The BICEP2 study was released a few weeks ago to great fanfare. The press release for the study announced breathlessly that it was evidence for the theory of cosmic inflation, the theory that the universe underwent exponential expansion during a fraction of its first second.
But the study involved at least two big fudges – cases in which curves were squashed or stretched unnaturally and unreasonably for the sake of getting observations to fit in with the favored storyline that evidence had been found for cosmic inflation. Before discussing each of the fudges, let me give a little background information.
The Difficulties of Looking for Primordial B-Mode Polarizations
The
idea behind the BICEP2 study is to look for a particular type of
radiation called b-mode polarization. Scientists predict that if a
period of cosmic inflation had occurred in the universe's first
second, it would have produced this type of radiation. But before the
BICEP2 study many scientists commented on the extreme difficulty of
finding evidence for cosmic inflation through such a process. The
main problem is that quite a few other astronomical phenomena can
produce this same type of b-mode polarization radiation. Among these
other phenomena are: various types of dust, synchrotron radiation and
gravitational lensing.
From the scientific paper here
The problem is that any lines that are higher-up in this graph are stronger signals that will drown out any signals that are lower in the graph (just as a 100-decibel sound of a passing motorcycle will completely drown out the 30-decibel sound of a child whispering). So if we use the curves in the chart above, there would seem to be basically zero chance of ever being able to confirm a theory of cosmic inflation by measuring b-mode polarizations (the technique used by the BICEP2 study).
To try to overcome such problems, the BICEP2 study resorted to some fudges I will now list.
Outrageous Fudge Number 1: Shrinking the Gravitational Lensing Model
The
BICEP2 study has a graph showing a projection of the expected amount
of b-mode polarization from gravitational lensing. But the projection is a shrunken, low-ball
projection. It is nowhere near as high as the projections made by
some previous scientists.
Here
is the BICEP2 graph in which they project gravitational lensing (the solid red line):
You
have to look closely at the little lines on this logarithmic graph to
figure out two things: the assumption being made about where
gravitational lensing starts, and the assumption being made about
where it peaks. The assumptions being made by the BICEP2 study are these:
Starting
point for gravitational lensing: 50 multipole ( l )
Peak
of gravitational lensing: .05 (close to 10-1).
The
problem is that this is a shrunken estimate, an extreme low-ball
projection. The first graph in this post (in which l is
the multipole) gives a very different, much-larger
projection:
Starting
point for gravitational lensing: 2 multipole
Peak
of gravitational lensing: .2
In
fact, most of the estimates that you will find (made prior to the
BICEP2 study) match this much larger estimate for gravitational
lensing. Indeed, the most recent POLARBEAR observations support these
larger estimates, by showing the peak of the gravitational lensing at
a much higher point than the peak in the BICEP2 graph.
Why
is this important? If the larger estimate of gravitational lensing is
correct, then all of the BICEP2 observations can be explained by
assuming gravitational lensing (not cosmic inflation) as the source
of the radiation. For example, if the larger projection of
gravitational lensing is correct, then we have a model of
gravitational lensing similar to the green line below, which can
explain all of the BICEP2 observations without requiring any cosmic
inflation in the universe's first second.
To
force their observations to fit a theory of cosmic inflation, the
BICEP2 study chose to present a shrunken, low-ball estimate of
gravitational lensing.
Outrageous Fudge Number 2: Shrinking the Dust Projection
When
they estimated the amount of b-mode polarization produced by cosmic
dust, the BICEP2 team almost admitted that they didn't have what they
needed to make an accurate projection:
The
team then present a graph showing some models they selected, models
that minimize the amount of dust, and suggest that dust is no big
problem when trying to measure signals from cosmic inflation. The
low-ball models selected are inconsistent with some previous
estimates, which estimate that dust should be blocking all or most of
any b-mode polarization produced by cosmic inflation.
See,
for example, this scientific paper, which on page 2 predicts a level
of dust polarization many times greater than the amount projected by
the BICEP2 study (as does this graph from a scientific conference). The relevant graph is shown at the beginning of
this blog post.
It
was almost rather like this:
Previous
scientists: How on earth can we find a signal from cosmic
inflation with all this cosmic dust all over the places we're looking
for the signal, dust that blocks what we're looking for?
BICEP2
scientists: Dust? What dust?
Of
course, the BICEP2 study needed to shrink and low-ball the dust
projections, to clear the field for their triumphant announcement of
evidence for cosmic inflation, and to try to rule out dust as the
source of their observations. This was another biased case of
artificially stomping on a data curve to get observations to fit a
favored explanation of the data.
In
an attempt to rule out dust as the source of their observations, and
bolster their case for cosmic inflation (in the universe's first second) as the source of their
observations, the BICEP2 team ran some simulations (using lots of
subjective, hand-picked inputs) that they say show that their
observations have characteristics “atypical” of dust. That is
very lame and unconvincing reasoning – rather like arguing that a
particular light seen in the sky is an alien spaceship because it has
characteristics “atypical” of an airplane. The graph of their
simulations (Figure 8 in the study) still shows a perfectly decent
chance that dust or synchrotron radiation is the source of their
observations, not cosmic inflation.
We
might expect with these two examples of curve fiddling that the end
results would match the favored model exactly. But no: even with
these heroic efforts, the BICEP2 graph below only shows 5 out of 9 data
points matching the favored model, with several of the data points
far off of the model.
When
a study is based on simple data observations, you can trust the
observer to record the observations correctly, unless you think he
might be careless or prone to fraud. For example, if a scientist
measures the temperature on a particular day, you pretty much have to
trust him, unless you think he might be faking it. But in a case
such as the BICEP2 study we have a very different situation. The
scientists took raw data, and subjected it to an extremely complicated
process of transformations, summaries, and modeling. The process was
almost like the process shown in the visual below:
Source: wikiuniversity, Howard Community College
Can
we be confident that the BICEP2 team got this extremely complicated
process right, and that the data points shown in their final graph
are correct? No, we cannot be. This is because it is rather clear
from these examples I have shown that the BICEP2 team had a strong
experimental bias. Evidently they wanted very much to make their
observations match a storyline that the observations came from
primordial cosmic inflation. Given this very strong partiality, there are
any number of ways in which things could have gone wrong because of
experimental bias. At any number of points in the incredibly
complicated data transformation process, the scientists may have made
decisions influenced by their desire to end up with results favoring
a theory of cosmic inflation, decisions that more objective and
impartial scientists would not have made.
We
need studies like the BICEP2 study to be performed by objective,
non-biased scientists without any favored agenda (scientists with an
attitude of “let the chips fall where they may”), rather than
scientists who seem determined to hammer square pegs into round
holes, in order to fit some desired preconceived storyline.
Postscript: See this link for a National Geographic story on how the BICEP2 results may be due to dust, not cosmic inflation. I'm not accusing any on the BICEP2 team of deliberately misleading anyone. I merely think that their desire to have an inflation-related result (good for their own careers) has influenced their paper, leading to some presentation and interpretation decisions that might not have been made by a more impartial set of writers.
Post-postscript: I didn't originally mention the pathetic way the BICEP2 paper handled the issue of synchrotron radiation. Synchrotron radiation is a widespread phenomenon that produces the same b-mode polarization observed by BICEP2. This type of radiation can be produced by many types of high-energy violent events inside and outside of our galaxy. Rather than making any substantial attempt to show that synchrotron radiation is not the source of their observations (which would require many pages), the BICEP2 paper (in sections 9.2 and 9.3) has the skimpiest treatment of the topic, using only 6 sentences to address it. This discussion calls section 9 of their paper "the most ridiculous handwave of all time in the whole history of physics."
Post-post-postscript: Today a physicist has on his blog a post that says the BICEP2 team made some big error in their dust projection (an error along the same lines as I insinuate in this post). He says, "However, at this point, there seems to be no statistically significant evidence for the primordial B-modes of inflationary origin in the CMB [cosmic microwave background]."
Postscript: See this link for a National Geographic story on how the BICEP2 results may be due to dust, not cosmic inflation. I'm not accusing any on the BICEP2 team of deliberately misleading anyone. I merely think that their desire to have an inflation-related result (good for their own careers) has influenced their paper, leading to some presentation and interpretation decisions that might not have been made by a more impartial set of writers.
Post-postscript: I didn't originally mention the pathetic way the BICEP2 paper handled the issue of synchrotron radiation. Synchrotron radiation is a widespread phenomenon that produces the same b-mode polarization observed by BICEP2. This type of radiation can be produced by many types of high-energy violent events inside and outside of our galaxy. Rather than making any substantial attempt to show that synchrotron radiation is not the source of their observations (which would require many pages), the BICEP2 paper (in sections 9.2 and 9.3) has the skimpiest treatment of the topic, using only 6 sentences to address it. This discussion calls section 9 of their paper "the most ridiculous handwave of all time in the whole history of physics."
Post-post-postscript: Today a physicist has on his blog a post that says the BICEP2 team made some big error in their dust projection (an error along the same lines as I insinuate in this post). He says, "However, at this point, there seems to be no statistically significant evidence for the primordial B-modes of inflationary origin in the CMB [cosmic microwave background]."
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.
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