ASTRONOMY 1040

Class Notes #9   ALL NOTES ARE SUBJECT TO CHANGE

Note: You are not specifically required to read Chapter 16. However, there is important material there and I have included notes below as a synopsis. You are responsible for the material in the notes.

Outline notes for Chapter 16 (Chapter 17 notes below)

Dark Matter
Astronomers and physicists have discovered that there is more matter (mass) in the Universe than can be accounted for with just the stars, galaxies and dust clouds we can see. Quite a bit more. The main reason for believing this is the unexpectedly fast revolution of many if not most galaxies. Based on the number of stars we can see and acount for, the galaxies should revolve at certain speeds. Instead, they turn considerably faster than expected. The only reasonable explanation is that the galaxies are more massive thatn we thought. Greater mass causes stronger gravity which requires the stars and dust clouds in the galaxies to orbit faster according to Kepler's laws. So we have evidence that  the additional matter is there, but we can't see it, so it is "dark."

Fritz Zwicky
Among the first to discovere evidence of what we now call "dark matter" was CalTech astrophysicist Fritz Zwicky. Zwicky also predicted neutron stars, discovered supernovae, was called by some "the father of the jet engine." He also was a colorful character, often quirky and sometimes intimidating to colleagues and students.

In the 1930s he compared masses of galaxy clusters based on their brightnesses, with the masses he derived by determining the orbital motions of the component galaxies. He found that the component galaxies were orbiting the cluster much faster than expected.  His explanation, generally dismissed at the time, was that the galaxy clusters contained up to 400 times more mass than could be detected visually. Again, the idea is that the more mass, the stronger the gravity. The stronger the gravity the faster the galaxies have to orbit the center of gravity of the cluster. So if we have a reasonable idea of the size of the cluster, and can measure the speed at which the galaxies are orbiting, then we can estimate the total mass. Since this mass is much greater than can be accounted for with the visible galaxies,  the concept of "dark matter" was born

Vera Rubin
She studied individual galaxies, not specifically clusters. In the early 1970s Dr. Rubin published studies of a number of galaxies showing that orbital speeds of gas clouds in the galaxies did not fall off with distance from the galactic core as expected. She was faced with a puzzle, as had been Zwicky, because this meant that either Newton and Kepler were wrong, or there is a lot of unseen mass in the outer regions of the galaxies, contributing to the overall mass (and gravity) and forcing the gas clouds and stars to orbit faster than expected. Both Newton and Kepler being well established and confirmed, the only logical conclusion was that the unseen mass was what we now call "dark matter"

Today, astronomers believe that there is perhaps 100 times more dark matter than visible matter

Composition
What is dark matter? Without any in the lab to examine, nor in fact any that we can observe from afar, it might seem presumptuous to even guess at what it is. However, we can pretty well say that it must fall into one of two categories, or perhaps a combination of both. The first category is "baryonic" matter. The term derives from a Greek word for "heavy," although in this case it really just means that it has mass composed of "normal" particles -- the same type that make up the Earth, the Sun and you and me. The other type, non baryonic matter, could be composed of uncommon, rare or just really difficult to detect particles, or even particles that so far are unknown to science:
  • Baryonic matter, which is what we are involved with every day, is made of protons and neutrons
    • Could be small dim stars, brown dwarfs, neutrons stars, etc.
    • Could be in the form of large dark planets and other non-stellar objects
  • Non-baryonic or rare particles, perhaps some exotic forms yet to be discovered
    • neutrinos, which are now known to have a small mass
    • "Exotic" particles with names like neutrilinos and axions, which interact only gravitationally
No firm conclusions have been drawn as to the exact nature of the dark matter

Implications of Dark Matter
It may not seem obvious, but the ultimate fate of the Universe could depend no the amount of dark matter in it. Basically, if there is too much dark matter, the Universe contains enough gravity that this will eventually slow down the universal expansion and cause the Universe to fall back in on itself (many billions of years from now), possibly ending in an inconceivably hot and violent fireball like the one we believe it came from.

If there is too little dark matter, there will not be enough gravity, and the expansion will continue forever, with all stars eventually burning out the the Universe freezing.

To help figure these things out, astronomers invented a parameter called Omega (Omega Parameter), the last letter in the Greek alphabet. Of course this is a very appropriate symbol, since it represents the ultimate fate of the Universe!

The Omega Parameter
Astronomers speak of the mass density of the Universe, which is a gauge on the total amount of matter in the Universe, call it Du (or "D sub U"). They also speak of the critical density, which is that density required to stop the expansion of the Universe, call it Dc (or "D sub C). The parameter Omega (Omega Parameter) is the ratio of  Du to  Dc or  Du / Dc 
  • If Omega Parameter > 1 (greater than 1), there is enough mass in the Universe to supply enough gravity to stop the Universal expansion and cause it to fall back in on itself. This is said to be a "Closed Universe"
  • If Omega Parameter < 1 (less than 1), there is too little mass to stop the expansion and the Universe will expand forever. This is said to be a "Open Universe" with no maximum size
  • If Omega Parameter = 1 , it is a seemingly highly unlikely situation (Something like standing a pencil on its point and expecting it to stand like this for thousands of years through earthquakes, hurricanes and so on. It would mean that the Universe would continue to expand forever, but continually slowing down, continually approaching but never reaching an effective maximum size. This is said to be a "Flat Universe"
Unlikely as it may seem, the Universe appears to be flat (Omega Parameter=1), or nearly so. This means that Omega had to have been 1 to about 60 decimal places at the Big Bang, a precision greater than shooting an arrow from Denver and hitting a bullseye in Los Angeles.

This so-called "Flatness Problem" seems to have been solved by the "Cosmic Inflation" addendum to the Big Bang model. Cosmic Inflation (mroe below) is an extra expansion of the Universe that happened at atiny, tiny fraction of a second after the Big Bang began. One of the natural consequences of cosmic inflation is that the Universe would just naturally be "flat."

The Fate of the Universe
Our current knowledge of the amount of matter in the Universe indicates that expansion will never stop. (Omega is less than one, but still close enough to one that it had to be incredibly close to one at the Big Bang.) Still, this question has not been conclusively answered.

In recent years, a complication has arisen in what astronomers call "dark energy," a kind of reverse gravity that pushes things apart rather than pulls them together. The evidence for this dark energy is in observations of some galaxies that appear to be receding from us at a greater than expected rate.

In any event, it currently appears that the Universe will expand forever.

Chapter 17 notes

Creation of the Universe
The Big Bang (see more details below) -- in essence, all of what we know started from a point smaller than an atom about 14 billion years ago. It was hot (trillions of degrees) and started expanding. As it expanded it cooled and matter began to form out of energy a bit like ice forming as water cools (remember E=MC2?). Eventually gravity condensed gas clouds into galaxies and stars, planets formed and at least in one tiny part of the Universe, Life arose.

The idea of the Big Bang can be said to be a natural consequence of Einstein's General Theory of Relativity, which he presented in 1915. Einstein himself originally believed that the Universe was static, neither expanding nor contracting, until Hubble's argument convinced him otherwise. In fact, the very fact of a universally attractive force of gravity meant that  the Universe could not easily be static was discussed at least as early as Newton, but it was not until the 1920s that the first conception of what we call today the Big Bang theory  arose. It was by Belgian astronomer-priest Georges Lemaitre in the 1920s. Other considerations were made before Lemaitre, but his is considered the beginning of the Big Bang as we know it. Many of the details,including some important predictions later shown to be true, were formulated in the late 1930s and 40s by Russian-American physicist, George Gamow. In particular, Gamow predicted the two most important lines of evidence in favor of the theory (other than the universal expansion shown by Hubble): the ratio of hydrogen to helium in the Universe, and the Cosmic Microwave Background radiation (see below).

The Steady State -- is now discredited, but is the only alternate to the Big Bang that ever had much of a following. Thought up by Fred Hoyle and others in the 1950s, Steady State calls for an eternal Universe with no beginning. As it expands, new matter is spontaneously created to maintain the same density, and to adhere to the Cosmological Principle (the requirement that the Universe look the same in all directions and from all locations, known as homogeneity and isotropy). In addition, Steady State required that it be the same throughout time, with a "Perfect" Cosmological Principle. The Steady State pretty well died when it failed to account for certain aspects of the observed Universe, most specifically the Cosmic Microwave Background Radiation (see below).

Others
Alfven's "ambiplasma" Universe - in which a combination of matter and antimatter somehow produce an expansion without an initialBig Bang-type event. [http://www.campusprogram.com/reference/en/wikipedia/p/pl/plasma_cosmology.html]

Ekpyrotic Universe - an idea that our universe basically resulted from a collision of two multi-dimensional membranes ("branes") in 5-dimensional space [http://www.space.com/scienceastronomy/astronomy/bigbang_alternative_010413-1.html]

The "Big Rip" - incorporating the fascinating "dark energy" now discussed by cosmologists, except having it increase with time to the point that it will eventually rip apart galaxies and the Universe itself.  [http://www.newscientist.com/news/news.jsp?id=ns99993461]

Needless to say, there have been many attempts to devise alternative cosmologies to supplant the Big Bang. Some claim to explain certain aspects of the observed evidence better than the Big Bang, but typically fail in other areas. Aside from the Steady State Theory and its more recent incarnations, none of the alternatives to the Big Bang have been fully successful in explaining all observed cosmological phenomena as well as Big Bang, and none have been widely accepted. Some, like the Inflationary Scenario, have done well in explaining certain aspects and have been incorporated into the Big Bang model.

Religious ideas
Hindu Creation - some aspects of the Hindu Creation myths seem to correspond well with aspects of the Big Bang model, especially in terms of the timeline.

Judeo-Christian Creation - Creation as outlined in the Bible, taken literally, does not correlate well with the physical evidence considered in the Big Bang. For instance, in Genesis, the Earth is created before the Sun and stars. Physical evidence suggests otherwise.

Mythologies - nearly every culture has a creation story, but none that I am aware of relates well to the Universe as currently understood in Cosmology.

"Scientific Creationism" - Attempts to interpret physical evidence to support the Biblical Creation story. However, it violates the scientific philiosophy by starting with a preconceived idea and attempting to "prove" it  (frankly, many scientists do the same -- usually to their detriment, but this fact does not in any way justify the same violation by Scientific Creationists).  Adherents to Scientific Creationism often ignore the preponderance of evidence that does not fit their preconceived idea, in favor of a small bit of evidence that may seem to support it.

Side note -- interestingly enough, a number of religious leaders have embraced the Big Bang concept  because it involves a creation event. In their minds, this requries the existence of a creator. This argument dates back at least to the ancient Greeks, and was well formulated by Saint Thomas Aquinas in the 13th Century. It is also called the "First Cause" argument, and a recent version is called the Big Bang Argument. In all cases the argument is simply that if the Universe started at a moment in time, there must have been some agency (the "First Cause") to start it, and by definition that "First Cause" was God. This is a philosophical and religious argument argument and is not addressed in any fashion int he Big Bang model. The Big Bang really has nothing to say about  it at all. In fact, many scientists feel that it is just as reasonable to assume that the Universe started out of nothing for no particular cause, since before the Universe began there was no cause and effect process anyway. Others note that even if the Universe came into being through some agent, which by definition could be called "God," that in no way implies that this force of creation was (or is) anything like the traditional view of an omniscient being involved in everyday affairs of the Universe, nor indeed even a conscious entity to which we can reasonably apply concepts such as good, evil, compassion and so on. This concept apparently is what Einstein referred to in his quotations about God (or "The Old One"), such as "God is not malicious, He is merely indifferent."

The Big Bang Scenario
Approximately 14 billion years ago, the Universe sprang into being, apparently out of nothing, as an incredibly small point (smaller than the nucleus of an atom) of pure energy. (In standard theory, we cannot know what happened before this time, and in fact cannot say anything definite about things that happened before 10-43 of a second after the beginning.) At this very early stage the one and only force in the Universe separated to form the force of gravity and the electronuclear force. This "symmetry breaking" provided the mechanism whereby this tiny point of energy began to expand. Temperatures were trillions of trillions of degrees.
  • At about 10-34 seconds, the Universe suddenly and temporarily increases the expansion rate a bit like a kernal of popcorn popping. This is due to the second symmetry breaking, that of the electronuclear force separating into the strong nuclear force and the electroweak force. This vastly increases the size of the "observable" Universe in a tiny fraction of a second, to about the size of the a beachball, and is known as "Cosmic Inflation." (The symmetry breaking released energy to fuel this sudden expansion. A similar release of energy [although far less violent] occurs when water freezes, which is another kind of symmetry breaking)
  • At about 10-32 seconds, quarks and electrons form and the temperature is about a quadrillion degrees!
  • At about 10-12 seconds, the electroweak force splits into the weak nuclear force and the electromagnetic force. Thus all four currently known forces of the Universe have come into being
  • At about 10-6 seconds, the temperature has dropped to about 10 trillion K, and quarks combine to form protons and neutrons
  • Within the first three minutes, all hydrogen nuclei (protons) and most helium nuclei in existence today had formed. At about 100 million degrees, it was still too hot for stable atoms (nuclei plus electrons) to form
  • In about 300,000 years (Bennett says 380,000 years), the Universe had grown to perhaps ten times or more than the current diameter of the Milky Way (that is, maybe 1 to 2 million light years). The temperature had dropped to about 3,000 degress and electons could bind with nuclei to form atoms. Prior to this time, the free electrons had gotten in the way of photons, preventing them from flying through space freely. Now that the photons could move freely, the Universe became "transparent." The "Cosmic Microwave Background Radiation" now observable in the Universe dates from this era.
  • Within about one billion years, the first galaxies begin to form from gravitationally contracting large gas clouds. Galaxy formation was a problem with the first big Bang models, which call for a very uniform spread of matter with no concentrations. However, it is now thought that small "quantum fluctuations" in the very early Big Bang spread and expanded with the Universe, providing a kind of "gravitational catalyst" along with gas clouds, galaxies and stars could form. The observable Universe was 4-5 billion light years across and the average temperature had dropped to aabout 20 degrees above absolute zero (~20K)
  • Galaxies, stars and planets (and ultimately life) have continued to evolve since then until now, about 14 billion years after the Big Bang. Today the observable universe is roughly 28 billion light years across and the temperature is about 2.735 degrees above absolute zero, on the average.

Big Bang Evidence - the Three "Smoking Guns"
  • The Universal Expansion -- the expansion of the Universe, as evidenced by red shifts of distant galaxies and following the Hubble Law, indicate strongly that the Universe started out small and has increased in size just as would be expected in a Big Bang scenario. The evidence for this can be seen in the Hubble Law (review):
    • Hubble and his assistant Milton Humason (former janitor turned astronomer) studied the motions of galaxies
    • They looked at the red shifts of galaxies and determined that most were moving away from the Milky Way
    • Hubble compared the red shifts with (only roughly known) distances to galaxies to formulate his law:
      • Galaxies are moving away from the Milky Way, and the farther they are, the faster they are moving
      • The upshot to this is the idea that the Universe is expanding
      • The uncertainty in this has always been our imprecise knowledge of the distances to galaxies
  • The Cosmic Microwave Background Radiation (CMBR) -- years before it was discovered, physicist George Gamow predicted that the Universe should have cooled to a particular temperature from the time of the Big Bang. With a few adjustments due to the currently estimated age of the Universe, Gamow's prediction has been strongly verified in the CMBR, which in essence measures the temperature of the Universe like a thermometer measuring the heat of cooling embers from a campfire.(This is sometimes referred to as the "Cosmic Microwave Background" or just CMB, or the "Cosmic Background Radiation" or CBR.)
  • The Ratio of Hydrogen to Helium in the Universe -- also years before it could be measured, Gamow predicted that the Universe should consist of about 75 percent hydrogen and 25 percent helium. This is the ratio of these elements formed in his model of the Big Bang. Aside from a little lihium and beryllium, all other elements are formed in stars and supenovaes, and even today represent only a tiny fraction of the elements in the Universe. The ratio of elements, measured years after Gamow's prediction, confirm the basic Big Bang model well.
Big Bang Problems - The Big Bafflers
  • The Horizon Problem - Today there are regions of space that are too far separated to ever have been in contact, even using signals traveling at the speed of light. Yet these areas have almost exactly the same physical characteristics including temperature. This is extremely unusual if these areas never had the chance to mix together (like mixing two cups of water together, each at a different temperature, and allowing them come to the same temperature). In essence, one part of the Universe can't look over the "horizon" to see what the other part is doing. Not to make it sound too anthropomorphic, but imagine that you are a plant superviser and headquarters requires that you keep your inventory of widgets the same as that in another plant 50 miles away, and that you can't communication with the other plant supervisor except through semaphore (visual signals with flags, lights or other devices). But the other guy is 50 miles away! Not even a telescope could help you because the other plant would be "beyond the horizon" and not visible at all. It would be pretty unlikely that you could keep the inventories exactly in step. Any yet the Universe is like that.
  • The Smoothness Problem - the original Big Bang scenario requires a completely "smooth" Universe, that is, on is which matter is completely uniformly distributed on large scales. The problem is that it is not smooth. If it were, there would be no galaxies, no stars, no planets, no life. On very large scales it is very nearly smooth, but thankfully not quite. Where did the "lumpiness" come from?
  • The Flatness Problem - thinking back to the Dark Matter situation, we noted that Omega is nearly one, meaning that this is nearly the exact amount of matter in the Universe to just cause it to slow down in its explansion, but never quite stop. This is a highly unusual situation that demands that the density of the universe equal the critical density at the time of the Big Bang to a phenomenal accuracy of about 60 decimal places. There is no other parameter in the Universe taht precise.
  • The Magnetic Monopole Problem - the Big Bang cosmology requires "Grand Unified Theories" or GUTs in which the nuclear forces and electromagnetism were once united. These theories call from a type of particle called a magnetic monopole, that have only one magnetic pole. Today we know only magnetic dipoles, like a bar magnet, that have both "north" and "south" poles. According to the theories, however, magnetic monopoles should exist today, but even after exhaustive searches, none have ever been found.

There are many other small questions that remain to be answered, as well as alternate explanations. For instance, there are been several suggestions of how the cosmological red shifts can be produced other than by expansion. None of these are considered to valid by many physicists or astronomers.

Cosmic Inflation - explains the problems above
  • The Horizon Problem is explained by assuming that in the very early stages of the Big Bang, everything was in contact with everything else, so for instance temperatures would all be exactly the same. The sudden expansion of Cosmic Inflation (just "inflation" below)  suddenly increased the size of the Universe at a rate greater than the speed of light, locking the uniformity in.
  • The Smoothness Problem is solved by inflation, as it locked a great deal of smoothness into the early Universe. However, it also carried with it tiny "quantum fluctations" and similarly blew them up in size. The quantum fluctuations in essence created "wrinkles" of stronger gravitational attractions in some locations. It is around these gravitational wrinkles that galaxies formed and the Large Scale Structure of the Universe is evident.
  • The Flatness Problem was solved by inflation as well. How it solves the flatness problem may be best explained with an analogy. Imagine that you are an ant on a partially inflated "magic" balloon. It is magic becuase it can be inflated without limit. Since the ballon is small and only partially inflated, it is obviously curved. Even a small but intelligent ant can tell it is round. Then suddenly the balloon is inflated to a billion billion times its original size. Assuming the ant survived the trip and didn't get flung off into space (!), the surface of the ballon would no longer look round or curved. It would appear completely flat. (The actual process is of course much more technical, but hopefully this gives you an idea.)
  • The Magnetic Monopole Problem is solved by inflation since by their very nature, only a small number of magnetic monopoles could have existed in the very early Universe before the Inflation event because the Universe was so small then. In the vast increase in size of the Inflation, the small and limited  number of monopoles was locked in yet spread out over an almost unimaginably larger area. Thus, while magnetic monopoles may still exist, our chances of finding one are very small, given that there are only a (relative) few in an enormous Universe.

The Big Bang Conclusion
Is the Big Bang Correct?
The Big Bang model is an attempt to connect the observed Universe of today to a scientifically feasible series of events leading back to its creation at some point in the past. But since it appears impossible to peer back in time to the event itself, the Big Bang will always retain some percentage of doubt. Today's model, with all its loose ends, seems the most plausible scenario for most scientists, but that does not mean that it is absolutely certain. It may be that the Big Bang model is a bit like Ptolemy's "Cycles and Epicycles," providing a measure of predicability in terms of observed phenomena, yet bearing no real resemblance to reality. Newton's gravity was thought to be a perfectly accurate mirror of reality for more than 200 years -- and it works incredibly well in everyday life -- yet Einstein showed that it was nothing more than a good but imperfect prediction device (more like Ptolemy' s models than a mirror of real life.) Most astronomers and physicists today believe that the Big Bang, or some process essentially similar to the Big Bang, really did happen. There are and will remain dissenters, still...

If it looks like a duck, and walks like a duck, and sounds like a duck .... well, you get the point.

After the Big Bang?
  • The Big Crunch - if there were enough mass in the Universe -- and no dark energy -- the expansion would stop and the Universe would fall back in on itself, possibly repeating the Big Bang scenario endless times. Currently this does not seem likely given the observational evidence.
  • The Big Freeze - today most astronomers believe that the Universe will expand forever, and eventually all stars will burn out, leaving the nothing but a few dark cinders in a cold, dark void. (Don't worry, though, as this is many billions of years in the future!)

END

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