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Tessa Baker

Growth Histories in Bimetric Massive Gravity - 1 views

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    Massive Gravity is this year's must-have theory of modified gravity. The concept is simple - what if the graviton had a (very small) mass? However, building a consistent and viable theory from this idea has proved very difficult. It has now been achieved for the background-level cosmology, and can fit the accelerated expansion (with the usual fine-tuning problems, of course!) This paper takes the first steps towards the perturbation theory that needs to be developed if we are to test Massive Gravity with measurements of structure growth, etc.
Tessa Baker

A practical approach to cosmological perturbations in modified gravity - 0 views

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    There's been a growing awareness that the 'free' functions used to parameterise modified gravity aren't really as arbitrary as one might first think. This methods paper suggests how to use these ideas - although the forecast isn't all that encouraging. Still, the proof of the pudding is yet to come.
Tessa Baker

[1204.1691] A tensor instability in the Eddington inspired Born-Infeld Theory of Gravity - 1 views

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    Celia visited us for several months last summer - this paper is the outcome of her work here. In EBI gravity there is an 'Eddington-dominated' epoch in the universe prior to radiation domination, which can avoid the need for a big bang singularity. However, it turns out that tensor perturbations in this early phase are unstable. It's particularly interesting that the instability only shows up at the perturbative level, whilst the background cosmology remains non-singular.
Tessa Baker

[1204.6044] Astrophysical Tests of Modified Gravity: Constraints from Distance Indicato... - 0 views

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    The chameleon mechanism suppresses the effects of a modified gravity theory in high-density environments. The authors of this paper suggest that this could lead to effects on stellar structure that would impact distance measures inferred from cepheid variables and red giant branch stars. Whilst it's not clear that the astrophysics involved is sufficiently well understood for such tests to be useful, I think there is some good thinking-outside-the-box here!
Kaiki Inoue

[1204.6608] Modified Gravity Spins Up Galactic Halos - 0 views

shared by Kaiki Inoue on 03 May 12 - No Cached
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    The authors numerically showed that angular momentum of isolated galactic halos with mass less than 10^11 solar mass is systematically larger in the f(R) gravity model since they are less affected by shielding via chameleon mechanism.
David Marsh

Tunneling and Rolling to False Vacua - 0 views

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    The authors construct exact instanton solutions for tunneling over very small barriers in the presence of gravity, and demonstrate matching between previous results, and with the flat potential and no gravity case. Confusingly, it seems that for consistency one should include the tunneling effect along side the rolling of a field on a flat potential, even when there is no barrier. I'm not sure quite what this means operationally, but I think it may have effects for models of quintessence where the asymptotic future is a big crunch. Here it seems we may not be able to consider simple scalar field rolling, but may also have to include the instanton effects. More excuses to go back and read Coleman-DeLuccia again are always good.
Timothy Clifton

Gravity, Entropy, and Cosmology: In Search of Clarity - 3 views

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    This paper is by David Wallace (a philosopher in Oxford, not the novelist). The idea seems to be to talk through some of the statements that are made about the treatment of entropy in gravitation. I found this to be a useful exercise, and there are some interesting thoughts in here, even if the cosmology is a bit hit and miss. In particular he points out that the formation of structure in the Universe does not necessarily imply that gravitational fields in the Universe have to carry large amounts of entropy at late times.
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    Potentially simple question alert: what is the entropy associated with a gravitational field? Is there a statistical physics (ie, about disorder vs order) interpretation to whatever thermodynamic integral is implied by Eintein's equations?
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    Good question. So far the only compelling definitions of gravitational entropy have been in stationary space-times (those that admit a time-like Killing vector). There are various suggestions for how to define entropy in other situations, most notably Penrose's Weyl curvature hypothesis, but nothing concrete has yet emerged.
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    Is there not a definition of gravitational entropy from the holographic principle?
Tessa Baker

Halo Scale Predictions of Symmetron Modified Gravity - 1 views

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    There are a number of `screening' mechanisms that are designed to suppress the effects of modified gravity below cluster scales. What are the characteristic radii at which this switch-off occurs in the different mechanisms?
David Marsh

Antigravity and the big crunch/big bang transition - 1 views

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    This group continue Steinhardt and Turok's on going interest in cyclic cosmology by using an anti-gravity phase to resolve a crunch/bang transition.
Tessa Baker

[1207.3804] Examining the evidence for dynamical dark energy - 0 views

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    Hints of something interesting or data artefact? Also: http://arxiv.org/pdf/1207.4781v1.pdf
Phil Marshall

Modified Gravity and Cosmology - 0 views

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    Tim Clifton's epic review
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    Not read it yet though ;-)
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