What if the Big Bang was really the “Big Bounce”? – Primordial inflation data may provide a clue to a unified quantum gravity. – by Chris Lee – Jul 9, 2014 5:30pm BST

Not so long ago, our very own Matthew Francis attended the press conference in which results were announced from Antarctic observatory BICEP 2. Researchers claimed that the instruments there had located the unmistakable signature of gravitational waves during primordial inflation—a period of time during which the Universe expanded at a furious rate.

But our initial article also hinted at trouble to come.

The BICEP 2 experiment measures the ratio between light scattered by gravitational waves and light scattered by everything else, which shows up in the polarization of the cosmic microwave background (CMB) radiation. BICEP 2, however, is not the only instrument that can measure the properties of the CMB. Scientists have used the Planck satellite to measure the same ratio of light scatters—and guess what? The value obtained from BICEP 2 data doesn’t agree with the value obtained from the Planck data.

Under these circumstances, we’re faced with two possibilities: either one set of experimental data has not been interpreted properly or the Universe plays by unexpected rules. These possibilities are not mutually exclusive, providing lots of room for an interesting range of explanations. Under these circumstances, theoretical physicists tend to get a bit wild around the eyes and start stocking up on food, water, paper, and pencils. Once they are in their safe place, they let their imaginations run wild…

Bounce house

A group of Chinese and Canadian physicists asked themselves if a bouncing Universe might explain both the BICEP 2 and Planck results. A bouncing Universe is a consequence of loop quantum gravity, an attempt to unify quantum mechanics and relativity. One neat feature of loop quantum gravity is that, when the Universe is dense, gravity becomes repulsive. This means that inflation occurs naturally and doesn’t require additional physics.

A secondary consequence of gravity becoming repulsive is that the Universe can’t collapse to a singularity. This implies that the Universe may not have started at the Big Bang, which (under this model) just represents the point at which the Universe was at its minimum size. At times before the Big Bang, the Universe was still collapsing from a previous expansion. In this view, the Universe is a bit like a jackalope: bounding and rebounding.

Could a bouncy Universe explain the discrepancy between BICEP 2 and Planck? As with all theoretical physics papers, the details are rather murky, but the core of the argument has to do with the fact that the Universe’s singularity is no longer a singularity.

If the Big Bang was truly a singularity, any trace of the Universe that existed before the Big Bang would have been erased in it; the singularity destroys all. However, in loop quantum gravity, the beginning of the Universe is not a singularity, and so some of the CMB has its origin in the contracting Universe that existed prior to the Big Bang.

Even this, by itself, cannot explain the discrepancy between BICEP 2 and Planck. But, according to this paper, the contraction of the Universe just before the Big Bang was slower than its expansion after the Big Bang. The contraction and expansion of the Universe modifies the spectrum of the CMB. As a result, the contribution to the CMB from before the Big Bang is different from the CMB generated by the Big Bang. Meaning that, to understand what we’re seeing, we need to separate these two contributions.

The researchers showed that the contribution from before the Big Bang suppresses the amount of power pushed into some features of the CMB (the lowest order multipoles) and, simultaneously, increases the degree to which these same modes are polarized by gravitational interactions.

You might be asking what a multipole is. If I understand it correctly, this is a way of describing the spatial distribution of the CMB. Basically, the CMB is very slightly irregular in space. Any irregular shape can be described by a series of wave-like shapes with different amplitudes and a regular frequency spacing. These amplitudes tell us how much power was radiated into a particular shape early in the Universe, which tells us a lot about what was happening at the earliest moments of the Big Bang.

The BICEP 2 experiment obtained data for the very high-order multipoles, but no data at all for the low-order multipoles. Planck, on the other hand, has great data for the high-order multipoles and very noisy data for the low-order multipoles.

The researchers claim that their model fits the two datasets better than some common standard models (known as lambda-cold dark matter models). But the lambda-CDM models are constrained by lots of data from other sources, and they simply cannot be twisted to fit the new information. This means that, if both the BICEP 2 and Planck results hold up, some of the lambda-CDM models are in trouble. Loop quantum gravity, by contrast, isn’t as well developed and has two completely unknown parameters. This gives the model the freedom to be tweaked to fit both data sets.

Unfortunately, this also means that loop quantum gravity requires some other source of data to constrain these two free parameters. At the moment, the researchers are in the position of stating that loop quantum gravity fits the existing data better and of simultaneously using that same data to determine the values of their free parameters. Once data from other sources comes in, the true test for loop quantum gravity will begin. In the meantime, I still love the idea of a bouncy cosmos.

Since its announcement, the BICEP 2 analysis has been called into question, leading Physical Review Letters to publish a warning alongside the letter reporting the BICEP 2 results. Why publish those initial results if they might be wrong? Because the work is a great first attempt, and everybody’s future results will only build on this. Science is mostly about putting down layers of bricks and mortar, rather than dropping pre-constructed buildings. The next set of results from BICEP 2, along with improved analysis of Planck data, should clarify things substantially.

Physical Review Letters, 2014, DOI: 10.1103/PhysRevLett.112.251301

Chris Lee / Chris writes for Ars Technica’s science section. A physicist by day and science writer by night, he specializes in quantum physics and optics. He lives and works in Eindhoven, the Netherlands.

Leave a Reply