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  1. wabbit

    Is de Sitter the only vacuum with positive CC

    Assuming a spacetime with zero Weyl curvature and an Einstein tensor proportional to the metric, is it true that in a finite neighborhood of any point, that spacetime must be isometric to a de Sitter vacuum, or are there other possible solutions, and if so how are they classified? Thanks
  2. wabbit

    Redshift of non-comoving galaxy

    Homework Statement In a flat FRW universe, for a source moving radially at velocity v (at emission time) relative to the local comoving frame, what is the redshift observed by a comoving observer? Homework Equations ##c=1## Proper time to cosmological time ratio...
  3. wabbit

    Observational difference between OS and FLRW

    Reading http://www.aei.mpg.de/~rezzolla/lnotes/mondragone/collapse.pdf The reasonning looks like it should generalize from pressureless dust to a matter-radiation mix, and presumably also works with a (small) cosmological constant thrown in, as in a Schwarzchild-de Sitter solution - is that a...
  4. wabbit

    Black hole horizon confusion

    I am confused about black hole horizons and such common statements as "light cannot escape from inside the horizon". The way I currently understand it is as follows : 1. Horizons are always relative to an observer, and what is called "the black hole horizon" is just a shorthand for "the black...
  5. wabbit

    Calculating the age of the universe without a model?

    This is a follow up to another thread (https://www.physicsforums.com/threads/something-about-calculating-the-age-of-the-universe.807250/), but I post it as a separate thread since it is not clear to me (a) this is actually correct, and (b) assuming it is, whether it actually provides a...
  6. wabbit

    Planck Stars and GR black and white holes

    Rovelli & Vidotto's Planck Stars describes a possible quantum black hole - white hole transition through a quantum bounce somewhat analogous to the LQC bounce. In another thread, @marcus pointed out to me that this was not necessarilly considered the most likely scenario for a QG black hole. Is...
  7. wabbit

    Why is the Universe (nearly) flat?

    My understanding is that the degree of flatness observed for the spatial universe is quite extraordinary, to the extent of begging for an explanation. If this is correct, then my question is, what are the (most) plausible mechanisms being considered as an explanation for either an exact, or...
  8. wabbit

    Why "Dark Energy density" instead of CC?

    Not worrying about the term DE exactly - my question really is, why is the CC described as an energy density, instead of just a nonlocal quantity ? Why bother with a constant scalar field when a constant would do ? After all we do not hear talk of a local gravitational strength density which...
  9. wabbit

    Quick question about LQG Self-Dual Black Holes

    This paper relies heavily of LQG self-dual black holes ; I saw an old thread here about didn't find the answer there so here's my question : On the face of it this looks like a somewhat bizarre/exotic theoretical solution, especially with the description refering to wormholes. However, since...
  10. wabbit

    Experimental Black Hole Physics

    (I don't know if this is the best place to ask this, if not please feel free to move it elsewhere or delete it.) A number of Quantum Gravity papers explore the evolution of Black Holes and their potential transition to White Holes, and some discuss the possibility of astronomical observation of...
  11. wabbit

    Time direction in LQC bounce

    My understanding is that, in the standard GR BB model, an expanding universe starts with infinite spacetime curvature but zero space curvature; but a collapsing universe ends with a diverging, spagghettifying spatial curvature - very different animals. The LQC bounce however connects, through a...
  12. wabbit

    Semiclassical Black Hole

    Hello. I'm thinking about Hawking radation and there's one thing I find a bit strange. The radiation is usually described as originating at he even horizon. The explanation makes sense and all, but still how can this locally non-special place become the seat of a local phenomenon(emitting...
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