Physics - Cosmology
Flashcards
Astronomical distance units
Where does the definition for $1 \text{au}$ come from?
The average earth-to-sun distance.
What does the definition for $1 \text{ly}$ come from?
It’s the speed light travels per year in a vacuum.
Arcminutes and arcseconds
The parsec
What is the formula for distance in parsecs?
What is the formal definition of a parsec?
The distance at which a $1\text{au}$ base line subtends an angle of $1$ arcsecond.
How can you work out the numerical value of $1 \text{pc}$ (parsec)?
The Doppler effect
What is the Doppler effect?
The relative change in frequency/wavelength of a wave when there is a relative velocity between observer and source.
What is true about the relationship between relative velocity and change in wavelength in the Doppler effect?
Red shift and blue shift
In red shift, what happens as the source is moving away from you?
The waves are spreading out.
In blue shift, what happens as the source is moving towards you?
The waves get bunched together.
The Doppler equation
What is the Doppler equation in terms of change in wavelength $\Delta \lambda$, original wavelength $\lambda$, velocity $v$ and the speed of light $c$?
What is the Doppler equation in terms of change in frequency $\Delta f$, original frequency $f$, velocity $v$ and the speed of light $c$?
How would you know the original wavelength when using the Doppler equation?
You’d measure the emission or absorptions lines of a source in a lab.
What is the rough limit of measurement due to stellar parallax?
Hubble’s law and the cosmological principle
What is Hubble’s law in terms of direct proportionality?
Because of Hubble’s law, what is generally true the further away a galaxy is?
It’s moving faster.
What is Hubble’s law in equation form, in terms of $v$, $d$ and $H _ 0$?
How can Hubble’s law work if galaxies like Andromeda are moving towards us?
Because it is applied to the whole universe on average, not on a case-by-case basis.
How can you work out the age of the universe given Hubble’s constant $H _ 0$?
What is the approximate value of Hubble’s constant in $\text{km s}^{-1}\text{Mpc}^{-1}$?
What is the cosmological principle?
The assumption that the universe is isotropic and homogenous.
What are the two assumptions made about the universe in the cosmological principle?
- Isotropic
- Homogenous
What does “the universe is homogenous” mean?
Matter is uniformly distributed across the universe.
Fate of the universe and cosmological puzzles
What happens in a flat universe?
The universe expands to a limit, but never reaches it.
If you universe is eternal and infinite, then what is Olbers’ paradox?
Why is the night sky dark?
Why can’t we know much about the early universe?
Because the energies and temperatures were so great that all our understanding breaks down.
Why is dark matter and energy needed?
To account for the fact that the universe is accelerating.
What explains the cosmic microwave background radiation?
The universe was initially full of high-energy gamma photons which have since been red-shifted into microwaves.
Hubble observations and the cosmic microwave background
What must you specify about the universe in order for Edwin Hubble’s observations to imply that the universe is expanding?
The universe began at a single point.
When determining the Hubble constant from the best fit of a straight-line graph, what is important?
That the line passes through the origin.
How does the temperature of the cosmic microwave background radiation count as evidence for the big bang?
It indicates that the universe was once very hot and due to expansion has cooled.
What is important you specify about galaxies in Hubble’s law?
They are _ distant _ .
Will the change in wavelength due to the Doppler effect be the same for all wavelengths or will it change?
It will increase with wavelength.
Evidence for the big bang and the early universe
Why might Hubble’s law only apply to distant galaxies?
Because there are stronger gravitational forces to nearby galaxies which causes clustering.
Why might Hubble’s law not be very accurate for very distant galaxies?
The rate of expansion may not have actually been linear.
What are two pieces of evidence for the big bang?
- Galaxies receding from each other
- CMBR
- The proportion of hydrogen and helium
How is the proportion of hydrogen and helium evidence for the big bang?
The big bang predicts neutrons decaying into protons faster than they are created, at a rate corresponding with temperature. The predicted abundance of neutrons vs protons matches observations of helium vs hydrogen in the universe.
What is the process of creating deuterium and helium called in the early universe?
Big bang nucleosynthesis.
What is the mnemonic for the evolution of the universe?
Bingo! Invalid men have tiny necks, cautiously spying.
“Bingo! Invalid men have tiny necks, cautiously spying” Can you turn this into the stages of evolution of the early universe?
- Big bang
- Inflation
- Mass
- Hadrons
- Temperature
- Nucleosynthesis
- Cooling
- Stars
What are the two relevant “temperature drops” in the evolution of the early universe?
- Temperature falling too low that pair production stops
- Temperature falling low enough that nuclei can capture electrons
How was most mass in the early universe created?
Pair production.
Why are almost no heavier elements than hydrogen (deuterium) or helium formed in the early stages of the universe?
The expansion was too rapid.
Evolution of the Universe
- The Big Bang: The universe begins as a singularity, infinitely dense and hot.
- Inflation: The universe expands rapidly and is full of EM radiation in the form of high-energy gamma photons.
- Mass: The first fundamental particles (quarks, leptons, etc.) gain mass.
- Hadrons: Quarks combine to form hadrons such as protons and neutrons. Pair production also occurs.
- Stop: Creation of matter stops as temperature drops.
- Nucleosynthesis: Protons and neutrons fuse together to form hydrogen, helium and small amounts of lithium and beryllium.
- Cooling: The universe cools enough that atoms can form. The nuclei capture free electrons.
- Stars: Stars appear, these create heavier elements through fusion.