Ch 20 Cosmology Flashcards
What is the cosmological principle?
- the universe is isotropic and homogenous
- laws of physics are universal
What does isotropic mean?
Universe is the same in all directions to every observer
Universe has no centre or edge
What does homogenous mean
Matter is uniformly distributed
Universe has a uniform density
What is the astronomical unit (AU)?
- average distance from the Earth to the Sun
- 1.5 x 10^11 m
When is AU most often used?
To express the average distance from the Sun and other planets in the Solar System
What is a light-year (ly)?
- distance travelled by light in a vacuum in a time of one year
- 9.46 x 10^15 m
When are ly mainly used?
When expressing distances to stars or other galaxies
What is an arcsecond?
1/3600 degree
What is an arcminute?
1/60 degree
What is a parsec (pc)?
The distance at which a radius of one AU subjects an angle of one arcsecond
3.1 x 10^16 m
Formula for a pc?
1 pc = 1 AU / tan (1 arcsecond)
Relationship between distance ‘d’ pc and angle subtended?
if point is at a distance ‘d’ parsec, angle subtended is 1/d arcsecond
When do we use stellar paradox?
To determine distances of stars that are relatively close to the Earth, distances less than 100 pc
What is 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.
What is the parallax angle ‘p’?
Angle of stellar parallax
Formula relating arcseconds with parsecs?
d (pc) = 1 / p (arcsecond)
What is the Doppler effect?
Whenever a wave source moves relative to an observer, the frequency and wavelength of the waves received by the observer change compared with what would be observed without relative motion.
How can the Doppler effect be used to determine the relative velocity of a distant galaxy?
- Absorption spectrum of a specific element is determined in the laboratory
- Since the same spectrum is observed in light from a distant galaxy, we can compare their absorption spectrums.
- If the observed wavelengths of the absorption lines are different, it is caused by the relative motion between the galaxy and the Earth.
Blue shift vs red shift?
Blue shift if the galaxy is moving towards the Earth, since the wavelength appears shorter
Red shift if the galaxy is moving away from the Earth, since the wavelength appears stretched
What is the Doppler equation?
Δ λ / λ = Δ f / f = v / c
Observations that led to Hubble’s law?
- light from the vast majority of galaxies was red-shifted
- the further the galaxy was the greater the observed red shift and so faster the galaxy was moving
What is Hubble’s law?
The recessional speed v of a galaxy is almost directly proportional to its distance d from the Earth.
v ∝ d
What is Hubble’s constant H₀?
2.2 x 10^-18 s^-1
What does the Big Bang theory state?
At some moment in the past, all the matter in the Universe was once contained in a single point.
Evidence for the Big Bang theory?
Hubble’s law
- if we ran time backwards, space would be receding into a point
Microwave background radiation
Explanation for Microwave Background Radiation?
- when the universe was young and extremely hot, space was saturated with high-energy gamma photons
- as Universe expanded, the wavelength of these photons stretched to become microwaves
OR - Universe can be treated as a black-body radiator
- at 2.7K (temperature of space), wavelengths would correspond to around 1mm, which is in the microwave region
Formula for the age of the universe?
t ≈ 1 / H₀
What is the estimated age of the universe?
~ 14 billion years
What is dark energy?
- hypothetical form of energy that explains the acceleration of the universe
- fills all of space and accelerates expansion of Universe
- should make up for 68% of energy in Universe
What is dark matter?
Theory for why the mass in the universe is spread out, instead of concentrated in the centre like galaxies are. Explanation as to why velocity of galaxy does not decrease the further from the centre of the Universe it is.
Should make up 27% of the mass in the Universe.
Does not interact with light therefore not seen in telescopes.
The Big Bang (Series)
Time and space are created
The Universe is a singularity - it is infinitely dense and hot
10^-35 s
Universe expands rapidly, phase of incredible acceleration called inflation.
No matter in Universe, full of electromagnetic radiation (high-energy gamma photons)
Temperature is around 10^28 K
10^-6 s
The first fundamental particles gain mass
10 ^-3 s
Quarks combine to form the first hadrons, such as protons and neutrons.
Most of the mass in the Universe was created within the first second
1 s
Creation of matter stops, temperature falls to around 10^9 K
100 s
Protons and neutrons fuse together to form deuterium and helium nuclei, around 25% of the matter in the Universe is helium nuclei.
380 000 years
The universe cools enough for the first atoms to form.
This stage’s electromagnetic radiation is now detected as Microwave Background Radiation.
30 million years
The first stars appear.
First heavy elements begin to form in these stars through nuclear fusion.
200 million years
Milky Way forms as gravitational forces pull clouds of hydrogen gas and existing stars together
9 billion years
The Solar System forms from the nebula left by the supernova of a larger star.
11 billion years
Formation of the Earth
12 billion years
Primitive life on Earth begins
13.7 billion years
Modern humans evolve. Temperature of the universe is 2.7 K
