AY C10 / L&S C70U Fall 2006Nicholas McConnell
1. What is the difference between and m ?
represents the overall content of matter (including dark matter) and dark energy in the universe (technically, it is a density divided by the “critical density”). m just represents the contribution of matter (including dark matter).
2. A Pringles potato chip is an example of what type of curvature?
Negative (hyperbolic)
3. If (hypothetically) m = 0.4 and = 0.8, what is the curvature of the universe?
In this hypothetical universe, = 0.4 + 0.8 = 1.2. Because > 1, the universe has positive curvature (like a globe).
4. What is one problem with the standard (no-inflation) Big Bang theory that is fixed by inflation?
The (almost perfect) uniformity of the Cosmic Background Radiation suggests that all parts of the observable universe ought to have been in contact with each other at some earlier time. Inflation is necessary to allow that—otherwise areas separated by more than 13.8 billion light years (half of the distance from one observable horizon to the opposite) would not have had time to communicate via light.
Also, the apparent flatness of the observable universe is very fragile (it is in unstable equilibrium, to be technical). Inflation would allow the overall universe to have a more stable non-flat curvature, but at such large scales that we would observe apparent flatness in our small region of it.
5. If the universe were slowing its expansion now, how would Hubble’s “constant” measured long in the past differ from its value now?
If the universe is slowing its expansion, presumably it was expanding faster in the past than it is now. This means that as we look into the past, we would observe a larger value of the Hubble constant. In fact, we observe the opposite, which suggests that our universe is expanding faster and faster.
6. Where was the oxygen we breathe originally created?
In stars. Only hydrogen, helium, and very small amounts of lithium were created during the nucleosynthesis phase following the Big Bang.
7. Why aren’t there any quasars in the nearest galaxies?
A quasar is created by matter spiraling into a super-massive black hole. Eventually the matter near the black hole is used up, and the quasar dies away. Quasars therefore should only appear in young galaxies. We view distant galaxies in their early stages because their light has taken a long time to reach us—nearby galaxies are seen when they are older, with no quasars.
8. Why isn’t the night sky filled with starlight in every direction?
Though the universe has a vast number of stars/galaxies spread in every direction, light from most of them has not had time to reach us yet. We can only see as far as light has traveled in the age of the universe (technically, our horizon is a little bit closer, where recombination first made the universe transparent). Therefore, we see a finite number of stars, within about 13.8 billion light years of us.
9. What is in the center of the Milky Way and other large galaxies?
A super-massive black hole, typically 1 million to 1 billion (106 – 109) times the mass of the sun.
10. Which is a more realistic model for the expanding universe: a balloon with dots drawn on in marker, or a balloon with paper dots taped on (dots represent galaxies or galaxy clusters)?
The balloon with paper is more realistic because the paper does not expand with the balloon. Likewise, galaxies, galaxy clusters, and smaller bound structures remain compact even as the rest of the universe expands.
11. Does the CBR spectrum resemble a blackbody?
Indeed. It is very close to a perfect blackbody, with a peak temperature of 2.73 Kelvin.
12. Is the universe currently undergoing inflation?
No. Inflation occurred for a short time interval (perhaps about 10-35 seconds) in the first instants of the universe’s existence (between 10-40 s and 10-35 s after the Big Bang).
13. What does it mean that our universe is “homogeneous and isotropic?”
“Homogeneous” means that it appears the same at all distances. A part of the universe that is 200 Mpc away from us should appear the same as a part of the universe 400 Mpc away. Note that light travel time complicates this. We expect that the universe looks the same at all distances now, but we see different distances at different times. So, for instance, the fact that we see quasars far away but not nearby does not violate the idea that the universe is homogeneous because that is an effect of time, not space.
“Isotropic” means that the universe appears the same in all directions. This is well-observed.
Note that for both of these, we are only considering the universe on large scales. Of course things look different inside the Milky Way than they do, say between the Milky Way and Andromeda. But on larger scales, when galaxies and galaxy clusters blur into larger structures, the universe is believe to appear the same at all distances and directions.
14. What value does analysis of CBR fluctuations suggest for
Analysis of CBR fluctuations suggests that space is flat (at least at the scale of the observable universe), so = 1 (or is very close to 1).
15. If a bright quasar formed in the first 10 years of the universe, would we be able to see it now (with a big enough telescope)?
No, because the universe was opaque before recombination, about 380,000 years after the Big Bang. We would not be able to see beyond the Cosmic Background Radiation to view this quasar.
16. Why doesn’t inflation violate the theory of relativity by exceeding the speed of light?
Relativity limits the speed at which an object can move through space. Space itself is not an object and can expand at any speed.
17. Does the universe seem to have more dark matter or dark energy?
Dark energy (about 73%). Dark matter is about 23% and ordinary matter is about 4%.
18. Einstein’s “cosmological constant” is most closely related to which feature of the universe as we understand it today?
Dark energy. Both Einstein’s constant and dark energy describe a force that opposes gravity and causes empty space to expand.
19. Cepheid variable stars and Type Ia supernovae are examples of what? Or if you can’t think of the vocabulary term, why are they useful?
The term is “standard candles.” A standard candle is an object that has a well-defined luminosity, so that a measurement of its brightness can yield its distance. (The formula relating distance, luminosity, and brightness is very important. If you don’t recall it, be sure to review it before the exam.) Cepheid variable stars are good standard candles which are visible in the nearest galaxies, whereas Type Ia supernovae are visible from much further away.
20. What is one piece of evidence that disfavors the steady state theory of the universe?
The Cosmic Background Radiation is a major blow against the steady state theory. The absence of quasars (whereas many exist in the older universe) is another argument against steady state theory.
21. How does quasars’ fast variability help confine our ideas for what they could be?
Because we observe quasars varying in brightness over short timescales (weeks to years), we can tell that their energetics must be confined to a small region (light weeks to light years, which in comparison to whole galaxy is very small). There are not many things other than a super-massive black hole that can create so much energy in such a small volume.
22. Astronomers observe two quasars with identical spectra closely flanking a large cluster of galaxies. What is actually happening here?
If the quasars have identical spectra, they are really the same quasar, which has been gravitationally lensed by the cluster into two images. The real quasar is behind the cluster, and its light gets bent around the massive cluster en route to our telescope, creating distortions or multiple images (or even a ring if the quasar and cluster are aligned just right).
23. Name two of the forces that a Grand Unified Theory would bring together.
A Grand Unified Theory unites the strong force, weak force, and electromagnetic force. A Theory of Everything additionally unites these forces with gravity.
24. A galactic rotation curve represents what versus what (what quantities are on the axes)?
Horizontal axis: distance from the center of a galaxy
Vertical axis: orbital speed around the galactic center