Physical cosmology: the study of the universe’s structure and evolution

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.

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.