Echoes | by Brian Koberlein



18 March 2014

Map of the sky by BICEP2.
BICEP2 Collaboration
Map of the sky by BICEP2.

Yesterday a analysis challenge generally known as BICEP2 introduced vital outcomes relating to cosmic inflation. The consequence facilities on what is called B-mode polarization within the cosmic microwave background. That is fairly massive information, however it’s also fairly advanced, so let’s have a look at what’s actually happening right here and why this issues.

A polarized light wave.
A polarized mild wave.

Let’s begin with the concept of polarization. Mild waves are a kind of wave generally known as transverse waves. This implies their oscillation is perpendicular to their path of journey. You possibly can see this in water waves, the place the up and down movement of the wave is perpendicular to its movement alongside the water’s floor. With mild waves (and electromagnetic waves typically), the vibration is that of the electrical and magnetic fields.

Due to this, transverse waves can have an orientation to them generally known as polarization. For those who think about a vibrating string, the waves may transfer up and down, or facet to facet, or at some angle. There are different kinds of polarization which might be extra advanced, however you get the concept.

Usually, the sunshine emitted by a scorching supply is randomly oriented, or “unpolarized”. Nonetheless if mild scatters off a cloth it may be polarized by the interplay. The ambiance does this to daylight a bit, which I’ve written about earlier than. This is the reason a great pair of polarized sun shades can remove glare. They block mild with horizontal polarization, which most reflections have. Polarization occurs in scattering when supply of sunshine isn’t uniform. For instance, polarized scattering happens within the ambiance as a result of daylight comes from the path of the Solar, not evenly throughout the sky. If daylight got here equally in all instructions, then you definitely wouldn’t see the scattering polarization.

The CMB has small variations in temperature.
The CMB has small variations in temperature.

So what does this should do with the cosmic microwave background (CMB)? The CMB is the sunshine from the earliest level the place the universe turned clear. That’s, when the universe lastly turned cool sufficient for atoms to kind. Earlier than that time, the electrons and nuclei have been too scorching to come back collectively, that means that they have been a thick plasma of charged particles. The sunshine would maintain scattering off the charged particles and couldn’t journey freely. Within the final moments of this plasma stage, when the electrons lastly began to bond with nuclei, the photons created within the massive bang would have one final scatter off an electron earlier than making its lengthy journey throughout the universe. Now if the early universe have been completely uniform, then we wouldn’t see any polarization from that final scattering. However there have been small variations within the early universe, so we might anticipate to see some polarization within the CMB. Since this final scattering occurred at a specific second within the early universe, cosmic polarization provides is a exact window into the earliest moments of the massive bang.

Different modes of CMB polarization.
Sky and Telescope
Completely different modes of CMB polarization.

Due to these small variations, the polarization of the CMB varies at totally different factors of the sky. To get an understanding of the construction of the early universe, we have to have a look at the general distribution of polarization throughout the sky. That is the place the totally different “modes” of polarization are available. You possibly can think about the polarization measurement at every level will be represented by a line. The place the traces seem to observe a path, or seem to radiate out from a specific area, then that is called E-mode polarization. The place the traces seem to twist relative to one another, that is called B-mode polarization. They’re so named as a result of their orientations are just like these easy electrical (E) or magnetic (B) fields.

These two modes are vital as a result of they’ve totally different causes. The E-mode polarization is brought on by the variations in density and temperature within the early universe, so it ought to observe the identical sample because the temperature variations seen within the cosmic background. This was first noticed in 2002 by the DASI interferometer, and the outcomes agreed with the temperature fluctuations as anticipated.

Gravitational lensing can twist E-mode into B-mode.
Gravitational lensing can twist E-mode into B-mode.

The B-mode polarization has two causes. The primary is because of gravitational lensing of the E-mode. The cosmic microwave background we see right this moment has travelled for greater than 13 billion years earlier than reaching us. Alongside its journey a few of it has handed shut sufficient to galaxies and the wish to be gravitationally lensed. This gravitational lensing twists the polarization a bit, giving a few of it a B-mode polarization. It’s this polarization that was noticed for the primary time this previous July.

However there’s one other mechanism for B-mode polarization that’s extra delicate, and requires extra information to look at. This mode is because of gravitational waves produced throughout the inflationary interval of the massive bang. You see, the cosmic background solely has small fluctuations, and we aren’t certain why. Given the scale and age of the universe at the moment, its temperature shouldn’t have been capable of even out.

The inflationary model.
Alan Guth
The inflationary mannequin.

We expect the answer is that the universe was extremely tiny in its earliest moments, however quickly entered an inflationary interval the place the observable universe expanded at a livid tempo. This is able to clarify not solely the evenness of the cosmic background, but additionally why the universe is flat. We’ve by no means seen direct proof of this inflationary interval, however we should always be capable of observe the impact of inflation in B-mode polarization. If the inflationary interval occurred, then it will have produced gravitational waves on a cosmic scale. Simply because the gravitational lensing produces B-mode polarization, the gravitational waves would produce a B-mode impact. Observe this delicate B-mode, and you’ve got direct proof of the inflationary interval.

The B-mode results. The solid line is expected for lensing alone. The dotted line is gravity waves at r = 0.2.
The B-mode outcomes. The stable line is predicted for lensing alone. The dotted line is gravity waves at r = 0.2.

Which brings us to this new work. What the staff has discovered is proof of extra B-mode polarization than anticipated by gravitational lensing alone. Since there are two methods during which B-mode polarization can happen, they wanted to point out that this “additional” was not simply resulting from lensing. They discovered that the additional B-mode polarization was seen at a five-sigma degree, which suggests (barring a 1 in 2 million fluke) the surplus is because of primordial gravitational waves. So we are able to say that primordial gravitational waves are actual.

They then analyzed the polarization to find out the energy of those waves, and that is the place issues get fascinating. Gravitational waves are measured when it comes to what is called an r issue, the place a bigger r means stronger gravitational waves and subsequently stronger inflation. Outcomes from the Planck area telescope decided that the higher restrict of r must be r < 0.11. What these new outcomes discover is r is between 0.15 and 0.27, with the very best consequence being r = 0.2. That is a lot increased than anticipated, and it conflicts with the Planck outcomes. Or at the very least it appears to. Analyzing this sort of information could be very tough, so this can be an impact of how the info is analyzed, or it might be an impact of actual physics.

So the place will we stand with all this? Though the analysis is now public, it has but to be peer reviewed. So it’s price being a bit cautious about these outcomes. The subsequent factor we’ll look in the direction of is new analysis from the Planck telescope staff. They’re presently analyzing their very own B-mode polarization information, and if this r=0.2 result’s legitimate Planck will see it as effectively. Then there’s the query of the best way to resolve the obvious battle between the preliminary Planck consequence and this new one. It also needs to be saved in thoughts that whereas these outcomes are in step with the inflationary mannequin, there are different much less well-liked fashions that haven’t been dominated out. Personally I discover the proof for inflation fairly convincing, however I could possibly be improper.

On the similar time, I don’t need to downplay this consequence an excessive amount of. The consequence could be very robust, and it factors fairly clearly at inflation. If it holds up, it means we are actually capable of examine the earliest moments of the universe. A lot sooner than we now have earlier than. In any case, the inflationary interval ended when the universe was about 10−32 of a second previous.

And we are able to see echoes of it from 13.8 billion years away.

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