v3.0.0 / chapter 10 of 10 / 01 feb 07 / greg goebel / public domain
* The discovery of the Big Bang was not the endpoint of the science of cosmology. Indeed, once having been proven, it almost immediately led to a set of other mysteries, whose investigation continues.
* Along with the open question of the ultimate fate of the Universe, the Big Bang theory also presented cosmologists with a number of puzzles:
Cosmologists express this issue in terms of a variable named "Omega", which is defined as the ratio of the gravitational energy of the Universe to the kinetic energy of the Universe due to its expansion. Omega is equivalent to the density of matter in the Universe, with a value of 1 defining the minimum density at which the cosmos will expand forever. A value of Omega of 1 is said to define a "flat" Universe.
A value of Omega more than 1 means the Universe expands forever, defining an "open", or "hyperbolic" Universe, where on a large scale the vertices of a triangle will add up to less than 180 degrees. A value of Omega less than 1 means it collapses, defining a "closed" or "spherical" Universe where on a large scale the vertices of a triangle will add up to more than 180 degrees. The fact that the Universe hasn't either dissipated or collapsed in the billions of years since the Big Bang means that the original value of Omega was within one part in 10^18 of exactly 1.
The fact that all three of these elements have permitted the creation of a Universe capable of being inhabited by humans and possibly other sentient life-forms has suggested to some that the Universe does show evidence of design by some higher power. (The comparable alignment of parameters at the scale of our Solar System to permit the development of life on Earth has been used to make the same point.) This is known as the "strong anthropic principle".
However, the use of the adjective "strong" also implies the alternate use of the adjective "weak". It can be and has been pointed out that if the Universe didn't begin in a state that wasn't either too uniform or too nonuniform, or had an Omega that differed a little bit more from a value of 1, then we wouldn't be around to wonder about it. In other words, by this "weak anthropic principle", we're about in the same position as someone who won the lottery: the odds of doing so might have been slight, but somebody won it and there was no cheating.
Cosmologists tend to avoid discussion of the anthropic principle, for the simple reason that once it's been discussed to the level provided here it's hard to think of anything to say that's more enlightening -- though Russian cosmologists have added the comment that though the anthropic principle made life possible, it's unfortunate that it doesn't make life perfect.
The fact is that cosmologists are looking for mechanisms, and neither the strong nor the weak anthropic principle provide them with much help in finding them. Scientists are also very reluctant to say anything that mixes up science and religion -- not necessarily out of any disrespect for religion, but because it's likely to create a noisy and useless controversy that will drain their time and energy, not to mention do nothing good for their professional reputations. Even Georges Lemaitre, a Catholic priest, was careful to keep his science and religion separate.
* In any case, puzzles like the value of Omega were the reasons why Fred Hoyle never gave up his opposition to the Big Bang model: he correctly identified that it had some holes in it that challenged its advocates. However, they did not avoid the challenge. In December 1979, Alan Guth (born 1947) of the Massachusetts Institute of Technology had a brainstorm, coming up with a theory that became known simply as "inflation" that explained why the Universe exists with these "just so" parameters.
Guth suggested that a hypothetical particle named an "inflaton" could have got into a highly energetic state in the Big Bang that essentially caused gravity to go into reverse, becoming repulsive instead of attractive, for an instant, about 10E-35 second, swelling the primeval fireball from a tiny size into cosmic dimensions. During this period of "inflation", the Universe would expand much faster than the speed of light. This short period of inflation would create a Universe that was flat or very close to flat, and smooth out nonuniformities in the state of the Universe, while simultaneously leaving subtle nonuniformities that would later give rise to galaxies and other structures.
* Inflation theory was refined by other astrophysicists, such as the Russian-American astrophysicist Andrei Linde (born 1948) and Japan's Katsuhiko Sato, and is widely admired for its elegance in explaining the previously unexplainable. There's one little problem with it: there's no proof that it really happened. One astrophysicist put it simply: "Inflation is a beautiful idea in search of a model." Worse, some cosmologists have suggested that inflation isn't provable, which to the strict would reduce it to a modern-day creation myth, not a scientific theory.
Inflation is still the main game in town for cosmologists at present, and it does have implications. One is that Omega has a value of 1, or something very, very close to it. Unfortunately, all attempts to survey the cosmos and take an inventory of the matter in it have come up with values much lower than 1.
Inventories of visible matter indicated it could only add up to a Omega of 0.01, and even adding all the "baryonic" matter, meaning protons and neutrons, calculated to have arisen in the Big Bang only gives an Omega of 0.06. This implies that most of the mass of the Universe must be a form of nonbaryonic matter that is very hard to detect. Astrophysicists now refer to this "missing mass" as "dark matter". Incidentally, this "cosmic" dark matter is not necessarily the same as the "galactic" dark matter associated with the unexplained rotation rates of galaxies discussed in an earlier chapter. They may have the same causes, but in the case of galactic dark matter there is no reason to rule out mundane sources such as brown dwarfs or large planets.
In any case, the first candidate for cosmic dark matter was the ghostly neutrino, which can zip through entire planets as if they weren't there. This was known as "hot" dark matter because neutrinos travel at high speeds. Recent observations of solar neutrinos suggest that neutrinos do have a slight amount of mass, but computer models using neutrinos as dark matter didn't seem to work very well. This led to consideration of hypothetical slow-moving or "cold" dark matter particles that were massive but very hard to detect, the WIMPs described in a previous chapter. The fact that nobody knew exactly what WIMPs might be was seen more as an opportunity than a problem, since theorists could then work backward from the evidence, or lack of it, to constrain what properties these particles could have. Candidate particles include the "neutralino", with a mass a thousand times that of the proton, and the "axion", with a mass a trillionth of that of the electron.
Neither of these "cold dark matter" particles have been detected, and the whole concept of cold dark matter ran into other difficulties. Measurements of the CMB showed it to be very uniform, but various sky surveys began to show that the Universe is even less uniform than previously believed, with huge cosmic filaments and sheets of galaxies -- though as mentioned, some suspect these structures are illusions. If they are for real, the problem is that the cold dark matter models cannot explain the emergence of such huge structures from a Universe that was originally very uniform. In order to explain the structures, some theorists came up with the concept of "cosmic strings" or "cosmic textures", defects in the structure of space that arose during the Big Bang, much as crystalline defects that arise at the boundaries when two regions of ice form and merge in an ice cube. Other theorists suggest that waves of supernovas in the early Universe could have led to the emergence of modern structures. Both theories have now fallen into disfavor and the issue remains open.
* Weaknesses in dark matter models have led some cosmologists to resurrect a concept that was very deliberately abandoned by Einstein a long time ago: the "cosmological constant", basically a "fudge factor" value that can be introduced into the cosmological equations to make them fit reality. This isn't a very clean solution to the problem and Einstein claimed it was his "biggest mistake".
The cosmological constant is denoted by "lambda" and is equivalent to a repulsive force that works over long ranges, a sort of "anti-gravity". Invoking a repulsive force might seen contradictory. If there's not enough mass in the Universe to constrain its expansion, adding an "anti-gravity" force would seem to make matters worse, causing the Universe to dissipate even faster. However, the expansion of the Universe may have been very slow in the distant past, and has been gradually picking up to the speed now observed. If it continues to pick up, then all the galaxies will eventually accelerate away from each other until they are lost from sight of all the others. They will then truly be "island Universes".
These days, the repulsive force defined by the cosmological constant is referred to, partly for lack of anything better, as "dark energy". There are possible candidates for dark energy. Modern quantum physics has shown that empty space is full of "zero-point energy" that could act as such a repulsive force. The major problem is that most analyses of the zero-point energy so far show that if it really does govern the expansion rate of the Universe, it is actually about 120 orders of magnitude weaker than analyses show it should be, otherwise the Universe would have dissipated long ago. Indeed, if something were not almost completely cancelling out the zero-point energy, space would be expanding so fast that the light from a object a meter in front of us could not be seen.
* Cosmologists are still tinkering with dark energy concepts, and in fact they coming up with seemingly endless variations of different cosmological theories. In essence, Guth touched off a "gold rush" to develop new schemes, some of which are perfectly mind-boggling. For example, while it has now become accepted that our Galaxy is only one of billions of galaxies in the Universe, some cosmologists claim that our Universe is only one endless numbers of them in an expanded "Multiverse". To absolutely no surprise, there are a number of competing Multiverse theories.
The original Multiverse theory arose out of a dilemma in quantum physics known as "superposition of states". Quantum mechanics states that the parameters or "state" of a particle are not only not known if the particle hasn't been observed, they actually undefined, or more properly, they exist across the complete range of values available to the particle, with a specific value obtained from an observation.
This is not merely a theoretical argument. It is observed in photon "self interference" experiments, where a single photon goes through two separate slits in parallel to interfere with itself. There is no serious dispute over the matter among physicists, a group of people who regard dispute as a fundamental part of their work. The well-known American physicist Dick Feynman (1918:1988), on being told by a student that superposition of states was unbelievable, replied that the student should go do the experiments until he did believe it.
For the most part, quantum physicists have simply shrugged and gone on with life, accepting that the rules of quantum physics apply to the microscopic world and don't have to make any sense in the macroscopic world in which we live. However, in the 1960s Hugh Everett III (1930:1982) of Princeton, bothered by the arbitrary way in which a specific state was selected, suggested that the simultaneous existence of all the states of a particle could be rationalized if the act of measurement split off a number of different universes, one for each possible value of the particle.
Some physicists have suggested this was a great deal of excess baggage to carry around and are skeptical, but there is no shortage of other multiverse theories. Andrei Linde has suggested that in the first instants after the Big Bang, the Universe was a "chaotic froth", with different regions with different properties, each undergoing inflation at different times. The regions became so thoroughly separated through the inflation process that they are now out of touch with each other, forming what amount to separate cosmos.
Linde added that new universes might also be spawned spontaneously off of existing universes by undergoing a burst of local inflation, with each universe having its own specific laws of physics, some dying out immediately but others surviving to spawn off other universes in turn -- in a sense, defining a Darwinian evolution of universes, with the emergence of our Universe with its anthropically-convenient parameters being just due to the right throw of the dice. Fred Hoyle has been judged wrong about the Steady-State theory, but though our Universe isn't eternal, the multiverse may well be.
In fact, Guth actually thinks that a separate universe could be created in the laboratory by compressing something the size of a bowling ball to the densities of a black hole, and then letting it expand again, much as our Universe did in the Big Bang. Guth says: "I like to think of it as an engineering problem, probably solvable by some future civilization."
* It is hard for laypeople to know how seriously many of the bizarre-sounding cosmological theories should be taken. In fact, it is hard to know how seriously some cosmologists take them. Astrophysicist Michael Turner of the University of Chicago has commented on the proliferation of such theories with surprising honesty: "Most of the theories are only attractive to the person who proposed them." Lev Landau once observed that "cosmologists are often in error but never in doubt."
Cosmologists now increasingly believe that new observational data is required to make sense of the chaos. One of the high-priority tasks is to get better estimates of the size of the Universe, which gives the proper value of the Hubble constant and by implication the age of the Universe. The higher the Hubble constant, the faster the expansion of the Universe, and the younger the Universe.
Cepheid variable stars can be used to obtain distance measurements to nearby galaxies, but they are useless for cosmological work, since the peculiar motions of nearby galaxies are much greater than any motion they have due to the expansion of the Universe. Even with the Hubble Space Telescope, Cepheids can be measured only out to a distance of 25 MPC. Galaxies that definitely show cosmological redshifts are at least 100 MPC away. However, Cepheids can be used to help calibrate other "distance indicators" to measure the distance to these galaxies. A number of such distance indicators are known:
The Tully-Fisher relation has been tested in a number of surveys, and it is generally though not universally regarded as a very accurate distance indicator. It can be used to estimate distance as far away as 300 million light-years, and it can be calibrated using Cepheid variables.
Type II supernovas, due to the collapse of a large star, can also be used as standard candles, though they are not regarded as a good a yardstick as Type Ia supernovas. Their absolute brightness varies considerably, but can be calibrated from the expansion velocity of the shell produced by the explosion.
The different methods tend to give somewhat different results. By the mid-1990s, they gave values of a Hubble constant in a range of about 50 to 100 KPS/MPC. The high value caused a problem, because dating of ancient stars in the globular clusters around our galaxy seemed to show that the stars were about 16 to 18 billion years old, well older than the Universe given a high Hubble constant.
During the 1990s, observations of 18 nearby galaxies by a team under Wendy Freedman of the Carnegie Observatories with the Hubble Space Telescope were used to box the value into a range of from 60 to 70 KPS/MPC. Improved measurements of the distances to Cepheid variables provided by the European Hipparcos star-position-mapping satellite had in the meantime shown that the Cepheid yardstick was "off" by about 10%. Applying the revised Cepheid yardstick to the old stars in globular clusters indicated that the oldest stars were only about 10 to 12 billion years old.
The age estimates of the Universe and the globular clusters seem to be falling in line, though the overlap in the error bars is still worrisome. However, cosmologists backing the "cosmological constant" theory pointed out that the acceleration of the Universe due to the cosmological constant would make the Universe appear younger than it was.
* Observations of the CMB have also been a high priority. In 1976, NASA flew an instrument on one of the agency's Lockheed U-2 aircraft, originally designed as a high-altitude spyplane but also well-suited to edge-of-space observations, to perform a crude partial map of the CMB. Results were intriguing and led to the development of an Earth-orbiting spacecraft to make a better map.
In 1989, NASA launched the "Cosmic Background Explorer (COBE)", the first satellite to map the CMB. Results of the mission were released in 1992. COBE's data showed that the background radiation is highly uniform, with a variation of less than 1 part in 100,000. This uniformity is consistent with Big Bang cosmology. It also demonstrated that the spectral curve of the CMB is highly consistent with that of a black body, as Big Bang cosmology predicts.
COBE project scientist George Smoot (born 1945) was the spokesman for a press conference that accompanied the release of the data and concluded his pitch by comparing the CMB map as being "like seeing the face of God." The remark appears to have been entirely casual but the press played it up, resulting in the predictable controversy, with Smoot being taken somewhat by surprise. In the end, however, he shrugged: "If my comment got people interested in cosmology, that's good, that's positive. Anyhow, it's done now, I can't take it back." Smoot and his colleague John C. Mather (born 1946) got the Nobel prize in 2006 for the CMB map.
The high uniformity of COBE's results was somewhat distressing to the cosmologists, since it made the emergence of cosmic structure difficult to explain. However, even COBE mission scientists warned that the COBE data was far from a perfect image of the CMB, since it had to extract that data from a sky full of confounding sources through averaging techniques. There was plenty of room for error, all the more so because COBE's map had a very coarse minimum resolution of 7 degrees of arc.
A number of other CMB mapping projects were conducted from the Earth and balloons during the 1990s, and in the summer of 2001, NASA launched a satellite, the "Wilkinson Microwave Anisotropy Probe (WMAP)", to obtain a better map of the CMB with an angular resolution of 0.3 degrees and substantially better temperature resolution as well. The spacecraft was named as an honor to the memory of Princeton physicist David Todd Wilkison (1935:2002), who had made major contributions to the development of COBE and WMAP. It was placed at the "L2" Lagrange point, positioned 1.5 million kilometers beyond the Earth in its orbit around the Sun. This was a stable position, given a little "station keeping" by the satellite, and provided an excellent view of the cosmos for a CMB mapping mission.
Initial WMAP results were announced in early 2003. The data implied that the Universe is 13.7 billion years old, with a composition of about 4.4% ordinary matter, 23% cold dark matter, and 73% dark energy. It also showed that the Universe is isotropic to 1 part in 10,0000 and Omega is close to 1, as consistent with inflation theory.
Additional analysis of the WMAP data provided more evidence of dark energy. Dark energy can be detected through what is known as the "Integrated Sachs-Wolfe (ISW)" effect, a phenomenon related to bending of light by its passage near massive objects. Einstein's theory of general relativity states that the distortion of space caused by a massive object will cause time to slow down, and so a photon passing near a such a massive object will be redshifted. If there was no dark energy, this would amount to nothing, since the photon would be blue-shifted back to its original wavelength once it passed away from the massive object. However, dark energy will stretch space while the photon is passing by the massive object, and so the "gravity well" that the photon climbs out of is shallower than the one it went into. The net effect is an increase in energy and a blueshifted photon. Researchers compared the WMAP survey of the CMB with the "Sloane Digital Sky Survey (SDSS)", a comprehensive survey of galaxies now in progress, and the results showed that on the average, photons were blueshifted by passing near massive objects.
Further missions are being considered to obtain even better CMB maps. The European Space Agency is now planning a mission named "Planck", which will be placed at the same location as MAP and is expected to provide even better resolution. Observational astronomers continue with their observations and cosmologists continue to refine their theories. Nobody is predicting the field will settle out any time soon.
* I'm not really a hard-core astronomy enthusiast. I once bought a nice telescope, and though it was one of the finest tools I ever had the pleasure to own, I got so little use out of it that I ended up selling it off. Nowdays I occasionally do a little sky-watching with binoculars on a clear night, but that's about all.
However, I always found astronomy an interesting element in science-fiction stories, and so I keep reasonably up-to-date on theoretical astronomy. I finally decided to organize what I knew in writing, and this document was the result. It was an interesting exercise, since there were things I'd known since I was a teenager and things that were surprises to me. This is why writing things down is so useful.
* I realize that professional astronomers use the Latin plural forms "nebulae" and "supernovae" instead of the English plural forms "nebulas" and "supernovas". However, this document is clearly not a professional work, and I chose to use the English plural forms for the sake of "user friendliness". For those who prefer the Latin forms, I beg forgiveness. For similar reasons, I have chosen to use light-years instead of parsecs as a unit of distance measurement. Since this is often done in popular works on astronomy, it should cause no difficulty.
* The original release of this document only covered stellar astronomy. The second major release covered galactic astronomy, extragalactic astronomy, and cosmology. I have taken a very cautious attitude towards cosmology: it's not that I think it's nonsense, it's just that I question that most cosmologists have the language skills to communicate some of their ideas to the lay public, and that the lay public may have very little reason to care even if they did.
Fans of the British DOCTOR WHO TV series will recall a running gag that perfectly illustrates the matter. DOCTOR WHO is about a scientist named the "Doctor" who travels the Universe with various companions in a time-space machine called the "Tardis" that looks like an old-fashioned British "police box" from the outside but is so huge inside that people get lost in it. The gag goes like this: "Why is the Tardis bigger on the inside than it is on the outside?"
"The Doctor says its because it's 'dimensionally transcendental'."
"What does that mean?"
Pause. "It means it's bigger on the inside than it is on the outside." A lot of cosmology comes across as much the same game. The cosmology discussion is as about as minimal as I can make it, and I keep looking for ways to cut it back.
* While the term "Big Bang" seems perfectly satisfying to most of us, there a few who feel that it is misleading or at least overly informal, and occasionally there are futile attempts to rename it. The marvelous little CALVIN & HOBBES comic strip of the 1990s, featuring an imaginative 6-year-old boy named Calvin and his stuffed toy tiger / imaginary friend Hobbes, came up with an even more colorful name: "the Horrendous Space Kablooie". So far, the name "Big Bang" seems to be hanging on handily.
* Sources include:
* Revision history:
v1.0 / 01 jul 00 / gvg / Originally released as THE STARS.
v1.0.1 / 01 jan 03 / gvg / Quick interim update.
v2.0.0 / 01 apr 03 / gvg / Cleanup, add chapters on galaxies, cosmology.
v2.0.1 / 01 mar 05 / gvg / Cleanup, tweaks, more on black holes.
v3.0.0 / 01 feb 07 / gvg / Thorough cleanup & restructuring.
The v1.0.1 release was the product of a screwup. I was working on the v2.0.0
release and somehow got the new, incomplete files scrambled with the old v1.0
files. Since I didn't have valid backup files for v1.0 and v2.0.0 wasn't
close to release at the time, I decided to throw together the v1.0.1 version
as fast as I could with what I had on hand. At least I had an opportunity to
correct a few bugs while I was at it.
The materials on fossil stars were originally released as a stand-alone document in September 1999. The material on gamma-ray bursters was also released as a stand-alone document in February 2000.
