It strikes me that the recent announcement of data from BICEP2 contains two really Big Deals:

  • the first evidence of gravitational waves
  • the first evidence of inflation.

Is there also a third?

  • the first observation that will require quantum gravity to explain?

Will the BICEP2 data, or data from any other experiment, require quantum gravity to explain? (I'm thinking here of other unexplained phenomena, like dark matter, etc )

Update, two days after OP:

Today Lawrence Krauss, Alan Guth, and John Kovac appeared on NPR's Science Friday. Krauss called the event a triple discovery, and a trifecta, providing first direct evidence of all three of these things. Guth said that he agreed. But I understand all the comments and answers; it seems what constitutes "direct evidence" is in the eye of the beholder.

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    $\begingroup$ I wouldn't say it's the first evidence of gravitational waves. For example, it was already known that the period of the Hulse-Taylor binary pulsar decreases 76 microseconds per year, which is in good agreement with the prediction form energy loss via gravitational waves. $\endgroup$
    – DavePhD
    Mar 19 '14 at 14:45
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    $\begingroup$ It's not the first evidence of gravitational waves or of inflation. It is the first evidence of gravitational waves from inflation, which implies a host of other things: a GUT-scale potential during inflation, a field range of more than M_Planck, extremely strong constraints on axions or other light scalar fields--I think it'll take quite a while to sort out all the implications. $\endgroup$
    – Matt Reece
    Mar 19 '14 at 15:31
  • $\begingroup$ There is a recent paper arguing that it primordial tensor modes would necessarily be an observation that requires quantum gravity to explain arxiv.org/abs/1309.5343 [of course there are many before that :)]. I think it's safe to say that if there were primordial gravitational waves from inflation, these have a quantum origin. $\endgroup$
    – Andrew
    Mar 19 '14 at 15:34
  • $\begingroup$ Sociology note: LIGO alone has spent more than a decade and the better part of a billion dollars looking for gravitational waves. Congress does not spend resources like that without there being some very good evidence already known. $\endgroup$
    – user10851
    Mar 20 '14 at 21:13
  • $\begingroup$ In 1981 Page and Geilker published "Indirect Evidence for Quantum Gravity" Phys. Rev. Let. vol. 27, 979-982, where they performed experiments with Geiger counters and a Cavendish balance, but I don't think this experiment has been widely accepted as evidence. $\endgroup$
    – DavePhD
    Mar 21 '14 at 14:11

As mentioned, this is not the first evidence for gravitational waves. The data from BICEP2 shows that there is a much higher amount of B-mode polarization than what is predicted by gravitational lensing alone. According to theory, this could only be due to higher amplitude tensor modes in the CMB than previously observed (or rather, lack of observed). These tensor modes are only caused by long wavelength gravitational waves that originated during inflation. Effectively, if it is right, this data rules out several alternative theories of inflation and theories on the nature of the early universe (it was my understanding that the Ekpyrotic model of the universe would be ruled out, but I may be mistaken so don't quote me on that)

So what this data points to is the first evidence for primordial gravitational waves. This agrees very well with elementary models of inflation and is only in slight tension with Planck data over the size of the slow-roll parameter for slow-roll inflation. This is not the first evidence for inflation. Rather, it is evidence that further agrees with inflation and rules out some of the non-inflation theories.

As far as I have seen, the results do not yet indicate that quantum gravity will be needed to explain them; however, I think there are some theories of quantum gravity that these result might support. Let me be clear because it was pointed out that what I said was misleading. Quantum gravity is the most promising method of explaining the results, but some models of inflation allow for other classical phenomena to generate primordial gravity waves. So while quantum gravity is preferred (especially among physicists that specialize in it), it is not yet required to explain it. That is, this is not solid evidence for quantum gravity since it does not rule out the possibility of classical mechanisms, but it can be taken optimistically as preliminary evidence. As for whether some other experiment will require quantum gravity to explain it, I can't speculate.

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    $\begingroup$ Look at this motls.blogspot.com/2014/03/… post by prof. Liam McAllister , implication3:"the tensor perturbations responsible for primordial B-mode polarization are the result of quantum fluctuations of the two polarization modes of the graviton. To belabor the point: the inflationary prediction is derived by promoting the fluctuations of the gravitational field to operators, imposing canonical commutation relations, specifying the vacuum state, and computing the correlation functions. The tensor fluctuations write quantum gravity on the sky. $\endgroup$
    – anna v
    Mar 20 '14 at 18:43
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    $\begingroup$ @annav Perhaps I should rewrite that part of my answer, but let me clarify here: We certainly can use quantum gravity to explain this, which is a good sign in itself for the field; however, as prof. McAllister pointed out in the next paragraph of his post, quantum gravity is not the only way to generate primordial gravity waves, it is simply the most promising at present. So when I answered the question of whether or not the results required quantum gravity to explain them, I wrote "no, it is not yet needed". I still say that is true; it is not needed but it is preferred. $\endgroup$
    – Jim
    Mar 20 '14 at 19:09

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