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Gravitational waves are a yet unproven idea... The lack of positive results from LIGO indicates these are still theoretical constructs not yet supported by experimental data. Is not the explanation of the BICEP2 analysis as primordial gravitational waves from inflation presuming the existence of a purely theoretical facet?


No one likes this... So it breaks down to the fact that it seems gravitational radiation is being used as an observational/experimental tool similar to electromagnetic radiation (EM) when the evidence of gravitational radiation is as of yet lacking (failure of LIGO's).


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closed as primarily opinion-based by Dilaton, Brandon Enright, Emilio Pisanty, Waffle's Crazy Peanut, Prahar Mar 27 at 16:30

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise.If this question can be reworded to fit the rules in the help center, please edit the question.

First sentence is entirely wrong: Hulse-Taylor binary. The 1993 Nobel prize was awarded for discovering gravitational waves. –  Kyle Kanos Mar 19 at 19:57
The primordial gravity waves that caused the B modes are far, far too faint ever to be detected by LIGO type experiments. However we do have indirect evidence for gravity waves such as the change in binary pulsar orbits. I don't think many people expected LIGO to detect anything because it's sensitivity was too low. We'd have needed some big event nearby and that isn't likely. However if Advanced LIGO fails to see anything there will be some furrowing of brows. –  John Rennie Mar 19 at 20:00
You should be very careful with the term "gravity wave", as it means something rather different. –  Emilio Pisanty Mar 19 at 20:04
@jaskey13: the BICEP2 experiment is indirect evidence just as the pulsar binary is indirect evidence. No-one is claiming any different. Of course, now we have two unrelated sets of indirect evidence ... –  John Rennie Mar 19 at 20:13
@jaskey13: no, that isn't true. Both experiments are explained by assuming the existance of gravitational waves. If the binary pulsar experiment had never been done we'd still be proposing gravitational waves as a hypothesis to explain the BICEP2 data. –  John Rennie Mar 19 at 20:19
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1 Answer 1

You are indeed correct that there are so far no direct detections of gravitational waves, and no serious scientists are claiming (apart from excusable moments of being carried away) that the BICEP result constitutes such a direct detection. However, there is absolutely no evidence that gravitational waves don't exist, and plenty of indirect evidence that they do. (Plenty? Try "GR has passed every test so far with flying colours" on for size.)

As regards the current epistemic character of the BICEP data, here's one way to phrase it. We have a complex model for a physical system - the Big Bang - which relies on a huge corpus of science. Some of it has been tested, like the mechanics of general relativity, some of it hasn't yet, like the existence of some form of quantum gravity, and some of it is only a vague conjecture, like the inflaton field. This model makes a number of predictions, and occasionally we can actually test those predictions.

The BICEP data is exactly such a test, and the data comes out in roughly good agreement with the models, though of course there is still a fair bit of leeway on the parameters. As it happens, to explain the constraints that the data place on the model, one must include gravitational waves into the mix, or one must really bend over backwards and produce some really contrived-looking model of inflation. (In fact, take that last sentence with a large grain of salt - I'm not sure at all that such contrived models do exist.) In light of that what the data means is roughly that

  • inflation is right, and gravitational waves exist, or
  • some other effect is going on within inflation that mimics gravitational waves as far as B modes are concerned, or even that
  • inflation is wrong, and there is after all some other model that explains all observations, which may not even need gravitational waves at all.

Now, I'm pretty sure that if you go on the arXiv these weeks you'll find plenty of papers exploring what other ways there could be to produce the observed B modes. It could also be that everything needs to be replaced, and if there's another model which truly fits all the data then believe me, cosmologists will love to hear of it, so bring it up if you have one. In the mean time, we try to make do with the models that we have and the data which we can measure, and we remain open to the possibility of a better model coming along. That's the way of science, more than your rather clumsy (if I may say so) attempts to define scientific rigour so far.

To put it shortly, the BICEP data simply say that the CMB does have primordial B modes. This can be extended into a solid estimate of how much energy was present in the tensor oscillations during inflation, which includes gravitational waves and more exotic phenomena. More generally, we regard it as confirmation of some aspects of the whole, complex model (which, again, contains more strictly-speaking untested science than just gravitational waves), and this makes us more confident that the model is generally correct. In particular, we hold that the detection of B modes in the CMB (and not of gravitational waves, as you suggest) constitutes good evidence of inflation. Finally, since the other ways to explain tensor modes within inflation are even more contrived, we take the data as indirect confirmation of the existence of gravitational waves, though of course we remain open to other possibilities and looking forward to experiments like Advanced LIGO which will hopefully tell for sure.

Note also, in particular, that this is the second independent indirect piece of evidence for the existence of gravitational waves. The Hulse-Taylor binary is the first, but to explain it one only needs a rather standard part of GR. This second experiment in no way relies on that evidence, and I really can't see how you think that.

It seems from the discussion in the comments that your mind is made up that scientific rigour has somehow been dropped, and it is not my intention to be drawn into a polemic with you. I post this answer in the hope that you'll accept that there are in fact subtleties which you have not yet considered, but that is in the end up to you. Your concern over the presentation of the issue to the general public is well grounded, and indeed any scientific development of this calibre tends to produce outreach articles with simplifications that are perhaps undue, and which do not reflect the current outlook of the scientific community. However, in such cases it is less helpful to simply decide that rigour has been dropped than it is to approach things with an open mind to learn what the outlook of the scientific community actually is. Just a thought...

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I like your answer.... but the first sentence in the last paragraph is dead wrong... and the association of the Hulse-Taylor binary was not presented by me nor misrepresented by me... my comments in regard to that concern the fact that the first comment to my question involved it as proof of gravitational waves... I only claimed one relied on the other in accordance to the first comment on my question as it implies relevance to what I ask. I merely attempted to dissuade that apparently much appreciated comment and misleading comment... –  jaskey13 Mar 22 at 19:22
and is the subtlety "I'm missing" that the evidence of B-mode polarization does not provide evidence of inflation unless the theoretical model of gravitational waves is taken into account? –  jaskey13 Mar 22 at 19:28
on a historical note the existence of the top quark was not claimed upon discovery of the tau and a then theoretical idea of the three generations of leptons... it took many more years before claims of the top quarks existence were made... the B-mode polarization is being presented as proof of inflation and gravitational waves... –  jaskey13 Mar 22 at 19:40
@jaskey13: the situations are not analogous. The standard model mostly works without the top quark. It is in there to make it less ugly. General relativity, which has made a large number of very precise measurements in a wide variety of circumstances, simply does not work without gravitational radiation. Indirect evidence that it exists carries more weight than indirect evidence of the top quark. I wouldn't even know how to start to reformulate GR without gravitational waves. If nothing else, fundamental causality issues would be created in any such theory. –  Jerry Schirmer Mar 22 at 20:44
Which isn't to say that LIGO (and LISA) isn't (aren't) worthwhile. Direct detection still is the gold standard. But it is, by far, the simplest explanation for gravitational phenomena that we have observed thus far to suppose that gravitational waves exist, and have not yet been observed. –  Jerry Schirmer Mar 22 at 20:46
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