Search

JWST's recent discovery of well-formed galaxies and supermassive black holes only a few hundred Myr after the big bang seriously challenges the timeline predicted by $\Lambda$CDM. Now, the latest identification of polycyclic aromatic hydrocarbons (PAHs) at $z=6.71$, together with these earlier inconsistencies, makes the time compression problem in this model quite overwhelming. We consider the timeline associated with the formation and growth of PAH grains based on current astrophysical models and argue that their appearance at $z=6.71$ favors the structure formation history in $R_{\rm h}=ct$ rather than that of Planck-$\Lambda$CDM. We estimate the time at which they must have started growing in each case, and then trace their history through various critical events, such as the end of the `dark ages,' the beginning of Pop III star formation, and the onset of reionization. Together, these three distinct discoveries by JWST, viz. high-$z$ galaxies, high-$z$ quasars and the surprisingly early appearance of PAHs, all paint a fully consistent picture in which the timeline in $\Lambda$CDM is overly compressed at $z\gtrsim 6$, while strongly supporting the expansion history in the early Universe predicted by $R_{\rm h}=ct$., Comment: 26 pages, 3 figures. Accepted for publication in PDU

JWST's discovery of well-formed galaxies and supermassive black holes only a few hundred Myr after the big bang, and the identification of polycyclic aromatic hydrocarbons (PAHs) at $z=6.71$, seriously challenge the timeline predicted by $\Lambda$CDM. Now, a recent analysis of reionization after JWST by Munoz et al. (2024) has concluded that the $\Lambda$CDM timeline simply cannot accommodate the combined JWST-Planck observations even if exotic fixes are introduced to modify the standard reionization model. In this paper, we argue that this so-called `photon budget crisis' is more likely due to flaws in the cosmological model itself. We employ the standard reionization model using the JWST-measured UV luminosity function in the early Universe and the timeline and physical conditions in both $\Lambda$CDM and $R_{\rm h}=ct$. We then contrast the predicted reionization histories in these two scenarios and compare them with the data. We confirm that the reionization history predicted by $\Lambda$CDM is in significant tension with the observations, and demonstrate that the latter are instead in excellent agreement with the $R_{\rm h}=ct$ timeline. Together, the four anomalies uncovered by JWST provide strong evidence against the timeline predicted by $\Lambda$CDM and in favor of the evolutionary history in $R_{\rm h}=ct$., Comment: 5 pages, 2 figures. Accepted for publication in A&A

Space-based missions studying the cosmic microwave background (CMB) have progressively refined the parameter space in conventional models of inflation shortly ($\sim 10^{-37}$ seconds) after the big bang. While most inflationary scenarios proposed thus far in the context of GR have since been ruled out, the basic idea of inflation may still be tenable, albeit with several unresolved conundrums, such as conflicting initial conditions and inconsistencies with the measured CMB power spectrum. In the new slow-roll inflationary picture, inflation arising in plateau-like potentials requires an initiation beyond the Planck time. This delay may be consistent with the cutoff, $k_{\rm min}$, measured recently in the primordial power spectrum. However, the actual value of $k_{\rm min}$ would imply an initiation time too far beyond the big bang for inflation to solve the horizon problem. In this paper, we also describe several other undesirable consequences of this delay, including an absence of well motivated initial conditions and a significant difficulty providing a viable mechanism for properly quantizing the primordial fluctuations. Nevertheless, many of these inconsistencies may still be avoided if one introduces non-conventional modifications to inflation, such as a brief departure from slow-roll dynamics, possibly due to a dramatic change in the inflationary potential, inflation driven by multiple fields, or a non-minimal coupling to gravity. In addition, some of these difficulties could be mitigated via the use of alternative cosmologies based, e.g., on loop quantum gravity, which replaces the initial big-bang singularity with finite conditions at a bounce-like beginning., Comment: 10 pages, 2 figures. Accepted for publication in ApJ

The absence of large-angle correlations in the temperature of the cosmic microwave background (CMB), confirmed by three independent satellite missions, creates significant tension with the standard model of cosmology. Previous work has shown, however, that a truncation, $k_{min}$, of the primordial power spectrum comprehensively resolves the anomaly and the missing power at $\ell\lesssim 5$ (the low multipoles). Since this cutoff is consistent with the hypothesized delay of inflation well beyond the Planck time, we are strongly motivated to consider its possible impact on other observational signatures. In this Letter, we analyze and predict its influence on the most revealing probe awaiting measurement by upcoming missions -- the B-mode polarization of the CMB, whose accurate determination should greatly impact the inflationary picture. We highlight the quantitative power of this discriminant by specifically considering the LiteBIRD mission, predicting the effect of $k_{min}$ on both the angular power spectrum and the angular correlation function of the B-mode, for a range of tensor-to-scalar ratios, $r$. While its impact on the latter appears to be negligible, $k_{\rm min}$ should have a very pronounced effect on the former. We show that for $r=0.036$, $k_{min}$'s impact on $C_{\ell}^{BB}$ at low $\ell$'s should be easily detectable by LiteBIRD, but will be largely hidden by the total uncertainty of the measurement if $r\lesssim 0.02$., Comment: 8 pages, 6 figures. Accepted for publication in PLB

Our concept of mass has evolved considerably over the centuries, most notably from Newton to Einstein, and then even more vigorously with the establishment of the standard model and the subsequent discovery of the Higgs boson. Mass is now invoked in various guises depending on the circumstance: it is used to represent inertia, or as a coupling constant in Newton's law of universal gravitation, and even as a repository of a mysterious form of energy associated with a particle at rest. But recent developments in cosmology have demonstrated that rest-mass energy is most likely the gravitational binding energy of a particle in causal contact with that portion of the Universe within our gravitational horizon. In this paper, we examine how all these variations on the concept of mass are actually interrelated via this new development and the recognition that the source of gravity in general relativity is ultimately the total energy in the system., Comment: 15 pages. Accepted for publication in Physica Scripta

Recent attempts at measuring the variation of $c$ using an assortment of standard candles and the redshift-dependent Hubble expansion rate inferred from the currently available catalog of cosmic chronometers have tended to show that the speed of light appears to be constant, at least up to $z\sim 2$. A notable exception is the use of high-redshift UV $+$ X-ray quasars, whose Hubble diagram seems to indicate a $\sim 2.7\sigma$ deviation of c from its value $c_0$ ($\equiv 2.99792458 \times 10^{10}$ cm s$^{-1}$) on Earth. We show in this paper, however, that this anomaly is due to an error in the derived relation between the luminosity distance, $D_L$, and $H(z)$ when $c$ is allowed to vary with redshift, and an imprecise calibration of the quasar catalog. When these deficiences are addressed correctly, one finds that $c/c_0=0.95 \pm 0.14$ in the redshift range $0\lesssim z\lesssim 2$, fully consistent with zero variation within the measurement errors., Comment: 6 pages, 3 figures, 3 tables. Accepted for publication in MNRAS

Spontaneous symmetry breaking in grand unified theories is thought to have produced an exceedingly large number of magnetic monopoles in the early Universe. In the absence of suppression or annihilation, these very massive particles should be dominating the cosmic energy budget today, but none has ever been found. Inflation was invented in part to dilute their number, thereby rendering their density undetectable by current instruments. Should the inflationary paradigm not survive, however, the ensuing disagreement between theory and observation would constitute a cosmological `monopole problem' and create further tension for any extension to the standard model of particle physics. But as is also true for all horizon problems, a monopole overabundance emerges only in cosmologies with an initial period of deceleration. We show that the alternative Friedmann-Lemaitre-Robertson-Walker cosmology known as the R_h=ct universe completely eliminates all such anomalies rather trivially and naturally, without the need for an inflated expansion. We find that the monopole energy density today would be completely undetectable in R_h=ct. Evidence continues to grow that the zero active mass condition from general relativity ought to be an essential ingredient in LCDM., Comment: 18 pages. Accepted for publication in Physics of the Dark Universe

The recent release of the final, complete survey of Lyman-alpha baryonic acoustic oscillation measurements provides the most significant and accurate data base for studying cosmic geometry at an effective redshift z_eff=2.334, which is inaccessible to other sources. In this Letter, we use these data to select among four distinct cosmologies: Planck LCDM, the R_h=ct universe, the Milne universe and Einstein-de Sitter. Given the breadth and depth of the Lyman-alpha study, this BAO measurement alone provides a strong model comparison, complementary to previous studies that combined Lyman-$\alpha$ data with measurements at lower redshifts. Though both approaches are useful, the latter tends to dilute the disparity between model predictions and the observations. We therefore examine how the models compare to each other strictly based on the BAO scale measured in the Lyman-alpha forest and background quasars. We find that Milne and Einstein-de Sitter are strongly ruled out by these data. There is also strong evidence disfavoring the standard model. The Lyman-alpha measurements are completely consistent with the cosmic geometry predicted by R_h=ct. As such, evidence continues to grow that the zero active mass condition from general relativity ought to be an essential ingredient in LCDM., Comment: 6 pages, 1 figure, 1 table. Accepted for publication in Europhysics Letters (EPL)

Studies of the Universe's transition to smoothness in the context of LCDM have all pointed to a transition radius no larger than ~300 Mpc. These are based on a broad array of tracers for the matter power spectrum, including galaxies, clusters, quasars, the Ly-alpha forest and anisotropies in the cosmic microwave background. It is therefore surprising, if not anomalous, to find many structures extending out over scales as large as ~2 Gpc, roughly an order of magnitude greater than expected. Such a disparity suggests that new physics may be contributing to the formation of large-scale structure, warranting a consideration of the alternative FLRW cosmology known as the $R_h=ct$ universe. This model has successfully eliminated many other problems in LCDM. In this paper, we calculate the fractal (or Hausdorff) dimension in this cosmology as a function of distance, showing a transition to smoothness at ~2.2 Gpc, fully accommodating all of the giant structures seen thus far. This outcome adds further observational support for R_h=ct over the standard model., Comment: 7 pages, 3 figures, 1 table. Accepted for publication in MNRAS

We use the dispersion measure (DM) and redshift measurements of 24 localized fast radio bursts (FRBs) to compare cosmological models and investigate the Hubble tension. Setting a flat prior on the DM contribution from the Milky Way's halo, $\mathrm{DM_{halo}^{MW}}\in[5,\;80]\;\mathrm{pc\;cm^{-3}}$, the best fit for flat $\Lambda$CDM is obtained with a Hubble constant $H_0=95.8^{+7.8}_{-9.2}\;\mathrm{km\;s^{-1}\;Mpc^{-1}}$ and a median matter density $\Omega_{\mathrm{m}}\approx0.66$. The best fit for the $R_{\mathrm{h}}=ct$ universe is realized with $H_0=94.2^{+5.6}_{-6.2}\;\mathrm{km\;s^{-1}\;Mpc^{-1}}$. We emphasize that the $H_0$ measurement depends sensitively on the $\mathrm{DM_{halo}^{MW}}$ prior. Since flat $\Lambda$CDM has one more free parameter, $R_{\mathrm{h}}=ct$ is favored by the Bayesian Information Criterion (BIC) with a likelihood of $\sim73\%$ versus $\sim27\%$. Through simulations, we find that if the real cosmology is $\Lambda$CDM, a sample of $\sim1,150$ FRBs in the redshift range $0

In standard inflationary cosmology, scalar and tensor perturbations grew as the Universe expanded and froze when their wavelengths exceeded the Hubble horizon, producing a tell-tale signature in the fluctuation spectrum and amplitude of the cosmic microwave background (CMB). But there are now very good reasons to examine whether structure formation could also have begun via the seeding of quantum fluctuations in a non-inflationary field. In this Letter, we study and compare the scalar and tensor modes produced in these two scenarios, and demonstrate that upcoming observations to measure the B-mode polarization of the CMB may be able to differentiate between them. Whereas both scalar and tensor modes should be observable if the field was inflationary, only scalar modes would be present in the CMB if it were not. Should gravity be purely classical, however, the tensor modes would have avoided canonical quantization in all cases, resulting in unmeasurably weak gravitational waves. A non-detection of B-mode polarization would thus not completely rule out inflation., Comment: 15 pages. Accepted for publication in Astroparticle Physics

The so-called `impossibly early galaxy' problem, first identified via the Hubble Space Telescope's observation of galaxies at redshifts z > 10, appears to have been exacerbated by the more recent James Webb Space Telescope (JWST) discovery of galaxy candidates at even higher redshifts (z ~ 17) which, however, are yet to be confirmed spectroscopically. These candidates would have emerged only ~ 230 million years after the big bang in the context of LCDM, requiring a more rapid star formation in the earliest galaxies than appears to be permitted by simulations adopting the concordance model parameters. This time-compression problem would therefore be inconsistent with the age-redshift relation predicted by LCDM. Instead, the sequence of star formation and galaxy assembly would confirm the timeline predicted by the R_h=ct universe, a theoretically advanced version of LCDM that incorporates the `zero active mass' condition from general relativity. This model has accounted for many cosmological data better than LCDM, and eliminates all of its inconsistencies, including the horizon and initial entropy problems. The latest JWST discoveries at z > 14, if confirmed, would add further support to the idea that the R_h=ct universe is favored by the observations over the current standard model., Comment: 6 pages, 3 figures, 1 table. Accepted for publication in MNRAS Letters

The sample of time-delay gravitational lenses appropriate for studying the geometry of the Universe continues to grow as dedicated campaigns, such as the Dark Energy Survey, the VST ATLAS survey, and the Large Synoptic Survey Telescope, complete their census of high-redshift sources. This catalog now includes hundreds of strong lensing systems, at least 31 of which have reasonably accurate time delay measurements. In this paper, we use them to compare the predictions of two competing Friedmann-Lemaitre-Robertson-Walker models: flat LCDM, characterized by two adjustable parameters (H_0 and Omega_m), and the R_h=ct universe (with H_0 as the single free variable). Over the past decade, the latter has accounted for the data better than the standard model, most recently the emergence of well-formed galaxies discovered by JWST at cosmic dawn. Here we show that the current sample of time-delay lenses favours R_h=ct with a likelihood of ~84% versus ~16% for the standard model. This level of accuracy will greatly improve as the ongoing surveys uncover many thousands additional lens systems over the next several years., Comment: 8 pages, 1 table, 4 figures. Accepted for publication in MNRAS

The search for alternative cosmological models is largely motivated by the growing discordance between the predictions of LCDM and the ever improving observations, such as the disparity in the value of H_0 measured at low and high redshifts. One model, in particular, known as the R_h=ct universe, has been highly successful in mitigating or removing all of the inconsistencies. In this picture, however, the anisotropies in the cosmic microwave background (CMB) would have emerged at a redshift z ~ 16, rather than via fluctuations in the recombination zone at z~1080. We demonstrate here that a CMB created in the early Universe, followed by scattering through a Pop III generated dust screen, cannot yet be ruled out by the current data. Indeed, the Planck measurements provide a hint of a ~2-4% frequency dependence in the CMB power spectrum, which would be naturally explained as a variation in the optical depth through the dust, but not a Thomson scattering-dominated recombination environment. Upcoming measurements should be able to easily distinguish between these two scenarios, e.g., via the detection of recombination lines at z~1080, which would completely eliminate the dust reprocessing idea., Comment: 14 pages, 1 table. Accepted for publication in ApJ

We consider several well-known f(R) cosmological models and constrain their parameters, namely the deviation parameter b and the cosmological parameters \Omega_m and h. We first obtain analytical approximations for the Hubble rate H(z) and the luminosity distance d_L(z) in terms of these parameters, and then test these against the observational expansion rate derived from cosmic chronometers and the distance modulus in the HII galaxy Hubble diagram, obtained in a model-independent way using Gaussian Processes (GP). We first optimize the models based solely on the cosmic chronometers and then repeat this process with a joint analysis using both the cosmic chronometers and HII galaxies., Comment: 16 pages

Spontaneously broken symmetries in particle physics may have produced several phase transitions in cosmology, e.g., at the GUT energy scale (~10^15 GeV), resulting in a quasi-de Sitter inflationary expansion, solving the background temperature horizon problem. This transition would have occurred at t~10^-36 to 10^-33 seconds, leading to a separation of the strong and electroweak forces. The discovery of the Higgs boson confirms that the Universe must have undergone another phase transition at the electroweak (EWPT) scale 159.5+/-1.5 GeV, about 10^-11 seconds later, when fermions and the W^+/- and Z^0 bosons gained mass, leading to the separation of the electric and weak forces. But today the vacuum expectation value (vev) of the Higgs field appears to be uniform throughout the visible Universe, a region much larger than causally-connected volumes at the EWPT. The discovery of the Higgs boson thus creates another serious horizon problem for LCDM, for which there is currently no established theoretical resolution. The EWPT was a smooth crossover, however, so previously disconnected electroweak vacuua might have homogenized as they gradually came into causal contact. But using the known Higgs potential and vev, we estimate that this process would have taken longer than the age of the Universe, so it probably could not have mitigated the emergence of different standard model parameters across the sky. The EWPT horizon problem thus argues against the expansion history of the early Universe predicted by standard cosmology., Comment: 15 pages. Accepted for publication in Physics of the Dark Universe

We use the age measurements of 114 old astrophysical objects (OAO) in the redshift range $0\lesssim z\lesssim 8$ to explore the Hubble tension. The age of the Universe at any $z$ is inversely proportional to the Hubble constant, $H_0$, so requiring the Universe to be older than the OAO it contains at any $z$ will lead to an upper limit on $H_0$. Assuming flat $\Lambda$CDM and setting a Gaussian prior on the matter density parameter $\Omega_{\rm m}=0.315\pm0.007$ informed by {\it Planck}, we obtain a 95\% confidence-level upper limit of $H_0<70.6 \rm{~km} \rm{~s}^{-1} \rm{~Mpc}^{-1}$, representing a $2\sigma$ tension with the measurement using the local distance ladder. We find, however, that the inferred upper limit on $H_{0}$ depends quite sensitively on the prior for $\Omega_{\rm m}$, and the Hubble tension between early-time and local measurements of $H_{0}$ may be due in part to the inference of both $\Omega_{\rm m}$ and $H_0$ in {\it Planck}, while the local measurement uses only $H_{0}$. The age-redshift data may also be used for cosmological model comparisons. We find that the $R_{\rm h}=ct$ universe accounts well for the data, with a reasonable upper limit on $H_{0}$, while Einstein-de Sitter fails to pass the cosmic-age test. Finally, we present a model-independent estimate of the spatial curvature using the ages of 61 galaxies and the luminosity distances of 1,048 Pantheon Type Ia supernovae. This analysis suggests that the geometry of the Universe is marginally consistent with spatial flatness at a confidence level of $1.6\sigma$, characterized as $\Omega_{k}=0.43^{+0.27}_{-0.27}$., Comment: 13 pages, 6 figures, 2 tables. Accepted for publication in ApJ

The evidence for a Big Bang origin of the Universe is truly compelling, though its cause remains a complete mystery. As the cosmic spacetime is revealed to us with ever improving detail, however, we are beginning to refine the range of its possible initial conditions -- at least within the framework of current physical theories. The Universe, it seems, is spatially flat, and here we discuss in clear, straightforward terms why this trait implies a cosmos with zero kinetic plus gravitational energy, though apparently not zero total energy. Such an outcome has far reaching consequences because of the possibility that the Universe may have begun its existence as a quantum fluctuation. Was this from `nothing,' or perhaps a pre-existing vacuum? A non-zero total energy would seemingly preclude the former scenario, but not necessarily the latter, though this would then raise the question of how a fluctuation with non-zero energy could have lived long enough, or classicalized, for us to see it 13.5 billion years later. The high-precision measurement of the Universe's spatial curvature may thus constitute the first tangible piece of evidence impacting a possible quantum beginning., Comment: 8 pages, 1 figure. Accepted for publication in Astronomische Nachrichten / Astronomical Notes

The most exciting future observation in cosmology will feature a monitoring of the cosmic expansion in real time, unlike anything that has ever been attempted before. This campaign will uncover crucial physical properties of the various constituents in the Universe, and perhaps answer a simpler question concerning whether or not the cosmic expansion is even accelerating. An unambiguous yes/no response to this query will significantly impact cosmology, of course, but also the standard model of particle physics. Here, we discuss -- in a straightforward way -- how to understand the so-called `redshift drift' sought by this campaign, and why its measurement will help us refine the standard-model parameters if the answer is `yes.' A `no' answer, on the other hand, could be more revolutionary, in the sense that it might provide a resolution of several long-standing problems and inconsistencies in our current cosmological models. An outcome of zero redshift drift, for example, would obviate the need for a cosmological constant and render inflation completely redundant., Comment: 15 pages, 4 figures. Accepted for publication in EJP

Several satellite missions have uncovered a series of potential anomalies in the fluctuation spectrum of the cosmic microwave background temperature, including: (1) an unexpectedly low level of correlation at large angles, manifested via the angular correlation function, C(theta); and (2) missing power in the low multipole moments of the angular power spectrum, C_ell. Their origin is still debated, however, due to a persistent lack of clarity concerning the seeding of quantum fluctuations in the early Universe. A likely explanation for the first of these appears to be a cutoff, k_min=(3.14 +/- 0.36) x 10^{-4} Mpc^{-1}, in the primordial power spectrum, P(k). Our goal in this paper is twofold: (1) we examine whether the same k_min can also self-consistently explain the missing power at large angles, and (2) we confirm that the of this cutoff in P(k) does not adversely affect the remarkable consistency between the prediction of Planck-LCDM and the Planck measurements at ell > 30. We use the publicly available code CAMB to calculate the angular power spectrum, based on a line-of-sight approach. The code is modified slightly to include the additional parameter (i.e., k_min) characterizing the primordial power spectrum. In addition to this cutoff, the code optimizes all of the usual standard-model parameters. In fitting the angular power spectrum, we find an optimized cutoff, k_min = 2.04^{+1.4}_{-0.79} x 10^{-4} Mpc^{-1}, when using the whole range of ell's, and k_min=3.3^{+1.7}_{-1.3} x 10^{-4} Mpc^{-1}, when fitting only the range ell < 30, where the Sachs-Wolfe effect is dominant. These are fully consistent with the value inferred from C(theta), suggesting that both of these large-angle anomalies may be due to the same truncation in P(k)., Comment: 9 pages, 4 figures, 1 table. Accepted for publication in A&A

Today we have a solid, if incomplete, physical picture of how inertia is created in the standard model. We know that most of the visible baryonic `mass' in the Universe is due to gluonic back-reaction on accelerated quarks, the latter of which attribute their own inertia to a coupling with the Higgs field -- a process that elegantly and self-consistently also assigns inertia to several other particles. But we have never had a physically viable explanation for the origin of rest-mass energy, in spite of many attempts at understanding it towards the end of the nineteenth century, culminating with Einstein's own landmark contribution in his Annus Mirabilis. Here, we introduce to this discussion some of the insights we have garnered from the latest cosmological observations and theoretical modeling to calculate our gravitational binding energy with that portion of the Universe to which we are causally connected, and demonstrate that this energy is indeed equal to mc^2 when the inertia m is viewed as a surrogate for gravitational mass., Comment: 12 pages. Accepted for publication in EPJ-C

The quantum to classical transition of fluctuations in the early universe is still not completely understood. Some headway has been made incorporating the effects of decoherence and the squeezing of states, though the methods and procedures continue to be challenged. But new developments in the analysis of the most recent Planck data suggest that the primordial power spectrum has a cutoff associated with the very first quantum fluctuation to have emerged into the semi-classical universe from the Planck domain at about the Planck time. In this paper, we examine the implications of this result on the question of classicalization, and demonstrate that the birth of quantum fluctuations at the Planck scale would have been a `process' supplanting the need for a `measurement' in quantum mechanics. Emerging with a single wavenumber, these fluctuations would have avoided the interference between different degrees of freedom in a superposed state. Moreover, the implied scalar-field potential had an equation-of-state consistent with the zero active mass condition in general relativity, allowing the quantum fluctuations to emerge in their ground state with a time-independent frequency. They were therefore effectively quantum harmonic oscillators with classical correlations in phase space from the very beginning., Comment: 14 pages, 2 figures. Accepted for publication in PLB

The EDGES collaboration has reported the detection of a global 21-cm signal with a plateau centered at 76 MHz (i.e., redshift 17.2), with an amplitude of 500^(+200)_(-500) mK. This anomalous measurement does not comport with standard cosmology, which can only accommodate an amplitude < 230 mK. Nevertheless, the line profile's redshift range (15 < z < 20) suggests a possible link to Pop III star formation and an implied evolution out of the `dark ages.' Given this tension with the standard model, we here examine whether the observed 21-cm signal is instead consistent with the results of recent modeling based on the alternative Friedmann-Lemaitre-Robertson-Walker cosmology known as the R_h=ct universe, showing that--in this model--the CMB radiation might have been rethermalized by dust ejected into the IGM by the first-generation stars at redshift z < 16. We find that the requirements for this process to have occurred would have self-consistently established an equilibrium spin temperature T_s~3.4 K in the neutral hydrogen, via the irradiation of the IGM by deep penetrating X-rays emitted at the termination shocks of Pop III supernova remnants. Such a dust scenario has been strongly ruled out for the standard model, so the spin temperature (~3.3 K) inferred from the 21-cm absorption feature appears to be much more consistent with the R_h=ct profile than that implied by LCDM, for which adiabatic cooling would have established a spin temperature T_s(z=17.2)~6 K., Comment: 9 pages, 1 figure. Accepted for publication in EPJ-C

In the standard model of cosmology, the Universe began its expansion with an anomalously low entropy, which then grew dramatically to much larger values consistent with the physical conditions at decoupling, roughly 380,000 years after the Big Bang. There does not appear to be a viable explanation for this `unnatural' history, other than via the generalized second law of thermodynamics (GSL), in which the entropy of the bulk, S_bulk, is combined with the entropy of the apparent (or gravitational) horizon, S_h. This is not completely satisfactory either, however, since this approach seems to require an inexplicable equilibrium between the bulk and horizon temperatures. In this paper, we explore the thermodynamics of an alternative cosmology known as the R_h=ct universe, which has thus far been highly successful in resolving many other problems or inconsistencies in LCDM. We find that S_bulk is constant in this model, eliminating the so-called initial entropy problem simply and elegantly. The GSL may still be relevant, however, principally in selecting the arrow of time, given that S_h ~ t^2 in this model., Comment: 12 pages, 1 Table. Accepted for publication in EPJ-C

The cosmic distance duality relation (CDDR), eta(z)=(1+z)^2 d_A(z)/d_L(z)=1, is one of the most fundamental and crucial formulae in cosmology. This relation couples the luminosity and angular diameter distances, two of the most often used measures of structure in the Universe. We here propose a new model-independent method to test this relation, using strong gravitational lensing (SGL) and the high-redshift quasar Hubble diagram reconstructed with a Bezier parametric fit. We carry out this test without pre-assuming a zero spatial curvature, adopting instead the value Omega_K=0.001 +/- 0.002 optimized by Planck in order to improve the reliability of our result. We parametrize the CDDR using eta(z)=1 + eta_0 z, 1 + eta_1 z + eta_2 z^2 and 1 + eta_3 z/(1+z), and consider both the SIS and non-SIS lens models for the strong lensing. Our best fit results are: eta_0=-0.021^{+0.068}_{-0.048}, eta_1=-0.404^{+0.123}_{-0.090}, eta_2=0.106^{+0.028}_{-0.034}, and eta_3=-0.507^{+0.193}_{-0.133} for the SIS model, and eta_0=-0.109^{+0.044}_{-0.031} for the non-SIS model. The measured eta(z), based on the Planck parameter Omega_K, is essentially consistent with the value (=1) expected if the CDDR were fully respected. For the sake of comparison, we also carry out the test for other values of Omega_K, but find that deviations of spatial flatness beyond the Planck optimization are in even greater tension with the CDDR. Future measurements of SGL may improve the statistics and alter this result but, as of now, we conclude that the CDDR favours a flat Universe., Comment: 10 pages, 9 figures, 1 table; accepted for publication in Monthly Notices of the Royal Astronomical Society

Inflation drives quantum fluctuations beyond the Hubble horizon, freezing them out before the small-scale modes re-enter during the radiation dominated epoch, and subsequently decay, while large-scale modes re-enter later during the matter dominated epoch and grow. This distinction shapes the matter power spectrum and provides observational evidence in support of the standard model. In this paper, we demonstrate that another mechanism, based on the fluctuation growth in the R_h=ct universe, itself an FLRW cosmology with the added constraint of zero active mass (i.e., rho+3p=0), also accounts very well for the observed matter power spectrum, so this feature is not unique to LambdaCDM. In R_h=ct, the shape of the matter power spectrum is set by the interplay between the more rapid decay of the gravitational potential for the smaller mode wavelengths and the longer dynamical timescale for the larger wavelengths. This combination produces a characteristic peak that grows in both amplitude and mode number as a function of time. Today, that peak lies at k approx 0.02 Mpc^-1, in agreement with the Ly-alpha and Planck data. But there is no need of an inflationary expansion, and a complicated epoch dependence as one finds in LambdaCDM., Comment: Accepted for publication in PDU

The recent measurement of a cutoff k_min in the fluctuation power spectrum P(k) of the cosmic microwave background may vitiate the possibility that slow-roll inflation can simultaneously solve the horizon problem and account for the formation of structure via the growth of quantum fluctuations in the inflaton field. Instead, we show that k_min may be interpreted more successfully in the R_h=ct cosmology, as the first mode exiting from the Planck scale into the semi-classical Universe shortly after the Big Bang. In so doing, we demonstrate that such a scenario completely avoids the well-known trans-Planckian problem plaguing standard inflationary cosmology., Comment: 7 pages. Accepted for publication in Astronomische Nachrichten

The geometry of the Universe may be probed using the Alcock-Paczynski (AP) effect, in which the observed redshift size of a spherical distribution of sources relative to its angular size varies according to the assumed cosmological model. Past applications of this effect have been limited, however, by a paucity of suitable sources and mitigating astrophysical factors, such as internal redshift-space distortions and poorly known source evolution. In this Letter, we introduce a new test based on the AP effect that avoids the use of spatially bound systems, relying instead on sub-samples of quasars at redshifts z < 1.5 in the Sloan Digital Sky Survey IV, with a possible extension to higher redshifts and improved precision when this catalog is expanded by upcoming surveys. We here use this method to probe the redshift-dependent expansion rate in three pertinent Friedmann-Lemaitre-Robertson-Walker (FLRW) cosmologies: LCDM, which predicts a transition from deceleration to acceleration at z ~ 0.7; Einstein-de Sitter, in which the Universe is always decelerating; and the R_h=ct universe, which expands at a constant rate. LCDM is consistent with these data, but R_h=ct is favoured overall., Comment: 5 pages, 5 figures, 1 table. Accepted for publication in MNRAS Letters

The notion that dust might have formed the cosmic microwave background (CMB) has been strongly refuted on the strength of four decades of observation and analysis, in favour of recombination at a redshift z ~ 1080. But tension with the data is growing in several other areas, including measurements of the Hubble constant H(z) and the BAO scale, which directly or indirectly impact the physics at the surface of last scattering (LSS). The R_h=ct universe resolves at least some of this tension. We show in this paper that---if the BAO scale is in fact equal to the acoustic horizon---the redshift of the LSS in this cosmology is z_cmb ~ 16, placing it within the era of Pop III star formation, prior to the epoch of reionization at 15 > z > 6. Quite remarkably, the measured values of z_cmb and H_0 = H(0) in this model are sufficient to argue that the CMB temperature today ought to be ~ 3 K, so H_0 and the baryon to photon ratio are not independent free parameters. This scenario might have resulted from rethermalization of the CMB photons by dust, presumably supplied to the interstellar medium by the ejecta of Pop III stars. Dust rethermalization may therefore yet resurface as a relevant ingredient in the R_h=ct universe. Upcoming high sensitivity instruments should be able to readily distinguish between the recombination and dust scenarios by either (i) detecting recombination lines at z ~ 1080, or (ii) establishing a robust frequency-dependent variation of the CMB power spectrum at the level of ~ 2-4% across the sampled frequency range., Comment: 24 pages, 3 figures, 2 tables. Accepted for publication in European Physical Journal Plus

Slow-roll inflation may simultaneously solve the horizon problem and generate a near scale-free fluctuation spectrum P(k). These two processes are intimately connected via the initiation and duration of the inflationary phase. But a recent study based on the latest Planck release suggests that P(k) has a hard cutoff, k_min > 0, inconsistent with this conventional picture. Here we demonstrate quantitatively that most---perhaps all---slow-roll inflationary models fail to accommodate this minimum cutoff. We show that the small parameter `epsilon' must be > 0.9 throughout the inflationary period to comply with the data, seriously violating the slow-roll approximation. Models with such an epsilon predict extremely red spectral indices, at odds with the measured value. We also consider extensions to the basic picture (suggested by several earlier workers) by adding a kinetic-dominated or radiation-dominated phase preceding the slow-roll expansion. Our approach differs from previously published treatments principally because we require these modifications to---not only fit the measured fluctuation spectrum, but to simultaneously also---fix the horizon problem. We show, however, that even such measures preclude a joint resolution of the horizon problem and the missing correlations at large angles., Comment: 13 pages, 4 figures. Accepted for publication in Proceedings of the Royal Society A

With the distance sum rule in the Friedmann-Lema\^{\i}tre-Robertson-Walker metric, model-independent constraints on both the Hubble constant $H_0$ and spatial curvature $\Omega_{K}$ can be obtained using strong lensing time-delay data and Type Ia supernova (SN Ia) luminosity distances. This method is limited by the relative low redshifts of SNe Ia, however. Here, we propose using quasars as distance indicators, extending the coverage to encompass the redshift range of strong lensing systems. We provide a novel and improved method of determining $H_0$ and $\Omega_{K}$ simultaneously. By applying this technique to the time-delay measurements of seven strong lensing systems and the known ultraviolet versus X-ray luminosity correlation of quasars, we constrain the possible values of both $H_0$ and $\Omega_{K}$, and find that $H_0=75.3^{+3.0}_{-2.9}$ km $\rm s^{-1}$ $\rm Mpc^{-1}$ and $\Omega_{K}=-0.01^{+0.18}_{-0.17}$. The measured $\Omega_{K}$ is consistent with zero spatial curvature, indicating that there is no significant deviation from a flat universe. If we use flatness as a prior, we infer that $H_0=75.3^{+1.9}_{-1.9}$ km $\rm s^{-1}$ $\rm Mpc^{-1}$, representing a precision of 2.5\%. If we further combine these data with the 1048 current Pantheon SNe Ia, our model-independent constraints can be further improved to $H_0=75.3^{+3.0}_{-2.9}$ km $\rm s^{-1}$ $\rm Mpc^{-1}$ and $\Omega_{K}=0.05^{+0.16}_{-0.14}$. In every case, we find that the Hubble constant measured with this technique is strongly consistent with the value ($\sim 74$ km $\rm s^{-1}$ $\rm Mpc^{-1}$) measured using the local distance ladder, as opposed to the value optimized by {\it Planck}., Comment: 11 pages, 5 figures, 2 tables. Accepted for publication in ApJ

We present a new model-independent method to determine the spatial curvature and to mitigate the circularity problem affecting the use of quasars as distance indicators. The cosmic-chronometer measurements are used to construct the curvature-dependent luminosity distance $D^{\rm cal}_{L}(\Omega_{K},z)$ using a polynomial fit. Based on the reconstructed $D^{\rm cal}_{L}(\Omega_{K},z)$ and the known ultraviolet versus X-ray luminosity correlation of quasars, we simultaneously place limits on the curvature parameter $\Omega_{K}$ and the parameters characterizing the luminosity correlation function. This model-independent analysis suggests that a mildly closed Universe ($\Omega_{K}=-0.918\pm0.429$) is preferred at the $2.1\sigma$ level. With the calibrated luminosity correlation, we build a new data set consisting of 1598 quasar distance moduli, and use these calibrated measurements to test and compare the standard $\Lambda$CDM model and the $R_{\rm h}=ct$ universe. Both models account for the data very well, though the optimized flat $\Lambda$CDM model has one more free parameter than $R_{\rm h}=ct$, and is penalized more heavily by the Bayes Information Criterion. We find that $R_{\rm h}=ct$ is slightly favoured over $\Lambda$CDM with a likelihood of $\sim57.7\%$ versus 42.3\%., Comment: 8 pages, 5 figures, 2 tables. Accepted for publication in ApJ

We use three different data sets, specifically $H(z)$ measurements from cosmic chronometers, the HII-galaxy Hubble diagram, and reconstructed quasar-core angular-size measurements, to perform a joint analysis of three flat cosmological models: the $R_{\rm h}=ct$ Universe, $\Lambda$CDM, and $w$CDM. For $R_{\rm h}=ct$, the 1$\sigma$ best-fit value of the Hubble constant $H_0$ is $62.336\pm1.464$ $\mathrm{km \ s^{-1} \ Mpc^{-1}}$, which matches previous measurements ($\sim 63$ $\mathrm{km \ s^{-1} \ Mpc^{-1}}$) based on best fits to individual data sets. For $\Lambda$CDM, our inferred value of the Hubble constant, $H_0=67.013\pm2.578$ $\mathrm{km \ s^{-1} \ Mpc^{-1}}$, is more consistent with the ${\it Planck}$ optimization than the locally measured value using $\mbox{Cepheid}$ variables, and the matter density $\Omega_{\rm m}=0.347\pm0.049$ similarly coincides with its ${\it Planck}$ value to within 1$\sigma$. For $w$CDM, the optimized parameters are $H_0=64.718\pm3.088$ $\mathrm{km \ s^{-1} \ Mpc^{-1}}$, $\Omega_{\rm m}=0.247\pm0.108$ and $w=-0.693\pm0.276$, also consistent with ${\it Planck}$. A direct comparison of these three models using the Bayesian Information Criterion shows that the $R_{\rm h}=ct$ universe is favored by the joint analysis with a likelihood of $\sim 97\%$ versus $\lesssim 3\%$ for the other two cosmologies., Comment: 20 pages, 4 figures, Physics of the Dark Universe, in press

It has been known for over three decades that the monochromatic X-ray and UV luminosities in quasars are correlated, though non-linearly. This offers the possibility of using high-z quasars as standard candles for cosmological testing. In this paper, we use a recently assembled, high-quality catalog of 1598 quasars extending all the way to redshift ~6, to compare the predictions of the R_h=ct and LCDM cosmologies. In so doing, we affirm that the parameters characterizing the correlation depend only weakly on the chosen cosmology, and that both models account very well for the data. Unlike LCDM, however, the R_h=ct model has no free parameters for this work, so the Bayesian Information Criterion favours it over LCDM with a relative likelihood of ~88 % versus ~10 %. This result is consistent with the outcome of other comparative tests, many of which have shown that R_h=ct is favoured over the standard model based on a diverse range of observations., Comment: 7 pages, 9 figures, 1 table. Accepted for publication in MNRAS

The growth of structure probes the re-ionization history and quasar abundance in the Universe, constituting an important probe of the cosmological predictions. Halos are not directly observable, however, so their mass and evolution must be inferred indirectly. Studies based on the assumption of a constant halo to stellar mass ratio M_h/M_* (extrapolated from z<4) reveal significant tension with LCDM---a failure known as "The Impossibly Early Galaxy Problem". But whether this ratio evolves or remains constant through redshift 4

The recently measured cutoff, k_min=[4.34(+/-)0.50]/r_cmb (with r_cmb the comoving distance to the last scattering surface), in the fluctuation spectrum of the cosmic microwave background, appears to disfavor slow-roll inflation and the associated transition of modes across the horizon. We show in this Letter that k_min instead corresponds to the first mode emerging out of the Planck domain into the semi-classical universe. The required scalar-field potential is exponential, though not inflationary, and satisfies the zero active mass condition, rho_phi+3p_phi=0. Quite revealingly, the observed amplitude of the temperature anisotropies requires the quantum fluctuations in phi to have classicalized at ~3.5x10^15 GeV, consistent with the energy scale in grand unified theories. Such scalar-field potentials are often associated with Kaluza-Klein cosmologies, string theory and even supergravity., Comment: 4 pages. Accepted for publication in EPJ-C Letters

We present a phenomenological method for predicting the number of Flat Spectrum Radio Quasars (FSRQs) that should be detected by upcoming Square Kilometer Array (SKA) SKA1-MID Wide Band 1 and Medium-Deep band 2 surveys. We use the Fermi Blazar Sequence and mass estimates of Fermi FSRQs, and gamma-ray emitting Narrow Line Seyfert 1 galaxies, to model the radio emission of FSRQs as a function of mass alone, assuming a near-Eddington accretion rate, which is suggested by current quasar surveys at z > 6. This is used to determine the smallest visible black hole mass as a function of redshift in two competing cosmologies we compare in this paper: the standard LCDM model and the R_h=ct universe. We then apply lockstep growth to the observed black-hole mass function at $z=6$ in order to devolve that population to higher redshifts and determine the number of FSRQs detectable by the SKA surveys as a function of z. We find that at the redshifts for which this method is most valid, LCDM predicts ~30 times more FSRQs than R_h=ct for the Wide survey, and ~100 times more in the Medium-Deep survey. These stark differences will allow the SKA surveys to strongly differentiate between these two models, possibly rejecting one in comparison with the other at a high level of confidence., Comment: 8 pages, 5 figures, 3 tables. Accepted for publication in MNRAS

The emergence of a highly improbable coincidence in cosmological observations speaks to a remarkably simple cosmic expansion. Compelling evidence now suggests that the Universe's gravitational horizon, coincident with the better known Hubble sphere, has a radius improbably equal to the distance light could have travelled since the Big Bang. The confirmation of this unexpected result would undoubtedly herald the influence of new physics, yet appears to be unavoidable after a recent demonstration that the Friedmann-Lemaitre-Robertson-Walker metric is valid only for the so-called zero active mass equation of state. As it turns out, a cosmic fluid with this property automatically produces the aforementioned equality, leaving little room for a cosmological constant. The alternative---a dynamical dark energy---would suggest an extension to the standard model of particle physics, and a serious re-evaluation of the Universe's early history., Comment: 23 pages, 9 figures, Review article accepted for publication in Modern Physics Letters A

The Universe has a gravitational horizon with a radius R_h=c/H coincident with that of the Hubble sphere. This surface separates null geodesics approaching us from those receding, and as free-falling observers within the Friedmann-Lemaitre-Robertson-Walker spacetime, we see it retreating at proper speed c, giving rise to the eponymously named cosmological model R_h=ct. As of today, this cosmology has passed over 25 observational tests, often better than LCDM. The gravitational/Hubble radius R_h therefore appears to be highly relevant to cosmological theory, and in this paper we begin to explore its impact on fundamental physics. We calculate the binding energy of a mass m within the horizon and demonstrate that it is equal to mc^2. This energy is stored when the particle is at rest near the observer, transitioning to a purely kinetic form equal to the particle's escape energy when it approaches R_h. In other words, a particle's gravitational coupling to that portion of the Universe with which it is causally connected appears to be the origin of rest-mass energy., Comment: 5 pages. Accepted for publication in IJMP-A

We present a model-independent measurement of spatial curvature $\Omega_{k}$ in the Friedmann-Lema\^itre-Robertson-Walker (FLRW) universe, based on observations of the Hubble parameter $H(z)$ using cosmic chronometers, and a Gaussian Process (GP) reconstruction of the HII galaxy Hubble diagram. We show that the imposition of spatial flatness (i.e., $\Omega_k=0$) easily distinguishes between the Hubble constant measured with {\it Planck} and that based on the local distance ladder. We find an optimized curvature parameter $\Omega_{k} = -0.120^{+0.168}_{-0.147}$ when using the former (i.e., $H_0=67.66\pm0.42 \, \mathrm{km}\,\mathrm{s}^{-1} \,\mathrm{Mpc}^{-1}$), and $\Omega_{k} = -0.298^{+0.122}_{-0.088}$ for the latter ($H_0=73.24\pm 1.74 \,\mathrm{km}\,\mathrm{s}^{-1} \,\mathrm{Mpc}^{-1}$). The quoted uncertainties are extracted by Monte Carlo sampling, taking into consideration the covariances between the function and its derivative reconstructed by GP. These data therefore reveal that the condition of spatial flatness favours the {\it Planck} measurement, while ruling out the locally inferred Hubble constant as a true measure of the large-scale cosmic expansion rate at a confidence level of $\sim 3\sigma$., Comment: 9 pages, 5 figures, accepted for publication in ApJ

We propose a model-independent way to determine the cosmic curvature using the Hubble parameter $H(z)$ measured with cosmic chronometers and the comoving distance $D(z)$ inferred from HII galaxies. We employ Gaussian processes to smooth the measure of distance and match it to $30$ values of $H(z)$. The curvature parameter $\Omega_k$ may be obtained individually for each such pair. The weighted average for the complete sample is $\Omega_k=-0.0013\pm0.0004$, suggesting a bias towards negative values. The accuracy of the curvature measurement improves with increased redshift, however, given possible systematic effects associated with local inhomogeneities. We therefore also analyze a high-redshift ($z>1.5$) sub-sample on its own, which is more likely to reflect the geometry of the Universe on large, smooth scales. We find for this set of data that $\Omega_k=-0.0111\pm0.0416$, consistent with zero to better than $1\sigma$. This result is in agreement with the spatially flat universe inferred from the cosmic microwave background observations. We expect this method to yield even tighter constraints on the curvature parameter with future, more accurate observations of HII galaxies at high $z$., Comment: 4 pages, 3 figures, submitted to ApJ

We examine whether a comparison between wCDM and R_h=ct using merged Type Ia SN catalogs produces results consistent with those based on a single homogeneous sample. Using the Betoule et al. (2014) joint lightcurve analysis (JLA) of a combined sample of 613 events from SNLS and SDSS-II, we estimate the parameters of the two models and compare them. We find that the improved statistics can alter the model selection in some cases, but not others. In addition, based on the model fits, we find that there appears to be a lingering systematic offset of ~0.04-0.08 mag between the SNLS and SDSS-II sources, in spite of the cross-calibration in the JLA. Treating wCDM, LCDM and R_h=ct as separate models, we find in an unbiased pairwise statistical comparison that the Bayes Information Criterion (BIC) favors the R_h=ct universe with a likelihood of 82.8% versus 17.2% for wCDM, but the ratio of likelihoods is reversed (16.2% versus 83.8%) when w_de=-1 (i.e., LCDM) and strongly reversed (1.0% versus 99.0%) if in addition k=0 (i.e., flat LCDM). We point out, however, that the value of k is a measure of the net energy (kinetic plus gravitational) in the Universe and is not constrained theoretically, though some models of inflation would drive k to 0 due to an expansion-enforced dilution. Since we here consider only the basic LCDM model, the value of k needs to be measured and, therefore, the pre-assumption of flatness introduces a significant bias into the BIC., Comment: 7 pages, 4 figures, 2 tables. Accepted for publication in EPL

Particle physics suggests that the Universe may have undergone several phase transitions, including the well-known inflationary event associated with the separation of the strong and electroweak forces in grand unified theories. The accelerated cosmic expansion during this transition, at cosmic time t~10^{-36}-10^{-33} seconds, is often viewed as an explanation for the uniformity of the CMB temperature, T, which would otherwise have required inexplicable initial conditions. With the discovery of the Higgs particle, it is now quite likely that the Universe underwent another (electroweak) phase transition, at T=159.5 +/- 1.5 GeV---roughly ~10^{-11} seconds after the big bang. During this event, the fermions gained mass and the electric force separated from the weak force. There is currently no established explanation, however, for the apparent uniformity of the vacuum expectation value of the Higgs field which, like the uniformity in T, gives rise to its own horizon problem in standard LCDM cosmology. We show in this paper that a solution to the electroweak horizon problem may be found in the choice of cosmological model, and demonstrate that this issue does not exist in the alternative Friedmann-Robertson-Walker cosmology known as the R_h=ct universe., Comment: 6 pages, 2 figures. Accepted for publication in the European Physical Journal C

A validation of the cosmic distance duality (CDD) relation, eta(z)=(1+z)^2 d_A(z)/d_L(z)=1, coupling the luminosity (d_L) and angular-diameter (d_A) distances, is crucial because its violation would require exotic new physics. We present a model-independent test of the CDD, based on strong lensing and a reconstruction of the HII galaxy Hubble diagram using Gaussian Processes, to confirm the validity of the CDD at a very high level of confidence. Using parameterizations eta(z) = 1 + eta_0 z and eta(z) = 1 + eta_1 z + eta_2 z^2, our best-fit results are eta_0 = 0.0147^{+0.056}_{-0.066}, and eta_1 = 0.1091^{+0.1680}_{-0.1568} and eta_2 = -0.0603^{+0.0999}_{-0.0988}, respectively. In spite of these strong constraints, however, we also point out that the analysis of strong lensing using a simplified single isothermal sphere (SIS) model for the lens produces some irreducible scatter in the inferred CDD data. The use of an extended SIS approximation, with a power-law density structure, yields very similar results, but does not lessen the scatter due to its larger number of free parameters, which weakens the best-fit constraints. Future work with these strong lenses should therefore be based on more detailed ray-tracing calculations to determine the mass distribution more precisely., Comment: 10 pages, 6 figures, 1 table. Accepted for publication in ApJ

Of all the distance and temporal measures in cosmology, the angular-diameter distance, d_A(z), uniquely reaches a maximum value at some finite redshift z_max and then decreases to zero towards the big bang. This effect has been difficult to observe due to a lack of reliable, standard rulers, though refinements to the identification of the compact structure in radio quasars may have overcome this deficiency. In this Letter, we assemble a catalog of 140 such sources with 0 < z < 3 for model selection and the measurement of z_max. In flat LCDM, we find that Omega_m= 0.24^{+0.1}_{-0.09}, fully consistent with Planck, with z_max=1.69. Both of these values are associated with a d_A(z) indistinguishable from that predicted by the zero active mass condition, rho+3p=0, in terms of the total pressure p and total energy density rho of the cosmic fluid. An expansion driven by this constraint, known as the R_h=ct universe, has z_max=1.718, which differs from the measured value by less than ~1.6%. Indeed, the Bayes Information Criterion favours R_h=ct over flat LCDM with a likelihood of ~81% versus 19%, suggesting that the optimized parameters in Planck LCDM mimic the constraint p=-rho/3., Comment: 6 pages, 3 figures, 1 table. Accepted for publication in EPL

In general relativity, a gravitational horizon (more commonly known as the "apparent horizon") is an imaginary surface beyond which all null geodesics recede from the observer. The Universe has an apparent (gravitational) horizon, but unlike its counterpart in the Schwarzschild and Kerr metrics, it is not static. It may eventually turn into an event horizon---an asymptotically defined membrane that forever separates causally connected events from those that are not---depending on the equation of state of the cosmic fluid. In this paper, we examine how and why an apparent (gravitational) horizon is manifested in the Friedmann-Robertson-Walker metric, and why it is becoming so pivotal to our correct interpretation of the cosmological data. We discuss its observational signature and demonstrate how it alone defines the proper size of our visible Universe. In so doing, we affirm its physical reality and its impact on cosmological models., Comment: 24 pages, 1 figure. Accepted for publication in AJP

Unlike other observational signatures in cosmology, the angular-diameter distance d_A(z) uniquely reaches a maximum (at z_max) and then shrinks to zero towards the big bang. The location of this turning point depends sensitively on the model, but has been difficult to measure. In this paper, we estimate and use z_max inferred from quasar cores: (1) by employing a sample of 140 objects yielding a much reduced dispersion due to pre-constrained limits on their spectral index and luminosity, (2) by reconstructing d_A(z) using Gaussian processes, and (3) comparing the predictions of seven different cosmologies and showing that the measured value of z_max can effectively discriminate between them. We find that z_max=1.70 +\- 0.20---an important new probe of the Universe's geometry. The most strongly favoured model is R_h=ct, followed by Planck LCDM. Several others, including Milne, Einstein-de Sitter and Static tired light are strongly rejected. According to these results, the R_h=ct universe, which predicts z_max=1.718, has a ~92.8% probability of being the correct cosmology. For consistency, we also carry out model selection based on d_A(z) itself. This test confirms that R_h=ct and Planck LCDM are among the few models that account for angular-size data better than those that are disfavoured by z_max. The d_A(z) comparison, however, is less discerning than that with z_max, due to the additional free parameter, H_0. We find that H_0=63.4 +\- 1.2 km/s/Mpc for R_h=ct, and 69.9 +\- 1.5 km/s/Mpc for LCDM. Both are consistent with previously measured values in each model, though they differ from each other by over 4 sigma. In contrast, model selection based on z_max is independent of H_0., Comment: 10 pages, 5 figures, 2 tables. Matches final, published version in MNRAS

In this paper, we use an unprecedentedly large sample (158) of confirmed strong lens systems for model selection, comparing five well studied Friedmann-Robertson-Walker cosmologies: LCDM, wCDM (the standard model with a variable dark-energy equation of state), the R_h=ct universe, the (empty) Milne cosmology, and the classical Einstein-de Sitter (matter dominated) universe. We first use these sources to optimize the parameters in the standard model and show that they are consistent with Planck, though the quality of the best fit is not satisfactory. We demonstrate that this is likely due to under-reported errors, or to errors yet to be included in this kind of analysis. We suggest that the missing dispersion may be due to scatter about a pure single isothermal sphere (SIS) model that is often assumed for the mass distribution in these lenses. We then use the Bayes information criterion, with the inclusion of a suggested SIS dispersion, to calculate the relative likelihoods and ranking of these models, showing that Milne and Einstein-de Sitter are completely ruled out, while R_h=ct is preferred over LCDM/wCDM with a relative probability of ~73% versus ~24%. The recently reported sample of new strong lens candidates by the Dark Energy Survey, if confirmed, may be able to demonstrate which of these two models is favoured over the other at a level exceeding 3 sigma., Comment: 19 pages, 2 figures, 2 tables. Accepted for publication in MNRAS