Wednesday, March 18, 2015

An alternative to cosmic inflation

On Friday Robert Mann gave a very nice colloquium at UQ, The Black Hole at the Beginning of Time. The video is below.

The (end of) the talk is based on the recent paper
Out of the white hole: a holographic origin for the Big Bang 
Razieh Pourhasan, Niayesh Afshordi, and Robert B. Mann

The key idea is to consider our universe as the 4-dimensional boundary (brane or hologram) of a 5-dimensional space-time in which there is a black hole.
In our universe one then has not just 4D gravity and matter, but also induced gravity and an effective fluid from the 5D "bulk".

(For better or worse) this work was recently featured on the cover of Scientific American.

Robert covered a massive amount of material moving through special relativity, general relativity, black holes, big bang, cosmology, recent results from the Planck satellite,  inflation, the multiverse,... and finally his alternative model.
I took several pages of notes.
He went overtime. I think this was one of the rare cases where I did not mind the speaker doing it.

Besides learning some interesting physics, what was most interesting to me was the refreshing way the work was presented. The tone was something like, "cosmology has some amazing successes but there are a few paradoxes, inflation is an interesting idea but also presents some problems, ...fine tuning is a challenge, ... so let me throw out a different idea.... it is a bit weird... but lets see where it goes ... it also has some strengths and weaknesses .... I am not sure this is better than inflation, but it is worth looking at." There was no hype or sweeping things under the rug.

Many in the audience were undergrads. I thought it was a great talk for them to hear. It was largely tutorial, there was some fascinating physics, connections to experiment were emphasised, healthy skepticism was modelled, and there was no hype.

I also liked the talk because it confirms my prejudice that people need to work harder, more creatively, and more critically, on foundational problems in cosmology. Dark matter, dark energy, inflation, and fine tuning are all really weird. They may be right. But they may not be. I think just accepting them as the only option and regressing to even weirder ideas like the multiverse is a mistake. [Of course, it is easy as an outsider to tell colleagues to work harder and more creatively.]

The physical model of the early universe that was presented was completely different to inflation. Yet it solves most of the same problems (horizon, flatness, and no monopoles). Its biggest problem is that it does not predict the observed 4 per cent deviation from scale invariance.

The most important and interesting bits are from about 52:00 to 58:00.


  1. The paper in which his theory is described, is well over a year old and has accumulated zero citations - yet somehow it was the subject of a note in Nature and a Scientific American cover story. It's based on an approach (DGP) inspired by string theory but which ignores other lessons from string theory about what does and what does not make sense. Its use of holography also looks very peculiar, and then on top of that further fudges are being introduced in order to make the thing resemble the real world. All the evidence suggests that this is second- or third-rate work that has been sold to the media by some first-rate hype. I also thought the talk was bad, it was more about putting on a show than it was about increasing the audience's understanding.

    1. Mitchell, thank you for your comment.

      1. I don't think that short term number of citations is a measure of the quality of research.

      A classic example is Weinberg's electroweak theory PRL that attracted about one citation per year for the first 5 years after publication.

      2. I don't think that what string theorists say works or does not "work" should be the sole criteria for deciding whether a piece of work is valid or important.

      3. The fact that this work got so much attention in the popular science media reflects what they like (exotica) rather than promotion by the authors and should not be used as criteria for deciding its validity or value.

      4. Robert was very transparent about the weaknesses of the theory. I found this refreshing.

      5. Readers can watch the video for themselves and judge the quality of the talk. I thought considerable effort was made to increase the audience's understanding.