scholarly journals Novel null tests for the spatial curvature and homogeneity of the Universe and their machine learning reconstructions

2021 ◽  
Vol 103 (10) ◽  
Author(s):  
Rubén Arjona ◽  
Savvas Nesseris
Keyword(s):  
Machine Learning ◽  
Spatial Curvature ◽  
The Universe

Unlocking the Universe with Astroinformatics

2017 ◽  
Vol 14 (S339) ◽  
pp. 201-201
Author(s):  
M. Lochner
Keyword(s):  
Machine Learning ◽  
Data Mining ◽  
Bayesian Statistics ◽  
Time Domain ◽  
Meaningful Information ◽  
Broad Field ◽  
Near Future ◽  
The Universe

AbstractIn the last decade Astronomy has been transformed by a deluge of data that will grow exponentially when near-future telescopes such as LSST and the SKA begin routine observing. Astroinformatics, a broad field encompassing many techniques in statistics, machine learning and data mining, is the key to extracting meaningful information from large amounts of data. This talk outlined Astroinformatics as a field, and gave a few examples of the use of machine learning and Bayesian statistics from my own work in survey Astronomy. The era of massive surveys in which we now find ourselves has the potential to revolutionise completely many fields, including time-domain Astronomy, but only if coupled with the powerful tools of Astroinformatics.

scholarly journals On the Necessity of Phantom Fields for Solving the Horizon Problem in Scalar Cosmologies

Universe ◽  
2019 ◽  
Vol 5 (3) ◽  
pp. 76 ◽  
Author(s):  
Davide Fermi ◽  
Massimo Gengo ◽  
Livio Pizzocchero
Keyword(s):  
Arbitrary Dimension ◽  
Energy Conditions ◽  
Solvable Models ◽  
Action Functional ◽  
Spatial Curvature ◽  
Horizon Problem ◽  
Particle Horizon ◽  
Perfect Fluids ◽  
The Universe ◽  
Phantom Fields

We discuss the particle horizon problem in the framework of spatially homogeneous and isotropic scalar cosmologies. To this purpose we consider a Friedmann–Lemaître–Robertson–Walker (FLRW) spacetime with possibly non-zero spatial sectional curvature (and arbitrary dimension), and assume that the content of the universe is a family of perfect fluids, plus a scalar field that can be a quintessence or a phantom (depending on the sign of the kinetic part in its action functional). We show that the occurrence of a particle horizon is unavoidable if the field is a quintessence, the spatial curvature is non-positive and the usual energy conditions are fulfilled by the perfect fluids. As a partial converse, we present three solvable models where a phantom is present in addition to a perfect fluid, and no particle horizon appears.

scholarly journals HOW MANY UNIVERSES DO THERE NEED TO BE?

2006 ◽  
Vol 15 (12) ◽  
pp. 2229-2233 ◽  
Author(s):  
DOUGLAS SCOTT ◽  
J. P. ZIBIN
Keyword(s):  
Cosmological Parameters ◽  
Current Data ◽  
Wilkinson Microwave Anisotropy Probe ◽  
Spatial Curvature ◽  
Observable Universe ◽  
Particle Horizon ◽  
Knowing That ◽  
Density Perturbations ◽  
Best Fit ◽  
The Universe

In the simplest cosmological models consistent with General Relativity, the total volume of the Universe is either finite or infinite, depending on whether or not the spatial curvature is positive. Current data suggest that the curvature is very close to flat, implying that one can place a lower limit on the total volume. In a Universe of finite age, the "particle horizon" defines the patch of the Universe which is observable to us. Based on today's best-fit cosmological parameters it is possible to constrain the number of observable Universe sized patches, NU. Specifically, using the new Wilkinson Microwave Anisotropy Probe (WMAP) data, we can say that there are at least 21 patches out there the same volume as ours, at 95% confidence. Moreover, even if the precision of our cosmological measurements continues to increase, density perturbations at the particle horizon size limit us to never knowing that there are more than about 105 patches out there.

scholarly journals Accelerated expansion of the universe from the perspective of inhomogeneous cosmology

2020 ◽  
Vol 35 (02n03) ◽  
pp. 2040037
Author(s):  
Irina Bormotova ◽  
Elena Kopteva ◽  
Mariia Churilova ◽  
Zdenek Stuchlik
Keyword(s):  
Deceleration Parameter ◽  
Spherical Symmetry ◽  
Accelerated Expansion ◽  
Spatial Curvature ◽  
Inhomogeneous Cosmology ◽  
Spatial Part ◽  
Expansion Of The Universe ◽  
The Universe ◽  
Special Case ◽  
Universe Evolution

We present a special case of the Stephani solution with spherical symmetry while considering different values of spatial curvature. We investigate the dynamics of the universe evolution in our model, build the R–T-regions for the resulting spacetime and analyze the behavior of the deceleration parameter. The singularities of the model are also discussed. The geometry of the spatial part of the obtained solution is explored.

A Timeless, Boundless, Equilibrium Universe

1982 ◽  
Vol 4 (4) ◽  
pp. 482-483 ◽  
Author(s):  
Grote Reber
Keyword(s):  
Steady State ◽  
Relative Motion ◽  
Big Bang ◽  
The Other ◽  
Expanding Universe ◽  
Spatial Curvature ◽  
Doppler Shifts ◽  
Spatial Dimensions ◽  
State Models ◽  
The Universe

For more than half a century the theory that the universe is expanding has dominated cosmology. All current cosmological theories, from the various Big Bang models to the various Steady State models, explicitly assume an expanding universe. The evidence in favour of an expanding universe is purely circumstantial, and is based on a “sheer assumption”, (Hubble 1936a) that red-shifts in the light received by an observer on Earth from distant objects are caused by relative motion and hence may be interpreted as Doppler shifts. Hubble (1936b) continues: “…the ever expanding model … seems rather dubious”, and “On the other hand, if the recession factor is dropped, if red-shifts are not primarily velocity-shifts, the picture is simple and plausible. There is no evidence of expansion and no restriction of time-scale, no trace of spatial curvature and no limitations of spatial dimensions.” (Hubble 1936c). These statements are as true today as they were in 1936.

scholarly journals Probing the unidentified Fermi blazar-like population using optical polarization and machine learning

2019 ◽  
Vol 486 (3) ◽  
pp. 3415-3422 ◽  
Author(s):  
I Liodakis ◽  
D Blinov
Keyword(s):  
Machine Learning ◽  
Predictive Power ◽  
High Energy ◽  
Point Sources ◽  
Low Energy ◽  
Optical Polarization ◽  
Sky Surveys ◽  
Γ Ray ◽  
Energy Processes ◽  
The Universe

ABSTRACT The Fermi γ-ray space telescope has revolutionized our view of the γ-ray sky and the high-energy processes in the Universe. While the number of known γ-ray emitters has increased by orders of magnitude since the launch of Fermi, there is an ever increasing number of, now more than a thousand, detected point sources whose low-energy counterpart is to this day unknown. To address this problem, we combined optical polarization measurements from the RoboPol survey as well as other discriminants of blazars from publicly available all-sky surveys in machine learning (ML, random forest and logistic regression) frameworks that could be used to identify blazars in the Fermi unidentified fields with an accuracy of >95 per cent. Out of the potential observational biases considered, blazar variability seems to have the most significant effect reducing the predictive power of the frameworks to ${\sim }80\hbox{-}85{{\ \rm per\ cent}}$. We apply our ML framework to six unidentified Fermi fields observed using the RoboPol polarimeter. We identified the same candidate source proposed by Mandarakas et al. for 3FGL J0221.2 + 2518.

scholarly journals QUINTESSENCE AND THE CURVATURE OF THE UNIVERSE AFTER WMAP

2004 ◽  
Vol 13 (01) ◽  
pp. 123-136 ◽  
Author(s):  
RALF AURICH ◽  
FRANK STEINER
Keyword(s):  
Equation Of State ◽  
Negative Curvature ◽  
Background Radiation ◽  
Spatial Curvature ◽  
Microwave Background ◽  
Effective Equation ◽  
Microwave Background Radiation ◽  
The Universe ◽  
Very High ◽  
Rule Out

We study quintessence models with a constant (effective) equation of state. It is investigated whether such quintessence models are consistent with a negative spatial curvature of the Universe with respect to the anisotropy of the cosmic microwave background radiation measured by the WMAP mission. If the reionization is negligibly small, it is found that such models with negative curvature are admissible due to a geometrical degeneracy. However, a very high optical depth τ to the surface of last scattering, as indicated by the polarization measurements of WMAP, would rule out such models.

scholarly journals THE VALUE OF THE COSMOLOGICAL CONSTANT

2011 ◽  
Vol 20 (14) ◽  
pp. 2875-2880 ◽  
Author(s):  
JOHN D. BARROW ◽  
DOUGLAS J. SHAW
Keyword(s):  
Wave Function ◽  
Cosmological Constant ◽  
Classical Limit ◽  
Constraint Equation ◽  
Einstein Equations ◽  
Spatial Curvature ◽  
Self Consistent ◽  
Curvature Parameter ◽  
The Universe ◽  
Planck Satellite

We make the cosmological constant, Λ, into a field and restrict the variations of the action with respect to it by causality. This creates an additional Einstein constraint equation. It restricts the solutions of the standard Einstein equations and is the requirement that the cosmological wave function possess a classical limit. When applied to the Friedmann metric it requires that the cosmological constant measured today, tU, be [Formula: see text], as observed. This is the classical value of Λ that dominates the wave function of the universe. Our new field equation determines Λ in terms of other astronomically measurable quantities. Specifically, it predicts that the spatial curvature parameter of the universe is [Formula: see text], which will be tested by Planck Satellite data. Our theory also creates a new picture of self-consistent quantum cosmological history.

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