# Expansion of the universe

Development stages of the universe (for illustration only, not to scale)

The expansion of the universe is the increase in the spatial expansion of the universe derived from observations . This is defined by the constant increase in the distance between objects that are far apart in space.

In accordance with the Big Bang theory, after initial inflation , the expansion of the universe slowed down for the first billion years of its existence. Since then the rate of expansion has increased. The explanation of this observed accelerated expansion is the subject of current research and has led to the concept of dark energy .

## Discovery story

Albert Einstein and Willem de Sitter described the universe for the first time in 1917 using the formalism of general relativity . However, they described a static, constant universe. De Sitter's description later turned out to be incorrect. In 1922 Alexander Friedmann gave the first relativistic description of an expanding or contracting universe ( Friedmann equations ). However, this publication was hardly noticed.

In 1912, the American astronomer Vesto Slipher was the first to discover the redshift of the spectral lines of light in distant galaxies . In 1925 Edwin Hubble published the distance to M 31, the nebula in Andromeda , which clearly showed that Andromeda lies far outside the Milky Way, in 1926 he published distances to other galaxies.

The expansion of the universe was discovered in 1927 by the Belgian Georges Lemaître . He discovered what Friedmann had found before him, that the basic equations of the theory of relativity result in a dynamic universe. From the observed galaxy flight , he concluded that the universe is expanding. He linked Slipher's redshifts of galaxies with Hubble's distances. In his publication in the Annales de la Société Scientifique de Bruxelles in 1927, Lemaître also indicated the relationship later known as the Hubble law , with a value for the so-called Hubble constant that was largely determined by the work of Hubble in 1929 has been confirmed. In October 2018, a majority of IAU members voted in favor of renaming the law the Hubble-Lemaître Act in the future. ${\ displaystyle v = H \ cdot r}$ ${\ displaystyle H_ {0}}$

Lemaître emphasized that the “flight” of the galaxies (also referred to in the context of the Shapley-Curtis debate with the term “ fog flight”, which is no longer used today ) should not be understood as movement in a fixed space, but rather in the sense of the general theory of relativity , as an expansion of the space itself.

Hubble himself found the relationship , i.e. the relationship between the distances of the galaxies and the redshifts interpreted as velocities ( Doppler effect ), in 1929. He did not interpret this as an expansion of the universe, but in the sense of de Sitters' 1917 proposed model of a static universe . Hubble never represented the model of the expanding universe and - judging from its publications - probably never believed in it. ${\ displaystyle v = H \ cdot r}$${\ displaystyle v}$

While Einstein had postulated a static universe in his theories, he revised his view in view of this then new theory of expanding space. Einstein had introduced a cosmological constant into the field equations in order to obtain static solutions of the universe. However, these solutions to the structure of the universe were unstable . Einstein later described the idea of ​​a cosmological constant, according to George Gamow, as the "greatest donkey of my life".

## State of research

Possible developments of the size of the universe in the past and future for a given current expansion rate H 0 are described by the Friedmann equation. The applicable case is the ΛCDM model (magenta).

According to the most common theory today, the cosmological redshift is not a Doppler effect in the strict sense, but is based on the general increase in distances in the universe over time. This leads to the assumption of the Big Bang , since the distances between the galaxies in this model disappear at a finite point in time in the past and therefore a state of infinitely high density is present.

For a long time it was unclear whether the expansion

• will continue indefinitely (open universe);
• becomes slower and slower, but will still reach an asymptotic limit state (flat universe);
• comes to a standstill at some point and returns to a contraction (closed universe).

In 1998, observations of distant Type Ia supernovae published as part of the Supernova Cosmology Project and High-Z Supernova Search Team , for whose analysis the astronomers Saul Perlmutter , Brian P. Schmidt and Adam Riess were awarded the Nobel Prize in Physics of 2011, show that the expansion of the universe is accelerating today . These results agree with studies of the cosmic background radiation , for example using the WMAP satellite . The cause is assumed to be dark energy , a generalization of the cosmological constant that varies over time. Dark energy has not yet been directly detected; their only currently observable effects relate to the expansion of the universe and the formation of structures in the universe.

The acceleration of the expansion is described with the Lambda CDM model .

Another hypothesis for the origin of the redshift is that of the distance-dependent photon aging . It came up with quantum theory, was based on the particle image of light and is now considered scientifically obsolete.

From some observations that cannot be understood in the context of the normal Friedmann-Lemaître-Robertson-Walker metric , one concludes that there was a phase of exponential expansion in the early days of the universe. These expansion theories are called inflation theories.

Attempts at explanations in the context of general relativity are also examined.

## literature

• Charles H. Lineweaver, Tamara M. Davis: The Big Bang - Myth and Truth. In: Spectrum of Science. May 2005, pp. 38-47, .
• Harry Nussbaumer: Eighty Years of Expanding Universe. In: Stars and Space. Volume 46, Issue 6, 2007, pp. 36-44, .
• Harry Nussbaumer: The worldview of astronomy. 2nd ext. and act. Edition, vdf Hochschulverlag, 2007, ISBN 978-3-7281-3106-5 .
• Harry Nussbaumer, Lydia Bieri: Discovering the Expanding Universe. Cambridge University Press, 2009, ISBN 978-0-521-51484-2 .