Physical cosmology

Cosmology (Greek κοσμολογία, kosmología, "the study of the world") deals with the origin, evolution, and fundamental structure of the cosmos, as well as with the universe as a whole. It is a branch of astronomy closely related to astrophysics. Its roots lie in cosmogonies, which first illustrated the origin of the world by means of mythical ideas, but with the pre-Socratics led to attempts to formulate abstract principles for it. Parmenides, for example, assumed a basic dualism that determined cosmic events according to "probability".

Today's cosmology describes the universe by applying physical theories, with general relativity being particularly important for the large scales and quantum physics for the smallest. The starting point for model building is astronomical observations of the distribution and properties of galaxies in the universe. The redshift of the spectral lines in the light of galaxies and their systematic increase with distance are interpreted as a growth in the size of the universe and lead to the idea that the universe emerged from an extremely dense and hot initial state and evolved from there to its presently observed state. Formally, the theory leads to a singularity, the Big Bang, which marked the beginning of the universe 13.75 billion years ago. However, beyond a certain magnitude and density of energies in the very early universe, the validity of known physical theories is exceeded. In particular, there is no valid theory of quantum gravity. While the beginning of the universe is therefore not accessible to current theories, there is a very successful standard model for the evolution of the universe, the Lambda-CDM model, which is in good agreement with a large number of observations.

Among the cosmologically relevant measurable objects of astronomy are the abundances of the lightest elements (hydrogen, helium and lithium) created by primordial nucleosynthesis, as well as the cosmic background radiation, which was released about 380,000 years after the Big Bang, when the temperature of the expanding universe had dropped enough for neutral atoms to exist. Subsequently, small density fluctuations due to the effect of gravity evolved into the large-scale distribution of galaxies and galaxy clusters, characterized by clumping, filaments, and intervening voids, which become increasingly homogeneous on the largest scales. Cosmology also records the small curvature of space measured on large scales, plus the spatiotemporal isotropy and homogeneity of the cosmos as a whole, the numerical values of the natural constants and the frequency distribution of the chemical elements.

All in all, this reveals a temporally forward evolution of the cosmos that proceeds in certain steps, the most prominent of which are called phase transitions, such as baryogenesis, primordial nucleosynthesis, or recombination.

Standard model

The standard or big bang model sees the beginning of the universe in a nearly infinitely dense state, from which it evolved in an expansion called the big bang to its present state, with the cosmos observable today inflating from a nearly point-like expansion to a radius of more than 45 billion light years. It is essentially based on the general theory of relativity and is supported by observations:

Density fluctuations

The density averaged over different length scales shows varying degrees of fluctuation. On the 10,000 megaparsec (Mpc) length scale, the variations are less than 1%, while on scales from 100 Mpc to 1 Mpc the structures become increasingly clumpy. Among the largest structures are the Sloan Great Wall, with a length of just over 400 megaparsecs, and the Hercules-Corona Borealis Great Wall, so far marked by only a dozen or so gamma-ray bursts (GRBs), with an extent of 2000 to 3000 Mpc.

The fluctuations observed today are thought to have evolved from quantum fluctuations during inflation, shortly after the beginning of time, with slower evolution on large scales than on smaller scales.

Frequency of the elements

In the primordial nucleosynthesis (Big Bang Nucleosynthesis) shortly after the Big Bang (10-2 s), the universe was so hot that matter had dissolved into quarks and gluons. The expansion and cooling of the universe created protons and neutrons. After one second, protons and neutrons fused to form the nuclei of light elements (^2H, ^3He, ^4He, ^7Li). This process ended after about three minutes. Thus the relative abundances of these light elements were largely established even before the formation of the first stars.

Cosmic background radiation

Postulated in 1946 by George Gamow, the cosmic microwave background (CMB) was discovered in 1964 by Arno Penzias and Robert Woodrow Wilson - with an average temperature of 2.725 Kelvin. The background radiation dates from 300,000 years after the Big Bang, when the universe was about one thousandth its present size. This is the time when the universe became transparent, before that it consisted of opaque ionized gas. Measurements, for example, by COBE, BOOMERanG, WMAP, Planck Space Telescope.

Expansion of the universe

In 1929, Edwin Hubble was able to prove the expansion of the universe, since galaxies show an increasing red shift in the spectral lines with increasing distance. The proportionality factor is the Hubble constant H, whose value is assumed to be 67.74 (± 0.46) km/s Mpc-1 (as of 2016). H is not a constant, but changes with time - inversely proportional to the age of the universe. We are not at the center of the expansion - space itself is expanding uniformly everywhere (isotropic universe). By counting back the expansion, the age of the universe is determined. If the Hubble constant (see Hubble time) is correct, it is about 13.7 billion years. Based on the data obtained so far by the WMAP probe and supernova observations, an open, accelerated expanding universe with an age of 13.7 billion years is now assumed.

Evolution of the universe

According to the standard model of cosmology, the sequence of events is roughly as follows.

• Planck era; to 10-43 seconds; all four forces still combined;
• Inflationary phase also GUT era; ends after 10-33s to 10-30 seconds; extreme expansion by a factor between 1030 and 1050;
• Quark era; up to 10-7 seconds; quarks, leptons, and photons are formed; the imbalance of matter and antimatter occurs in baryogenesis;
• Hadron era; up to 10-4 seconds; protons, neutrons and their antiparticles are created; also muons, electrons, positrons, neutrinos and photons;
• Lepton era; up to ten seconds; muons decay, electrons and positrons annihilate;
• Primordial nucleosynthesis; up to three minutes; hydrogen, helium, lithium are formed;
• Radiation era; about 300,000 years;
• Matter era; until today; universe becomes transparent, galaxies are formed.

Today, important instruments for exploring the universe are carried by satellites and space probes: the Hubble Space Telescope, Chandra, Gaia and Planck.

To explain the observed expansion and the flat geometry of the universe at large, the big bang model is supplemented today according to ideas of Alan Guth that a symmetry breaking in the early times of the universe led to a very strong short-time expansion, which explains the uniformity of the universe at the edge of the observable range (horizon). The biggest challenge to cosmological theory is the mismatch between observable matter and its distribution and the observed mean speed of expansion of the universe. The usual explanation makes dark matter (with 23 %) and dark energy (with 73 %) responsible for the parts of the required matter density not observable by means of electromagnetic radiation.

These proportions are time-dependent: The radiation-dominated era in the early days of the universe was followed by the matter era, in which matter made up the largest fraction. This era ended when the universe was about 10 billion years old; since then, dark energy has made up the largest fraction. Accordingly, the time course of the expansion changed: Until the end of the matter era, it was slowed down; since then, the expansion has been accelerated. This transition can be traced directly and independently of the model by observing supernovae over a wide range of distances.