Module 5 Cosmology Flashcards
What makes the line spectrum of elements different?
electron energy levels different for each chemical element
Define an energy level
the discrete energy with which electrons orbit the nucleus
Why are electron levels considered negative?
Energy is required to remove an electron from the atom
What is ionisation energy?
minimum energy required to remove electron that is closest to the nucleus (ground state)
What is the ground level?
n=1, first energy level in an atom (the most negative one)
What is excitation?
The process by which an electron moves from a lower to a higher energy level (by absorbing a photon)
Describe the process of excitation
Electron must receive the exact amount of energy
Usually by absorbing a photon
Electron gains this energy and moves to a higher energy level
What is de-excitation or relaxation?
the process by which an electron moves to a lower energy level by losing energy through emitting a photon
How do you find the number of possible wavelength that can be observed when firing a beam of electrons at a given KE at atoms using an energy level diagram?
Find the energy of electron in same units as energy diagram
Add this value to the value of the ground level (you should still get a negative number)
Take all the number of relaxations possible below this value (line below this value)
That is the number of wavelengths that can be observed ie the number of photons of different energies
What are the three main spectra?
Continuous
Emission line spectra
Absorption line spectra
What is continuous spectrum?
a spectrum containing all colours, wavelengths, and frequencies
How is a continuous spectrum produced?
viewing a white light through a diffraction grating
Describe an emission line spectrum?
a series of narrow, bright, coloured lines on a dark background
Explain how is an emission spectrum produced in terms of movement of electrons?
Each line is due to a photon of specific energy being emitted as electrons move to lower energy levels through relaxation
Gaps between levels have fixed energy therefor photons have fixed energy and wavelengths
Describe an absorption spectrum
A series of narrow dark lines on a continuous spectrum background
In simple terms, describe how an emission spectrum is produced
electron moves to a lower energy level by emitting a photon
How is an absorption spectrum produced?
When a continuous spectrum (from passing white light through a diffraction grating) passes through a cool gas
The gas absorbs certain photon energies producing excitation of electrons
Each photon absorbed (gap in spectra) corresponds to a gaps between energy levels
Producing a series of narrow dark bands on a continues background
How can an absorption line spectrum be used to identify elements
Each black line corresponds with a wavelength of photons being absorbed
Also corresponds to gaps between energy levels
Each element has a unique set of energy levels, thus can be compared to absorption spectrum
What is a diffraction grating?
Usually a piece of metal containing thousands of tiny slits of equal thickness which are equally spaced
State the diffraction grating equation
dsin0=nλ
Where are angles measured from when calculating diffraction gratings
from the central fringe (straight through position)
In the diffraction grating formula, what does d stand for?
distance between the gaps in the diffraction grating
When given a number of lines in a distance for a grating, how do you calculate d (use N=250 mm^-1)?
Take the unit of length over the number of divisions
1x10^-3/250
Explain how you calculate the largest number of orders that can be observed
Rearrange for n
sin90=1
n_max=d/λ
Truncate ie round down to nearest whole number
Explain why when white light is diffracted, does blue light appear on the centre and red on the outside of the diffraction patterns?
Blue light has a shorter wavelength
rearrange for sin0=nλ/d
If λ is smaller so is the angle
Explain why a white light is seen in the centre of a diffraction grating
All wavelengths combine and interfere forming white light
because at centre, path difference is 0xwavelengths for all wavelengths
How was redshift first observed?
Through comparing the absorption line spectra of hydrogen from distant galaxies to laboratory experiments
What is the doppler effect?
the apparent change in wavelength due to relative motion between a source and the observer
What is redshift?
the stretching of wavelengths when a source of EM radiation is moving away from the observer
What is blueshift?
the compression of wavelengths when a source of EM radiation is moving towards an observer
Explain how you would find the speed of a galaxy in relation to earth using the wavelengths of given spectra
Δλ/λ=Δf/f=v/c
Δλ is calculated by (observed λ-actual λ)
to find v=Δλ/λ*c
Explain why longer wavelengths produce a larger Doppler shift Δλ?
speed of galaxy is the same
Δλ will be greater if λ is larger
What is recessional speed/velocity?
the apparent movement of galaxies away from the observer due to the expansion of the universe
State Hubble’s law
the speed of recession of distance galaxies is directly proportional to their distance from the observer
Describe how the Hubble constant is measured?
plotting a graph of recessional speed against distance
straight line through origin.
Gradient is the Hubble contant
How can the the change in wavelength from a distance galaxy and it’s distance can be used to find the Hubble Constant?
Δλ/λ=Δf/f=v/c
to find v=Δλ/λ*c
Speed of recession is proportional it’s distance
So Hubble constant H_0 = v/d
What are the units for the Hubble constant?
s^-1
or
kms^-1*Mpc^-1
Hubble constant - how to convert kms^-1*Mpc^-1 to s^-1?
Multiply by 10^3 to get from km to m
divide by 10^6x3.1x10^16 (to remove Mpc)
Explain how Hubbles constant can be used to find the age of the universe
t=d/v
v=H_0d
v/d=H_0
d/v=1/H_0
therefore t = 1/H_0 (Hubbles constant must be in seconds) giving age of universe in seconds
Explain how Hubble’s Law can be used as evidence for the big bang theory
galaxies moving away from each other, implying they initially were located at the same point
What happens in the first instance of the the big bang?
high energy photons produce quarks and leptons from pair production
What happens during the first 100s of the big bang
quarks combine to form hardons - universe contains leptons and photons
What is the state of the universe during the first few thousand years?
protons and neutrons fuse to form simple nuclei (helium and hydrogen), universe is a plasma of nuclei, electrons and photons
What is the last stage of the evolution of the universe (not how it ends)
Expansion of universe causes photons to loose energy
Allowing atoms form from nuclei and electrons
Photons no longer have enough energy to ionise so they are free to travel and form CMBR
What initiates the formation of stars and galaxies in the early universe?
tiny variations in density in the universe lead to gravity forming stars and planets
What is CMBR?
cosmic microwave background radiation
What temperature does CMBR correspond with?
Background temperature of the universe 2.7K (now)
CMBR are photons from the big bang, originally gamma radiation. Explain what would have caused this radiation to cool to the levels that it can be detected today
Decrease in energy = increase in wavelength
This is due to the expansion of the universe causing the stretching of wavelengths
What causes variations in CMBR?
tiny variations in density in the universe lead to gravity forming stars and planets
What is the cosmological principle?
on a large scale, the universe is homogenous and isotropic to universal laws of physics
In terms of the universe, what does homogenous mean?
universe has uniform density on a large scale
In terms of the universe, what does isotropic mean?
universe looks the same in all directions on a large scale
What are hadrons
mainly protons and neutrons formed when quarks combine
What are leptons?
mainly electrons
What is the current composition of the universe?
5% ordinary matter
25% dark matter
70% dark energy
What is dark matter?
the substance which provides the extra gravitational attraction needed to stop galaxies from falling apart
(what affects the rotational speed of galaxies leading to all stars rotating around the centre of the galaxy at the same speed))
Why is dark matter so hard to observe and study?
does not absorb or emit electromagnetic radiation
What is dark energy?
the force which is thought to cause the rate of expansion of the universe to increase
What causes the rate of universe expansion to increase?
dark energy
What makes scientists believe dark energy exist?
force of gravity should slow down the rate of expansion, whereas measurements show that the rate is increasing
What are the different time stages to the evolution of the universe?
First instance
First 100 seconds
Thousand years
Present
