Big bang: The point in time 13.7 billion years ago at which the universe was at its highest density and temperature.

The hypothesis that a big bang occurred was disputed into the 1970s. However, many astronomical observations can be explained by this hypothesis, so that by now it has achieved the status of a scientific fact. The redshift — the observed fact that all distances within the universe increase — indicates that all galaxies must once have been in the exact same spot. Though this event occurred a long time ago, there are still clear traces of it — the cosmic background radiation, the distribution of matter, the mass ratio of isotopes, and the concentration of helium and hydrogen — that enable us to reconstruct it. And while the current standard model of physics does not provide an explanation of the big bang itself, it does allow us to determine the events following it with a high degree of precision.

In the millennium simulation conducted by the Virgo research group, the development of the universe was recalculated with high exactitude in Spring 2005 on a parallel computer by the Max Planck Institute in Garching near Munich (Germany). The end result of this simulation perfectly matches the distribution of matter in our current universe, giving the simulation considerable credibility. Here is what it tells us about how things transpired:

Time zero: What exactly triggered the big bang, and how precisely it took place, are still matters of conjecture, since the standard model of physics is not suited for such brief periods of time. One thing that is quite certain is that the universe did not develop out of an explosion at a single spot as some textbooks still maintain.

The physicist Gabriele Veneziano developed an interpretation of string theory according to which the world is an infinitely large, eternal, cold, ten-dimensional space. At first, according to this interpretation, all ten ►dimensions were equivalent. An overlapping of two pulsating multi-dimensional membranes within this space caused our familiar three spatial dimensions to split off from the others at time zero. At the time of the split-off, the remaining dimensions coiled up into minuscule sizes and space underwent a process of extreme densification and heating. According to this theory, big bangs are recurrent events, each resulting in the temporary creation of a three or four- dimensional universe.

Whether or not this is true, all subsequent events can be derived from the standard model:

10-43 seconds:* The universe begins to evolve in an extremely dense and hot state. There are no atoms or atomic nuclei at this point, only electromagnetic radiation (thus, the Bible was surprisingly close to the mark with its "Let there be light!"). One liter of big bang weighs 1094 kilograms and has a temperature of 1032 degrees Celsius.** At that temperature the four basic forces in contemporary physics — gravity, strong and weak nuclear forces, and electromagnetic force — are still combined in a joint primordial power.

Space immediately begins to expand.*** The expansion decreases the radiation's density and temperature. Then gravity splits off from the combined primordial power to become the first autonomous physical force; the reason is that under a certain temperature, gravity is not subject to the same law as the other three forces. At this point, the radiation is still so energy-laden that it makes radiation particles turn constantly and spontaneously into anti-matter particles and back into radiation particles. During this process, a tiny amount of surplus matter — tiny compared to the amount of anti-matter — is created due to an asymmetry (CP violation). This surplus amount of barely 0.0000000001 percent is the basis of the entire amount of matter currently in the universe.

10-36 seconds: The radiation temperature has sunk to 1027 degrees Celsius. Now the strong nuclear force also splits off as an independent force. The separation of forces triggers a phased transition in the force fields not unlike the process by which water freezes and becomes ice. Energy is thereby released, leading to an "inflationary" expansion of space, during which space expands within a very short time by a factor of 1030. In the process, the region corresponding to what is now the observable part of the universe (►Hubble volume) rapidly reaches the size of a tennis ball. This extremely accelerated inflation of space is the cause of the even distribution of matter and radiation that we can now observe in the universe.

Due to this extreme expansion, the radiation cools down radically to 1016 degrees Celsius (a number we can even pronounce: ten thousand trillion degrees Celsius). At this point, the electromagnetic force and the weak nuclear force also split off. This completes the division of the primordial force into the four elementary forces with which we are familiar today.

10-16 seconds: The hot plasma consisting of radiation and particles that fills space now generates various sorts of heavy elementary particles by a process of agglomeration of quarks and antiquarks. As the temperature decreases, the heaviest of these particles decay until only the protons and neutrons (which will later be the component particles of atomic nuclei), along with their anti-particles, are left. Most of these protons and neutrons go on to destroy one another in collisions with their anti-particles, leaving only the tiny surplus of matter already mentioned.

Now the radiation energy is no longer sufficient to produce heavy particles. Only light elementary particles such as electrons and their anti-particles, the positrons, can evolve. A one-liter volume of space now weighs only 10 billion kilograms at a temperature of one billion degrees Celsius.

10 seconds: the temperature is now so low that the remaining protons and neutrons can unite to form stable ►atomic nuclei without immediately being torn apart by the radiation. While 75% of the protons float about as hydrogen nuclei, the other 25% become helium (which consists of 2 protons and 2 neutrons) while 0.001% become deuterium (1 proton, 1 neutron). Even today, the oldest stars still consist of exactly this mixture.

It is to be noted that the expansion of space does not lead to an expansion of the atomic nuclei themselves. Only the distances between them become larger. After five minutes, the density of matter has decreased so much that no further atomic nuclei are produced. The remaining neutrons are unstable and decay within the next few minutes. After that nothing notable happens for thousands of years.

10,000 years: Due to the steadily decreasing temperature, the radiation's energy and mass constantly decline. At this point there is more matter than radiation in the universe. Below 2700 degrees Celsius (3000 Kelvin), the positively charged atomic nuclei are able to "catch" negatively charged electrons and combine with them to form stable hydrogen atoms. These atoms are electrically neutral; as a result, they barely interact with the radiation particles. Light can now, approximately 380,000 years after the big bang, spread freely in the universe. The universe becomes transparent. With its radiation temperature of 3000 Kelvin, the light that filled the universe at that time can still be observed today in the form of background radiation.

1 million years: Since radiation no longer exerts pressure on matter, the latter is now increasingly under the impact of gravity, leading to reciprocal attraction between the particles. Previously, matter had been almost completely evenly distributed, with the exception of some minor density fluctuations that emerged during the aforementioned inflation phase 10-36 seconds after the big bang. These density fluctuations are now transformed into large clusters. In this process the atoms behave just like blowflies: the more of them are accumulated in one cluster, the more they attract others. This results in mega-clusters of hydrogen and helium atoms.

1 billion years: due to gravity, the mega-clusters undergo a process of contraction, which continuously increases their density. Eventually they turn into black holes so massive that they curve all space around them, turning that space into a closed-off region. Large clouds of helium and hydrogen gas are caught by gravitational attraction and rotate around them. Gas streams tumble into the black holes and disappear forever, releasing enormous amounts of radiation. These radiation sources — the ►quasars no longer exist today, but we can still see them at the periphery of the observable region of the universe.

Within the rotating gas clouds, the first stars and star clusters are created out of locally densified aggregations. Up to this point the world knew only hydrogen and traces of other light elements; now all heavy elements, including iron, start building up in the stars due to the melting of atomic nuclei. The larger stars explode as supernovas after only a few million years. The explosions create elements heavier than iron and catapult them into outer space. All heavy elements were originally incubated inside stars and in supernova explosions; since we ourselves are made up of such elements, we can literally say that we consist of stardust!

Once the black holes have attracted and swallowed up most of the gas clouds in their immediate vicinities, the gas streams pouring into them, and with them also the quasar radiation, run dry. The black holes come to rest and grow largely invisible. They become the centers of galaxies, which build up around them as stars are formed.

9 billion years: A cloud of gas and dust accumulates at the periphery of an otherwise not very notable spiral galaxy that also contains heavy elements from previous supernova explosions. Under the influence of gravity this cloud eventually coalesces into a solar system with nine planets.

13.7 billion years: The temperature of the all-embracing radiation has now decreased to under -270 degrees Celsius, barely 2.7 degrees above the absolute zero-point. This is the temperature measured today for the cosmic ►background radiation. On the third planet of the solar system mentioned above, small herds of creatures equipped with hand tools come crawling out of the jungle. They glance into the night sky and at once start pondering ►infinity...

What happens then? Since there is no reason to halt the computer simulation in the here and now, we can let it keep running until the end of the universe. You will find the result under Universe.

* = 0.000000000000000000000000000000001 seconds; scientific notation, see numbers.

** Every radiation possesses a certain mass (according to Einstein's formula E = mc2) and a certain temperature (according to Planck's radiation law), and both depend on the density and frequency of the radiation particles. Radiation can spontaneously turn into matter and anti-matter of the same mass. If matter comes into contact with anti-matter it decays again.

*** It does so even if it is already infinitely large. To picture how this can be, imagine that space consists of infinitely many small rubber cubes, all of which expand equally. It is still expanding today, though at a much slower rate; its expansion currently amounts to no more than 6 x 10-5 each million years. There are signs, however, that the expansion rate may be increasing again.

Links Related to the Topic:

■ The Myth of the Beginning of Time

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