Wednesday, June 25, 2014

Bicep flexing 2 : After the dust up

This week saw the actual publication of the BICEP2 result on the detection and analysis of the polarisation signature of inflationary behaviour on the cosmic wave background in the final arbiter of all physics research, Physical Review Letters. The paper does an admirable job of explaining  the problem, its background, the data, the analysis, the results and the limitations, if not to the lay reader (not the purpose of PRL, anyway), at least to physicists in other fields. Here is a short summary of what it says.

First, the background. The central paradigm for the origin and evolution of the universe, is the Big Bang Theory, first proposed by Alpher, Bethe and Gamow. The validity of the Big Bang theory was established by the discovery of the cosmic microwave background by Penzias and Wilson. One of the most fruitful areas of research in recent times has been the observation of the anisotropies in the cosmic microwave background, and the consequent refinement of the cosmological models. The `standard model' of cosmology is known as `Lambda-CDM' model, where the Lambda refers to the cosmological constant and CDM stands for `cold, dark, matter'.  This model can account for observed properties like the existence and structure of the cosmic microwave background, the large scale structure in the distribution of galaxies, the abundances of gases like hydrogen, helium and lithium, and the accelerating expansion of the universe. It is particularly important to point out that cosmic microwave background measurements now have precision over angular scales ranging from the whole sky to arcminutes. As a result, the parameters of the Lambda-CBM model are constrained to a precision of less than 1 %.

Inflationary scenarios, including the first one proposed by Alan Guth,  extend the model above by adding an early period wherein a metastable state corresponding to a local minimum of the potential energy undergoes  nearly exponential expansion, which leads to the subsequent big bang.
The addition of inflation to the early scenario explains features like the flatness and isotropy, which cannot be explained within the standard framework above. Inflation also explains that the Universe's primordial perturbations  originate in quantum fluctuations stretched by this exponential expansion, which invokes quantum effects in curved space time at energy and time scales of the order of 10^16 GeV and 10^(-32) seconds, which are completely inaccessible in the real world, both in nature and the laboratory. It is therefore important to have a direct and clear test of this theory. The BICEP2 experiment provides such a test, and therein lies its importance.

 The signature is as follows. While a number of inflationary scenarios exist, they all have a common prediction. Inflation would have produced gravitational waves that would cause characteristic distortions in the cosmic microwave background, with a unique  signature in the radiation field.  The quadrupole nature of the  gravitational waves lead to a polarisation in the radiation field with the `B-mode' pattern characteristic of a curl on angular scales of the order of a degree. Such a pattern cannot be produced by density fluctuations which produce a  `E-mode' pattern characteristic of a gradient on smaller angular scales. It is the `B-mode' pattern seen at exactly the expected angular scales in the BICEP2 experiment that provides the unique signature of inflation. The ratio of the amplitude of the tensor perturbations  characteristic of gravitational waves, to the scalar modes associated with density perturbations, is used to identify the strength of the B-mode, and also to predict the energy density during the inflationary phase.

Since the `B-mode' pattern provides the signature of inflation, it is important to eliminate the effect of foreground sources  which can produce such a pattern. One of these is the effect of gravitational lensing. However, this is much weaker than the effect seen in the BICEP2 experiment. Hence this effect has been ruled out. One serious objection raised since the results were announced, is that such a pattern could be due to the impact of the galaxy's dust. Unfortunately, there is no convincing data on the distribution of dust. Hence the BICEP2 analysis has estimated the effects of cosmic dust based on theoretical models, and concluded that dust could not reproduce the magnitude of the observed signal. However,the validity of this conclusion depends on the accuracy of  hitherto experimentally unvalidated theoretical models. Critics of the BICEP2 result claim that the new data released by the PLANCK satellite is not incompatible with dust levels that lead to polarisation signatures  which are of the order of the BICEP2 signal. A number of measurements that are scheduled within the coming year are expected to resolve this ambiguity.

The BICEP2 experiment also leads to a variety of exciting theoretical implications. More on these can be read here. For the present, there is no doubt that the BICEP2 experiment constitutes an important landmark in understanding the mechanisms that lead to the formation of the universe and its subsequent evolution. We look forward to its further validation.

This blog post is by Neelima Gupte and Sumathi Rao.
Here is the outlook    after the latest analysis of the Planck data. The BICEP2 result begins to look dusty. More later.

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