The actual paper (pdf) is very heavy in error quantification - and rightly so. They presented an experiment result that is statistically extremely difficult to obtain. But for the rest of us, conclusion is the most important part. The abstract says:
The observed B-mode power spectrum is well fit by a lensed-$\lambda$CDM + tensor theoretical model with tensor/scalar ratio r = 0.20
As far as I can tell, this is their conclusion. To start to understand, we have to get a grasp on the component theories.
- Lambda-CDM_model is a cosmology model, basically
- The Cosmic Microwave Background (CMB) is the light from the last scatter from when the early universe was still opaque
- The scalar density perturbations (fluctuations) of the CMB are from quantum fluctuations in the energy universe, and these were the "seeds" that allowed galaxies to form, giving rise to the structure we live among.
- The tensor/scalar ratio (variable r) in the paper is a measure of the magnitude of tensor fluctuations of the CMB relative to the already-measured scalar fluctuations. I satisfy myself by saying the tensor fluctuations are "vector" fluctuations.
- B-mode is the type of polarization signal. Apparently another type of this signal was discovered earlier, but this is beyond my understanding.
The paper is also very clear that the r value is not 0. Their experiment proves this fact to $5.9 \sigma$. By my standards, that makes the proposition true.
The physicist who predicted this was visited by someone who worked on it, and there's a video of it online. First words said was that "it's 5 sigma at .2". The 5 sigma just means it's right. The .2 is referring to the r value above. That was the shock that the media is referencing. The fact r=.2 is new information to science here.
The blog post by another user's blog is also very informative. It's also much more heavy on the implications of the discovery. For instance, the discovery gives us much better information on the energy at which the inflation epoch took place. However, the impacts of the discovery are extraordinarily far reaching. So, I would say that the specific discovery at hand here is that r does not equal zero, and is close to 0.2.
Here are some quotes from the first part of the paper which hint at the motivations for the experiment in the first place. Emphasis mine:
Inflation predicts that the quantization of the gravitational field coupled to exponential expansion produces a primordial background of stochastic gravitational waves with a characteristic spectral shape (Grishchuk 1975; Starobinsky 1979; Rubakov et al. 1982; Fabbri & Pollock 1983; Abbott & Wise 1984; also see Krauss
& Wilczek 2013).
The wording here is crucial, note "quantization of the gravitational field". This is quantum gravity, and a theory-based prediction that led to a measured result. To me, this fact is even more incredible than getting direct evidence for gravitational waves. In fact, from my reading, this seems to be from treating the graviton's properties within the context of quantum fields.
For more detail:
Though unlikely to be directly detectable in modern instruments, these gravitational waves would have imprinted a unique signature upon the CMB. Gravitational waves induce local quadrupole anisotropies in the radiation field within the last-scattering surface, inducing polarization in the scattered light (Polnarev 1985). This polarization pattern will include a “curl” or B-mode component at degree angular scales that cannot be generated primordially by density perturbations.
This is going over how to get from gravitational waves to the polarization. I'm still a little iffy on exactly what property of gravitational waves leads to this. However, their visuals page gives a helpful hint for me. See their depiction of polarization. The "density wave" is what I had typically associated with a gravity wave. However, I recognize that a more complicated alternative is also possible. This is trivially true because general relativity uses tensors. It's the difference between pushing a slinky forward-and-back versus side-to-side. If we're talking about those side-to-side modes, then I would expect that to polarize things passing through... as opposed to just redshifting them and blueshifting them back.
For more on that:
Gravitational lensing of the CMB’s light by large scale structure at relatively late times produces small deflections of the primordial pattern, converting a small portion of E-mode power into B-modes.
This looks like it covers some of the more fine detail, but also explains why this work is set apart from previous experiments that are said to have results regarding both the E-mode and B-modes. The polarization effect, as long as it's sufficiently ancient, would seem to necessarily have come from quantum gravity effects.
I have 3 even more detailed points that I have found from various writeups of this event. These relate to what was measured, what makes BICEP2 different from other experiments, and why the experiment is so important. These specific details are:
- The pattern sought was a 45$^{\circ}$ polarization relative to the temperature (?) gradient
- Setting a lower bound on the value of r was the novel contribution of BICEP2
- A relationship called the Lyth bound calculates the time/energy of inflation from this r value
The first bullet comes from a youtube video by minute physics. They state that the density, motion, and temperature of matter at the genesis of the CMB impacts its polarization. Making no reference to gravity waves, we expect polarization at 0$^{\circ}$ and 90$^{\circ}$ relative to the temperature gradient (note: there is some confusion in the video whether this is density of temperature graident, they say one thing, but write another). They go on to say that the BICEP2 result is that about 15% of the polarization comes from the 45$^{\circ}$ "jiggles", which are tale-tale signs of gravity waves. It's much more difficult to explain why this should be true.
Next bullet - let's clarify why this matters when the same thing has been measured by previous experiments. Other experiments have estimated the r value, but those estimates are inherently clustered around r=0. This still constrains the value, but it is not effective to determine if it is non-zero, which has value for theorists. Without a doubt, this is related to my first bullet - that the critical measurement involves measuring polarization angle relative to the density gradient of a scalar field.
Third bullet, something called the Lyth bound/relationship is oft-quoted in discussions of this subject. For more reading, there is discussion on Quora. The equation is:
$$ \Delta \phi = m_p N_e \sqrt{ \frac{ r}{8} } $$
The variable $N_e$ has been measured by previous experiments. It is being cited in various places, including follow-up academic articles, that the BICEP2 result narrows down the above equation to $\Delta \phi \approx 9.6 M_{pl}$. The remaining variable is just the Planck mass. I believe that this number is interpreted to be the energy that inflation "traversed". In more practical terms, this gives us the energy/time at which inflation happened. This is where people are coming from when they mention how this result allows us to look further into the past.
