arXiv subjects

Brett Bochner

Publications and source records attributed to Brett Bochner.

11 recordsLinked to original sources

arXiv2026

Exploring Cosmological Tensions with Hubble Parameter Tomography via Linear Cosmography

Given the persistence of various tensions in the "Cosmic Concordance" -- such as the "Hubble Tension", and possible departures from LambdaCDM time evolution -- seen from combinations of complementary data sets (e.g., Cosmic Microwave Background, Baryon Acoustic Oscillations, Type Ia Supernovae), it remains an ongoing possibility for these to have a real cosmological origin. If one assumes such deviations to be real, a model-independent formalism (cosmography) is useful for locating the source of the problem with concordance cosmology. The extraordinarily good fit of LambdaCDM to CMB data shows that it was a successful model of the universe at high redshift. Yet at lower redshift -- when the dark energy density becomes significant, and its precise physical nature becomes important -- the universe may have gone off the track of simple LambdaCDM. Here we use linear cosmography fits to binned Supernova data to reconstruct the detailed temporal history of the Hubble parameter, probing for interesting time-dependent behaviors of the expansion rate during and after the onset of cosmic acceleration. Using combined Type Ia supernovae from the Dark Energy Survey 5-Year data release and the Union2.1 compilation, we find intriguing hints of oscillations in the Hubble parameter during the acceleration era. While these hints are of limited statistical significance, and not robust under different redshift binnings, we present this work as a proof-of-concept demonstration of this method for reconstructing the Hubble parameter evolution, which may be useful for the voluminous Supernova data sets anticipated to become available during the next few years.

astro-ph.CO↗

arXiv2021

Electromagnetic wave propagation in general Kasner-like metrics

The curved spacetime Maxwell equations are applied to the anisotropically expanding Kasner metrics. Using the application of vector identities we derive 2$^\textrm{nd}$-order differential wave equations for the electromagnetic field components; through this explicit derivation, we find that the 2$^\textrm{nd}$-order wave equations are not uncoupled for the various components (as previously assumed), but that gravitationally-induced coupling between the electric and magnetic field components is generated directly by the anisotropy of the expansion. The lack of such coupling terms in the wave equations from several prior studies may indicate a generally incomplete understanding of the evolution of electromagnetic energy in anisotropic cosmologies. Uncoupling the field components requires the derivation of a 4$^\textrm{th}$-order wave equation, which we obtain for Kasner-like metrics with generalized expansion/contraction rate indices. For the axisymmetric Kasner case, $(p_{1}, p_{2}, p_{3}) = (1,0,0)$, we obtain exact field solutions (for general propagation wavevectors), half of which appear not to have been found before in previous studies. For the other axisymmetric Kasner case, $\{p_{1}, p_{2}, p_{3}\} = \{(-1/3),(2/3),(2/3)\}$, we use numerical methods to demonstrate the explicit violation of the geometric optics approximation at early times, showing the physical phase velocity of the wave to be inhibited towards the initial singularity, with $v \rightarrow 0$ as $t \rightarrow 0$.

gr-qc↗

arXiv2015

Testing Lambda and the Limits of Cosmography with the Union2.1 Supernova Compilation

We present a cosmographic study designed to test the simplest type of accelerating cosmology: a flat universe with matter and a cosmological constant ($Λ$). Hubble series expansions are fit to the SCP Union2.1 supernova data set to estimate the Hubble Constant ($H_{0}$), the deceleration parameter ($q_{0}$), and the jerk parameter ($j_{0}$). Flat $Λ$CDM models always require $j_{0} = 1$, providing a single-parameter test of the entire paradigm. Because of convergence issues for $z \gtrsim 1$, we focus on expansions using the newer redshift variable $y$, and to estimate the effects of "model-building uncertainties" -- the dependence of the output results upon the fitting function and parameters used -- we perform fits using five different distance indicator functions, and four different polynomial orders. We find that one cannot yet use the supernova data to reliably obtain more than four cosmological parameters, and that cosmographic estimates of $j_{0}$ remain dominated by model-building uncertainties, in conjunction with statistical and other error sources. While $j_{0} = 1$ remains consistent with Union2.1, the most restrictive bound that we can place is $j_{0} \sim [-7.6,8.5]$. To test the future prospects of cosmography with new standard candle data, ensembles of mock supernova data sets are created, and it is found that the best way to reduce model-building uncertainties on lower-order Hubble parameters (such as $\{H_{0}, q_{0}, j_{0} \}$) is by limiting the redshift range of the data. Thus more and better $z \lesssim 1$ data, not higher-redshift data, is needed to sharpen cosmographic tests of flat $Λ$CDM.

astro-ph.CO↗

arXiv2012

Apparent Acceleration and an Alternative Concordance from Causal Backreaction

A phenomenological formalism is presented in which the apparent acceleration of the universe is generated by cosmic structure formation, without resort to Dark Energy, modifications to gravity, or a local void. The observed acceleration results from the combined effect of innumerable local perturbations due to individually virializing systems, overlapping together in a smoothly-inhomogeneous adjustment of the FRW metric, in a process governed by the causal flow of inhomogeneity information outward from each clumped system. After noting how common arguments claiming to limit backreaction are physically unrealistic, models are presented which fit the supernova luminosity distance data essentially as well as $Λ$CDM, while bringing several important cosmological parameters to a new Concordance. These goals are all achieved with a second-generation version of our formalism that accounts for the negative feedback of Causal Backreaction upon itself due to the slowed propagation of gravitational inhomogeneity information.

astro-ph.CO↗

arXiv2012

Cosmic Acceleration from Causal Backreaction with Recursive Nonlinearities

We revisit the causal backreaction paradigm, in which the need for Dark Energy is eliminated via the generation of an apparent cosmic acceleration from the causal flow of inhomogeneity information coming in towards each observer from distant structure-forming regions. This second-generation formalism incorporates "recursive nonlinearities": the process by which already-established metric perturbations will then act to slow down all future flows of inhomogeneity information. Here, the long-range effects of causal backreaction are now damped, weakening its impact for models that were previously best-fit cosmologies. Nevertheless, we find that causal backreaction can be recovered as a replacement for Dark Energy via the adoption of larger values for the dimensionless `strength' of the clustering evolution functions being modeled -- a change justified by the hierarchical nature of clustering and virialization in the universe, occurring on multiple cosmic length scales simultaneously. With this, and with one new model parameter representing the slowdown of clustering due to astrophysical feedback processes, an alternative cosmic concordance can once again be achieved for a matter-only universe in which the apparent acceleration is generated entirely by causal backreaction effects. One drawback is a new degeneracy which broadens our predicted range for the observed jerk parameter $j_{0}^{\mathrm{Obs}}$, thus removing what had appeared to be a clear signature for distinguishing causal backreaction from Cosmological Constant $Λ$CDM. As for the long-term fate of the universe, incorporating recursive nonlinearities appears to make the possibility of an `eternal' acceleration due to causal backreaction far less likely; though this does not take into account gravitational nonlinearities or the large-scale breakdown of cosmological isotropy, effects not easily modeled within this formalism.

astro-ph.CO↗

arXiv2012

Cosmic Acceleration from Causal Backreaction in a Smoothly Inhomogeneous Universe

A phenomenological formalism is presented in which the apparent acceleration of the universe is generated by large-scale structure formation, thus eliminating the coincidence and magnitude fine-tuning problems of the Cosmological Constant in the Concordance Model, as well as potential instability issues with dynamical Dark Energy. The observed acceleration results from the combined effect of innumerable local perturbations, due to individually virialized systems, overlapping together in a smoothly-inhomogeneous adjustment of the FRW metric, in a process governed by the causal flow of inhomogeneity information outward from each clumped system. We discuss several arguments from the literature claiming to place sharp limits upon the strength of backreaction-related effects, and show why such arguments are not applicable in a physically realistic cosmological analysis. A selection of simply-parameterized models are presented, including several which are capable of fitting the luminosity distance data from Type Ia supernovae essentially as well as the best-fit flat $Λ$CDM model, without resort to Dark Energy, any modification to gravity, or a local void. Simultaneously, these models can reproduce measured cosmological parameters such as the age of the universe, the matter density required for spatial flatness, the present-day deceleration parameter, and the angular scale of the Cosmic Microwave Background to within a reasonable proximity of their Concordance values. We conclude by considering potential observational signatures for distinguishing this cosmological formalism from $Λ$CDM or Dark Energy, as well as the possible long-term fate of such a universe with ever-spreading spheres of influence for its increasingly superposed perturbations.

astro-ph.CO↗

arXiv2012

Cosmic Acceleration and a New Concordance from Causal Backreaction

A phenomenological formalism is presented in which the apparent acceleration of the universe is generated by large-scale structure formation, thus eliminating the magnitude and coincidence fine-tuning problems of the Cosmological Constant in the Concordance Model, as well as potential instability issues with dynamical Dark Energy. The observed acceleration results from the combined effect of innumerable local perturbations due to individually virializing systems, overlapping together in a smoothly-inhomogeneous adjustment of the FRW metric, in a process governed by the causal flow of inhomogeneity information outward from each clumped system. After explaining why arguments from the literature claiming to place restrictive limits upon backreaction are not applicable in a physically realistic cosmological analysis, we present a selection of simply-parameterized models which are capable of fitting the luminosity distance data from Type Ia supernovae essentially as well as the best-fit flat $Λ$CDM model, without resort to Dark Energy, any modification to gravity, or a local void. Simultaneously, these models can reproduce measured cosmological parameters such as the age of the universe, the matter density required for spatial flatness, the present-day deceleration parameter, and the angular scale of the Cosmic Microwave Background to within a reasonable proximity of their Concordance values. A potential observational signature for distinguishing this cosmological formalism from $Λ$CDM may be a cosmic jerk parameter significantly in excess of unity.

astro-ph.CO↗

arXiv2007

Looking for signs of Anisotropic Cosmological Expansion in the High-z Supernova data

Several problematical epochs in cosmology, including the recent period of structure formation (and acceleration), require us to understand cosmic evolution during times when the basis of FRW expansion, the cosmological principle, does not completely hold true. We consider that the breakdown of isotropy and homogeneity at such times may be an important key towards understanding cosmic evolution. To study this, we examine fluctuations in the high-z supernova data to search for signs of large-scale anisotropy in the Hubble expansion. Using a cosmological-model-independent statistical analysis, we find mild evidence of real anisotropy in various circumstances. We consider the significance of these results, and the importance of further searches for violations of the cosmological principle.

astro-ph↗

arXiv2007

Perturbations to the Cosmological Expansion in a Grainy Universe

The matter content of the Universe is generally regarded as a perfect fluid on sufficiently large scales, for all epochs. But the recent cosmological matter distribution, consisting of an (ideally) random distribution of gravitationally collapsed structures, is more accurately described as a collection of discrete grains, than as a fluid. It is well known that granular materials may have very different macroscopic properties than fluids; analogously, we investigate the possibility that pervasive small-scale inhomogeneities in the recent Universe may lead to perturbations of the cosmological expansion on intermediate and/or large scales.

astro-ph↗

arXiv2003

Grid-based simulation program for gravitational wave interferometers with realistically imperfect optics

We describe an optical simulation program that models a complete, coupled-cavity interferometer like those used by the Laser Interferometer Gravitational-Wave Observatory (LIGO) Project. A wide variety of interferometer deformations can be modeled, including general surface roughness and substrate inhomogeneities, with no a priori symmetry assumptions about the nature of interferometer imperfections. Several important interferometer parameters are optimized automatically to achieve the best possible sensitivity for each new set of perturbed mirrors. The simulation output data set includes the circulating powers and electric fields at various points in the interferometer, both for the main carrier beam and for its signal-sideband auxiliary beams, allowing an explicit calculation of the shot-noise-limited gravitational-wave sensitivity of the interferometric detector to be performed. Here we present an overview of the physics simulated by the program, and demonstrate its use with a series of runs showing the degradation of LIGO performance caused by realistically-deformed mirror profiles. We then estimate the effect of this performance degradation upon the detectability of astrophysical sources of gravitational waves. We conclude by describing applications of the simulation program to LIGO research and development efforts.

astro-ph↗

arXiv2003

Simulating a dual-recycled gravitational wave interferometer with realistically imperfect optics

We simulate the performance of a gravitational wave interferometer in the Dual Recycling (DR) configuration, as will be used for systems like Advanced-LIGO. Our grid-based simulation program models complex interferometric detectors with realistic optical deformations (e.g., fine-scale mirror surface roughness). Broadband and Tuned DR are modeled here; the results are also applied qualitatively to Resonant Sideband Extraction (RSE). Several beneficial properties anticipated for DR detectors are investigated: signal response tuning and narrowbanding, power loss reduction, and the reclamation of lost power as useful light for signal detection. It is shown that these benefits would be limited by large scattering losses in large (multi-kilometer) systems. Furthermore, losses may be resonantly enhanced (particularly for RSE), if the interferometer's modal resonance conditions are not well chosen. We therefore make two principal recommendations for DR/RSE interferometers: the DR/RSE cavity must be modally nondegenerate; and fabricated mirror surfaces and coatings must be as smooth as is practically feasible.

astro-ph↗