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The novel ‘Cold Hydrogen Dark Matter’ (CHDM) theory is summarized in this chapter. Special attention is paid to the fact that current technology prevents us from directly observing extremely cold ground state atomic hydrogen when it is of sufficiently low density in deep space locations. A number of very recent observations in support of this theory are summarized, including cosmic dawn constraints on dark matter. The importance of the Wouthuysen-Field effect as a probable mechanism for CMB decoupling of hydrogen at cosmic dawn is also stressed. This mechanism does not require a non-baryonic dark matter intermediary. Several predictions for this theory are made for the coming decade of observations and simulations.

The famous Dirac sea idea can be resurrected if one replaces the concept of positive and negative matter mass with positive and negative energy. Utilizing this concept, the perpetually spatially-flat matter-generating FSC model can be shown to be a realistic Milne “empty universe” model. Furthermore, this may be why Rh = ct cosmological models like FSC show an excellent statistical fit with the accumulated data of the Supernova Cosmology Project.

Theory and observations concerning the cosmic reionization epoch are briefly discussed in the context of recent observations attributed to dark matter. A case is made that cold ground state interstellar atomic hydrogen of average density of about one atom per cubic centimeter (1.67 × 10-21 kg·m-3 or 1.67 × 10-24 g·cm-3) appears to be the most likely candidate to explain these observations.

Realistic FLRW cosmic coasting models which contain matter now appear to be a reasonable alternative in explaining the accumulated Supernova Cosmology Project data since 1998. In sharp contrast to the unrealistic original classic Milne universe, which was entirely devoid of matter, these modified Milne-type models containing matter, often referred to as realistic linear Rh = ct models, have rapidly become the primary competition with standard cosmology. This paper compares the expected relative luminosity distances and relative angular diameter distances for given magnitudes of redshift within these two competing models. A simple ratio formula is derived, which explains how expected luminosity distances and angular diameter distances for given magnitudes of redshift within a realistic Milne-type cosmic expansion could create the illusion (for standard model proponents) of cosmic acceleration where none exists.

There have been a number of observational surprises with respect to galactic dark matter-to-visible matter ratios. These surprises confirm our continued lack of understanding of the fundamental nature of dark matter. Because of their apparent close ties with galactic gravitational entropy, at least four recent observations appear to provide the first evidence in support of Verlinde’s theory of gravity, dark energy and dark matter as emergent properties. They also appear to correlate with Roger Penrose’s gravitational entropy concept, as well as entropy defined in the Flat Space Cosmology (FSC) model. Given the observational support, two different testable versions of a “Dark Matter Index” (DMI) are introduced in this paper, and its utility is discussed in terms of potentially achieving a better understanding of the fundamental nature of dark matter.

Cosmologists have long ignored a stipulation by quantum field theorists that the vacuum pressure p corresponding to the zero-state vacuum energy must always be equal in magnitude to the vacuum energy density ρ (i.e., p=ρ). Although general relativity stipulates the additional condition of proportionality between the vacuum gravitational field and (ρ+3p), the equation of state for the cosmic vacuum must fulfill both relativistic and quantum stipulations. This paper fully integrates Flat Space Cosmology (FSC) into the Friedmann equations containing a cosmological term, with interesting implications for the nature of dark energy, cosmic entropy and the entropic arrow of time. The FSC vacuum energy density is shown to be equal to the cosmic fluid bulk modulus at all times, thus meeting the quantum theory stipulation of (p=ρ). To date, FSC is the only viable dark energy cosmological model which has fully-integrated general relativity and quantum features.

Following recent Cosmic Microwave Background (CMB) observations of global spatial flatness, only two types of viable cosmological models remain: inflationary models which almost instantaneously attain cosmic flatness following the Big Bang; and non-inflationary models which are spatially flat from inception. Flat Space Cosmology (FSC) is the latter type of cosmological model by virtue of assumptions corresponding to the Hawking-Penrose conjecture that a universe expanding from a singularity could be modeled like a time-reversed black hole. Since current inflationary models have been criticized for their lack of falsifiability, the numerous falsifiable predictions and key features of the FSC model are herein contrasted with standard inflationary cosmology. For the reasons given, the FSC model is shown to be superior to standard cosmology in the following eleven categories: Predictions Pertaining to Primordial Gravity Waves; Cosmic Dawn Early Surprises; Predicting the Magnitude of CMB Temperature Anisotropy; Predicting the Value of Equation of State Term w; Predicting the Hubble Parameter Value; Quantifiable Entropy and the Entropic Arrow of Time; Clues to the Nature of Gravity, Dark Energy and Dark Matter; The Cosmological Constant Problem; Quantum Cosmology; Dark Matter and Dark Energy Quantitation; Requirements for New Physics.

The purpose of this paper is to show how the dark matter predictions of FSC differ with respect to the standard cosmology assertion of a universal dark matter-to-visible matter ratio of approximately 5.3-to-1. FSC predicts the correct ratio to be approximately 9-to-1, based primarily on the universal observations of global spatial flatness in the context of general relativity. The FSC Friedmann equations incorporating a Lambda Λ cosmological term clearly indicate that a spatially flat universe must have equality of the positive curvature (matter mass-energy) and negative curvature (dark energy) density components. Thus, FSC predicts that observations of the Milky Way and the nearly co-moving galaxies within 100 million light years will prove the 5.3-to-1 ratio to be incorrect. The most recent galactic and perigalactic observations indicate a range of dark matter-to-visible matter ratios varying from essentially zero (NGC 1052-DF2) to approximately 23-to-1 (Milky Way). The latter ratio is simply astonishing and promises an exciting next few years for astrophysicists and cosmologists. Within the next few years, the mining of huge data bases (especially the Gaia catalogue and Hubble data) will resolve whether standard cosmology will need to change its current claims for the cosmic energy density partition to be more in line with FSC, or whether FSC is falsified. A prediction is that standard cosmology must eventually realize the necessity of resolving the tension between their flatness observations and their assertion of dark energy dominance. The author makes the further prediction that FSC will soon become the new paradigm in cosmology.

Flat Space Cosmology (FSC) is a mathematical model of universal expansion which has proven to be remarkably accurate in comparison to observations. Temperature scaling is redefined in this paper in terms of a new “Universal Temperature” Tu scale according to Tu = T 2, where T 2 is in K2. This rescaling puts FSC cosmic temperature, time, total matter mass, and Hubble radius on the same scale, covering roughly 60.63 logs of 10 from the Planck scale to the present scale. This paper focuses on the relatively subtle temperature curve differences between the FSC model and standard cosmology. These changes become more pronounced in the early universe. Recent observational studies of the early universe, particularly with respect to the “cosmic dawn” epoch, the first stars and first galaxies, have surprised standard model proponents as to how soon these events have occurred following the Big Bang. This paper suggests that, because the FSC model temperature/time curve is lower at each stage of cosmic time, FSC may actually be a better fit for the timing of these events.

This paper integrates the Flat Space Cosmology (FSC) model into the Friedmann equations containing a cosmological term. The Lambda term within this model scales according to 3H2t/c2 and 3/R2t. Use of the Bekenstein-Hawking definition of closed gravitational system total entropy provides for FSC cosmic parameter definitions in terms of . Cosmic time, radius, total matter mass-energy and vacuum energy in this model scale in exactly the same way as . This analysis opens the way for understanding gravity, dark energy and dark matter as being deeply connected with cosmic entropy. The recent theoretical work of Roger Penrose and Erik Verlinde is discussed in this context. The results of this FSC model analysis dovetail nicely with Verlinde’s work suggesting gravity as being fundamentally an emergent property of cosmic entropy. This emergent-property-of-entropy definition of gravity, if true, would also indicate that gravitational inertia, dark matter and dark energy are simply manifestations of cosmic entropy. Thus, they would likely have no identifiable connection to quantum physics, including the standard particle model.

This paper briefly discusses existing problems with the theory of general relativity despite remarkable accuracy in most of its applications. The primary focus is on existing problems in the field of cosmology, particularly those pertaining to expectations of global cosmic space-time curvature in the absence of observational proof. The discussion centers on Krogdahl’s recent Lorentz-invariant flat space-time cosmology and its superiority to general relativity with respect to accounting for global cosmic space-time flatness and dark energy observations. The “cosmological constant problem” is briefly addressed as a problem for general relativity with respect to particle physics and quantum field theory. Finally, two very specific validation predictions in favor of Krogdahl’s flat space-time cosmology are made with respect to ongoing studies, including the dark energy survey (DES).

Deriving an acceptable quantum field theory of gravitation from general relativity has eluded some of the best scientific thinkers. It is gradually becoming more apparent that general relativity’s classical assumptions are simply incompatible with quantum mechanics. For instance, simultaneous certainty of the location and momentum of any moving body, regardless of size, is a fundamental feature of general relativity. And yet, special relativity and quantum mechanics (thru Heisenberg’s uncertainty) reject the very notion of simultaneity. Since special relativity is already fully integrated into quantum field theory concerning the other forces of nature, were it possible to remove the confounding smoothly curved space-time fabric of general relativity and replace it in the form of a new and improved Lorentz-invariant (flat space-time) gravitational theory, final unification might well be achievable. This brief review paper further informs the reader as to why Krogdahl’s recent Lorentz-invariant relativity model of gravitation improves on general relativity, thus providing a deeper understanding of black holes, the cosmological flatness problem and dark energy. Most importantly, since the smoothly curved space-time of general relativity may well have been the road block to unification, Krogdahl’s flat space-time model is predicted to lead to an acceptable quantum theory of gravitation (i.e., “quantum gravity”) and unification (i.e., a so-called “theory of everything”).