Midterm Exam #2 Tuesday, April 20. Closed book Will cover Lecture 15 (Stellar Evolution) through Lecture 21 (Galaxy Evolution) only If a topic is in the book, but was not covered in class, it will not be on the exam! Some combination of multiple choice, short answer, short calculation
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To answer, we need to know:
How much energy is in the form of mass (including dark matter)
How much energy is in the form of light
How much “weird” energy (not mass, not light) is there
If there’s nothing especially weird about the energy components to the universe, then the fate of the universe will depend on the balance of Kinetic Energy and (Gravitational) Potential Energy
Kinetic Energy: energy of motion (due to big bang)
(Gravitational) Potential Energy: energy that is stored when two massive objects are located at some distance from each other (after about 10 sec the universe has substantial amounts of mass in it; objects with mass attract each other)
What happens if you throw a ball upwards?
What happens if you fire a rocket at 11 km/s?
No mass = universe has no brakes! (constant expansion rate)
“Little bit” of mass = universe has weak brakes (weak deceleration)
“Lots of mass” = universe has strong brakes (strong deceleration)
Depending on how much mass there is in the universe you (naively) expect 3 possible fates for the universe…
halt, no collapse
halt, no collapse
halt, no collapse
weird: initially decelerating
What’s the evidence for Dark Matter and Dark Energy - should you take this stuff seriously????
The stars and gas in the outer parts of the disks of spiral galaxies are moving too quickly to be explained by the amount of mass contained in the luminous material of the galaxies, therefore the majority of the mass in galaxies must be “dark”.
v = (GM / r)1/2 is the rotation speed
G is Newton’s constant, M is the mass contained within a radius r
Rotation speeds, v, of planets in solar system decrease, proportional to (1/r)1/2
Implication: “M” above is a constant in our solar system = Msun
Measure rotation speed from the Doppler shift
Rotation looks nothing like we see in the solar system!
In spiral galaxies, v isapproximately constant at all radii
Rearrange the previous formula and you’ll get:
M = (v2 r) / G
If v = constant, then we must have that the total mass contained within a radius r, M increases with radius:
M( r ) = constant x r
BUT, the star light decreases sharply with radius!!!!
Dark Matter in Elliptical Galaxies: NGC 4649
Some elliptical galaxies have hot, X-ray gas in them. The gas is so hot (i.e., it’s moving so quickly) that the only way it can be bound to the galaxy by gravity is for the mass of the galaxy to be mostly DARK MATTER.
In a typical galaxy (like our own) there is at 50 to 100 times more dark matter than luminous matter.
A large galaxy cluster
Hot X-ray gas in galaxy clusters and gravitational lensing by galaxy clusters leads to the conclusion that galaxy clusters contain a great deal of dark matter.
In a typical big galaxy cluster there is 500 to 800 times more dark matter than luminous matter.
Turns out that it cannot be made up of the ordinary chemical elements (not enough time to make it in the Big Bang).
Best candidate is a type of particle called a WIMP (Weakly Interacting Massive Particle).
Pros of WIMPs:
Computer models show excellent agreement with observations
Grows the right number of galaxies, galaxy clusters and gives right “structure” to the universe
Cons of WIMPs:
Nobody has (yet) seen a WIMP - they are currently hypothetical particles (it’s possible they may be detected for the first time within 5 years)
“frothy”, “lacework” pattern to the observed galaxy distribution
WIMP universes reproduce the observed “structure” to the universe, grows the right number of galaxies of the right size, and grows the right number of galaxy clusters of the right size (big success).
Big problem is that WIMPS are currently hypothetical particles!
Fair Question: Do we really understand how gravity should behave in the limit of very weak gravity?
Einstein showed us that Newton was wrong in the limit of very strong gravity. Maybe Newton was also wrong in the limit of very weak gravity (i.e., maybe Newton’s law of gravity only works in the “middle range”).
At its peak brightness, a supernova is typically as bright as the whole galaxy in which it lives.
White Dwarf Supernovae (known as a Type-Ia supernova) can be used to make very accurate measurements of the distances to galaxies.
Type-Ia supernovae are easily identified from details in their spectra and light curves.
HST image of a supernova in a nearby spiral galaxy
HST images, before and during supernova in an extremely distant galaxy
Who’s awake out there?
Why is the big spiral galaxy in this picture ORANGE?
The spirals we saw in other classes were more blue in color.
Fair question: Do we really understand supernovae well enough to make this statement?
Maybe stars blew up very differently in the past than they do today. Maybe something else (not accelerating universe) is causing the distant supernovae to look fainter than we expect.
Answer: Everywhere! (they make up about 90% of the radiant energy of the universe at the present day, but you can’t see them with your eyes…)
MICROWAVE photons (very long wavelength, very low energy)
The CMBR should appear as a (nearly) uniform “hiss” of microwave radiation on the sky, and it should have a black body spectrum.
blue = “cold”,red = “hot”
Note the big temperature range! (100o C)
CMBR Temperature “Fluctuations”MicroKelvin (10-6 K) deviations from universal average temperatureColor scale goes from -3x10-6K (blue) to +3x10-6K (red)
The “lumps and bumps” in the CMBR aren’t random. They’re correlated and the pattern tells us that there is dark energy in the universe.
Neither the CMBR nor the supernovae have anything to do with each other, but they both give the same value of the amount of dark energy in our universe!!