Week 7 Instructor Notes

The material for this unit is both the most difficult intellectually and (to me) the most interesting. In just the past few years, clues about the behavior of early-stage galaxies have flooded astronomers. The bottom line - small galaxies formed early, collisions (or interactions as they are sometimes called) stimulate galaxy growth, starburst activity, and energy emission by the massive black hole located at the center of just about every major galaxy. The black hole energy emission occurs when new material (provided by the galaxy collision dynamics) can stream into a previously relatively quiet black hole. The early universe was relatively small and packed with small galaxies, and galaxy collisions were frequent compared with today’s conditions. Thus, just about all the galaxies with active nuclei (especially quasars) are at "cosmological" distances - meaning near the limit of our ability to see out into the universe, or several billion light years distant. Furthermore, this "active" stage of active galactic nuclei is temporary, perhaps lasting only a few hundred million years.  Some questions remain about just how the supermassive black holes formed, and about how the actual clumping of mass occurred to form the earliest galaxies, and NASA’s Next Generation Space Telescope (aka the James Webb Telescope due for launch around 2012) will try to address those questions.

The text summarizes what astronomers know about unusually high-luminosity galaxies, i.e., galaxies for which most of the light output is not from stars, but from material falling toward a central supermassive black hole.  Astronomers have some rule-of-thumb data for luminosity (power output)  from various kinds of active galaxies, as well as some estimates of supermassive black hole mass consumption rates that correspond to those luminosities.
*  For reference, Milky Way optical luminosity ~ E37 Watt (from stars), radio luminosity ~ E31 Watt
Approximate consumption rates corresponding to various supermassive black hole luminosities:
    E36 Watt corresponds to about 1 solar mass consumed every 1000 years
    E38 Watt corresponds to about 1 solar mass consumed every 10 years
    E40 Watt corresponds to about 10 solar mass consumed every year
    E42 Watt corresponds to about 1000 solar mass consumed every year.
The various types of active galaxies produce luminosities in the following approximate ranges:
    Seyferts - from E36 Watt to E38 Watt
    Lobe Radio Galaxies - E36 Watt to E38 Watt
    Blazars - about E41 Watt
    Quasars - E38 Watt to E42 Watt
These luminosities are total throughout the electromagnetic spectrum, and for many active galaxies much of the luminosity is in the radio range.  In all cases, the spectrum is decidedly not starlike.  Refer to your text for observed characteristics of the different types of active galaxy.

One point worth remembering regarding General Relativity is that large masses (such as galaxies or clusters of galaxies) DO NOT BEND LIGHT BEAMS. LARGE MASSES BEND SPACE AND TIME, AND LIGHT BEAMS MOVE IN THIS WARPED SPACE/TIME; IN FACT, THE PATH OF LIGHT IN WARPED SPACE/TIME DEFINES WHAT THEORETICIANS MEAN BY "STRAIGHT LINES", OR SHORTEST DISTANCE BETWEEN 2 POINTS. This idea has its roots in something that is easy to understand. Suppose you compare your perception of your weight (which is the force you feel as you stand or sit, and is the result of Earth’s gravity) for two situations:

1. You are sitting in a hard chair in your house with all windows covered (so you can’t peek outside) on Earth’s surface at sea level, and

2. You are on the same hard chair in a space ship ACCELERATING with an acceleration of one "g" (i.e., the same acceleration you would experience during free fall on Earth), again with windows covered.

Would you be able to do any type of experiment to determine which situation you are really in? Einstein argued that you could not, and inferred from this answer that a mass such as that of Earth will bend (or warp) space/time, so that light beams would travel in apparent curved fashion. He went through this critical thinking process by himself over a period of years, completely without any guide from prior experimental observations, and in my opinion this still stands as the most impressive scientific intellectual achievement by one individual of all time. Since 1915, when his general relativity theory became common knowledge, all experiments have agreed with his original theory, and all related theoretical work has branched from that original theory. Understanding the physics associated with neutron stars, black holes, binary star mergers, and cosmology (to name a few) rely heavily on general relativity, a field many physicists and astronomers still understand only in a rudimentary way (including me).

While it is easy to state that mass results in curved space/time, we cannot imagine how that looks because we live in an approximately "flat" space/time. That is, space looks as though we can locate things with absolutely straight lines, and time appears to progress nicely in one direction at a constant rate, and light beams appear to move through space in straight lines. Our intuition therefore screams "flat" and that's as far as our brain can go.  The best we can do is imagine a two-dimensional curved space, because we are familiar with saddles, balloons, and other 2-dimensional shapes. Therefore, the analogies to curved 3-dimensional space that work best are the 2-dimensional shapes, and inclusion of time remains awkward. Your text uses the raisin pudding analogy for expansion of the universe. I also like the saddle or balloon analogy. Imagine a balloon with dots on it, with each dot representing a galaxy. The surface of the balloon IS the universe in this analogy, and this is a closed, finite universe analogy. Light traveling in this universe moves on the surface of the balloon, and everything in the universe is on the surface of the balloon. As you imagine the balloon blowing up to larger size, the separation of the dots increases, or the distance between any pair of galaxies increases. Furthermore, the farther apart any pair of galaxies is, the greater will be the increase in separation - exactly what we see in Hubble’s law. If you imagine light moving on the surface of the balloon, then its wavelength increases as the balloon grows larger - exactly the correct explanation of red-shift for distant galaxies. One of the discussion question assignments will give a brief glimpse into the difficulty of REALLY understanding what is meant by the following: A distant galaxy lies at a distance of 15 billion light years. Understanding this is tough not only because of the changing curvature of the space of our universe (as time has gone on), but also because of the rate at which time passes and how that has changed. Confused? I hope so - because if you think you can imagine a curved 4-dimensional coordinate system (3 space dimensions plus time) and its implications you belong in the cosmology graduate school hall of fame (or you need some serious psychological counseling).

The following table lists results of a calculation of galaxy recession speed (relative to lightspeed), present galaxy distance, and light travel time (aka "look-back time") versus redshift (the measured quantity) for today's model of the universe, with a Hubble constant of about 70 km/s/Mpc.  Note the increasing difference between the numbers in the two right-most columns with increasing redshift.  Remember, a redshift of 0.1 means a 10% increase in wavelength, or a factor of 1.1 times the original wavelength.  Similarly, a shift of 5.0 refers to a 500% increase in wavelength, corresponding to a multiplicative factor of 6 times the original wavelength.  The redshift of the Cosmic Microwave Background Radiation (CMBR) is about 1000, and corresponds therefore to a lookback time of very close to 13,700 years.
REDSHIFT velocity
lightspeed		 PRESENT DISTANCE
   (MILLION L.Y.)		 LOOK-BACK TIME
(MILLIONS OF YEARS)
0.000 0.000 0 0
0.010 0.010 137 137
0.025 0.025 343 338
0.050 0.049 682 665
0.100 0.095 1350 1290
0.200 0.180 2640 2410
0.250 0.220 3260 2920
0.500 0.385 6140 5020
0.750 0.508 8640 6570
1.0 0.600 10800 7730
1.5 0.724 14400 9320
2.0 0.800 17100 10300
3.0 0.882 21100 11500
4.0 0.923 23800 12100
5.0 0.946 25900 12500
6.0 .960 27500 12700
10.0 .984 31500 13200
50.0 .999 40100 13600
100.0 1.000 42200 13700
INFINITE 1.000 47500 13700

As you study cosmology (chapter 15), try to focus on the observational clues that support the theory of an inflationary, expanding universe as described in your text - and also take note of the data gaps that define the direction of today's research activities. If you plan to hang onto your text, you might want to pencil in some epochs on Figure 15-11, page 336. Your text describes these, and they are as follows:

EPOCH TIME (after zero)

            MAIN EVENTS

Planck 0 to E-41 sec Unknown physics, quantum gravity, random fluctuations of everything?
GUT E-41 to E-35 sec Unified strong, weak, E/M forces
Inflation (??) E-35 to E-32 sec Universe inflates size tremendously, faster than lightspeed (??)
Hadron E-32 to E-4 Production of high-mass particles (protons, neutrons)
Lepton E-4 to 4 sec Production of light particles (electrons, neutrinos)
Nuclear
reactions		 to 2000 sec Formation of deuterium and helium by fusion of protons, neutrons

Era of
nuclei

2000 sec to ~380,000 years

 Protons, helium-4 nuclei, electrons zipping around & scattering photons;
photons have short mean free path between scattering events
Atomic ~380,000 to 2.8E8 years Starts with formation of atoms (electrons combine with nuclei);
release of CMBR (photons no longer have free charged particles
to scatter from)
Galactic 2.8E8 to E9 years Formation of first stars, galaxies, and large-scale structure
Stellar to present Ongoing star formation & evolution, galaxy interactions & evolution


The times listed are highly approximate, and you will see different numbers in different texts - don’t worry about these minor differences. Everyone seems to agree on the sequence and basic physics. When atoms formed (electrons combining with nuclei), a key event occurred - the so-called decoupling of electromagnetic radiation. In simpler words, photons could then travel freely throughout the universe without interacting with charged particles. Those photons are still all around us, and we now refer to that radiation as Cosmic Microwave Background Radiation (CMBR), which your text describes well. This is one of the clearest signals from the early universe (and the main observation supporting the Big Bang theory), and study of that radiation tells us about conditions at a time corresponding to about 380,000 years after the universe began, as well as initial conditions around the time of inflation.

Another thing to note from the table of epochs is that physicists can theoretically reconstruct (based on the clues just mentioned) the universe’s history with known physics all the way back through production of the matter of the universe and (before that) the separation of the basic forces, BUT NOT EARLIER. Another area that remains speculative is the possible existence of an inflationary epoch that would have occurred (if it did) between about E-35 and E-32 seconds, just prior to the hadron epoch. The inflationary epoch would also include unknown physics, although there have been proposals that might eventually lead to acceptance and understanding.

In units 6/7, you learn about gravitational and light lensing evidence for the existence for large amounts of dark matter. In this week’s reading material, you also learn why this is such a big deal to astrophysics. The amount of dark matter is critically linked with the past and future expansion rates of the universe, and the future of the universe depends on the mass density of dark matter (as well as on dark energy). The present age of the universe also depends on the mass density of dark matter and dark energy. To further complicate matters, during the past few years cosmologists have begun to accept the existence of dark energy that is somehow responsible for accelerating the rate of expansion of the universe (based on observations of type Ia supernovae in very distant galaxies). Give yourself plenty of slack in case you come away from the week feeling somewhat in the dark (har!).

In closing, I suggest you adopt an attitude for this week’s reading as follows (pretty much my attitude):

1. We can understand the physics associated with production of the known stuff of the universe (photons and protons and electrons and helium and atoms and galaxies and stars), BUT

2. Some of this history requires a wild imagination, a sense of adventure and humor, and a skeptical, wait-and-see attitude - I'm thinking mainly of inflation, dark matter, and dark energy.