Thursday, February 16, 2017

Pending problems in the standard cosmological model

The standard cosmological model, prevailing since 1998, is called LCDM and is based on the following statements:
  • The universe began with a Big Bang, after which there was a phase of accelerated expansion (inflation), which then declined to levels close to the current ones. Ordinary matter appeared later, formed essentially by hydrogen and helium.
  • The average curvature of the cosmos is close to zero (flat universe): three-dimensional space is approximately Euclidean.
  • The average density of matter in the cosmos is equivalent to about 30% of the critical density (which separates an open, unlimited expanding cosmos from a closed cosmos that would contract again). Since the ordinary density of matter detected so far represents less than 5% of critical density, the remainder (over 25%) must be an unknown form (dark matter). In fact, it would be what is called cold dark matter, which explains the initials CDM in the name of the model. I talked about dark matter in an earlier post.
  • The accelerated expansion of the universe is explained by resurrecting Einstein's cosmological constant, to which I devoted another post. It would represent an unknown interaction (dark energy) that would have to be added to the four we know: gravity, electromagnetism and strong and weak nuclear interactions. Its effect would be equivalent to 70% of the critical density, which added to 30% of the matter density, adds exactly to the critical value. As the cosmological constant is represented by the L symbol, the name of the standard model is explained.
The standard model is opposed by a series of alternative models that receive little consideration by the most famous cosmologists (those who have received the Nobel Prize). They can be classified as follows:
  1. Derivations from the now abandoned steady state theory, which replace the Big Bang by many Small Bangs.
  2. Modifications to Einstein's theory, which in its present state predicts the existence of singularities and seems to make the Big Bang inevitable, as Roger Penrose and Stephen Hawking proved in 1970.
  3. Models based on unknown properties that could explain the redshift of galaxies without resorting to the expansion of the universe, such as the theory of tired light, which argues that light loses energy as it travels enormous distances. This would lead to a redshift.
  4. Observations that could be interpreted in the sense that the universe is not isotropic (equal in all directions in its large scale structure), contrary to what the standard model implies.
In favor of the Big Bang and the standard model, two correct predictions dating back to the late 1940s and an observation made at the end of the 20th century are adduced:
  • The proportion of light elements in the universe (especially He4) predicted in 1948 by Alpher and Gamow.
  • Cosmic background radiation, with a temperature of about 5°K (actually it is 2.73°K), predicted in 1948 by Alpher and Herman.
  • The accelerated expansion of the universe, signaled in 1998 by two different research teams.
Martín López Corredoira
In a recent article, Martín López Corredoira, who works in the Canarias Astrophysical Institute, has made a complete revision of the problems still faced by the standard cosmological model, and how the different alternative models address them. These problems can be summarized as follows:
  1. Problems related to the redshift. The expansion of the universe has been studied through various observations: experiments on temporal dilation, use of cosmic chronometers, Tolman test on the surface brightness of galaxies, the number of galaxies as a function of their magnitude, and so on. A table in the article compares models based on the reality of cosmic expansion with static models, and finds that some of the tests favor the former, while others seem to be better suited to the latter. The results, therefore, are inconclusive.
  2. Problems related to the cosmic background radiation. There are discrepancies regarding the predictions of its temperature, its origin, and the anisotropies that have been detected in this radiation after the analysis of the data provided by the COBE, WMAP and Planck satellites. There are also problems with the effect of the dust in our galaxy, as what happened in 2014, when a circular polarization apparently detected in the cosmic background radiation, which was initially taken as a confirmation of the inflationary theory, turned out to have been produced by this dust.
  3. Problems related to the abundance of light elements in the universe. There are discrepancies about the abundances of He4 and Li7 depending on the methods used to estimate them.
  4. Problems related to the formation of galaxies. Some observations on the large scale structure of the universe detect the presence of clusters, voids and periodicities in the galaxy distribution that do not seem compatible with the standard model and with the universal isotropy it predicts.
  5. Problems related to the age of some galaxies. From the composition of the stars in several very distant galaxies, it seems that some of them should have a greater age than the universe, which is obviously absurd.
  6. Problems related to dark matter, whose abundance and distribution is different when computed using the standard model, or the behavior of stars in galaxies, which was the initial reason to predict its existence. So far, all experiments designed to detect dark matter have yielded negative results.
  7. Problems related to dark energy. The interpretation of the data that led to the theory of the accelerated expansion of the universe might not be correct. Other explanations have been proposed.

Einstein's cosmological equation
It will be observed that the problems indicated make a varied collection. Some go against the Big Bang theory. Others are rather directed against the standard model (LCDM) without significantly affecting the fundamental theoretical basis (Einstein’s cosmological equation). In short, things are not as clear as we are usually made to believe, especially by the media. The theory of everything and the end of science are very far from us. Fortunately, new mysteries are suggested whenever new discoveries are made.

Manuel Alfonseca

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