Chapter 20 - Cosmology Flashcards
Doppler Effect
The change in detected wavelength and frequency of a wave as it moves relative to an observer
(you appear to see more frequent wavefronts as they come from gradually closer)
Star moving closer
Shorter wavelength, higher frequency, blue-shifted
Star moving away
Longer wavelength, lower frequency, red-shifted
Doppler shift in starlight
The shift in the spectral lines of an absorption spectra of a galaxy compared to a sample in a lab are due to the relative motion of the galaxy compared to Earth
Doppler effect equation
Δλ/λ0 = v/c
where Δλ is the shift in wavelength and λ0 is the original wavelength
Astronomical Unit (AU) and value
The average distance from the Earth to the Sun
1.5 x 10^11 m
Light Year (ly)
Distance travelled by light in a vacuum in 1 year (9.46 x 10^15 m)
Parsec (Pc)
The distance at which a radius of 1 AU subtends an angle of 1 arcsecond (3.1 x 10^16 m)
1 arcminute
(1/60)°
1 arcsecond
(1/3600)°
Angle in arcseconds formula
θ = 1/d (where d is the distance in parsecs)
Parallax
The apparent shift in position of a relatively close star compared to a background of much more distant stars as the Earth orbits the Sun
Using stellar parallax to determine the position of the star
- When the Earth is at opposite sides of the Sun, a star is viewed
- The parallax angle is the angle between one of these and the line from the star to the Sun (1/2 x the difference between the angles)
- The distance in parsecs is 1/p where p is the parallax angle
Hubble’s observations from absorption spectra
- Light from the majority of galaxies was red shifted
2. In general, the red-shift observed was greater for more distant galaxies
Hubble’s law
The recessional speed of a galaxy from Earth (v) is directly proportional to its distance from the Earth (d)
Hubble’s law equation
v = H d
0
Cosmological Principle
When viewed at a large enough scale, the Universe is both homogenous and isotropic and the laws of physics are universal
Homogenous
Matter is evenly distributed across the universe. For a very large volume, the density of the universe is uniform. This means that the same type of structures are seen everywhere
Isotropic
The universe looks the same in all directions to every observer. This means that there is no centre or edge to the universe
Big Bang Theory
Suggests that all matter was contained in an infinitely dense and hot singularity that expanded outwards
Cosmic Microwave Background Radiation (CMBR)
Emitted by the plasma that made up the universe 13.5 billion years ago
Space was saturated with high-energy gamma photons that have red-shifted to microwaves as the universe expanded
The universe is around 2.7K, giving a wavelength of approximately 1mm
Observed to have uniform intensity
Age of the universe from Hubble’s constant
≈ 1/H0
First second after the big bang
The universe expands rapidly, a phase of incredible acceleration called inflation - full of gamma photons at 10^29K
The first fundamental particles gain their mass and the quarks then combine to form the first hadrons in pair-production
After 1 second the creation of matter stops, 10^9K
More than 1 second after the big bang
Protons and neutrons fuse into deuterium and helium nuclei + some lithium and beryllium, too fast to form any heavier elements
The universe eventually cools and atoms form, the nuclei capture electrons and CMBR is emitted
The first stars appear allowing heavier elements to fuse before clouds of hydrogen are pulled together to form galaxies
Accelerating Universe
Type 1A supernovae were observed to produce a less intense kind of light than expected. There must be a type of energy to produce this “dark energy”
Dark matter
The velocity of stars wasn’t observed to decrease as they moved from the centre of a galaxy
This suggests that the mass of the galaxy is not concentrated in the centre as the behaviour was not observed in other gravitational systems
Dark matter must be spread through the galaxy - cannot be seen by telescopes and neither absorbs nor emits light
