FW’s Cosmology Unwrapped 2021: a year of papers in Review [Ongoing]

A summary of papers I keep in a special Mendeley folder: stuff that I consider big news and major developments in the field of cosmology and nearby areas.

I will do one every year from now on. This is aimed at a general physics audience, and the stories are not presented in any particular order.

01. BICEP3/Keck Array’s New Release of CMB Data

Phys. Rev. Lett. 127 (2021) 15, 151301
arxiv: 2110.00483

strongest constraints to date on primordial gravitational waves

In the widely-accepted theory of cosmic inflation, our universe underwent a period of exponential growth in size over a very brief period of time shortly after the Big Bang. Inflation solves many conundrums in cosmology: it is able to explain why our spacetime’s flatness, the uniformity in the temperature of the cosmic background, and provides a viable pathway to understanding the origin of matter.

It is commonly supposed that a scalar field was responsible for the inflation process. It had noise — random fluctuations of quantum mechanical nature called the Primordial Scalar Fluctuations1— that is forcibly scaled into macroscopic inhomogeneities, far removed from their original quantum domain. These tiny fluctuations then seeded matter accretion and structure formation, attracting (dark) matter to become galaxies and galaxy clusters.

There may also have been fluctuations in space-time itself, a phenomenon permitted by the inflation model but not yet observed Primordial Tensor Fluctuations. After inflation has completed all its directives, these fluctuations can be preserved in the current cosmos as Primordial Gravitational Waves2.

The ratio of amplitudes of the tensor to the scalar fluctuations is called our universe’s tensor-scalar ratio, denoted by r. The denominator has been measured both by matching observations of galaxy clusters with comprehensive cosmological evolution models and by directly looking at cosmic background anisotropies. On the other hand, the numerator has proven technically challenging. On this front, BICEP (Background Imaging of Cosmic Extragalactic Polarization) has been a pioneering project to find primordial gravitational waves.

The first light that shone through the universe after neutral atoms formed — the Cosmic Microwave Background (CMB) — is theorized to possess various polarisation modes due to various physical processes, the E- and B- type polarisations, so named to represent how we typically decompose vector fields into curl-less (here “E” for electric) and divergence-less (“B” for magnetic) components.

Crucially, E-type polarisations arise as the CMB scatters off interstellar matter (scalar fluctuations), and B-type polarisations have two distinct origins: 1) primordial gravitational waves, and 2) gravitational lensing of (unknown) foreground objects, such as matter in our own galaxy.

Detecting B-polarised CMB photons and ascertaining their origin to me is one of the most arduous statistical exercises ever conducted by scientists on this floor of the science building. In the interest of keeping this essay succinct and other-physicist-friendly, the reader is referred to the main citation and references therein for technical details of the measurement and improvements made in BICEP3.

Various inflation mechanisms predict different r values, and so a good measurement of this ratio may open doors for humanity to probe into the physics at energies billions of times higher than our colliders can hope to achieve on earth. While still inconclusive, at a reported r-value of 0.036 (lowered from 0.07 a few years ago), the results in BICEP3 already ruled out a few inflation models.

Optimistic estimates exist that we can measure r accurate to +- 0.01 in the next decade.

  1. For a more comprehensive and technical overview, see The primordial power spectrum https://ned.ipac.caltech.edu/level5/Sept02/Kinney/Kinney4_6.html
  2. BICEP2 data release was during my freshman year of high school. I was quite excited about the popsci coverage back then.