## Friday, June 30, 2017

### Requirements for Gravitational Theories

In this episode Jim and Randy talk about how to evaluate alternative gravity theories. What sort of things do we want them to explain, what experiments do they have to predict, and what theoretical requirements do they have to meet. This is in some ways a continuation of Episode 9 - f(R) Theories of Gravity, but the discussion is relevant to all attempts to amend gravitational theory.

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Notes:

1. "f(R) Theories of Gravity, by Thomas Sotiriou and Valerio Faraoni". The basis for this discussion and the last program's, as well.

2. The list Randy refers to in the episode. Probably just above this one. 3. Our subreddit.

### List of Requirements for Gravitational Theories

Here it is without fanfare. If you have any questions about any part of it, please comment in the comments:

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To be theoretically consistent and compatible with experiment, a theory of gravitation must:

(1) Predict correct cosmological dynamics

a. Big bang nucleosynthesis

(2) Produce the correct evolution of cosmological perturbations

a. Cosmic microwave background

b. Large scale structure

(3) Have the correct weak-field limits

a. Reproduces Newtonian Mechanics

b. Predicts post-Newtonian experiments in weak field

c. Produces stable solutions

i. Matter-side instabilities (Dolgov-Kawasaki)

1. Ground states should be highly symmetric

ii. Gravity-side instabilities

1. Stable de Sitter solutions

iii. Stability of the first loop in quantum gravity

iv. Stability in the face of inhomogeneous but isotropic perturbations

v. Black hole nucleation

(4) Not contain any ghost fields

(5) Admit a well-posed Cauchy problem

(6) Reasonable theory of gravity waves

-------------------------------

To be theoretically consistent and compatible with experiment, a theory of gravitation must:

(1) Predict correct cosmological dynamics

a. Big bang nucleosynthesis

(2) Produce the correct evolution of cosmological perturbations

a. Cosmic microwave background

b. Large scale structure

(3) Have the correct weak-field limits

a. Reproduces Newtonian Mechanics

b. Predicts post-Newtonian experiments in weak field

c. Produces stable solutions

i. Matter-side instabilities (Dolgov-Kawasaki)

1. Ground states should be highly symmetric

ii. Gravity-side instabilities

1. Stable de Sitter solutions

iii. Stability of the first loop in quantum gravity

iv. Stability in the face of inhomogeneous but isotropic perturbations

v. Black hole nucleation

(4) Not contain any ghost fields

(5) Admit a well-posed Cauchy problem

(6) Reasonable theory of gravity waves

## Friday, June 2, 2017

### f(R) Theories of Gravity

Jim and Randy discuss gravitational theories that modify general relativity by changing the action using a polynomial dependence on the Ricci scalar. Although not physically motivated, some of these theories produce effects similar to those of dark matter, dark energy, and cosmological constants.

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Notes:

1. "f(R) Theories of Gravity, by Thomas Sotiriou and Valerio Faraoni". The basis for this discussion and the next program's, as well.

2. Our subreddit.

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