CP and Distance Ladder Flashcards
What is the current picture of the universe?
it is expanding
it is aging
there is a relic of a hot dense early era
What time in the early universe did the atomic nuclei form?
3 mins
When did the first stars and galaxies form?
18Myrs
What is the cosmological principle?
On average, the universe is the same everywhere and what we see from our position is what anyone else anywhere in the universe would also see
it allows us to use Newtonian mechanics and simple thermodynamics
What does the CP imply about the universe?
it is isotropic: looks the same in all directions
it is homogeneous: looks the same in all places (galaxy distribution)
(they are independent of one another)
What evidence is there of isotropy?
the CMB (temperature looks the same in all directions at T = 2.7K)
What evidence is there of homogeneity?
Redshift surveys of galaxies show that density is constant
When looking outwards, fewer galaxies are seen because they are harder to be seen not due to their lack of existence so separate regions of universe need to be observed (so CP still holds)
How are galaxies identified?
Using spectra (measure lamda observed) which also gives redshift
What is used as a proxy for distance?
redshift
If z = 1, is the object moving faster than c?
No as the redshifting is doing work against gravity so v is not greater than c
(for higher z don’t use v, only z)
What do galaxies show?
Absorption lines (dips in spectra) from atmosphere of stars and some show emission lines (lines of increased intensity)
How are galaxies found?
Using colour (especially young galaxies)
Red, green, blue, infrared filters (on brightness vs wavelength graph)
young galaxies emit UV light
What is the Lyman limit?
Where all photons gets ionised by neutral hydrogen and become unobservable as non of these photons escape to us
at 91.2nm
ground state to infinity transition of hydrogen
What is the photo-ionisation potential?
13.6eV
How are cosmological models tested?
By comparing how the redshift and distance relate to each other
(as redshift and distance are not the same)
What is bootstrapping?
Measuring real distances to galaxies indirectly
What is the only direct method of determining distances?
parallax: a star at a distance of 1 parsec from the Sun has a parallax of 1 arcsec
due to effects of atmosphere, parallax is found using satellites
only measures out to a few kpc currently
What is parallax used to do?
Calibrate “secondary” methods on the distance ladder
What do bluer stars indicate?
Hotter stars
(red stars are colder)
What is main sequence fitting?
Luminosity (found using parallax) of stellar clusters are compared to main sequence of distant clusters
Can’t apply to distant stars as they begin to merge at a distance
What are cepheids?
Evolved massive stars which pulsate regularly (due to gravity) on the instability strip
period is proportional to luminosity
Why is it is difficult to use cepheids?
Cepheids need to be well established to be used
Stars have different metallicities which changes period-luminosity relations
Dust causes objects to appear redder and fainter
Only seen out to 10Mpc (methods become less accurate beyond this)
accuracy of distance +- 10%
How are cepheids (standard candle) used?
Their luminosity is found using period and flux and distance can be found
(they are partially calibrated from parallax, mostly from main sequence cluster fitting)
What are Type I and Type II Cepheids?
Type I are stars with normal metallicity (in our galaxy)
Type II are stars with lower metallicity
What is a standard candle?
Any source whose intrinsic luminosity can be measured and takes measured flux to get distance
What is used to find the distance to distant sources?
Supernovae Type Ia as they are very bright at about z = 2
(in a binary system as mass is transferred from primary to secondary star, critical mass is reached and system is destroyed causing bang and a lot of light emitted)
How are Supernovae Ia found?
Host galaxy is imaged
Then host galaxy and supernova
Then final shot of host galaxy is taken
The brightness varies with time so both the rise and fall need to be observed due to variation (this standard candle is less accurate)
What is measured to find Supernovae Ia?
The luminosity vs time from peak luminosity
the duration is the width of the peak as luminosity is proportional to duration, distance can be found
Both sides of peak need to be observed (rise time is only a few days)
How are Supernovae Ia distinguished from other supernovae curves?
Spectrum proves it and gives the redshift
Peak also indicates them
Supernovae show that universe is accelerating
What are the pros and cons of using Supernovae Ia?
evolutionary differences (e.g metallicity) have small effect
Not in dusty regions so dust does not obscure them
Sufficient nearby SN Ia to check properties
Show what’s happening in late universe (last few billion years)
Shows as far as we can accurately see
