OTROS PASIVOS

Th e fundamental knowledge of physics explained in the last three chapters will now be used to explore the universe in more detail. To begin with, let me relate the second important landmark of cosmology: the discovery of cosmic microwave background

radiation, or CMB for short.

In 1963, Arno Penzias and Robert Woodrow Wilson, two radio astronomers working at the Bell Laboratories in Holmdel, New Jersey, USA, decided to use the 20-foot horn-refl ector antenna, originally built to receive signals bounced off the Echo satellite, to study astronomy. Th ey found an annoying noise which they could not get rid of no matter how hard they tried, including cleaning off the pigeon droppings left on the antenna and getting rid of the two pigeons. Th is unexpected electromagnetic noise turned out to be cosmic in origin, left behind by the reverberations from the original Big Bang. Th is discovery is so important that they were awarded the Nobel Prize for Physics in 1978.

Th e original and the subsequent measurements reveal that the wavelength distribution of this electromagnetic noise obeys a black-body spectrum with a temperature of 2.725 K. It is the same in all directions, to an accuracy of several parts in a thousand. Th e dominant wavelengths at this temperature are in the microwave

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and the far infrared region, which are partly absorbed by the earth’s atmosphere. To improve accuracy of the data, one needs to find a locale where atmospheric effects are minimized, in the arctic region or above the earth’s atmosphere. This is best done with satellites, where the accuracy reaches parts in a million at the present.

The first dedicated satellite, COBE (COsmic Background Explorer), was launched in 1989. At the time of writing in 2007, the satellite up there is the WMAP (Wilkinson Microwave Anisotropy Probe), launched in 2001. A third satellite, Planck, is scheduled to be launched some time in 2008.

Already with an accuracy of one part in a thousand, obtained by flying a U2 spy plane high above the atmosphere in 1976 and 1977, one began to see a slightly different temperature in two opposite directions in the sky. Half the sky is on average slightly warmer than the other half, an effect due to the motion of our sun in the universe. In the direction towards which the sun is moving, light waves are compressed, their wavelengths shortened, and that corresponds to a higher temperature. In the opposite direction, waves are stretched and wavelengths lengthened, corresponding to

Figure 35: Penzias (right) and Wilson (left) in front of the antenna they used to discovered the CMB.

Figure 36: The perfect blackbody spectrum of the CMB taken from the COBE satellite,[1] _{corresponding to 2.725 degrees Kelvin. The error bars are smaller}

than the width of the line.

Figure 37: Temperature fluctuation of CMB in the sky from the 2.725 degree

background, three-year data taken by the WMAP satellite.[2] _{Blue regions have}

slightly higher temperature than red regions.

3-year

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a lower temperature. This kind of bipolar distribution is called a

dipole.

With an accuracy of parts per million, obtained by the COBE and WMAP satellites, the anisotropy (differences in different directions) becomes much more complicated, but that complicated pattern can be interpreted as an acoustic oscillation set up by quantum fluctuations at the beginning of the universe. This complicated but small fluctuation turns out to be very important, because much information about the universe can be extracted from it, as will be discussed in Chap. 17.

I will discuss now how the cosmic microwave background radiation comes about. A more detailed discussion, especially about the fluctuation seen in Fig. 37, can be found in Chap. 17.

Just after the Big Bang, the universe was very small, but very hot. Atoms and complex nuclei could not exist because they were all torn apart by the intense heat into nucleons and electrons.

This equal mixture of positively charged and negatively charged particles is called a plasma.

Since photons can easily be absorbed and emitted by charged particles, in the dense environment of an early universe, no photon could last very long or travel very far before being reabsorbed. Although the universe was very bright at that time, it was also very opaque because the photons reaching your eyes must come from only a very short distance away.

This opaqueness is present even at the much lower temperature of the sun. The photons reaching us all come from a surface layer of the sun, known as the photosphere. The 6,000 degree temperature of the sun quoted before is only the temperature at its surface. The temperature at the center of the sun is some 15 million degrees, but because of the opaqueness we cannot see it directly.

About 400,000 years after the Big Bang when the temperature of the universe was reduced to approximately a quarter of an electronvolt, corresponding to a scale factor of about a thousandth and a redshift of about 1,000, the universe finally became sufficiently cool for electrons to be captured by the nuclei to form neutral atoms. This allowed the trapped photons to escape, which come to us today as the observed CMB.

The time when neutral atoms were formed and photons finally came out unimpeded is known either as the time of recombination (to form neutral atoms), or decoupling (of the photon from matter). We will come back in Chap. 17 to discuss what happened at that time and the wealth of information about the universe the CMB gives us.

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A Short History of the