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"Horizon"
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Ultimate horizons : probing the limits of the universe
In the last hundred years, modern physics and cosmology have shown that there exist regions of the universe forever beyond our reach, hidden by truly ultimate horizons. Such regions exist in those remote parts of the universe where, from our point of view, space expands faster than the speed of light. They are found in black holes, where the gravity is strong enough to retain even light within its field of attraction. And in the realm of the very small, quarks must remain forever confined to their world of extreme density and can never be removed from it. The aim of this book is to describe these ultimate horizons, how they were discovered, how they shape our view of the world, and what clues we have about a world beyond them.
Book
Black holes, Cauchy horizons, and mass inflation
Event horizons and Cauchy horizons are highly idealized mathematical constructions that do not fully capture the key physics of either Hawking radiation or mass inflation. Indeed, because they are teleological, both event horizons and Cauchy horizons are (in a precise technical sense) not physically observable. In contrast, by inspecting the quasi-local behaviour of null geodesics, long-lived apparent horizons (or more generally long-lived quasi-local horizons) are in principle physically observable, and are “good enough\" for then pragmatically redefining a black hole, and “good enough” for generating Hawking radiation. Furthermore it is now also clear that long lived apparent horizons (quasi-local horizons) are also “good enough\" for generating mass inflation. These observations suggest that one should be somewhat careful when trying to extrapolate rigorous mathematical theorems, which often embody mathematical idealizations that do not necessarily correspond to what a finite resource astronomer can actually measure, into the astrophysical realm.
Journal Article
A first comprehensive census of fungi in soil reveals both hyperdiversity and fine-scale niche partitioning
Fungi play key roles in ecosystems as mutualists, pathogens, and decomposers. Current estimates of global species richness are highly uncertain, and the importance of stochastic vs. deterministic forces in the assembly of fungal communities is unknown. Molecular studies have so far failed to reach saturated, comprehensive estimates of fungal diversity. To obtain a more accurate estimate of global fungal diversity, we used a direct molecular approach to census diversity in a boreal ecosystem with precisely known plant diversity, and we carefully evaluated adequacy of sampling and accuracy of species delineation. We achieved the first exhaustive enumeration of fungi in soil, recording 1002 taxa in this system. We show that the fungus:plant ratio in Picea mariana forest soils from interior Alaska is at least 17:1 and is regionally stable. A global extrapolation of this ratio would suggest 6 million species of fungi, as opposed to leading estimates ranging from 616000 to 1.5 million. We also find that closely related fungi often occupy divergent niches. This pattern is seen in fungi spanning all major functional guilds and four phyla, suggesting a major role of deterministic niche partitioning in community assembly. Extinctions and range shifts are reorganizing biodiversity on Earth, yet our results suggest that 98% of fungi remain undescribed and that many of these species occupy unique niches.
Journal Article
Gulf oil spill
Describes the events of 2010 after an oil rig toppled over in the Gulf of Mexico, releasing millions of gallons of oil into the water.
Book
Comparison of Polarized Radiative Transfer Codes Used by the EHT Collaboration
Interpretation of resolved polarized images of black holes by the Event Horizon Telescope (EHT) requires predictions of the polarized emission observable by an Earth-based instrument for a particular model of the black hole accretion system. Such predictions are generated by general relativistic radiative transfer (GRRT) codes, which integrate the equations of polarized radiative transfer in curved spacetime. A selection of ray-tracing GRRT codes used within the EHT Collaboration is evaluated for accuracy and consistency in producing a selection of test images, demonstrating that the various methods and implementations of radiative transfer calculations are highly consistent. When imaging an analytic accretion model, we find that all codes produce images similar within a pixel-wise normalized mean squared error (NMSE) of 0.012 in the worst case. When imaging a snapshot from a cell-based magnetohydrodynamic simulation, we find all test images to be similar within NMSEs of 0.02, 0.04, 0.04, and 0.12 in Stokes I, Q, U, and V, respectively. We additionally find the values of several image metrics relevant to published EHT results to be in agreement to much better precision than measurement uncertainties.
Journal Article
Black hole chasers : the amazing true story of an astronomical breakthrough
Presents the true story of one of the Event Horizon Telescope team's reveal of the first image of a super massive black hole\"--Provided by publisher.
Book
Investigating Loop Quantum Gravity with Event Horizon Telescope Observations of the Effects of Rotating Black Holes
A mathematically consistent rotating black hole model in loop quantum gravity (LQG) is yet lacking. The scarcity of rotating black hole solutions in LQG substantially hampers the development of testing LQG from observations, e.g., from the Event Horizon Telescope (EHT) observations. The EHT observation revealed event horizon-scale images of the supermassive black holes Sgr A* and M87*. The EHT results are consistent with the shadow of a Kerr black hole of general relativity. We present LQG-motivated rotating black hole (LMRBH) spacetimes, which are regular everywhere and asymptotically encompass the Kerr black hole as a particular case. The LMRBH metric describes a multi-horizon black hole in the sense that it can admit up to three horizons, such that an extremal LMRBH, unlike the Kerr black hole, refers to a black hole with angular momentum a > M. The metric, depending on the parameters, describes (1) black holes with only one horizon (BH-I), (2) black holes with an event horizon and a Cauchy horizon (BH-II), (3) black holes with three horizons (BH-III), or (4) no-horizon spacetime, which we show is almost ruled out by EHT observations. We constrain the LQG parameter with the aid of the EHT shadow observational results of M87* and Sgr A*, respectively, for inclination angles of 17° and 50°. In particular, the VLTI bound for Sgr A*, δ ∈ (−0.17, 0.01), constrains the parameters (a, l) such that for 0 < l ≤ 0.347851M (l ≤ 2 × 106 km), the allowed range of a is (0, 1.0307M). Together with the EHT bounds of Sgr A* and M87* observables, our analysis concludes that a substantial part of BH-I and BH-II parameter space agrees with the EHT results of M87* and Sgr A*. While the EHT M87* results totally rule out BH-III, but not that by Sgr A*.
Journal Article
Quasi-local black hole horizons: recent advances
While the early literature on black holes focused on event horizons, subsequently it was realized that their teleological nature makes them unsuitable for many physical applications both in classical and quantum gravity. Therefore, over the past two decades, event horizons have been steadily replaced by quasi-local horizons which do not suffer from teleology. In numerical simulations event horizons can be located as an ‘after thought’ only after the entire space-time has been constructed. By contrast, quasi-local horizons naturally emerge
in the course of
these simulations, providing powerful gauge-invariant tools to extract physics from the numerical outputs. They also lead to interesting results in mathematical GR, providing unforeseen insights. For example, for event horizons we only have a qualitative result that their area cannot decrease, while for quasi-local horizons the increase in the area during a dynamical phase is quantitatively related to
local
physical processes at the horizon. In binary black hole mergers, there are interesting correlations between observables associated with quasi-local horizons and those defined at future null infinity. Finally, the quantum Hawking process is naturally described as formation and evaporation of a quasi-local horizon. This article focuses on the
dynamical
aspects of quasi-local horizons in classical general relativity, emphasizing recent results and ongoing research.
Journal Article