In the media storm following the BICEP announcement that they had measured the polarization of the cosmic microwave background due to gravitational waves, Chao-Lin Kuo, member of the BICEP team was widely quoted with saying:
“This is the first direct image of gravitational waves across the primordial sky.”
As of lately, it’s been debated whether BICEP has signals from the early universe at all, or whether their signal is mostly produced by matter in our own galaxy that hasn’t been properly accounted for. This isn’t my area of research and I don’t know the details of their data analysis. Let me just say that this kind of discussion is perfectly normal to have when data are young. Whether or not they actually have seen what they claimed, it is worthwhile to sort out exactly what it would mean if the BICEP claims correct, and that is the purpose of this post.The BICEP2 results have variously been reported as the first direct evidence of cosmic inflation, direct proof of the theory of inflation, indirect evidence for the existence of gravitational waves, the first indirect detection of the gravitational wave background [emphasis theirs],the most direct evidence of Albert Einstein’s last major unconfirmed prediction, and evidence for the first detection of gravitational waves in the initial moments of the universe.
What is a direct measurement?
A direct measurement of a quantity X is if your detector measures quantity X.
One can now have a philosophical discussion about whether not human senses should account for as the actual detector. Then all measurements with external devices are indirect because they are inferred from secondary measurements, for example the reading off a display. However, for what physicists are concerned the reading of the detector by a human is irrelevant, so if you want to have this discussion, you can have it without me.
An indirect measurement is if your detector measures Y and you use a relation between X and Y to obtain X.
A Geiger-counter counts highly energetic particles as directly as it gets, but once you start thinking about it, you’ll note that we rarely measure anything directly. A common household thermometer for example does not actually measure temperature, it measures volume. A GPS device does not actually measure position, it measures the delay between signals received from different satellites and infers the position from that. Your microphone doesn’t actually measure decibel, it measures voltage. And so on.
One problem in distinguishing between direct and indirect measurements is that it’s not always so clear what is or isn’t part of the detector. Is the water in the Kamiokande tank part of the detector, or is the measurement only made in the photodetectors sourrounding the water? And is the Antarctic part of the IceCube detector?
The other problem is that in many cases scientists do not talk about quantities, they talk about concepts, ideas, hypotheses, or models. And that’s where things become murky.
What is direct evidence?
There is no clear definition for this.
You might want to extend the definition of a direct measurement to direct evidence, but this most often does not work. If you are talking about direct evidence for a particle, you can ask for the particle to hit the detector for it to be direct evidence. (Again, I am leaving aside that most detectors will amplify and process the signal before it is read out by a human because commonly the detector and data analysis are discussed separately.)
However, if you are measuring something like a symmetry violation or a decay time, then your measurement would always be indirect. What is commonly known as “direct” CP violation for example would then also be an indirect measurement since the CP violation is inferred from decay products.
In practice whether some evidence is called direct or indirect is a relative statement about the amount of assumptions that you had to use to extract the evidence. Evidence is indirect if you can think of a more direct way to make the measurement. There is some ambiguity in this which comes from the question whether the ‘more direct measurement’ must be possible in practice or in principle, but this is a problem that only people in quantum gravity and quantum foundations spend sleepless nights over...
BICEP2 is direct evidence for what?
BICEP2 has directly measured the polarization of CMB photons. Making certain assumptions about the evolution of the universe (and after subtracting the galactic foreground) this is indirect evidence for the presence of gravitational waves in the early universe, also called the relic gravitational wave background.
Direct measurement of gravitational waves is believed to be possible with gravitational wave detectors that basically measure how space-time periodically contracts and expands. The slowing down of the rotation period in pulsar systems is also indirect evidence for gravitational waves, which according to Einstein’s theory of General Relativity should carry away energy from the system. This evidence gave rise to a Nobel Prize in 1993.
Evidence for inflation comes from the presence of the gravitational wave background in the (allegedly) observed range. How can this evidence for inflation plausibly be called “direct” if it is inferred from a measurement of gravitational waves that was already indirect? That’s because we do not presently know of any evidence for inflation that would be more direct than this. Maybe one day somebody will devise a way to measure the inflaton directly in a detector, but I’m not even sure a thought experiment can do that. Until then, I think it is fair to call this direct evidence.
One should not mistake evidence for proof. We will never prove any model correct. We only collect support for it. Evidence – theoretical or experimental – is such support.
Now what about BICEP and quantum gravity?
Let us be clear that most people working on quantum gravity mean the UV-completion of the theory when they use the word ‘quantum gravity’. The BICEP2 data has the potential to rule out some models derived from these UV-completions, for example variants of string cosmology or loop quantum cosmology, and many researchers are presently very active in deriving the constraints. However, the more immediate question raised by the BICEP2 data is about the perturbative quantization of quantum gravity, that is the question whether the CMB polarization is evidence not only for classical gravitational waves, but for gravitons, the quanta of the gravitational field.
Since the evidence for gravitational waves was indirect already, the evidence for gravitons would also be indirect, though this brings up the above mentioned caveat about whether a direct detection must not only be theoretically possible, but actually be practically feasible. Direct detection of gravitons is widely believed to be not feasible.
There have been claims by Krauss and Wilzcek (which we discussed earlier here) and a 2012 paper by Ashoorioon, Dev, and Mazumdar that argues that, yes, the gravitational wave background is evidence for the quantization of gravity. The arguments in a nutshell say that quantum fluctuations of space-time are the only way the observed fluctuations could have been large enough to produce the measured spectrum.
The problems with the existing arguments is that they do not carefully track the assumptions that go into it. They do for example make assumptions about the coupling between gravity and matter fields being the usual coupling. That is plausible of course, but these are couplings at energy densities higher than we have ever tested. They also assume, rather trivially, that space-time exists to begin with. If one has a scenario in which space-time comes into being by some type of geometric phase transition, as is being suggested in some approaches to quantum gravity, one might have an entirely different mechanism for producing fluctuations. Many emergent and induced gravity approaches to quantum gravity tend not to have gravitons, which raises the question of whether these approaches could be ruled out with the BICEP data. Alas, I am not aware of any prediction for the gravitational wave background coming from these approaches, so clearly there is a knowledge gap here.
What we would need to make the case that gravity must have been perturbatively quantized in the early universe is a cosmic version of Bell’s theorem. An argument that demonstrates that no classical version of gravity would have been able to produce the observations. The power of Bell’s inequality is not in proving quantum mechanics right - this is not possible. The power of of Bell’s inequality (or measuring violations thereof respectively) is in showing that a local classical, ie “old fashioned”, theory can not account for the observations and something has to give. The present arguments about the CMB polarization are not (yet) that stringent.
This means that the BICEP2 result is strong support for the quantization of gravity, but it does not presently rule out the option that gravity is entirely classical. Though, as we discussed earlier, this option is hard to make sense of theoretically, it is infuriatingly difficult to get rid of experimentally.
The BICEP2 data, if it holds up to scrutiny, is indirect evidence for the relic gravitational wave background. It is not the first indirect evidence for gravitational waves, but the first indirect evidence for this gravitational wave background that was created in the early universe. I think it is fair to say that it is direct evidence for inflation, but the terminology is somewhat ambiguous. It is indirect evidence for the perturbative quantization of gravity, but cannot presently rule out the option that gravity was never quantized at all.