Thursday, February 09, 2017

New Data from the Early Universe Does Not Rule Out Holography

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It’s string theorists’ most celebrated insight: The world is a hologram. Like everything else string theorists have come up with, it’s an untested hypothesis. But now, it’s been put to test with a new analysis that compares a holographic early universe with its non-holographic counterpart.

Tl;dr: Results are inconclusive.

When string theorists say we live in a hologram, they don’t mean we are shadows in Plato’s cave. They mean their math says that all information about what’s inside a box can be encoded on the boundary of that box – albeit in entirely different form.

The holographic principle – if correct – means there are two different ways to describe the same reality. Unlike in Plato’s cave, however, where the shadows lack information about what caused them, with holography both descriptions are equally good.

Holography would imply that the three dimensions of space which we experience are merely one way to think of the world. If you can describe what happens in our universe by equations that use only two-dimensional surfaces, you might as well say we live in two dimensions – just that these are dimensions we don’t normally experience.

It’s a nice idea but hard to test. That’s because the two-dimensional interpretation of today’s universe isn’t normally very workable. Holography identifies two different theories with each other by a relation called “duality.” The two theories in question here are one for gravity in three dimensions of space, and a quantum field theory without gravity in one dimension less. However, whenever one of the theories is weakly coupled, the other one is strongly coupled – and computations in strongly coupled theories are hard, if not impossible.

The gravitational force in our universe is presently weakly coupled. For this reason General Relativity is the easier side of the duality. However, the situation might have been different in the early universe. Inflation – the rapid phase of expansion briefly after the big bang – is usually assumed to take place in gravity’s weakly coupled regime. But that might not be correct. If instead gravity at that early stage was strongly coupled, then a description in terms of a weakly coupled quantum field theory might be more appropriate.

This idea has been pursued by Kostas Skenderis and collaborators for several years. These researchers have developed a holographic model in which inflation is described by a lower-dimensional non-gravitational theory. In a recent paper, their predictions have been put to test with new data from the Planck mission, a high-precision measurement of the temperature fluctuations of the cosmic microwave background.

In this new study, the authors compare the way that holographic inflation and standard inflation in the concordance model – also known as ΛCDM – fit the data. The concordance model is described by six parameters. Holographic inflation has a closer connection to the underlying theory and so the power spectrum brings in one additional parameter, which makes a total of seven. After adjusting for the number of parameters, the authors find that the concordance model fits better to the data.

However, the biggest discrepancy between the predictions of holographic inflation and the concordance model arise at large scales, or low multipole moments respectively. In this regime, the predictions from holographic inflation cannot really be trusted. Therefore, the authors repeat the analysis with the low multipole moments omitted from the data. Then, the two models fit the data equally well. In some cases (depending on the choice of prior for one of the parameters) holographic inflation is indeed a better fit, but the difference is not statistically significant.

To put this result into context it must be added that the best-understood cases of holography work in space-times with a negative cosmological constant, the Anti-de Sitter spaces. Our own universe, however, is not of this type. It has instead a positive cosmological constant, described by de-Sitter space. The use of the holographic principle in our universe is hence not strongly supported by string theory, at least not presently.

The model for holographic inflation can therefore best be understood as one that is motivated by, but not derived from, string theory. It is a phenomenological model, developed to quantify predictions and test them against data.

While the difference between the concordance model and holographic inflation which this study finds are insignificant, it is interesting that a prediction based on such an entirely different framework is able to fit the data at all. I should also add that there is a long-standing debate in the community as to whether the low multipole moments are well-described by the concordance model, or whether any of the large-scale anomalies are to be taken seriously.

In summary, I find this an interesting result because it’s an entirely different way to think of the early universe, and yet it describes the data. For the same reason, however, it’s also somewhat depressing. Clearly, we don’t presently have a good way to test all the many ideas that theorists have come up with.


Phillip Helbig said...

Peter Coles has a more sceptical take on this.

עמיר ליבנה בר-און said...

As Multivac would say in their 1950's manner, THERE IS INSUFFICIENT DATA FOR A MEANINGFUL ANSWER.

Topher said...

This and a report you tweeted on the vorticity of quark-gluon plasma reminded me of a talk I heard in the early 2000's. The claim was that the then-recently-measured viscosity of quark-pluon plasma at RHIC was very low, but just above a kind of lower limit predicted by string theories in a very general way.

I haven't heard any updates on this idea, but at the time it seemed like a genuine test of string theory which could have failed, but did not. Sadly it was not the kind of test that would distinguish string theories from the Standard Model, but still it seemed noteworthy. Has there been any recent news around this?

Matthew Rapaport said...

I don't understand why "fitting the data" stands as evidence of anything in particular here. We are talking about mapping *information* here and many information mappings can fit the same data. You have only to look at all the various undecidable interpretations of QM to see this at work.

So assuming this vision of reality to be true in some sense, how does it affect the rest of physics? What for example does it do to the standard model?

Unknown said...

Seems holographic principle just a computational device?
If there's no difference in predictive or explanatory ability between holography and "conventional" descriptions, let Occams Razor prevail and we "really" live in an ordinary 3-D physical universe.

An example within some engineering fields is use of the complex number plane for certain calculations. Some analyses involving AC voltages, currents, and circuit elements are easier to both calculate & visualize. But no one ever thinks there is some physical reality represented by sqrt(-1).

-- TomH

Pfogle said...

I totally agree that it is extraordinary that these two, so different concepts, can come up with a model that even approximately matches the data. But I might be being naive, as I've no way of knowing how flexible these parameters make the models.

Also, does the positive cosmological constant undermine the holographic model for being mathematically inconsistent? In which case what is the status of the model used in the study? Is this the origin of the divergence of the model from Concordance at higher multipoles?

To me, the value of the idea would seem to be the discovery that the information contained in a finite volume varies as the surface area of the boundary. I find that extraordinary, and quite counter-intuitive. This seems to be the beauty of duality!

Sabine Hossenfelder said...


Not very flexible as there aren't all that many parameters. This does not factor in however, how much flexibility there was in choosing this model to begin with, which is much harder to quantify.

The model is (needless to say) not known to be mathematically inconsistent. The positive cosmological constant doesn't undermine its consistency, it brings in additional guesswork in the assumptions. No, the origin of the divergence at low multipoles is that one uses an approximation which is no longer valid in this regime and a method to circumvent the problem is not known. Best,


Sabine Hossenfelder said...


It didn't work because the predictions didn't fit with the LHC data, so most string theorists seem to have given up on it and don't want to be reminded of it. I wrote about this here. It's kinda interesting that this did not make headlines. Best,


Sabine Hossenfelder said...


I agree with what Peter wrote in his blogpost, the title of that news story is very misleading. As you can see, I've picked my title to capture what the paper actually says. (The result is, needless to say, that few people care about it - that's the problem with fake news, they're designed to spread.)

Having said that, he forgets to mention though that there are good reasons to exclude the low multipoles in the data analysis. That still doesn't result in "evidence for" of course. Best,


KSkenderis said...

In planet in a far away galaxy a high profile murder is committed. The police rounds up a number of suspects and after a while there is enough circumstantial evidence to single out suspect A from all the others and put him on trial. New technology becomes available and DNA testing of genetic material left in the scene reveals a close match with that of suspect A. The prosecution presents this evidence to the court as the definitive proof that suspect A committed the crime. Front-page titles with "GUILTY" appear in the news. Everyone is now convinced that A is guilty, despite a number of gaps in the prosecution's case, and it seems a matter of time before the court proceeding conclude with a "Guilty" verdict. Meanwhile the defence team uncovers a new suspect B, who wasn't investigated previously, and who has different DNA than that of suspect A, but nevertheless the match with the genetic material from the murder scene is of equal quality with that of suspect A.

How should the defence team present this to the court and to the media?

It is definitely the case that suspect B is not ruled out as the murderer but more is true: based on the DNA test both suspects are equally likely to have committed the crime.

The law firm of the defence team issues a press release with title:

"Substantial evidence that suspect B committed the crime"

and the text says there is substantial evidence against B, equal to that against A.

Is this misleading?

Matthew Rapaport said...

I think truth table values are same but misleading because "against" here is ambiguous between "for guilt" and "for innocence". But in science looking for "evidence against" meaning falsifying, or "evidence for" meaning confirming, are both legitimate. Of course your paper should make explicit which direction you are going

KSkenderis said...

The scientific case is unambiguous. Both models provide an excellent fit to the data (i.e. the models explain the structure seen in the data), and as such the data provide evidence for both theories. Having established this, the next question is whether the data prefer the one or the other theory. The main scientific tool for addressing this is to compute the so-call Bayesian Evidence. This computation essentially shows that both models are equally likely to describe the early universe. More data are needed to discriminate between the two models (and more theoretical work in the case of the holographic model (which is in progress), if one is to use it for the very low multipoles).

The issue at hand is how to best communicate these results to the general public. I don't claim to have the answer to this. Clearly, one has to use a broad brush picture as the general public lacks the scientific background to understand the technical details. But such board brush picture can be prone to misinterpretation. While one could debate whether different phrasing in the press release would be more appropriate, I do not think it was actually misleading: it said that there is evidence for the holographic model, which is equal to that of the concordance model. What is misleading here?