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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
Soil resource status affects the responses of nitrogen processes to changes in temperature and moisture
The mechanisms of the different responses of soil gross nitrogen (N) transformation to increasing temperature or moisture in different types of soils are still unclear. Here, we conducted two 15N tracing experiments to investigate the effects of increasing temperature (15 °C and 25 °C) or moisture (30%, 45%, and 60% water-filled pore space (WFPS)) on soil gross N transformation rates for two soils (organic (O) and mineral (A) horizon soils) in a temperate forest. As the temperature increased from 15 to 25 °C or moisture increased from 30 to 60% WFPS, total mineralization rates increased by 4.5-fold and 2.5-fold respectively, total NH4+ immobilization rates increased by 173.2-fold and 7.6-fold respectively, and autotrophic nitrification rates increased by 0.7-fold and 0.6-fold respectively in the O horizon. Under the same treatment, the changes in autotrophic nitrification rates, NH4+ immobilization rates, and mineralization rates were much smaller in the A horizon than in the O horizon. We propose that the difference between the two horizons in the responses to changing temperature and moisture was due to the different resource status. The O horizon could provide more substrates than the A horizon, resulting in a stronger response of N processes to changing temperature and moisture. Resource status also affected the competition for substrates between NH4+ immobilization and autotrophic nitrification. The N transformation rates were higher in the O horizon than in the A horizon, consistent with higher microbial biomass N, microbial respiration rate, and amoA gene abundance in the O horizon. Our results suggest that the activity switch of microbes and the competition for resources are important biotic factors regulating potential responses of soil N cycling processes to changing abiotic factors.
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
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
Distinct fungal successional trajectories following wildfire between soil horizons in a cold-temperate forest
• Soil fungi represent a major component of below-ground biodiversity that determines the succession and recovery of forests after disturbance. However, their successional trajectories and driving mechanisms following wildfire remain unclear.
• We examined fungal biomass, richness, composition and enzymes across three soil horizons (Oe, A1 and A2) along a near-complete fire chronosequence (1, 2, 8, 14, 30, 49 and c. 260 yr) in cold-temperate forests of the Great Khingan Mountains, China. The importance of soil properties, spatial distance and tree composition were also tested.
• Ectomycorrhizal fungal richness and β-glucosidase activity were strongly reduced by burning and significantly increased with ‘time since fire’ in the Oe horizon but not in the mineral horizons. Time since fire and soil C : N ratio were the primary drivers of fungal composition in the Oe and A1/A2 horizons, respectively. Ectomycorrhizal fungal composition was remarkably sensitive to fire history in the Oe horizon, while saprotroph community was strongly affected by time since fire in the deeper soil horizon and this effect emerged 18 years after fire in the A2 horizon.
• Our study demonstrates pronounced horizon-dependent successional trajectories following wildfire and indicates interactive effects of time since fire, soil stoichiometry and spatial distance in the reassembly of below-ground fungal communities in a cold and fire-prone region.
Journal Article