The universe or universe in astronomy, or the cosmos in cosmology, are synonyms for all matter and energy within the entire space-time continuum in which man evolves.
Astronomy distinguishes between the visible and the theoretical universe:
The visible universe comprises that part of the universe from which, since the "beginning of time", light could reach the earth, and which is therefore perceptible to man. At the given speed of light, the part of the universe which, measured from the "beginning of time", is observable to man is finite.
The term "theoretical universe" refers to several cosmological models, which theoretically describe possible, inclusive structures in which the visible universe may be "embedded". These models are investigated in the different string theories and in the theory of a possible multiverse.
The Big Bang
Current science assumes that the universe was formed as described in the big bang theory, which states that the universe was formed about 13.75 billion (± 1%) years ago.
Belgian astronomer Georges Lemaître introduced the universe expansion theory in 1927, which explained that distant galaxies show a redshift. Two years later, American astronomer Edwin Hubble found experimental evidence for Lemaître's theory. Hubble proved that all galaxies move away from Earth at a rate proportional to their distance from Earth, a law of physics now called Hubble-Lemaître's Law. This fact implies a primordial beginning from a certain point, and in 1931 Lemaître hypothesized that the universe must have arisen from the explosion of a "primordial atom". This theory was subsequently called "big bang theory" by British astronomer Fred Hoyle in disdain, a term still widely used with (today) a neutral connotation. The main evidence for the theory came in 1964 from Arno Allan Penzias and Robert Woodrow Wilson, who received the Nobel Prize in Physics for this. They discovered by accident, they weren't looking for it, the cosmic microwave background radiation predicted by the big bang theory.
From the most distant galaxies, the wavelength of light reaching Earth has increased by more than a factor of 6. This indicates that the Universe has been expanding by this factor since the light left these galaxies after the Big Bang.
Based on the temperature of the cosmic microwave background radiation measured by the "Cosmic Background Explorer" (COBE), it was possible to calculate how old the universe is. The moment of the big bang was finally determined to be 15.556 billion years ago, with a margin of error of (relatively only) 24 million years. However, more recent research with the Wilkinson Microwave Anisotropy Probe (WMAP) indicated an age of 13.7 billion years with an uncertainty of 1% (about 137 million years). ESA's measurements with Planck Observatory, released in 2013, indicate an age of 13.8 billion years.
The visible universe
The large-scale structure
In order to study the large-scale structure of the universe, attempts are being made to examine the faintest possible luminous galaxies in different regions of the sky by taking long exposures with several telescopes at the same time. Examples include the Hubble Extreme Deep Field and the Lockman Hole. There are also projects to determine the redshift, and thus the distance, of as many galaxies as possible, for example the Sloan Digital Sky Survey.
According to today's knowledge, the visible universe is made up of large groups of superclusters and clusters that, together with strings of galaxies (filaments), form a filamentous network between which huge super cavities are located. These clusters in turn are made up of hundreds to thousands of galaxies. Such a cluster can have a diameter of just