Big Bang Theory

Today, it has become widely accepted that our universe originated with the so-called big bang. This event marks the beginning of time and the further evolution of our universe. In the standard model of cosmology, this origin arises in a space-time singularity. The question what was “before” the big bang cannot be answered, because no physical description is available. More accurately, with reference to the singularity the concepts of time and space lose their meaning, as the scales involved (called Planck time and Planck length) are so incredibly small that spacetime can no longer be considered a continuum. Quantum gravitational effects come into play, and no consistent quantum gravitational theory has yet been developed. Physical description and understanding are therefore possible only after this period, which is called the Planck era. It is generally held that the big bang occurred about 15 billion years ago.

The idea of the big bang goes back to the Belgian mathematician Georges Edouard Lemaître (1894–1966). Many of its predictions are supported by astrophysical observations. The theory predicts that one fourth of all baryonic material should be made out of helium. Here, baryonic matter means mass consisting of “normal” nonexotic particles, like hydrogen, helium, and other nuclei, together with electrons. Observations of metal-poor objects in the universe indicate that this is indeed the case: Their chemical composition yields a helium amount of about 24%. By using this measured abundance of helium, it is possible to derive the different kinds of neutrinos (electrically neutral particles). It was theorized that three different kinds should exist, and this has since been confirmed by accelerator experiments.

Another important hint was the detection of the cosmic microwave background (CMB) by Arno Penzias and Robert Wilson in the 1960s. The astrophysicist George Gamow predicted this thermal blackbody radiation in 1946. Actual measurements by the satellites COBE and WMAP confirm this exact blackbody spectrum. Due to the expansion of the universe the temperature of the CMB has decreased from 3000 Kelvin (i.e., 2726.85 degrees Celsius) to a value of about 2.73 Kelvin (-270.42 degrees Celsius). Yet one more important point refers to the structures in the universe we observe today. They originate from small fluctuations in the density. These fluctuations should be imprinted on the CMB as fluctuations in temperature, which have been detected by COBE and WMAP. Altogether, the theory of the big bang is very promising.

Briefly after its birth, our universe underwent several different epochs. It is assumed that in the beginning, all natural forces were united into one elementary force.

The expansion of the universe led to a decrease in temperature and therefore shortly to the decoupling of the forces into the four basic forces: the gravitational force, the strong force, the weak force, and the electromagnetic force. Following the Planck era, the first one that split from the rest was gravitation. The remaining forces comprise the so-called GUT force. (GUT stands for grand unified theory.) The continued decrease in temperature then led to the separation of the others, starting with the strong force (which acts between the protons and neutrons in atomic nuclei), followed by the electroweak force, which divides into the electromagnetic force (describing the behavior of magnetic and electric fields) and the weak force (which acts between electrons and light particles). This happens after a time span of about only 10−12) seconds!

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