B-Mode Polarization Findings May Not Be Real After All - So Primordial Gravitational Waves & Cosmic Inflation Aren't Either?

 

                                           Cosmic microwave background image.

According to the Big Bang “sub-theory” of cosmic inflation, the universe rapidly “stretched” in size during its initial expansion phase, and by that we mean the expansion was “faster than light”. One would therefore say  the inflation mode is entirely in the realm of 'false vacuum' in which a large and negative valued cosmological constant is assumed. In the context of Einstein's theory of general relativity, the 'false vacuum' may be thought of alternatively as contributing a cosmological constant about 10-100 times larger than it can possibly be today. It is this peculiarity which generates a corresponding 'repulsive' force, causing the universe to inflate on an exponential scale.

Note that this sort of thing (implying velocities of expansion > c) isn't particularly novel. In fact, a number of cosmological models posit that proper distances, i.e. between clusters of galaxies, may increase faster than the speed of light.  To fix ideas here, if the initial size of the universe was 1 fm (Fermi = 10 -¹⁵ m or about the size of a proton) then the inflating cosmos would have attained a scale of about eight times the Earth-Sun distance (8 astronomical units) after 90 doubling times, with periods ranging from 10 -⁴³ secs to 10 -³⁶ secs.

But is the phenomenon of inflation real, and by extension primordial gravitational waves? The problem appeared at first solved in the positive mode with the BICEP2 (Background Imaging of Cosmic Extragalactic Polarization – see link at bottom) observations published in 2014 in the Journal Nature.  

It was based on the supposition that the cosmic microwave background (or CMB, see image at top) ought to have a characteristic “swirling” polarization known as a B-mode. Light is polarized when its electromagnetic waves are preferentially oriented in a particular direction.  Other forms of polarization include:

Linearly or horizontally polarized: I.e. the E- vector is confined to one  (horizontal) plane

---------à E

vertically polarized: I.e. the E- vector is confined to one  (vertical) plane

^ E

!
!
!

Circular: The E-vector rotates through 360 deg

Elliptic: any polarization not circular or plane.

B-mode polarization is much more complex and illustrated below. 

Here, the E-vector (seen in line of sight) can curl either clockwise or counter-clockwise into spiral patterns, or curliques.  But hold the phone. We have since learned that the team of astronomers that reported evidence for gravitational waves from the early Universe has evidently now withdrawn the claim. A joint analysis of data recorded by the team's BICEP2 telescope at the South Pole and by the European spacecraft Planck has revealed that the signal can be entirely attributed to dust in the Milky Way rather than having a more ancient, cosmic origin.

To grasp how significant the orginal finding was, consider this:  No telescope exists that can see anything older than the oldest light which comes from 380,000 years after the Bjg Bang or long after inflation ended.  But the BICEP2 team discovered that this light, called the CMB or “cosmic microwave background”  has a major defect or “scar” from the cosmos’ early violent expansion. This is by way of the B-mode curlicue polarization. And cosmologists had already predicted that inflation -generated gravitational waves could be detected if the right polarization was found.

Lo and behold, this B-mode version was precisely this type of pattern that the BICEP 2 detected suggesting that both cosmic inflation and primordial gravitational waves are real (since the latter are the only things known that can give rise to the B-mode polarization)  But a wrench got hurled into the mix.

Turns out that dust in the Milky Way also emits polarized light that can have the same curlicue pattern imprinted on it. And let's be clear astronomers have suggested several times since the original work that the BICEP2 team had been fooled by that Galactic signal (see 'Full-Galaxy dust map muddles search for gravitational waves').  All of which underscores again the importance of confirmation and reproducibility in any scientific endeavor. 

 Of course, since this paper there have been other instances - in other areas of science - in which results were prematurely released then withdrawn. See, for example:

Two Tessier-Lavigne papers retracted on his last day as president (stanforddaily.com)

And:

Retracted coronavirus (COVID-19) papers – Retraction Watch

And:

Top science publisher withdraws flawed climate study (phys.org)

 See Also:

BICEP2/Keck Array XI: Beam Characterization and Temperature-to-Polarization Leakage in the BK15 Data Set - IOPscience

And:

BICEP | Center for Astrophysics (harvard.edu)