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Steinhardt 16 Dec on inflation (+alts) and spirited debate

  1. Dec 21, 2014 #1


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    Several 30-40 minute videos from a great conference on cosmology and the Planck mission results are very much worth watching!
    The conference was called The Primordial Universe after Planck and was held in Paris 15-19 Dec.
    On the second day, 16 Dec, there was a debate about what the Planck data results mean for inflation. IIRC it was the convenor who reminded the conferees of a bumper sticker he had seen which read "My Karma Ran Over My Dogma"
    As introduction to the debate, to get things started, Steinhardt gave a critical review:

    Here is the debate ( Steinhardt, Mukhanov, Linde, Brandenberger) that followed Steinhart's (powerful) critique.

    Interestingly all four members returned several times to the topic of bounce cosmologies.
    Brandenberger was invited to join the panel at the last minute, and he and his group at McGill have been focusing their research for some time on a type of bounce cosmology.

    BTW at the start of Thursday, 18 Dec, later in the conference, Brandenberger gave a 45 minute review talk on Alternatives, which was followed by P. Peters talk on Bouncing models.
    and another by Nelson Pinto-Neto with a similar title: Bouncing models and Planck.​

    During the 4-way debate, Linde made the point that in order to make a difference people would need to come up with something better than the Inflation scenario and that the way to do that, he thought, was to resolve the "singularity". If the singularity could be removed (for example, he said, by a bounce, or by fluctuation out of an indefinite something else) then that would open up new possibilities which might cure some of the problems with inflation mentioned by Steinhardt.

    So the idea kept surfacing. People responded to Steinhardt's critique (by and large) by saying we don't need to abandon the inflation paradigm altogether, we just have to understand it better, what came before it. This could explain, for example, some of the apparent fine-tuning. So there's an interesting trend that manifests itself in different ways.

    Incidental note: I sampled a 4-person panel discussion on Future observational prospects and found parts of it interesting. How much farther can we reduce uncertainty? On average spatial curvature for example. How can observations be refined? What might be the consequences of determining, for example, that the overall spatial curvature was positive? The first speaker on the panel addressed this.
    Last edited: Dec 21, 2014
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  3. Dec 21, 2014 #2


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    The link to Robert Brandenberger's talk, "Alternatives to Inflation", currently is not working. I think the correct link is:
    But it just connects one back to the main "webcast" index.

    Too bad, his group at McGill has been doing a lot of research in that area. If you google "matter bounce cosmology" most of the first 10 hits are to papers reflecting the approach pursued at McGill.
    https://www.google.com/?gws_rd=ssl#q=matter bounce cosmology
    That Brandenberger "alternatives" link is now working and I had time to watch the first 25 minutes of the 50 minute segment. It turns out to be a broad survey. He makes a case for inflation and then covers a range of alternatives but does not overly stress the "matter bounce" which many of the McGill group's papers seem to be about lately. He touches on stringy alternatives: "string gas" and "D brane". So although he does discuss bounce alternatives to inflation he is careful not to seem to be promoting them. He stresses ways that the various alternatives could be distinguished by observations, and the difference between pre- and post-dictions (made by various alternatives.)

    Hi Skydive :^) I think you are right about a lot of the Loop bounce cosmic models including inflation, so not being an alternative! Several papers have argued that the Loop bounce picture gets adequate inflation with high probability without fine tuning, so it avoids the weaknesses that Steinhardt was criticising. That could have been a serious omission.

    Skydive, in answer to your question in next post: I agree there are those two versions being worked on, and as far as I could see they only discussed the former. There's a kind of blind spot in the community.
    Last edited: Dec 22, 2014
  4. Dec 22, 2014 #3
    Very interesting , thanks for posting Marcus. hope I will get time to watch that soon.
    I get the impression there are in some sense two classes of bounce models. Those that try and replace inflation and solve the puzzles of cosmology by introducing a pre big bang era that then bounces. Then there are the bounce models that actually include inflation as happening after the bounce. Brandberger seems to me to be pushing the former class , so Im wondering do you agree and were both classes of bounce models contrasted or just the former discussed?
  5. Dec 29, 2014 #4
    How powerful can it be, seeing how Planck's polarization data nails the HBB plus many more tests of inflation, so it is stronger than before? Theoretical problems is all he is left with, and he is lucky BICEP2+PLANCK is still outstanding. [Disclaimer: I haven't watched this round of Steinhardt's gripes with a successful physics theory...]
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