I keep wanting to start a new topic, so I’ll combine three minor topics into a single thread.
1. Supernova standard candle origin solved – or is it?
http://www.abc.net.au/science/articles/2015/05/21/4239232.htm
The origins of cosmology’s standard candles, Type 1a supernovae, just got a boost from two studies that came to opposite conclusions.
Type 1a supernovae involve the explosive destruction of a white dwarf in a close binary orbit with another star. However, we’ve kind of had this dirty little secret going on, in that we really don’t know what the companion star is.
In the first of two models used to explain the origin of thermonuclear supernovae the companion star is another white dwarf, and the pair merge triggering the supernova explosion.
In the other model the companion star is either a normal star, or an old bloated star at the end of its life, called a red giant. In this model, the white dwarf’s gravity draws matter from the companion star onto its own surface, eventually reaching a point where the pressure and heat of all this added material triggers the supernova explosion.
The new studies give evidence for both of these. But according to mollwollfumble the problem is still not solved, because counts of white dwarfs show that there are not nearly enough white dwarf pairs to account for even a small fraction of the rate of Type 1a supernovae. Perhaps it’s little green men.
2. The universe is said to be metastable, but what exactly do they mean by that?
The background metric of our universe, ignoring the presence of gravity from its mass, is known as deSitter space dS. It is flat, has zero curvature. But there’s a different metric known as Anti-de Sitter space AdS, that is used on quantum mechanics, and has a smaller “vacuum energy”. AdS space has negative curvature, is hyperbolic.
A bubble of AdS space that is formed inside our universe would expand to take over the whole of our universe resulting in universal destruction, or would it? A new paper http://arxiv.org/pdf/1505.04825.pdf confirms that it would, unless it is hidden inside the event horizon of a black hole. If reading this, it helps to skip to section 3.3 “Bubble evolution in de Sitter spacetime”.
In the literature one finds conflicting statements about the evolution of AdS regions. One point of view, based on flat-space
intuition, is that AdS regions should expand because their interior has lower energy than the exterior. A different point of view is that, since AdS space eventually contracts, regions in which the Higgs lies at its true minimum will shrink, possibly leaving some
almost point-like relics, which are nevertheless efficiently diluted, and thus made harmless, by the inflationary expansion of space.
We will show that addressing the question about the fate of AdS regions involves a number of non-trivial and counter-intuitive issues raised by general relativity. First, gravitational energy contributes to the total energy budget. Second, an AdS region might expand, while remaining hidden behind a black-hole horizon. Third, the interior AdS space is dynamically unstable: it reaches a ‘big-crunch’ singularity in a finite amount of internal time. If space is empty, this is just a coordinate singularity; if space is filled by a background field (for example the Higgs field), its fluctuations grow until the energy density becomes infinite. Furthermore,
the AdS geometry has a timelike boundary, such that the evolution cannot be predicted unless information flows in from infinity. This confirms the expectation that the AdS bubble is unstable.
mollwollfumble finds their Figure 7 interesting. “Figure 7: Penrose diagram describing an AdS bubble that expands in a dS
space.” The destruction of our universe starts slower than the speed of light from what is essentially a white hole, or at least a size much smaller than the Planck length, and then speeds up towards the speed of light.
3. What is the law of gravity governing the gravitational interaction between dark matter and normal matter? Although the gravitational law governing normal matter has been confirmed many times to be consistent with General Relativity, this hadn’t been confirmed for dark matter. The paper http://arxiv.org/pdf/1505.04789.pdf confirms that the gravity from dark matter behaves according to General Relativity, to quite high accuracy. To better than one part in a million.
“Another interesting question that may be answered using cosmological data is whether dark matter has the same gravitational interaction as the ordinary baryonic matter. So far we have seen extensive evidence for the existence of dark matter, such as galactic rotation curves and gravitational lensing. There is no doubt that dark matter has gravitational interactions, although we have not found additional forces felt by dark matter, despite a large effort to understand dark matter particle properties. This serves as motivation to understand the dark matter gravitational interaction better. In this paper, we will use the current cosmological data to constrain the properties of the dark matter gravitational interaction.”