The cosmological constant problem in codimension-two brane models

We discuss the possibility of a dynamical solution to the cosmological constant problem in the contaxt of six-dimensional Einstein-Maxwell theory. A definite answer requires an understanding of the full bulk cosmology in the early universe, in which the bulk has time-dependent size and shape. We comment on the special properties of codimension two as compared to higher codimensions.Comment: 11 page

Making Sense of the Great Barrier Reef's Mysterious Green Donuts

On the outer shelf of Australia’s Great Barrier Reef (GBR), under 20–50 meters of water, lies a broad expanse of giant green “donuts.” These seafloor circles, each several hundred meters across, aren’t the result of a rogue offshore baking experiment. Rather, they consist of the remnants of countless generations of green calcareous algae from the Halimeda genus. The green color comes from the current generation of Halimeda living atop these bioherms, as scientists call this type of mounded deposit with a sunken center.The actively accumulating Halimeda bioherms on the northern GBR shelf cover more than 6,000 square kilometers and are the most extensive of such deposits in the world [Whiteway et al., 2013; McNeil et al., 2016]. These globally significant bioherms have complex morphologies that are not yet explained, and compared with the adjacent coral reef systems, little is known about the fundamental processes that control their distribution anddevelopment. Much also remains unknown about the biogeochemical cycling associated with the bioherms, their role as key habitats for benthic (bottom-dwelling) species between the coral reefs and the Australian coast, and how they may be affected by climate change. In August and September 2022, we were part of a multidisciplinary team of scientists that set out on a research voyage on Australia’s R/V Investigator to better understand these enigmatic structures by mapping and sampling them in breathtaking detail. The mission of Project HALO (Halimeda Bioherm Origins, Function and Fate) was to illuminate how the bioherms formed over the past 12,000 or so years (i.e., the Holocene) and their importance in biogeochemical nutrient cycling and as modern habitats amid one of Earth’s most critical but vulnerable biodiversity hot spots

Early Holocene sea level in the Canadian Beaufort Sea constrained by radiocarbon dates from a deep borehole in the Mackenzie Trough, Arctic Canada

Deglacial and Holocene relative sea level (RSL) in the Canadian Beaufort Sea was influenced by the timing and extent of glacial ice in the Mackenzie River corridor and adjacent coastal plains. Considerable evidence indicates extensive ice cover in this region of northwestern Canada during the Late Wisconsinan. However, no absolute ages exist to constrain maximum RSL lowering before the late Holocene (4.2–0 ka). In 1984, the Geological Survey of Canada drilled an 81.5‐m‐deep borehole in the western Mackenzie Trough at 45 m water depth (MTW01). The lower 52.5 m of the borehole was interpreted as a deltaic progradational sequence deposited during a period of rising sea level. The upper 29 m was described as foraminifer‐bearing marine sediments deposited after transgression of the site, when RSL rose above ~−74 m. Here, we present radiocarbon measurements from MTW01, acquired from benthic foraminifera, mollusc fragments and particulate organic carbon in the >63 μm fraction (POC>63 μm) in an attempt to constrain the chronology of sediments within this borehole and date the timing of transgression. The deepest carbonate macrofossil was acquired from 8 m above the transgressive surface (equivalent to 21 m b.s.l.), where mollusc fragments returned a date of 9400 +180–260 cal. a BP (2σ). This provides the oldest constraint on Holocene sea‐level lowering in the region, and implies that transgression at this site occurred prior to the early Holocene. Ages obtained from the lower 52.5 m of the borehole are limited to POC>63 μm samples. These indicate that progradational sediments were deposited rapidly after 24 820 +390–380 cal. a BP (2σ). Due to the incorporation of older reworked organic matter, the actual age of progradation is likely to be younger, occurring after Late Wisconsinan glacial ice retreated from the coast