C20 - Cosmology (The Big Bang) Flashcards
What is an arc minute?
1/60 th of a degree.
A minute of arc.
1° = 60 arcminutes
What’s an arc second?
1/3600 th of degree.
A second of arc.
1 arc minute = 60 arc seconds
What is an astronomical unit, AU?
A measurement of distance from the Earth to the Sun
What’s a light year, ly?
The distance travelled by light in a vacuum in one year.
“The distance a photon if light would travel through space in one year”.
What’s a parsec, pc?
The distance at which a radius of one AU subtends an angle of one arcsecond.
“The distance of an object that would have a parallax of 1 arcsec when observed from Earth.”
How are distance and the parallax angle related?
d = 1/p
Where d is in metres and p is the parallax angle in arcseconds.
What’s stellar parallax?
A technique used to determine the distance to stars that are relatively close to the Earth.
Parallax is the apparent shift in the position of a relatively close star against the backdrop of much more distant stars as the Earth orbits the Sun.
What’s parallax?
The apparent shift in the position of a relatively close star against the backdrop of much more distant stars as the Earth orbits the Sun.
How is stellar parallax used to determine distances?
As the Earth orbits the Sun, when the Earth is in position A, the parallax angle of a nearby star is determined.
6 months later (when Earth is at its opposite end of orbit) another parallax angle of the star is determined (against the backdrop of distant stars).
These are then used to calculate the distance:
d = 1/p
What’s the Doppler effect?
The change in wavelength and frequency of waves when a wave source moves relative to an observer.
How can absorption spectra be used to analyse the Doppler effect and motion of galaxies from star light?
Any difference in wavelength of absorption lines must be caused by the relative motion between the galaxy and Earth. (Red shift analysed)
If moving towards the Earth, it will appear blue shifted.
If moving away from the Earth, it will be red shifted.
How can the Doppler effect in starlight be analysed mathematically?
Using the Doppler equation:
Δ λ/λ = Δf/f = v/c
How is red shift calculated? (z)
Red shift = z = Δ λ/λ
What is Hubble’s law?
The recessional speed, v, of a galaxy is almost directly proportional to its distance from the Earth.
vd = (Hubble’s) constant
What 2 key observations were made when Hubble analysed the Doppler shift?
1) majority of galaxies were red-shifted.
2) the further the galaxy was, the greater the red shift and so the faster the galaxy was moving.
What is the expanding universe model?
A model explains that space and time is expanding in all directions.
How is Hubble’s law’s graph plotted?
Distance along the x axis (in Mpc - mega parsecs)
Recessional speed along y axis (in km/s)
Straight line with positive gradient (= to Ho)
What is Hubble’s law’s graph’s gradient equal to?
Hubble’s constant - Ho
What’s the cosmological principle?
The assumption that, when viewed in a large enough scale, the universe is homogeneous, isotropic and the laws of physics are universal.
Homogeneous - matter is distributed uniformly across the universe (uniform density).
Isotropic - the universe looks the same in all directions to every observer. There’s no edge or centre.
What evidence supports the Big Bang theory?
Cosmic microwave background radiation
Red shift
Abundance of lighter elements
Hubble’s law
How was microwave background radiation detected?
When scientists attempted to detect signals from objects in space, they detected a uniform microwave signal which they couldn’t account for.
How can the existence of CMBR be explained (2 ways)?
1) When the universe was young and very hot, space was saturated with high E gamma photons.
The expansion of the universe stretched the wavelengths of these high E photons so now it’s observed as microwaves.
2) The universe was extremely hot and dense when young. Expansion reduced the temperature to 2.7K. The universe may be treated as a black body radiator - at this temperature the peak wavelength would correspond to the microwave region.
How can the age of the universe be determined?
Age of universe = t = 1/Ho
What’s dark energy?
A hypothetical form of energy that fills all of space and would explain the accelerating expansion of the universe.
What’s dark matter?
A hypothetical form of matter spread throughout the galaxy that neither emits no absorbs light - it could explain the differences between predicted and observed velocities of stars in galaxies.
What’s a black body?
An idealised object that absorbs all the electromagnetic radiation incident on it and, when in thermal equilibrium, emits a characteristic distribution of wavelengths at a specific temperature.
What happened after the Big Bang? (As T decreases - time after…)
Big Bang - time and space created. Infinitely dense and hot.
10^-35s: rapid expansion and acceleration (known as inflation). There’s no matter, only high E gamma photons.
10^-6s: fundamental particles gain mass
10^-3s: quakes combine to form hadrons.
1s: creation of matter stops
100s: proteins and neutrons fuse to form deuterium and He nuclei.
380,000 yrs: universe cools for first atoms to form. Nuclei capture electrons. EM radiation at this stage is later detected as CMBR.
30mil yrs: first stars appear, thus producing elements heavier than Li.
200mil yrs: Milky Way forms due to gravitational forces.
9bil yrs: solar system forms from the nebula of a supernova of a larger star. Sun formed > Earth formed > primitive life begins
13.7bil yrs (now): modern humans evolve
What is the universe made up of?
5% matter
27% dark matter
68% dark energy
What can be a cause of the acceleration of the universe?
Dark energy.
A supernova was observed but light detected was less than predictions, suggesting the universe is accelerating.
