Physics - Recall for Chapter 20; Cosmology
Flashcards
2022-04-18
What is an astronomical unit (AU)?
The average distance from the Earth to the Sun.
What is a light year?
The distance light travels in a vacuum in one year.
What is an arcminute?
An angle equal to $1/60\text{th}$ of $1^\circ$.
What is an arcsecond?
An angle equal to $1/60\text{th}$ of $1’$.
What is stellar parallax?
The apparent shift of nearby stars against the background of distance stars due to the Earth’s motion around the Sun.
What is a parsec?
The distance at which a radius of one AU subtends an angle of one arcsecond.
What is the parallax angle of a star?
Half of the angular shift observed in the apparent position of a nearby star over a period of six months.
When there are multiple six month periods where the star has different angular shifts in position, how do you choose which one to use in determining the parallax angle of the star?
You pick the largest shift.
What is $d$ in the equation
\[d = \frac{1}{p}\]
?
The distance to a nearby star in parsecs.
What is $p$ in the equation
\[d = \frac{1}{p}\]
?
The parallax angle of a nearby star in arcseconds.
What is the Doppler effect?
The apparent change in frequency of waves measured due to relative motion of the source and the observer.
How is the red or blue shift of a galaxy measured?
The absorption spectrum of the galaxy is compared to those of elements observed on Earth.
What is blue shift?
The shift towards shorter wavelengths of absorption lines of a galaxy moving towards the Earth.
What is red shift?
The shift towards longer wavelengths of absorption lines of a galaxy moving away from the earth.
How the approximate velocity of a galaxy calculated from its red shift?
What two observations did Hubble make about distant galaxies?
- Most galaxies are red shifted
- The red shift tends to increase with distance
What is Hubble’s law in words?
The recessional speed of distant galaxies is proportional to their distance from us.
What is the Cosmological Principle?
The universe is homogenous and isotropic on a large scale.
What is meant by the homogeneous nature of the universe?
The idea that matter is distributed uniformly across the universe.
What is a consequence about the density of the universe under the assumption that it is homogenous?
It is constant.
What is meant by the isotropic nature of the universe?
The idea that the universe looks the same in all directions to every observer.
What is the Big Bang?
The leading theory to describe the origin of the universe in which space and time expanded from a singularity approximately 13.7 billion years ago.
What is the cosmic microwave background radiation (CMBR)?
The largely uniform microwave radiation with the black body spectrum of an object at $2.7K$ detected in every direction in the sky.
What is the temperature of the cosmic microwave background radiation (CMBR)?
How does the Big Bang theory explain the cosmic microwave background radiation?
Short wavelength photons at the beginning of the universe have been red shifted as space expanded.
How can you determine the Hubble constant?
The gradient of the best-fit line for a plot of recessional speed against distance for a set of galaxies.
How can the age of the universe be estimated using the Hubble constant $H _ 0$?
Approximately what percentage of the universe is thought to be composed of normal matter?
What process lead to the conversion of radiation to matter in the early universe?
Pair production.
What was the composition of the early universe after fusion ended at around $t = 100s$?
- 75% hydrogen
- 25% helium
- Trace amounts of lithium and beryllium
What were the two trace elements present in the universe at around $t = 100s$?
Lithium and beryllium.
Why was light able to freely propagate in the universe after around $380,000$ years?
Nuclei cooled enough that they could capture electrons and form neutral atoms.
What is dark matter?
A hypothetical form of matter that doesn’t emit or absorb light.
What does anomaly would dark matter explain?
The difference between the expected and observed orbits of stars around galactic centres.
What evidence suggests the expansion of the universe is accelerating?
Light from very distant supernovae is less intense than predicted.
What is dark energy?
A hypothetical form of energy postulated to explain the apparent acceleration in the expansion of the universe.