Inflation (cosmology)

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Temporal and spatial progression of the expansion of the universe , not to scale. Note the inflation phase on the left edge of the yellow area.

As cosmological inflation phase is extremely rapid expansion of the universe referred assumes from the one that it immediately after the Big Bang has occurred. This very short period of time is also called the GOOD era.


In cosmology ,  the term GUT era is used for the very first time after the Planck period from 10 −43 s. During this era, inflation started at around 10 −35  s and lasted between 10 −33  s and 10 −30  s after the Big Bang. GUT stands for 'Grand Unified Theory', in German ' Great Unified Theory '. This would unite the strong nuclear force , the weak nuclear force and the electromagnetic force . High-energy experiments at particle accelerators indicate that at an energy of around 2 × 10 16  GeV (gigaelectron volts) the three mentioned forces can no longer be distinguished from one another. Above this energy there would only be one force called the GOOD force. This would be a state of higher symmetry. At energies below this value, this symmetry breaks and the three mentioned forces become visible. It is assumed that the universe has expanded by at least a factor of 10 26 during this time . The universe then continued to expand under the standard Big Bang model, as described by Friedmann's equations .

The hypothesis of this inflationary expansion was proposed by Alan Guth in 1981 and is not part of the original Big Bang model. Preliminary work on the development of the inflation theory was carried out by Andrei Linde in the 1970s , for which he was awarded the Gruber Prize for Cosmology in 2004. The reason was the observation that the relativistic cosmology to explain some fundamental observations (see below) requires a fine tuning (“fine tuning”) of cosmological parameters, which in turn required an explanation. The inflation hypothesis offers a physical mechanism for this, from which some fundamental properties of the universe result directly.

According to this, the cause of this expansion is the change in state of a scalar field with an extremely flat potential. This scalar field, called the inflaton field, has an equation of state with negative pressure. According to the general theory of relativity , this leads to a repulsive force and thus to an expansion of the universe. The change in state of the field during the inflationary phase is comparable to a first order phase transition . Within the framework of the great unified theory , the conditions under which the phase transition occurs are determined by Higgs fields .

The assumption of such an inflationary expansion appears on the one hand arbitrary, on the other hand it elegantly solves several larger cosmological problems:

  • The universe visible today contains essentially similar structures everywhere. On the other hand, it consists of areas that were only able to interact causally with one another very late in the case of a standard expansion, since they initially moved away from one another too quickly immediately after the Big Bang. The fact that one observes a high degree of homogeneity of the universe and isotropy of the cosmic background radiation is called the horizon problem and cannot be explained in the context of a standard expansion. If, on the other hand, an inflationary expansion had existed, all areas of the universe visible today would have already been temporarily interacting before this inflation.
  • The area of ​​the universe visible today shows no measurable curvature of space . As part of a standard expansion, an extremely precise coordination of matter density and kinetic energy would have been required immediately after the Big Bang, for which there is no explanation. In the event of an inflationary expansion, on the other hand, the observed flatness of the space would only be a consequence of its enormous expansion, since the universe visible today would only represent a tiny section.
  • The inflation hypothesis also explains the density fluctuations, from which the galaxies and galaxy clusters emerged, as a result of quantum fluctuations in the inflaton field. The extreme expansion increased these fluctuations to a corresponding macroscopic size, which a standard expansion would not have been able to achieve to a sufficient extent.
  • According to certain theories, magnetic monopoles were also supposed to have arisen during the Big Bang , but these have eluded experimental evidence to this day. During an inflationary expansion, however, the particle density of these monopoles would have decreased to such an extent that the probability of finding individual ones in the range of the universe visible today would be extremely low - in accordance with the experimental data.

Field dynamics

To explain the dynamics of inflation, a scalar quantum field is required that is spatially homogeneous and has a finite energy density. If the field changes slowly enough over time (namely in the direction of a reduction in energy density), it has negative pressure and effectively behaves like a cosmological constant , thus leading to an accelerated expansion of the universe. The expansion is exponential when the energy density of the quantum field is the dominant component in the universe. No concrete candidate for this quantum field is currently known. The name for a quantum field that causes inflationary expansion is the inflaton field with the inflaton as the mediator particle .

The lowest energy state of the inflaton field does not have to be, but can be different from zero. That depends on the density of the potential energy of the field given as a parameter . Before the expansion period, the inflaton field was in a higher energy state. Random quantum fluctuations triggered a phase transition , with the inflaton releasing its potential energy in the form of matter and radiation as it switched to the lower energy state. This process created a repulsive force that accelerated the expansion of the universe.

A simple model for an inflaton field is through the potential

given, where the temperature dependence comes about through the interaction with the thermal fluctuations of the other particles and fields in the universe. At high temperature this potential has a single minimum at . If the temperature falls below a first critical temperature due to the expansion of the universe , a second local minimum appears in the potential function . First of all, the potential at this secondary minimum has a higher value than in the global minimum in which the field is located. If the temperature falls below a second critical value , the potential in the secondary minimum has a lower value than in the primary minimum. The global minimum of the potential function is called the true vacuum and the local minimum is called the false vacuum.

In order to pass from the false to the energetically preferred true vacuum, the field has to overcome an energy barrier or tunnel through it (this is possible through the quantum mechanical tunnel effect ). Since the energy density of the false vacuum does not change even with an expansion of space, provided that the quantum mechanical tunneling process runs slowly enough, the pressure of the false vacuum must be negative and, according to Friedmann's equations, leads to an exponential expansion.


The hypothesis of inflationary expansion is a research area in which numerous variants are still being discussed. In particular, the nature of the particles or fields that could have caused the required vacuum state is still completely unexplained.

Whether there actually was an inflationary phase in the early days of our universe has to be decided by observation; this is the subject of current research. Current observations of, for example, temperature fluctuations in the cosmic background radiation by the US space probe WMAP are compatible with the inflation hypothesis, but do not yet allow a final judgment.

The current accelerated expansion of the universe, which is inferred in particular from observations of distant supernovae , is attributed to the presence of dark energy with negative pressure and thus to a physical mechanism that is related to the actual inflation in the early days of the universe.

Despite the complexity of this theory, it is widely recognized by most scholars as it provides an initial logically understandable hypothesis.

According to the current model, gravitational waves arose shortly after the big bang in the inflation phase , which imposed a characteristic polarization pattern on the cosmic background radiation . In principle, this would allow the theories of inflation to be tested experimentally , but an observation reported by BICEP2 in 2014 turned out to be premature.

Criticism and alternatives

The inflation model requires its own scalar field, the inflaton. Nevertheless, it can initially only insufficiently describe the development of the universe. Depending on the variant, either additional fields or additional parameters are required. The problem of the small cosmological constant is also unexplained in the inflation model. The current value differs by 120 orders of magnitude from that of the inflationary phase.

Big bang and inflation enable a multiverse with properties that are randomly distributed across the universes. Steven Weinberg complains that, apart from the anthropic principle, the size of physical constants or dark energy in our universe can in principle no longer be explained in a reasonable way. For similar reasons, the co-founder Paul Steinhardt rejects the inflation theory and considers it non-falsifiable due to its prognostic arbitrariness.

Models competing with inflation theory include: a .:

  • Loop quantum gravity : This does not require any additional fields. According to David Sloan and Abhay Ashtekar, the inflation phase results from the assumption of a quantized spacetime in this model . Sloan and Ashtekar explain that inflation in the standard cosmologies, based on general relativity, requires very special initial conditions and is unlikely (according to Neil Turok , Gary Gibbons ), but is very likely according to loop quantum cosmology.


Web links

Individual evidence

  1. Harald Lesch: Astronomy: the cosmic perspective. 2010, accessed September 2014.
  2. Andreas Müller: GUT era. In: Astro-Lexicon., accessed in September 2014 .
  3. Andreas Müller: Inflation. In: Astro-Lexicon., accessed on December 8, 2016 .
  4. ^ Steven Weinberg: Physics: What We Do and Don't Know. In: The New York Review of Books. November 7, 2013, accessed December 8, 2016 .
  5. ^ John Horgan , Physicist Slams Cosmic Theory He Helped Conceive. In: Scientific American. December 1, 2014, accessed on December 8, 2016 (English, "inflation is very flexible (parameters can be adjusted to give any result) and generically leads to a multiverse [...] in which any outcome is possible. [...] It rules out nothing and can never be put to a real test. " ).
  6. Anil Ananthaswamy, Big bounce cosmos makes inflation a sure thing , New Scientist, October 13, 2010
  7. ^ Gibbons, Turok, Measure problem in cosmology, Phys. Rev. D, Volume 77, 2008, p. 063516, abstract
  8. Ashtekar, Sloan, Loop quantum cosmology and slow roll inflation, Phys. Lett. B, Volume 694, 2010, pp. 108-110, abstract