20 - Cosmology Flashcards
Define the astronomical unit (AU).
The average distance from the Earth to the sun.
1.5 x10^11 m
Define the light-year (ly).
The distance travelled by light in a vacuum in a time of one year.
9.46 x10^15 m
Define the parsec (pc).
The distance at which a radius of one AU subtends an angle of one arcsecond.
- 1x10^16 m
(3. 26 ly)
What is 1 arc second in degrees?
1/3600 degrees
What can we use when calculating a parsec?
Small angle approximation
Tan θ = θ
How do we calculate a parsec?
1pc = 1AU / tan(1arcsecond)
What is stellar parallax?
A technique used to determine the distance to stars that are relatively close to Earth, < 100 pc.
What is parallax?
The apparent shift in position of a relatively close star against a backdrop of much more distant stars as the Earth orbits the sun.
Why does parallax occur when viewing stars?
Because if we view a star at one point, 6 months later our relative position has changed so the observer position of the star changes.
How can we calculate the parallax angle?
d = 1 / θ
d: distance in parsecs
θ: parallax angle
Describe the Doppler Effect.
When a wave source moves relative to an observer, the frequency and the wavelength of the waves received by the observer change compared to what would be observed without relative motion.
What is blue shift?
When a galaxy is moving towards Earth, the wavelength appears shorter and absorbtion lines are blue shifted.
How does the wavelength and frequency change when an object is moving away from an observer?
The wavelength increases and the frequency decreases.
How does the wavelength and frequency change when an object is moving towards an observer?
The wavelength decreases and the frequency increases.
What is the Doppler equation?
∆λ/λ ≈ ∆f/f ≈ v/c
∆λ: change in wavelength
λ: source wavelength
What is the Doppler equation?
∆λ/λ ≈ ∆f/f ≈ v/c
∆λ/∆f: change in wavelength / frequency
λ/f: source wavelength / frequency
What is the Cosmological Principle?
An assumption that, when viewed on a large enough scale, the universe is homogeneous and isotropic and the laws of physics are universal.
What does homogeneous mean?
Matter is distributed uniformly across the Universe.
For a very large volume, the density of the Universe is uniform.
What does isotropic mean?
The Universe looks the same in all directions to every observer.
It has no centre or edge.
What does it mean that the laws of physics can be applied to everything within the universe all over time and space?
Models and theories tested on Earth can be applied to everything within the universe all over time and space.
What is Hubble’s Law?
Recessional speed of a galaxy is almost directly proportional to the distance from the Earth.
v ≈ H₀d
H₀: Hubble’s constant
When can the doppler equation only be used?
For galaxies travelling at speeds much lower than the speed of light.
What two observations did Hubble make are evidence for the Big Bang?
- Light from the majority of galaxies is red shifted - so the galaxies are moving away from the Earth
- The further away a galaxy was the greater the observed red shift and therefore the faster the galaxy was moving
What is Hubble’s constant?
- 8 kms⁻¹ Mpc⁻¹
2. 2 x10⁻¹⁸ s⁻¹
How can we use Hubble’s law to estimate the age of the universe?
1 / 2.2x10⁻¹⁸ s⁻¹ = 4.55x¹⁷ seconds
= 14.6 billion years
What are the main pieces of evidence for the Big Bang theory?
- Hubble’s law
- Microwave background radiation
What does Hubble’s law prove?
The universe is expanding.
Because all galaxies are receding away from each other because space itself is expanding.
What would the Universe be like, according to Hubble’s law, if we rewound time back to the start of the universe?
It would be much smaller, denser, hotter and would eventually reach a single point.
How is background microwave radiation evidence for the Big Bang?
If the universe started off extremely hot, it would been saturated with high energy gamma radiation.
This would have stretched as the universe expanded and we now detect them as microwaves.
How is the temperature of the universe evidence for the Big Bang theory (links with background radiation)?
The universe began very hot and dense. Expansion over billions of years has cooled it down to 2.7 Kelvin.
If the universe is treated as a black body, the peak wavelength for this temperature is a microwave.
Why is the estimation of the age of the universe not accurate?
Because recent observations have shown that the universe is expanding not expanding uniformly.
What percentage of the universe does dark energy make up?
Around 68.3%
What percentage of the universe does dark matter take up?
Around 26.8%
What percentage of the universe does matter make up?
Around 4.9%
Why can we not observe dark energy or dark matter?
Because they do not interact with the electromagnetic spectrum
What is dark matter?
Extra matter that cannot be observed in the universe but which must exist.
How can dark matter be proven?
By observing the outer stars of galaxies which move faster than they should, meaning they must have more mass than we can detect.
What is dark energy?
The energy that would be needed to overcome gravity and cause the Universe’s expansion to accelerate.
Describe the evolution of the universe after the Big Bang?
- BIG BANG - Infinitely dense and hot singularity
- RAPID EXPANSION - no matter only gamma radiation
- FIRST PARTICLES - involves the Higgs Boson, quarks formed protons + neutrons
- PRIMORDIAL HELIUM - Protons and neutrons form helium nuclei and a small amount of lithium and beryllium nuclei
- FIRST ATOMS - universe cools enough for atoms to form
- FIRST STARS - first heavy elements formed in nuclear fusion
- MILKY WAY - our galaxy forms
- SOLAR SYSTEM - our solar system forms from a nebula left from a larger star than the Sun
How was it discovered that the expansion of the Universe is expanding?
The light from a supernova was found to be less intense than it should have been, proving that dark energy must exist and the universe is expanding.
