Once Before Time - Martin Bojowald [63]
16. Two sides of space, which, in many regards, behave like mirror images or inversions of each other. (Universo positivo [Positive Universe], 2002–06; Universo negativo [Negative Universe], 2002–04. Sculptures and photograph: Gianni Caravaggio.)
SO, WHAT ACTUALLY HAPPENED BEFORE THE BIG BANG?
THE ULTIMATE QUESTION
No direct observation of the universe before the big bang is possible. In the extremely compact state near its turning point, the early universe is simply too opaque for light or other forms of electromagnetic radiation to reach us. The earliest moment visible in this way was after about 380,000 years of expansion. At that time, the universe had grown enough to cool down and dilute matter, making space transparent. In a universe still at very high temperatures, about 4,000 degrees Celsius, matter radiated like a hot, glowing body akin to the surface of a star. Before that time, matter in the universe was even denser and hotter and radiated in the same way. But the radiation was directly reabsorbed into the fiery plasma of the early universe. Just as one can see only the surface of the sun and not the interior, we cannot glance back arbitrarily deeply into the big bang.
IMPENETRABILITY: THROUGH THE FOG OF OLD
Only after sufficient dilution and cooling of the still hot matter was it possible for some of the radiation to escape and start traveling through the expanding universe to us. As drops of rain condense in a dense fog and slowly improve the view, below a temperature of 4,000 degrees Celsius, neutral atoms formed in the early universe. Without a net electric charge, atoms scatter light much more weakly than unbounded electrons and protons. Traces of the radiation released after sufficient expansion can be observed with sensitive antennae as cosmic background radiation. They are one of the most important sources of information about the early universe, as we will encounter in more detail in the next chapter.
All this happens long after the big bang and can at best give indirect indications for the form of the universe before it. There are other harbingers not of an electromagnetic nature, with a potential to show us traces of the big bang. With suitable information carriers interacting with matter less emphatically than electromagnetic radiation, one could possibly see back to earlier times. On the other hand, these messengers must be sufficiently sturdy and long-lived so as not to decay on the long trip to us. Only two possibilities then remain: neutrinos and gravitational waves.
Neutrinos are elementary particles that, in contrast to electrons, are electrically neutral and have almost no mass. They are easily created in radioactive decays and also played a role in the early universe. Unlike photons, the carriers of electromagnetic radiation, neutrinos are hardly absorbed by matter. One can see this impressively by the fact that neutrinos produced by nuclear fusion deep in the sun can reach Earth, while light can reach us only from the surface, where the temperature is too low for fusion. Most neutrinos even fly right through the whole planet: One can see them on the night side as well as the day side facing the sun, in clear contrast to light. On the other hand, owing to this extremely weak interaction with matter we notice only a small fraction of all neutrinos. From known production rates of neutrinos we can estimate that 1 cubic meter of space, everywhere in the universe, including on Earth, contains about 30 million of those particles, of which we notice nothing. Due to their small mass, all neutrinos