Cosmic Microwave Backgroundby Cyrus Ance
Cosmic Microwave Backgroundby Cyrus Ance
Cosmic Microwave Background
My first column a> stirred up some controversy about the Big Bang. There are three observations that raise the Big Bang above theory or model. The observation by Edwin Hubble 1929 that all the galaxies are flying apart, implying that the Universe is expanding, is clear evidence that the Universe was much smaller at earlier times. When the Big Bang was first proposed as the origin of the Universe, essentially by Einstein in 1917, the thought that the Universe is expanding was thought absurd. He even put in a fudge factor, the so called Cosmological Constant, to counteract the expansion and hold the Universe in a steady state. Others besides Hubble and Einstein were crucial in the early development of the Big Bang model: Dutch astronomer Willem deSitter took Einstein's initial model in unexpected directions and predicted the Hubble observation but dismissed it as illusary; Georges Lemaitre was the first to predict the expanding Universe; Aleksander Friedmann, a Russian mathematician, analyzed the Einstein model without a Cosmological Constant and was the first to note that the Universe could end in a Big Crunch, Steady State, or a Big Chill. Einstein dismissed Lemaitre's and Friedmann's work until Hubble's observation. Einstein would later call the Cosmological Constant the greatest mistake of his life. Today we call it the Einstein-deSitter Big Bang. See here for more on the development of the Big Bang theory.
The second observation supporting the Big Bang is that the atoms in the Universe are mostly Hydrogen with about 24% Helium, and the other elements showing up at below the 0.01% level. If the Universe started too hot for a nucleus to form it stands to reason that as the Universe expanded and cooled down that the first nuclei to form would be the simplest ones, such as Hydrogen and Helium. Even better the exact proportions of the lighter elements agrees with the Big Bang prediction. This process goes under the name of Big Bang Nucleosynthesis and happened when the Universe was about one second old. The formation of the heavy elements will be covered in a future column.
The third observation supporting the Big Bang is the Cosmic Microwave Background (CMB), which is the subject of this column. The latest experiment to study this, Wilkinson Microwave Anisotropy Probe (WMAP), announced its first results just days before my first Bleeding Edge column appeared. Much of this column is based on the very excellent WMAP web site, so I will skip the usual footnotes and urge the reader to explore that site and its links to learn more. A few illustrative links to specific WMAP pages appear below. Basically as the fireball from the Big Bang expands and cools at a certain point, when neutral atoms form and the the Universe was about 400,000 years old, light energy can propagate freely rather than being absorbed by free electrons. This light traveling in all directions fills the Universe and should be observed today as microwaves.
The CMB as a consequence of the Big Bang was first predicted in the late 1940's. It was not observed until 1965 by Penzias and Wilson at Bell Labs in New Jersey. They made the observation inadvertently. They had noise in a radio receiver they were testing that they at first attributed to a slight radioactivity in pigeon droppings that covered the outer surface of their receiver. After cleaning the noise was still there. At nearby Princeton a group of researchers including Dave Wilkinson, the W in WMAP, had been planning a search for the CMB. When they heard of the problems with the radio receiver at Bell Labs they knew that the CMB had been found. It was at this point that serious discussion of other models for the origin of the Universe came to an end. The Big Bang model predicted the expanding Universe and the CMB which were later observed; predictions of other models, the Steady State model was the leading competitor, are in contradiction with observations. The abundance of Hydrogen, Helium, and other light elements was a severe test of the predictions of the Big Bang and it passed it during the 80's and 90's. Penzias and Wilson were awarded the Nobel Prize in 1978 for the observation of the CMB.
Nevertheless a great many details remain to be filled in. Exactly how did the Universe evolve from something small, hot, and dense, to what we see today, large, cold, and mostly empty? The CMB is a great tool for studying this. When the light gets free from the Universe it has a memory of the local density of the Universe. Denser regions would be hotter and this would result in slightly more energetic light than from colder, less dense regions. Since the light is traveling in all directions it is not possible to build a map of the density of the early Universe, but we can map the energy fluctuations of the CMB. We can tell the difference between a Universe that is lumpy, or one that is smooth. This was realized in the 70's and a series of balloon borne experiments started to fill in the map. The first definitive result came from the COBE mission in the early 1990's.
COBE did a spectacular job of showing that the CMB was exactly as the Big Bang prediction. There is a large feature at the level of one in a hundred that shows that the Earth is moving through the CMB. The radiation coming towards us looks more energetic than the radiation coming from behind. Taking out that effect the CMB is mostly uniform. COBE observed that the fluctuations were below the level of a few in ten thousand. Also COBE did not have good angular resolution, about 7 degree, that is it had a hard time telling the direction of the radiation it was observing.
WMAP is an improved version of COBE. It is sensitive to fluctuations in CMB energy at a few parts in a hundred thousand and has angular resolution of a few tenths of a degree. Both of these are more than ten times better than COBE. It sits at the L2 Lagrange point of the Earth-Sun system, the one beyond the Earth's orbit and thus gets an undisturbed view of the Universe over the course of one year. Results from the first year of data were announced on 12 February 2003. Here is a great comparison of the COBE and WMAP maps of the CMB fluctuations showing the improved resolution of WMAP.
The results are spectacular. Here is a list of what has been learned from the initial data:
The WMAP results have swept away any doubts that had been lingering about Dark Energy. The next challenge is to understand it, and WMAP has taken the first step on that road. The results also point the way to other new and renewed mysteries. It has been clear for a long while that there was some mysterious stuff called Dark Matter, and that it was a large fraction of the Universe. WMAP narrows what it could be. Inflation seems to offer the best explanation for the evolution of the very early Universe. We have no clue what drives that expansion.
Finally the WMAP results make it clear that we are living in a Golden Age of Cosmology, the study of the origin and structure of the Universe. Before the COBE result the field was noted for the vagueness of its predictions and a dearth of meaningful results. Since then it has gone from triumph to triumph. COBE set limits on the lumpiness of the Universe, which led to predictions about inflation, Big Bang Nucleosynthesis, and the nature of Dark Matter. Big Bang Nucleosynthesis was shown to be an accurate description of the abundances of the light elements. A number of observations were made of the effects of Dark Matter. The observation of the increasing acceleration of the expansion rate of the Universe by Dark Energy remains a shock. Now the improved data from WMAP has resulted in stack of results that no one can deride as vague or meaningless. Let us all hope that future results will shed light on the nature of Dark Energy, Dark Matter, and inflation.