Once Before Time - Martin Bojowald [70]
In fact, the apparent sizes of the moon and the sun in the sky are nearly equal, and impressive eclipses can result with a visible solar corona. As is well known, such events have often played important roles in ancient cultures. And by stimulating the desire to predict future eclipses, they have fostered support for astronomical research. Today, however, there is no doubt that the correspondence occurred by chance. Moreover, the moon is slowly moving away from the earth because the tides caused by its presence in orbit transfer energy from the earth to the moon. (Cosmic expansion does not play a role in this context, since its effects are overwhelmed by the gravitational binding of the earth-moon system.) The agreement of the apparent sizes of the moon and the sun is thus doubly random, the result of the unimportant properties of the earth and moon orbits in the solar system as well as of the period in time when we do our observations.
Issues are similar with many examples in cosmology. But starting a few years ago, observational cosmologists have become very clever in perusing various kinds of complementary measurements, and by now achieved unprecedented precision. This is the topic of the present chapter, with an emphasis on properties related to quantum gravity.
A further difference between measurements in cosmology and in the rest of physics is the fact that a cosmologist as observer is always part of the system under investigation: the universe. In other branches of physics, by contrast, the observer is separate from the prepared and measured system (except for rare self-experiments). Especially in combination with quantum physics, or quantum cosmology, this often leads to difficulties in understanding the theory, and to apparent paradoxa. We will come back to this issue in chapter 9, which addresses the uniqueness of cosmological solutions.
THE TRIAD OF OBSERVATIONAL COSMOLOGY:
A MIGHTY TRIUMVIRATE
Cosmological insights into the large-scale structure of the universe and its expansion are currently founded on three rather different kinds of observation:
(i) measurements of the already mentioned cosmic background radiation;
(ii) wide-ranging mappings of matter as it is collected on large scales in galaxies;
(iii) precise determinations of distances and velocities of violent star explosions called “supernovae type Ia.”
In addition, there are data on material decomposition, especially the relative abundances of light elements such as hydrogen and helium, that give information about an early phase in the universe called nucleosynthesis.
THE COSMIC MICROWAVE BACKGROUND: HEAVENLY DISTRIBUTIONS
A painting of you: not on the wall, on heaven itself, over it all.
—RAINER MARIA RILKE, The Book of Hours
Electromagnetic radiation once emitted as light has been traveling through space since early times, starting less than half a million years after the big bang,1 quite early in comparison with the total age of the universe of slightly less than 14 billion years. The radiation is rather faint, but it can now be measured in many details such as the distribution of its varying intensity over the sky. As already mentioned, this radiation had its origin in the hot universe near the big bang and began its travels to us when matter became transparent, at a temperature of about 4,000 degrees Celsius. At that time, matter had not yet been able to concentrate in galaxies despite gravitational attraction, for the radiation pressure of the hot universe would immediately have pulverized any denser aggregate.
Although they were emitted very early in the universe, the electromagnetic waves still exist as so-called background radiation. Unlike directed radiation from a single burst, such as a star explosion, which would eventually pass us and quickly fade away, the background