Archive for Holographic Universe

A Challenge for Inflationary Cosmologists

Posted in The Universe and Stuff with tags , , on February 6, 2017 by telescoper

A few days ago I wrote a very sceptical post about an alternative to the present standard cosmological which is called the holographic universe. After an interesting discussion thread on that post I thought I’d pose a challenge here. It might be a bit specialist as it is for inflationary theorists and model-builders (a club to which I do not belong) but I thought I’d try it as it might prove education for me as for other readers.

Anyway, the point is that in the inflationary paradigm there is a fairly generic prediction that the primordial scalar power spectrum (related to the spectrum of density fluctuations) takes the form of a power law:

equation-1

The wavenumber is denoted q. There are two free parameters here: the spectral index ns (which is usually close to unity); and an overall normalization amplitude parametrised here at an arbitrary “pivot” scale q*.

In the holographic model the functional form of the spectrum is quite different:
equation-2

This has two different free parameters: g and β, both of which relate to properties of a dual Quantum Field Theory which appears in the model.

The second model is motivated by very different considerations from those behind the inflationary model, but my suspicion is that in fact one could create a version of inflation that produces a spectrum of the form (2) rather than (1). There is an imtimate relationship between the scalar perturbation spectrum and the inflationary dynamics which means that there is considerable freedom to “design” the perturbation spectrum by building features into the potential.

Anyway, that’s the challenge. Would any cosmologists out there with time on their hands please make me an inflationary model that produces the spectrum (2). Alternatively, if this can’t be done, give me a proof why it can’t!

Fake News of the Holographic Universe

Posted in Astrohype, The Universe and Stuff with tags , , , , , , on February 1, 2017 by telescoper

It has been a very busy day today but I thought I’d grab a few minutes to rant about something inspired by a cosmological topic but that I’m afraid is symptomatic of malaise that extends far wider than fundamental science.

The other day I found a news item with the title Study reveals substantial evidence of holographic universe. You can find a fairly detailed discussion of the holographic principle here, but the name is fairly self-explanatory: the familiar hologram is a two-dimensional object that contains enough information to reconstruct a three-dimensional object. The holographic principle extends this to the idea that information pertaining to a higher-dimensional space may reside on a lower-dimensional boundary of that space. It’s an idea which has gained some traction in the context of the black hole information paradox, for example.

There are people far more knowledgeable about the holographic principle than me, but naturally what grabbed my attention was the title of the news item: Study reveals substantial evidence of holographic universe. That got me really excited, as I wasn’t previously aware that there was any observed property of the Universe that showed any unambiguous evidence for the holographic interpretation or indeed that models based on this model could describe the available data better than the standard ΛCDM cosmological model. Naturally I went to the original paper on the arXiv by Niayesh Ashfordi et al. to which the news item relates. Here is the abstract:

We test a class of holographic models for the very early universe against cosmological observations and find that they are competitive to the standard ΛCDM model of cosmology. These models are based on three dimensional perturbative super-renormalizable Quantum Field Theory (QFT), and while they predict a different power spectrum from the standard power-law used in ΛCDM, they still provide an excellent fit to data (within their regime of validity). By comparing the Bayesian evidence for the models, we find that ΛCDM does a better job globally, while the holographic models provide a (marginally) better fit to data without very low multipoles (i.e. l≲30), where the dual QFT becomes non-perturbative. Observations can be used to exclude some QFT models, while we also find models satisfying all phenomenological constraints: the data rules out the dual theory being Yang-Mills theory coupled to fermions only, but allows for Yang-Mills theory coupled to non-minimal scalars with quartic interactions. Lattice simulations of 3d QFT’s can provide non-perturbative predictions for large-angle statistics of the cosmic microwave background, and potentially explain its apparent anomalies.

The third sentence (highlighted) states explicitly that according to the Bayesian evidence (see here for a review of this) the holographic models do not fit the data even as well as the standard model (unless some of the CMB measurements are excluded, and then they’re only slightly better)

I think the holographic principle is a very interesting idea and it may indeed at some point prove to provide a deeper understanding of our universe than our current models. Nevertheless it seems clear to me that the title of this news article is extremely misleading. Current observations do not really provide any evidence in favour of the holographic models, and certainly not “substantial evidence”.

The wider point should be obvious. We scientists rightly bemoan the era of “fake news”. We like to think that we occupy the high ground, by rigorously weighing up the evidence, drawing conclusions as objectively as possible, and reporting our findings with a balanced view of the uncertainties and caveats. That’s what we should be doing. Unless we do that we’re not communicating science but engaged in propaganda, and that’s a very dangerous game to play as it endangers the already fragile trust the public place in science.

The authors of the paper are not entirely to blame as they did not write the piece that kicked off this rant, which seems to have been produced by the press office at the University of Southampton, but they should not have consented to it being released with such a misleading title.