vol IV chap 14 sect 3

Previous: 14.2. Neutrino oscillations, background radiations, accelerating expansion, and exoplanets.

14.3. Contextualization of learning about the universe.¶

Cosmological knowledge has been created by organizing what has been seen through specific windows for watching and registering celestial objects: first through the human eye and optical telescopes and afterwards with the support of systems of modern telescopes placed on Earth or in satellites external to the terrestrial atmosphere. Three steps characterize the evolution of cosmology:

(1) According to astrology and the development of prescientific astronomy by eye, it was believed that the positions of planets, stars and constellations defined the destiny of human beings. However, careful maps of their positions served as useful guides for navigation. According to this approach, the universe was a stationary and eternal scenario explained in terms of mysterious agents and miraculous circumstances.

(2) Optical telescopes enlarged and deepened what could be seen with the naked eye because what was observed, measured, and described was communicated and interpretated in scientific terms. Then, the universe was conceived as a complicated scenario dynamically active and composed of many celestial objects among which the planet Earth was not central. This was a critical shift from philosophical speculations towards scientific explanations.

(3) What now is called physical cosmology deals with the study and use of the emission, transport, and reception of signals different from visible light, like those produced by neutrinos or by gravitational waves.

Three elements characterize contextualization of learning: the proposition of a problematic situation, the elaboration of generating questions which address the main issues of the theme to be contextualized, and the description of learning and evaluation activities to answer those questions. In what follows we apply this contextualization of learning procedure to analyze the content of Peebles´ Nobel Lecture.

Firstly, we indicate that the problematic situation consists in the explanation of what cosmological knowledge is according to Peebles´ Nobel Lecture. Then we consider the following generating questions (GC):

The third component of contextualization of learning consists in a set of answers (Ans) for each one of the previous questions. The answers we suggest are expressed in terms of quotes taken from Peebles’ Lecture. Each quote is identified by a short line describing the content of the corresponding selected paragraph (Prg) and a number indicating the numerical order it has in the Lecture.

Problematic situation: explain what Physical Cosmological is according to Peebles´ Nobel Lecture How Physical Cosmology Grew.

GQ1: What characterizes the research life of James Peebles?

Ans1.1. Role of Robert Dicke.

Doctoral dissertation under the advice of Dicke [Prg01].

“I began studying the large-scale nature of the universe in 1964, on the advice of Professor Robert Henry Dicke at Princeton University. Bob guided my doctoral dissertation and from then on, I counted on him as my professor of continuing education.”

Integration into the Gravity Research Group [Prg04].

“In 1964 Bob Dicke explained to three junior members of his Gravity Research Group, Peter Roll, David Wilkinson, and me, why he thought the universe might have expanded from a hot dense early condition”. …” Bob suggested that Peter and David build a microwave radiometer that would detect the radiation, if it’s there, and he suggested that I think about the theoretical implications of the result.”

Dicke directed the search for the radiation [Prg07].

“Bell Laboratories showed us in Princeton credible evidence that we are in a sea of microwave radiation, and that the radiation is close to uniform because the excess noise is close to the same wherever in the sky the antenna points. It proves to be what Dicke had suggested we look for, a fossil from the hot early stages of expansion of the universe.” … “Why did the Nobel committee not name Dicke with Penzias and Wilson for the identification of this radiation?” … “Bob directed the search for the radiation that explains the Bell Labs anomaly that so puzzled Penzias and Wilson.”

Dicke suggests to Peebles thinking about the existence of the microwave radiation [Prg08].

“At Bob Dicke’s suggestion I had been thinking about the significance of finding or not finding a sea of radiation. A negative result, a tight upper bound on the radiation temperature, would have suggested an interesting problem. …. So, I proposed a way out: postulate a sea of neutrinos with degeneracy energy large enough to have prevented electrons from combining with protons.” …

The Nobel Prize Committee did not recognize Dicke’s contributions [Prg41].

“I confess to having been unhappy with the Nobel Prize Committee for not recognizing Bob Dicke’s deep influence in the development of gravity physics and cosmology. The committee had their reasons, of course; their considerations can be complicated. But I am satisfied now because my Nobel Prize is closure of what Bob set in motion, his great goal of establishing an empirically based gravity physics, by the establishment of the empirically based relativistic cosmology.”

Ans1.2. Opinions on cosmology and science.

Cosmology as an extension of the reach of well-tested physical science [Prg39].

“The establishment of cosmology is a considerable extension of the reach of well-tested physical science, and the story is simple enough that it offers a good illustration of the ways of physical science. …. And I picture the broad general advance of physical science as a spreading wave that touches many and might be expected to trigger any particular idea more than once, apparently independently.”

The established social constructions of science and Lambda cold dark matter [Prg40].

“Meanwhile, let us not forget the great lesson that the established social constructions of science are buttressed by rich and deep webs of evidence. Surely there is a better more complete cosmology than ΛCDM. But we may be confident that the better theory will predict a universe that is a lot like ΛCDM, with something analogous to its cosmological constant and dark matter, because the universe has been examined from many sides now and found to look a lot like ΛCDM.”

NOTE: ΛCDM means Lambda cold dark matter.

Ans1.3. Publication of Peebles´ books.

First book: Physical Cosmology [Prg35].

“I have written four books on the state of research in cosmology. I meant the title of the first, Physical Cosmology, to indicate that I did not intend to get into the subtleties of what might be termed astronomical cosmology: evidence from stellar evolution ages and the extragalactic distance scale.” … “At about the time of publication, in Peebles (1971), Steve Weinberg (1972) published his book, Gravitation and Cosmology. It is more complete in the mathematical considerations. Mine is more complete in the considerations of phenomenology and of how the phenomenology might be related to physical processes.”

Second book: The Large-Scale Structure of the Universe [Prg36].

“My second book on cosmology, The Large-Scale Structure of the Universe, published in 1980, is a sort of catalog of the statistical measures I had devised and applied, the methods of analyses of how these measures might be expected to have evolved in an expanding universe, and the observational consequences of the evolution.” … “I meant this book to be a working guide to how we might proceed in research in physical cosmology.” …” Writing this book helped me introduce what came to be known as the Cold Dark Matter cosmological model, in 1982.”

Third book: Principles of Physical Cosmology [Prg37].

“My third book, Principles of Physical Cosmology, is much larger than the second, which in turn is much larger than the first. This one was published in 1993, at about the end of the time when it was practical to aim to present in one volume a reasonably complete assessment of the state of research in the physical science of cosmology…. Research in cosmology in the mid-1990s was an active turmoil of multiple ideas and promising-looking but confusing results from model fits to measurements in progress. That situation quite abruptly changed at the end of the decade, when research converged on a well-tested standard model, the ΛCDM cosmology.”

Fourth book: Cosmology´s Century [Prg02].

“The usual thinking at the time was that the universe is homogeneous in the large-scale average, and that it is expanding and evolving as predicted by Einstein’s general theory of relativity. The schematic nature of this cosmology, and its scant observational support, worried me. But I saw a few interesting things to look into, the results suggested more, and that continued through my career. I review my story at length in the book Cosmology’s Century (Peebles 2020). Here I recall a few of the steps along the path to the present standard and accepted cosmology that is so much better established than what I encountered in the early 1960s.”

GQ2: What is the cosmic microwave background radiation?

Ans2.1. Discovery of the cosmic microwave background radiation.

What does it prove the Cosmic Microwave Background radiation (CMB)? [Prg03].

“Cosmology became more interesting with the discovery that the universe is filled with a near uniform sea of microwave radiation with a thermal spectrum at a temperature of a few degrees Kelvin. This CMB (for cosmic microwave background radiation) proves to be a remnant from the hot early stages of expansion of the universe. Theory and observations in this great advance converged in a complicated way.”

Experiments showing a departure from exact isotropy in the microwave radiation [Prg05].

Description of a photography of a radiometer used to obtain a map of the variations of the radiation intensity across the sky, as an attempt to identify the presence of the sea of microwave radiation.

Detection of CMB radiation in 1950 and initial investigations at Bell Labs until 1964 [Prg06].

“The evidence I know is that the sea of microwave radiation was first detected in the late 1950s as unexpected excess noise in experiments in microwave communication at the Bell Telephone Laboratories.” … “The unexplained excess consistently appeared in later experiments. It remained a “dirty little secret” at Bell Labs until 1964, when Arno Penzias and Robert Wilson, both new to the Bell Radio Research Laboratory at Crawford Hill, New Jersey, resolved to look into the problem. They carefully searched for the explanation of this puzzling excess microwave noise, whether originating in the instrument or somehow entering from the surroundings.”

Independent consideration of the same problem at the same time [Prg08].

… “So, I proposed a way out: postulate a sea of neutrinos with degeneracy energy large enough to have prevented electrons from combining with protons. In the Soviet Union Yakov Zel’dovich saw the same problem with a cold big bang and he offered the same solution, lepton degeneracy. … The interesting thing is that we saw the problem at essentially the same time, independently.

Recognition that the early universe had a rapid expansion and cooling [Prg09].

“I saw that a universe hot enough to have left a detectable sea of thermal radiation would have tended to leave the abundances of the elements in a mix characteristic of the rapid expansion and cooling of the early universe. In an unpublished preprint in late 1964, I estimated that a reasonable upper bound on the primeval helium abundance requires a lower bound on the CMB temperature, \(T_0 ≥ 10 K\), in the absence of degeneracy.”

Calculation of a lower bound on the CMB temperature \(T_0\) [Prg10].

“We might pause to review why I had a lower bound on \(T_0\). During the course of expansion of the early universe, when the temperature fell through the critical value \(T_c ≥ 10^9 K\), detailed balance would have switched from suppression of deuterons by photodissociation to accumulation by radiative capture.” … ”So, my 1964 bound on the CMB temperature is a factor of three high. I have not attempted to discover why.”

Ans2.2. Calculations of a fossil radiation from a hot big bang theory.

Theory of a hot big bang by Gamow [Prg11].

“After I had worked out these considerations, I learned that George Gamow already presented the physics of element buildup in a hot big bang in two memorable papers published in 1948 (Gamow 1948a, b). Gamow had earlier proposed that the chemical elements were produced in the hot early stages of expansion of the universe by successive neutron captures, beta decays keeping the atomic nuclei in the valley of stability.”

Abundances of elements coming out of a hot big bang [Prg12].

… “But Fermi and Terkevich at the University of Chicago soon worked the first computation of the buildup of element abundances in a hot big bang using realistic nuclear reaction rates. They established that there would be little element buildup beyond helium, a result of Alpher’s mass-5 gap.” … Soon after that we realized there is a sea of microwave radiation, and after that I published a better computation in Peebles (1966).”

Calculations in the Soviet Union [Prg13].

“… Zel’dovich knew Gamow’s ideas but thought they must be wrong because the theory predicts an unacceptably large primeval helium abundance. To check the prediction, he asked Yuri Smirnov (1964) to compute element production in the hot big bang model, along the same lines I was taking in the USA.”

Calculations in the UK [Prg14].

“In the UK, Hoyle and Tayler (1964) knew the evidence that the helium abundance in old stars is large, and not inconsistent with Gamow’s (1948a, b) ideas.” … “Tayler (1990) recalls that in 1964 he and Hoyle realized that Gamow’s theory predicts the presence of a sea of thermal radiation, a fossil from the early hot conditions, but they supposed it would be obscured by all the radiation produced since then.”

Description of the situation in 1964 [Prg15].

… “In the USSR, Zel’dovich thought Gamow’s hot big bang theory is wrong because it overpredicts the helium abundance. In the UK, Hoyle knew the evidence that the prestellar helium abundance is large, and maybe consistent with Gamow’s theory. But Hoyle expected the fossil radiation that would accompany it would be uninterestingly small. In the USA, I did not know about Gamow yet, but I knew there was a chance of detecting fossil radiation from a hot big bang that made helium because the foreground at microwave frequencies looked likely to be small. Also, in the USA, 30 miles from Princeton, Penzias and Wilson had a clear case of detection of microwave radiation of unknown origin.”

Ans2.3. On the space distribution of galaxies.

Observations of the distributions of matter and radiation [Prg16].

“Yet another multiple was the recognition of the role of the sea of thermal radiation in the gravitational growth of the galaxies, independently by Gamow, Zel’dovich and his group, and me. I hit on what might be a singleton, the analysis of the effect of the dynamical interaction of matter and radiation in a hot big bang cosmology (in Peebles 1965 and many later papers).”… “The termination of acoustic oscillations is a boundary condition that favors discrete wavelengths. That imprints distinctive patterns on the distributions of matter and radiation. The effects became known as BAO, for baryon acoustic oscillations.” … “I developed the basic ideas of the modern approach to the growth of cosmic structure that describes the radiation by its distribution in phase space.”

The distributions of matter and radiation and the BAO theory [Prg17].

“It took some time to connect BAO theory to observations of the effect in the distributions of matter and radiation. The BAO effect in the angular distribution of the CMB was discovered and well measured at the turn of the century, and at the time there was a hint of detection in the galaxy space distribution (as reviewed in Peebles 2020).” …. “Anyway, I consider the connection of theory and observation of the effect of BAO to be a multiple.”

NOTE: CMB means Cosmic Microwave Background.

Possible departure from the equilibrium condition in the CMB [Prg18].

“For most of the time between BAO theory and observation it was not at all clear to me that there would be a detection. The BAO theory assumes standard physics, including the general theory of relativity. That is an extrapolation from the tests in the solar system and smaller, on scales \(≥ 10^{13} cm\), to the scales of cosmology \(≥ 10^{28} cm\).” ….. “The theory also assumes cosmic structure grew out of departures from homogeneity associated with small near scale-invariant spacetime curvature fluctuations.” … “That might mean violent events in the early universe released a lot of energy, contributing some of it to the CMB and some to rearranging the matter. Or maybe the universe is not very close to homogeneous; maybe we observe a mix of radiation temperatures from different regions.”

Experiments establishing that the CMB spectrum is very close to thermal [Prg19].

“This uncertain situation was resolved in 1990 by two brilliant experiments, one carried by the USA NASA satellite COBE, the other by the Canadian University of British Columbia rocket COBRA. Both established that the spectrum is very close to thermal.”

NOTE: COBE means Cosmic Background Explorer and COBRA Co-optimized Booster for Reusable Applications.

Statistical measures of the galaxy distribution and motions [Prg20].

… “so while awaiting clarification of the spectrum measurements I turned to another program, statistical measures of the galaxy distribution and motions relative to the mean homogeneous expansion of the universe. There were several catalogs of galaxy positions ready and waiting for analyses.” … “They counted galaxies in small cells in the sky, logging some one million galaxies by scanning photographic plates with a traveling microscope. This heroic effort took them ten years. Converting to data suitable for computation of statistical measures was a considerable effort too.”

Results concerning the space distribution of galaxies [Prg21].

“Since I like images, I was pleased with the map we made of the largescale galaxy distribution.”

Determination of galaxy position correlation functions [Prg22].

“The Lick and other catalogs are compilations of galaxy angular positions with approximate distances. The statistical measures I used are N-point position correlation functions and their Fourier or spherical harmonic transforms.” … “It showed that we had reliable measurements of the low order galaxy position correlation functions at separations from a few tens of kiloparsecs to a few megaparsecs.”

NOTE: parsec means parallax of one arc second. It is an astronomical unit measuring distances: 1 parsec = 3,2616 light years = 3,0857 \(\times 10^{16}\) (30 856 804 799 935 500 meters).

Methods and results of the calculation program [Prg23].

… “And I had the vague feeling that the results might offer a hint to how the galaxies and their clumpy space distribution got to be the way they are.” ….

Existence of nonbaryonic matter in the empirically based cosmology [Prg24].

“By 1980 it had become clear that the sea of microwave radiation is far smoother than the space distribution of the galaxies. But the mass concentrations in galaxies and groups and clusters of galaxies were supposed to have grown by gravity out of the initially close to homogeneous early universe of the hot big bang theory. How could this growth of mass concentrations have so little disturbed the CMB?” …. “Suppose the baryonic matter that stars and planets and people are made of is only a trace element, and that most matter is dark and interacts weakly if at all with radiation and our type of baryonic matter.” … “The CMB would slip freely through this nonbaryonic dark matter, allowing mass concentrations to grow while disturbing the CMB only by the weak effect of gravity and by the interaction with a modest amount of our baryonic matter.” … “I also knew the relativistic prediction of the gravitational disturbance to the radiation produced by the departure from a homogeneous mass distribution.”

Variation in the temperature caused by the observed matter distribution. [Prg25].

“The model I put together from these pieces predicts that the disturbance to the CMB caused by the formation of the observed matter distribution would cause the CMB temperature to vary across the sky by a few parts per million. That is much less than the upper bounds from the CMB anisotropy measurements we had when I published this prediction in Peebles (1982). The CMB anisotropy was detected some 15 years later and found to agree with my computation within the modest uncertainties.”

GQ3: What is dark matter?

Ans3.1. On cold dark matter.

Proposition of a new form of matter to be called cold dark matter [Prg26].

“The new form of matter in my 1982 proposal became known cold dark matter, or CDM, the “cold” meaning the dark matter pressure in the early universes was small enough not to have excessively smoothed the primeval mass distribution. I added the assumption that general relativity survives the immense extrapolation to the scales of cosmology, and that mass concentrations grew out of primeval spacetime curvature fluctuations.”

The weakly interacting massive particles (WIMP) as a new class of neutrinos [Prg27].

“There was a remarkable multiple in 1977. Five groups, independently as far as I can tell, introduced the idea of a new class of neutrinos with rest mass ~ 3 GeV. They became known as WIMPs, for weakly interacting massive particles.” ….. “Yet the WIMP idea appeared not long after the astronomers had good evidence of subluminal mass around galaxies, and not long before I needed nonbaryonic cold dark matter to account for the smoothness of the CMB.”

Simplicity of the cold dark matter cosmology (CDM) [Prg28].

“In particular, my 1982 paper assumed for simplicity that the universe is expanding at escape velocity, but by that time I already knew what I considered to be reasonably good evidence that the expansion is faster than that.”

Analyses of data related to the measurements of galaxy redshifts [Prg29].

“Expansion at escape velocity, in the relativistic Einstein-de Sitter cosmological model that assumes space curvature and Einstein’s cosmological constant may be ignored, would mean that whenever we happened to flourish and take an interest in the expanding universe, we would find that the rate of expansion is at escape velocity.” … “The results in Davis and Peebles (1983) surprised me by suggesting that we do flourish at a special epoch.”

Determination of the mean mass density in the formation of galaxies [Prg30].

“These redshift data yielded a probe of the relative motions of the galaxies. That gave a measure of galaxy masses, which indicated that the mean mass density is less than required for escape velocity.” … "Davis and I found consistent galaxy mass estimates from the relative motions of galaxies over a range of a factor of ten in separation.” …. “But if galaxy formation were suppressed in low density regions then galaxies that did manage to form there ought to show signs of a deprived youth: irregulars or dwarfs.”

Expansion of the universe faster than escape velocity [Prg31].

“From the early 1980s through the mid-1990s I played the role of Cassandra, emphasizing the growing evidence that the universe is expanding faster than escape velocity to people who for the most part would rather not think about it.” … “The evidence was reasonably good then, and it is well established now, that we flourish at a special time in the course of evolution of the universe, as the rate of expansion is becoming significantly more rapid than escape.”

NOTE: according to Greek mythology, Cassandra was a priestess who suffered the malediction that nobody will belief on her predictions.

Ans3.2. The ΛCDM cosmology as a well-tested standard model.

Einstein’s cosmological constant Λ in the ΛCDM theory [Prg32].

“In 1984 I introduced the accommodation to the low mass density that proves to work: add Einstein’s cosmological constant, Λ (Peebles 1984), in what became known as the ΛCDM theory.” … “Einstein wrote his constant as λ. I don’t know who introduced the change to Λ. I believe Michael Turner, the University of Chicago, introduced the change of name to dark energy.” … “Anyway, I think I was the first to present actual computations of the effect of adding Λ.”

Measurements of CMB anisotropy confirmed characteristic of the ΛCDM theory [Prg33].

“In the years around the mid-1990s I again acted in my self-appointed role of Cassandra, because I was not at all confident that the ΛCDM theory is a good approximation. The tests were not yet all that tight, and I could think of other models that fit the data about as well. In the late 1990s I was finishing my latest and maybe most elegant alternative to ΛCDM when I learned that the CMB anisotropy measurements revealed features characteristic of the theory Jer Ju and I had worked out a quarter century earlier.”

The ΛCDM theory passes demanding tests [Prg34].

“I remain surprised and impressed at how well ΛCDM passes ever more demanding tests. But I continue to hope that challenges to ΛCDM will be found and help guide us to a still better more complete theory.”

Observational programs converging on a well-tested standard model [Prg38].

“The convergence was driven by three great observational programs. One is the tight measurement of the redshift-magnitude relation that reveals the departure from the linear low redshift limit.” … “Second is the precision measurement of the cosmic microwave radiation anisotropy spectrum. “ … “The third, the measurement of the cosmic mean mass density, was the main focus of empirical research in cosmology from the early 1980s through the mid1990s.” …. “The three made the case for a cosmology that is hard to resist. I have once again given in to the impulse to write a book.”

NOTE: Peebles refers to his fourth book Cosmology’s Century published in 2020.

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