General Relativity and Cosmology: Unsolved Questions and Future Directions [CL]

I missed this when it appeared on the arXiv last week, but now that I’ve read it I couldn’t resist reblogging this nice review of the current state of General Relativity and its alternatives, with an emphasis on the cosmological ramifications.


For the last 100 years, General Relativity (GR) has taken over the gravitational theory mantle held by Newtonian Gravity for the previous 200 years. This article reviews the status of GR in terms of its self-consistency, completeness, and the evidence provided by observations, which have allowed GR to remain the champion of gravitational theories against several other classes of competing theories. We pay particular attention to the role of GR and gravity in cosmology, one of the areas in which one gravity dominates and new phenomena and effects challenge the orthodoxy. We also review other areas where there are likely conflicts pointing to the need to replace or revise GR to represent correctly observations and consistent theoretical framework. Observations have long been key both to the theoretical liveliness and viability of GR. We conclude with a discussion of the likely developments over the next 100 years.

Read this paper…

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13 Responses to “General Relativity and Cosmology: Unsolved Questions and Future Directions [CL]”

  1. Here’s a perhaps unsolved question for the pundits here. An accelerated charge radiates. A charge at rest in a gravitational field does not. Does this violate the equivalence principle? If yes, is that a problem? If not, explain how it does not, since it allows one to distinguish, even in the absence of non-uniform fields and tidal effects, between the two cases of uniform acceleration and rest in a gravitational field.

    There is a bit of literature on this, but has the community arrived at a consensus?

  2. Sorry Philip, my eyes! Are you sure about the ball on your desk not radiating at all? The earth is accelerating away from the sun (now about 15 cm per year and increasing) and the moon is accelerating away from the earth (now about 4 cm per year and increasing) etc.

    • This makes it more complicated, as I am accelerating along with it. Perhaps it is radiating, but only one photon every few years.

      The simpler problem: at rest in static gravitational field vs. acceleration in Minkowski space. Why does it radiate in one case and not in another (there is no debate about this) if the equivalence principle is valid?

      • I understand the question but I don’t see the problem. Because 1. I don’t think events in Minkowski space lead to other results than in Helbig space or van Dijk space, 2. I don’t think that at rest in static gravitational field exists.

  3. I am a beginner in Relativity but that subject is a very interesting and hard one to me now. It is adressed in some textbooks on relativity:

    “but the proper solution seems to have been first recognized by Ehlers: It is necessary to restrict the class of experiments covered by the EP to those that are isolated from bodies or fields outside the cabin. In the case of the charges discussed above, their electric field extends beyond the cabin and is, in fact, ‘anchored’ outside; since radiation is a property of that whole field, it follows that these ‘experiments’ lie outside the scope of the EP.” Rindler, Relativity 2006, page 23.


    “Einstein Equivalence Principle: Any local physical experiment… In this case ‘local’ means that the experiment does not involve fields, such as electric fields, that may extend over large regions and therefore extend outside the domain of validity of the local inertial frame.” Schutz, A first course in General Relativity, 2009, page 173.

    The comments of Ted Bunn appear here (thread “gravitationally accelerated electron”)

  4. An accelerated charge radiates. A charge at rest in a gravitational field does not. Does this violate the equivalence principle?

    No. Because gravity is not a force in the Newtonian sense. Gravity doesn’t do any work on a falling body. It merely converts internal kinetic energy into external kinetic energy. When you drop an electron it isn’t accelerating like the electron in the collider. No energy is being added. When you’re in free fall you feel no force because there isn’t any. There is however a force when you hit the ground. In similar vein the electron radiates when you stop it falling, leaving you with a mass deficit. After that it doesn’t continue to radiate because it isn’t accelerating. You feel a force on your feet because standing on the Earth is like accelerating, but it isn’t actually an acceleration. No energy is being added. Being motionless in inhomogeneous space is like accelerating through homogeneous space, but it isn’t the same. That’s why the equivalence principle only applies to a region of infinitesimal extent. A region of no extent. To no region at all.

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