Orbits and The Wider Universe Flashcards
Kepler’s First Law
Each planet moves in an ellipse with the sun at one focus
Kepler’s Second law
The line joining a planet to the center of the sun sweeps out equal areas in equal times
Kepler’s Third Law
The square of the period, of the planets orbital motion is proportional to the orbital radius cubed
Explain how we know dark matter exists
The measured rotational velocity of stars further out from the center of the galaxy is higher than expected. This implies that the mass is greater than what we can see, this suggests dark matter exists
When using Kepler’s third law, what is something that we assume
We assume that a small mass is orbiting a larger mass
What are three that we can infer about a system where two masses orbit a center of mass
- The center of mass must be in line with the two masses
- The center of mass must be between the objects, as centripetal acceleration must be towards it
- The angular velocities of the objects must be identical
If a star/galaxy is moving towards us what happens to the wavelength
It decreases, this is known as blue shift
If a star/galaxy is moving away from us what happens to the wavelength
It increases, this is known as red shift
Radial velocity, in Doppler shift equation
The component of the objects velocity along the line joining the observer to the object
What is the equation for Doppler shift in words
Delta lambda over lambda = radial velocity over the speed of light
Radial Velocity, When related to the Doppler shift equation
The Radial velocity is the component of the objects velocity along the line joining the observer to the object
What did Hubble notice with the red shift from stars and galaxies
That the redshift of the stars and galaxies was greater the further from earth they were.
What is the equation taken from the graph Hubble created using data from redshift of galaxies
Hubble’s constant = Velocity over distance
Hubble’s law
The speed of the recession of a galaxy is directly proportional to its distance from the earth
What is the value of Hubble’s constant
2.2x10^-18s^-1
What does Hubble’s constant give a measure of
The expansion rate of the universe, how fast objects appear to be moving away from our galaxy as a function of distance
What do we have to assume about radial velocity of galaxies to calculate the age of the universe
That the radial velocity has been constant since the beginning of the universe
What is the equation for the age of the universe
1 over Hubble’s constant
What are the two problems with Hubble’s constant
- Uncertainty in the value, since it is determined using redshift and distance to the stars.
- The assumption that the radial velocity of the galaxies has been constant
What are the three fates of the universe
Open
Close
Flat
Describe and open Universe, in terms of denisty
It is where the density of the universe is less than the critical density, so the universe will always expand
Describe a closed universe in terms of density
It is where the density of the universe is greater than the critical density, so the universe will eventually stop expanding and collapse back in on itself.
What is the equation used for critical density with Hubble’s law in
Critical Density = (3 x Hubble’s constant squared) over ( 8 times pi times the gravitational constant)
What is the value of the gravitational constant
6.67x10^-11 Nm^2Kg^-2
Critical Density
The average density of matter required for the universe to just halt its expansion, but only after an infinite time
Red shift is … earth
away from
Blue shift is … earth
towards
