History and Problems of the Standard Model in Cosmology

Martín López-Corredoira
Published in EdgeScience 53

Since the beginning of the 20th century, a continuous evolution and perfection of what we today call the standard cosmological model has been produced, although some authors like to distinguish separate periods within this evolution. A possible historical division of the development of cosmology into six periods was proposed by Luminet (2008): (1) the initial period (1917–1927); (2) the period of development (1927–1945); (3) the period of consolidation (1945–1965); (4) the period of acceptance (1965–1980); (5) the period of enlargement (1980–1998), and (6) the period of high-precision experimental cosmology (1998–).

The Initial Period (1917-1927)

At the beginning of the 20th century, two great achievements in physics and astronomy initiated the journey toward the standard cosmological model as we know it today. The first was the observational evidence for the existence of many galaxies separated by very large distances—much larger than the usual distances managed by astronomers previously—the Milky Way thus being only one galaxy among many. It was definitively established after a period of discussion that finished with the Great Debate in 1920 between the American astronomers Heber D. Curtis (1872–1942), who defended the hypothesis that some nebulae (now called galaxies) were not part of the Milky Way but were located at very large distances from it, and Harlow Shapley (1885–1972), who claimed that these nebulae were part of the Milky Way. This achievement gave rise to the subsequent development of extragalactic astronomy, and, implicitly, a new cosmological vision was emerging out of this scenario: a vision of a Universe of vast spaces, impossible to imagine, where galaxies are the fundamental components in a larger-scale structure.

The other great achievement came from physics in the form of Albert Einstein’s (1879–1955) general relativity. Certainly, his earlier theory of special relativity was also very important, but for astronomy, particularly from the perspective of cosmology, general relativity was the long-awaited breakthrough. Newton’s magnificent achievements had blocked the free expansion of cosmological ideas because of the problems in solving the stability of systems without an eventual collapse and having recourse to godly intervention.

The models that would constitute the basis of our present standard cosmology came a little later. The basic idea assumed is that the current Universe is homogeneous on a large scale and that the distances among all the different objects are currently growing owing to the expansion of the Universe, a recession of objects with respect to one another on a large scale. On small scales, different objects could cluster together because their gravitational attraction overcomes the expansion. The Russian physicist Alexander Friedmann (1888–1925) developed the basic aspects of the application of general relativity to a cosmological model (Friedmann 1922, 1924).

The Period of Development (1927–1945)

In 1924, the German astronomer Carl Wirtz (1876–1939) noted a correlation between the faintness of a galaxy and its redshift. Edwin P Hubble (1889–1953) and Milton Humason (1891–1972) measured the distance of a number of galaxies during the same year and would later find the famous Hubble–Lemaître law of the linear relationship between radial velocities and distances. The redshifts were interpreted as proof of the expansion of the Universe (Hubble, 1929). Prior to Hubble’s publication in 1927, the Belgian Catholic priest, physicist, and astronomer Georges Lemaître (1894–1966) developed a theoretical model of an expanding Universe in an extension of the work of Friedmann. The work by Lemaître (1927) was published in French in a small Belgian journal, and also tells us about the recession of galaxies and the recession rate in the linear velocity–distance relationship, including an analysis of observational data, as rediscovered later by Hubble in 1929.

Another line of development of the cosmological model was suggested by the Japanese physicist Seitaro Suzuki, who suggested that the observed helium–hydrogen ratio might be explained “if the cosmos had, at the creation, the temperature higher than 10⁹ degrees” (Suzuki, 1928). Lemaître, in 1931, with the expansion and the arrow of time from the second law of thermodynamics in mind, developed his concept of the ‘primeval atom’ (Lemaître, 1931), the first version of what later would be called the “Big Bang”. According to him, the initial state of matter in the Universe might be thought of as a sea of neutrons. Lemaître thought that cosmic rays were relics of primordial decays of atoms, which was later demonstrated to be wrong. Moreover, his ideas on stellar evolution were also demonstrated to be wrong during the 1930s. So, by the end of the decade, the primeval-atom hypothesis had been generally rejected by the scientific community.

The Period of Consolidation (1945–1965)

After World War II, George Gamow (1904–1968), a Russian physicist who emigrated to the US in 1934, compared the detonation of an atomic bomb with the origin of the Universe and popularized the Big Bang theory (Gamow, 1947). In fact, the name “Big Bang” was not given by Gamow, but by one of the opponents of his theory, Fred Hoyle (1915–2001), who dubbed Gamow’s primeval atom theory as the “Big Bang”, in order to ridicule it. Gamow and one of his students, Ralph Alpher (1921–2007), published a paper in 1948. Gamow, who had a certain sense of humor, decided to put the reputed physicist Hans Bethe (1906–2005) as the second author, even though he had not participated in the development of the paper, so the result was a paper by Alpher, Bethe, and Gamow (Alpher et al., 1948), to rhyme with “alpha, beta and gamma”. Later, Robert Herman (1914–1997) joined the research team, but—according to Gamow—he stubbornly refused to change his name to “Delter.”

. . . the name “Big Bang” was not given by Gamow, but by one of the opponents of his theory, Fred Hoyle (1915–2001), who dubbed Gamow’s primeval atom theory as the “Big Bang”, in order to ridicule it.

Alpher and Herman (1949) and Gamow (1953) also predicted an early stage of the Universe that would produce relic radiation that could be observed at present as a background in microwave wavelengths, corresponding to the epoch of decoupling of matter and radiation. The first published recognition of relic radiation as a detectable microwave phenomenon was in 1964 by the Russian cosmologists Andrei Doroshkevich (1937–) and Igor Dmitriyevich Novikov (1935–) (Doroshkevich & Novikov, 1964). Then came the official discovery of the cosmic microwave background radiation by Arno Allan Penzias (1933–) and Robert Woodrow Wilson (1936-) (Penzias & Wilson, 1965), although this same radiation had been previously directly or indirectly observed by other researchers.

The Period of Acceptance (1965–1980)

More evidence supporting the standard model of the expanding Universe came from Malcolm Longair (1941–) and Martin Ryle (1918–1984), who argued that the data indicate that the Universe must be evolving (Longair, 1966; Ryle, 1968). The galaxies at high redshift—that is, at great distance—showed distributions and properties different from those at low redshift. Since at larger distances we are observing the past Universe, given the limited speed of light, this implies that the distant galaxies belong to an epoch of the Universe that was much earlier than the present one.

The confirmation of the predicted microwave radiation and evolution of the Universe gave confidence to those cosmologists who supported the standard model. Many hitherto skeptical physicists and astronomers became convinced they now had a solid theory. By the mid-seventies, cosmologists’ confidence was such that they felt able to describe in intimate detail events of the first minutes of the Universe (Weinberg, 1977).

The Period of Enlargement (1980–1998)

Nonetheless, there were problems that remained to be solved, such as why the Universe appeared to be the same in all directions (isotropic), why the cosmic microwave background radiation was evenly distributed, and why its anisotropies were so small. Why was the Universe flat and the geometry nearly Euclidean? How did the large-scale structure of the cosmos originate? Clearly, work on the fundamental pillars of the cosmological edifice remained to be done. In the 1970s and 1980s, proposals were brought forth to solve these pending problems, with inflation as the leading idea in the solution of cosmological problems at the beginning of the Universe, and the idea of non-baryonic dark matter as a new paradigm that allows the theory to fit the numbers of some observations. Grand Unified Theories of particle physics would also support the existence of CP violation (asymmetry of matter and antimatter) or non-baryonic dark matter. Also, the joining of cosmology and particle physics and scenarios containing baby universes, wormholes, superstrings, and other exotic ideas were born. This excess of theoretical speculation, not based on observations, has led some authors to call this epoch the era of post-modern cosmology (Bonometto, 2001). This union between cosmology and particle physics is due in part to the halting of particle physics experiments because of their escalating cost, a situation that led many particle physicists to move over into cosmology, wishfully contemplating the Universe as the great accelerator in the sky (Disney, 2000; White, 2007). Alas, particle physicists lack the necessary astronomical background—complained Mike Disney—to appreciate how soft an observational, as opposed to experimental science, necessarily has to be.

In the 1990s, a third patch was applied to the theory in an effort to solve new inconsistencies with the data in the form of dark energy, which supposedly produced acceleration in the cosmic expansion. The problems to be solved were basically the new Hubble–Lemaître diagrams with type Ia supernovae as putative standard candles, the numbers obtained from cosmic microwave background radiation anisotropies, and especially estimates of the age of the Universe, which were inconsistent with the calculated ages of the oldest stars.

The renovated standard model, including these ad hoc elements, would come to be called the ΛCDM cosmological model, where Λ stands for dark energy, and CDM stands for cold dark matter, the favored subgroup of models of non-baryonic dark matter. Some cosmologists referred to it as “concordance cosmology: to emphasize that this model is in agreement with all the known observations. Other authors, critical of the standard model, prefer to call it “consensus cosmology,” wishing to emphasize that this new cosmology is, above all, a sociological question of agreement among powerful scientific teams in order to establish the orthodoxy of a fundamental dogma. This agreement would be mainly between two powerful cosmological groups, the teams dedicated to the analysis of supernovae and the cosmic microwave background, who found a rough coincidence in the necessary amount of dark energy, although with large error bars, that reinforced their belief that they had discovered an absolute truth, thus compelling the rest of the community to accept this truth as a solid standard, while at the same time discarding the results of other less powerful cosmological groups that presented different values of the parameters. Talking about consensus cosmology, Rudolph (‘Rudy’) Schild (1940–) once queried, “Which consensus? Do you know who consented? A bunch of guys at Princeton who drink too much tea together” (Unzicker & Jones, 2013, ch. 3).

Talking about consensus cosmology, Rudolph Schild once queried, “Which consensus? Do you know who consented? A bunch of guys at Princeton who drink too much tea together.

The Period of High-Precision Experimental Cosmology (1998–)

Rather than major discoveries or proposals, this epoch is characterized by a lack of discussion on the fundamental ideas in cosmology, when it becomes a tenet of belief that all the major problems have been solved. This state of complacency has resulted in excess confidence in the robustness and superiority of the standard model, with little consideration for alternative models. Certainly, some minor topics are being debated, such as the equation of the state of dark energy, and the types of inflation or the coldness or hotness of dark matter, but these are subtleties (Byzantine arguments) within the major fundamental scheme. This is the epoch in which the main enterprise of cosmology consists of spending big money on megaprojects that will achieve accurate measurements of the values of the cosmological parameters and solve any small problems that remain to be explained.

This is also the epoch of the highest social recognition of cosmology: Not only do schools, museums, and popular science journals talk about the Big Bang as well established, to be compared to Darwin’s evolution and natural selection theory, but cosmology now occupies a privileged ranking among the most prestigious natural sciences. For instance, cosmology and its four dark knights (CP violation, inflation, non-baryonic dark matter, and dark energy) have been awarded Nobel Prizes in Physics in 2011 and 2019, respectively, for the putative discovery of the dark energy that produces the acceleration of the expansion, and the inclusion of the dark components in our understanding of the Universe. One may wonder whether unconfirmed quasi-metaphysical speculations should properly form part of the body of the recognized knowledge of physics, leaving behind the conservative tradition of Nobel committees not awarding prizes for speculative proposals. Einstein did not receive either of his Nobel Prizes for his discovery of special and general relativity; neither did Curtis for his definitive recognition of the true nature of galaxies in the Great Debate of 1920. Neither Lemaître nor Hubble received the Nobel Prize for their discovery of the expansion of the Universe, but we now have committees that give maximum awards for highly speculative proposals, such as the acceleration of the expansion of the Universe, the reality of which has yet to be confirmed. We certainly do live in a very special time for cosmology.

However, this brand of epistemological optimism has declined with time, and the expression “crisis in cosmology” is stubbornly reverberating in the media. The initial expectation of removing the pending minor problems arising from the increased accuracy of measurements has backfired: the higher the precision with which the standard cosmological model tries to fit the data, the greater the number of tensions that arise, the problems proliferating rather than diminishing. Moreover, there are alternative explanations for most of the observations.

At the Anomalies in Modern Astronomy Research online symposium organized by the Society of Scientific Exploration (October 22nd, 2022), Prof. Pavel Kroupa presented anomalies from galactic to Gpc scales (large-scale structures), including some examples of 5σ tensions and some mention of Modified Newtonian dynamics (MOND) as an alternative to standard gravity and dark matter. We can complement the range of anomalies in cosmology with further cases of Cosmic Microwave Background Radiation, nucleosynthesis, tests of expansion, CP violation, inflation, and other topics. There is no space in the present text to discuss in detail these topics; the reader interested in these anomalies and tensions can read the recent literature on the collections of problems of the standard model: (Perivolaropoulos & Skara, 2022; Abdalla et al., 2022; Melia, 2022; López-Corredoira, 2017, 2022).

CP violation has problems; There is no experimental evidence for a finite lifetime of a proton below 10³⁴ years (Tanaka et al., 2020). Inflation has problems; Some authors have argued that the inflation necessary to explain a flat Universe is highly improbable (Iljas et al., 2017). Hubble–Lemaître diagrams with type Ia supernovae can be explained without dark energy (López-Corredoira & Calvo-Torel, 2022); also, dark energy can be avoided in other observations.

The standard interpretation of the redshifts of galaxies is that they are due to the expansion of the Universe plus peculiar motions, but there are other explanations, such as the “tired light” hypothesis, which assumes that the photon loses energy owing to some unknown photon–matter process or photon–photon interaction when it travels some distance. Different observational tests give different results, although none of them so far provides strong proof in favor of a static Universe (López-Corredoira, 2017; López-Corredoira, 2022, ch. 4). The discussion on anomalous redshifts is also inconclusive.

Doubt is cast upon that precision cosmology derived from Cosmic Microwave Background Radiation analysis, owing to the difficulties in making maps totally free from foreground contamination. Moreover, many alternative explanations of its origin are found in the technical literature, and certain observed anomalies, such as the lack of low multipole signal, alignment of quadrupole and octupole, and others, are at odds with the standard model (Schwarz et al., 2016), which opens the door to possible fundamental errors in the standard cosmological description of this radiation.

In the standard model, it is claimed that helium-4, lithium-7, and other light elements were created in the primordial Universe, and the existence of these elements was used as proof for the necessity of a hot Universe in its first minutes of life. However, only helium-4 has had successful direct confirmation of the predictions, although at the price of requiring a baryon density raises other problems. The observed abundance of lithium-7 is 3 to 4 times lower than predicted (Coc et al., 2012). The other light elements are affected by uncertainties in the theoretical model or by later creation or destruction associated with stellar nucleosynthesis, cosmic rays, or other astrophysical processes, so they cannot be used to corroborate cosmological predictions. Moreover, there are alternatives to primordial nucleosynthesis to explain the observed abundances, even for helium-4 (Adouze et al., 1985; Burbidge & Hoyle, 1998).

Cosmology is not a science like others since it contains more speculative elements than is usual in other branches of physics, with the possible exception of particle physics. The goal of cosmology is also more ambitious than routine theories in physics: cosmology aims to understand everything in our Universe without limit. However, cosmological hypotheses should be very cautiously proposed and even more cautiously received. This skepticism is well-founded. There are scientific, philosophical, and sociological arguments to support this claim (López-Corredoira, 2022).

Some of the material for these articles was excerpted from the book Fundamental Ideas in Cosmology. Scientific, Philosophical and Sociological Critical Perspectives (López-Corredoira, 2022).

Author’s Bio

Martín López Corredoira received a PhD in Physics at the Univeristy La Laguna (Tenerife, Spain) in 1997 and a PhD in Philosophy at the University of Seville (Spain) in 2003. Since 2011, he has been a permanent staff researcher at the Instituto de Astrofísica de Canarias in Tenerife (Canary Islands, Spain). He is the author of around a hundred papers on galaxies and cosmology in international refereed scientific journals, half of them as first author, and more than 50 articles on philosophy and humanities or social topics. He is a visiting scientist in 2023 under the President’s International Fellowship Initiative of the Chinese Academy of Sciences (grant nr. 2023VMB0001) at Purple Mountain Observatory, Nanjing, and the National Astronomical Observatories, Beijing.

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