BICEP2 Study Has Not Confirmed Cosmic Inflation

The BICEP2 study was released yesterday, and found some evidence of something called b-mode polarization in the early universe. Advocates of the theory of cosmic inflation were quick to trumpet these results, with many of them claiming that the study had finally confirmed the theory of cosmic inflation. This theory maintains that the universe underwent a period of exponential inflation during a fraction of its first second.

But there are several reasons why the BICEP2 study does not confirm cosmic inflation or even provide substantial evidence for it.

The first reason is that a single scientific study rarely proves anything. Having followed scientific developments closely for more than four decades, I have lived through many a case of scientific announcements that did not stand the test of time. I remember back around 1980 an announcement in which scientists announced a fate for the universe (collapse) that is the exact opposite of the fate they now predict for it (unending expansion). I also remember the famous “life on Mars” announcement in the 1990's which did not pan out. At this web site a scientist says that there is only a 50% chance that the results from this BICEP2 study will hold.

Another reason for doubting this BICEP2 study is that it makes an estimate for an important cosmological ratio called the tensor-to-scalar ratio, and that estimate is about twice the maximum possible value, according to the estimate from a very definitive source on the topic, the Planck team of scientists (larger than the BICEP group). Apparently some group of scientists are in serious error in regard to this matter, and there is a 50% chance that it is the team that made yesterday's BICEP2 study. If they are the ones who are wrong, it throws much of their study into doubt.

A third reason why the BICEP2 study does not confirm the theory of cosmic inflation is that BICEP2's results do not match well with the predictions of that theory.

The supporters of the inflation theory are citing the graph below from the BICEP2 study. The black dots are the BICEP2 observations, with the vertical lines being error bars (representing uncertainty in the data). The bottom red dashed line is what we expect from the cosmic inflation theory.

Considering just what is predicted from the theory of cosmic inflation, the results do not match well at all. The little black dots show a rise in the line exactly where the cosmic inflation theory predicts a fall in the line.

To patch up this embarrassing discrepancy, the authors add a “gravitational lensing” factor, which seems like quite the little fudge factor. Gravitational lensing is a very exotic effect that is not an easy thing to predict or nail down with any certainty. Estimating the amount of gravitational lensing that occurred long ago is like estimating the total tonnage of asteroids that have struck in the past billion years – very much a type of estimate that involves a huge amount of uncertainty. The BICEP2 authors seem to have got an estimate for gravitational lensing by making inputs to a 3-year old computer program called LensPix. There are lots of ways to go wrong there, either in the inputs or in the software (and the site for the software says “there are almost certainly bugs” in this software).

What is interesting, however, is that even if you accept as gospel truth this estimate of the amount of gravitational lensing, the data from BICEP2 ends up strongly diverging from the expected results produced from the estimated amount of gravitational lensing and cosmic inflation.

It is very hard to tell how big this discrepancy is from the graph shown above, because it uses two sneaky data presentation techniques to make the discrepancy look much smaller than it is. The techniques are: (1) the graph unnecessarily includes a whole load of irrelevant data in the top half of the graph, causing the scale of the graph to be unnecessarily large; (2) the graph uses a logarithmic scale (a type of scale that often tends to make two data items look closer than they are).

I can use exactly the same techniques to make a graph that makes it look like a dishwasher makes almost the same amount of money as a Wall Street bond trader.

But, thankfully, buried within the BICEP2 scientific paper is a nice simple non-logarithmic graph that shows just how great the difference is between the BICEP2 results and the results predicted from the cosmic inflation theory. The graph is below.

In the graph above the black dots are the new BICEP2 observations. The vertical lines are uncertainties in the data. The bottom red dotted line is the prediction from the theory of cosmic inflation. The solid red line is the estimated gravitational lensing factor. The upper red dashed line is the result predicted given a combination of the gravitational lensing factor and the theory of cosmic inflation.

Notice the big difference between the observed results and the expected result. Even if we include this highly uncertain gravitational lensing fudge factor, the predicted results from the cosmic inflation theory do not closely match the observed results. Note that the sixth and seventh black dots are way above the top dashed red line.

Therefore these results are far from being a confirmation of the theory of cosmic inflation. They can't even be called good evidence for cosmic inflation.

I may also note that there are numerous non-inflationary cosmological models that might produce the type of polarization observations that BICEP2 has produced. If there is currently a shortage of such models, it is largely because cosmic inflation speculations have almost monopolized the activities of theoretical cosmologists during recent decades.

The BICEP2 observations can be explained by the decay of exotic particles, or by some noninflationary exotic phase transition. Or it could be that all of the observed effect is produced by gravitational lensing and none of it produced by cosmic inflation. Scientists are already assuming that most of the observed effect is being produced by gravitational lensing; it's a short jump from “most” to “all” (particularly given the many uncertainties involved in estimating the amount of gravitational lensing).

If cosmologists spend as much time producing non-exponential non-inflationary models of the early universe as they do producing models that involve inflationary exponential expansion, they will probably find that non-exponential non-inflationary models are able to explain the observed BICEP2 results just as well, and perhaps even better.

Because the effect observed by the BICEP2 study can be produced by gravitational lensing, and because we will for many decades be highly uncertain about how much gravitational lensing has occurred in the past, it is very doubtful that any study like the BICEP2 will ever be able to provide real evidence for a theory of cosmic inflation. Just as UFO photographs rarely prove anything (because there are so many ways in which lights in the sky can be produced), a study like BICEP2 doesn't prove cosmic inflation (because there are other ways, such as gravitational lensing, that the observed polarization effect can be produced).

The case for the theory of cosmic inflation theory is much weaker than many think. In a nutshell the standard sales pitch for the theory is that it solves two cosmological problems: one called the flatness problem and the other called the horizon problem. The flatness problem is an apparent case of cosmic fine-tuning, and the horizon problem is an example of cosmic uniformity. The weakness in trying to solve these problems with a theory of cosmic inflation is that we have many other apparent cases of cosmic fine-tuning and many other cases of astonishing cosmic uniformity (including laws of nature and constants that are uniform throughout the universe). Inflation theory claims to solve only one of these many cases of apparent cosmic fine-tuning, and only one of the many cases of cosmic uniformity. That puts it in not a very good position, rather like a theory of the origin of species that only explains the origin of lions and tigers without explaining the origin of any other animals. I will explain this point more fully in a later blog post.

What is particularly ironic is that the theory of cosmic inflation claims to help in getting rid of some cosmic fine-tuning, but the theory itself requires abundant fine-tuning of its own to work, as many parameters in the theory have to be adjusted in just the right way to get a universe that starts exponentially inflating and stops inflating in a way that matches observations. 

Postscript: The chart below (in which I have added a green line) shows one way we can explain the BICEP2 observations without requiring any cosmic inflation.  We simply imagine a slightly higher amount of gravitational lensing (shown in the green line). The shape of this line matches the shape of the gravitational lensing estimated by the BICEP2 study (solid red line). Because the green line passes through all of the vertical error bars, it is consistent with the BICEP2 observations. 

BICEP2 graph with an added trend line (green)

Post-postscript: at this link cosmologist Neil Turok says, "I believe that if both Planck and the new results agree, then together they would give substantial evidence against inflation!"

Post-post-postscript: See the post here for a discussion of wishful thinking and cherry picking involved in the main graph shown above.   

Post-post-post-postscript: See this link for a National Geographic story on how the BICEP2 results may be caused by dust, not cosmic inflation. 

Yet another postscript: see this post for a discussion of a talk at Princeton University in which a scientist gives a presentation that gives a devastating blow to the inflated claims of the BICEP2 study. The scientist gives projections of dust and gravitational lensing which show how such common phenomena (not from the Big Bang or cosmic inflation) can explain the BICEP2 observations.  

Yet another postscript: In this article in the scientific journal Nature, it is explained that two recent scientific papers have concluded that there is no significant evidence the BICEP2 signals are from cosmic inflation or gravitational waves, with dust and cosmological lensing being an equally plausible explanation.