you expect the velocity to do as distance from centre increases?

Answer 41

As it increases, would expect the velocity to decrease using Kepler third law thing

Question 42

So what do we expect the velocities ti be if every galaxy as distance from centre increases, and why? However Whars happenign and why?

Answer 42

We expect it to decrease, as we observe same effect for smaller mass systems like solar system and Jupiter and moons However for entire GALAXIES, the effect observed is different

Question 43

Why is the effect not observed in entire galaxies?

Answer 43

Becaus we assume that all mass is concentrated in the centre, if it was it would work Therefore all the mass IS NOT CONCENTRATED AT THE CENTRE

Question 44

Therefore if the mass of a galaxy is nkt cintrrsted in the centre, what and where is ti?

Answer 44

This is DARK matter As the velocity of the planters / stars doesn’t decrease the fur here away from the galaxy it is, there must be another type of matter that we CANT SEE THATS MAKING THE SPEEDS HIGHER this is dark matter

Question 45

So what is dark matter and it’s prorptied

Answer 45

Is matter that’s present in galaxies, and thought to be conetrsted at extremities which allows the stars furthered away to still rotate at same speed as the ones closest to the centre - does not emit or absorb light - does not interact with matter, thus is not antimatter

Question 46

What does expected vs observed VELOCOTY for stars in galaxies away from the centr?

Answer 46

Expected is the VELOCOTY goes up as it’s close to centre, then goes down Observed is it goes up, then stays up

Question 47

Okay let’s summarise dark energy and dark matter then

Answer 47

1) dark energy is thought to be the energy that propels the universe into acceleration 2) dark matter is though ti be matter that is spread around the universe - this is because if the galaxy followed systems where mass was concentrated in the centre, like solar system, then further away from centre with distance, you’d get less VELOCOTY of orbit ( y Kepler 3rd law) - as this doesn’t acc happen and velocities are maintained there’s thought to be dark matter which we can’t see which is cinctrsted throughout universe that caused these the move fast. - does not emit or absorb em radiator , interact with matter thud is not anti Matter either

Question 48

Proper good explanation for CMBR microwaves and why they are here

Answer 48

When the universe was though to have been a dense concentrated area of high gamma photons, since then it’s expanded, where the universe cooled, and gamma photons have red shifted to become microwaves