A Was the LIGO team over-hasty to claim black holes confirmed?

[MaxWallis]

Since the thread In LIGO’s pulse, how much comes from BH merging/ inspiraling where I questioned the late ‘ringdown’ part of the LIGO signal, scientists have pointed out that the main pre-merging signal could indicate various types of binary compact objects, including gravastars of similar mass (~30 solar masses). In discussing this, Physics World , quotes Prof Sathyaprakash from Cardiff’s LIGO team saying that "Our signal is consistent with both the formation of a black hole and a horizonless object – we just can't tell." Now, Remo Raffini (of the Rees-Ruffini-Wheeler textbook) co-authors an arXiv paper (arXiv:1605.04767v1 [gr-qc] 16 May 2016) saying that unfortunately the signal of the merging “occurs just at the limit of the sensitivity of LIGO (so is) not sufficient to determine the astrophysical nature of GW 150914, nor to assess that it was produced by a binary black-hole merger leading to a newly formed black-hole." The Editors have invited me to start this new thread, now it’s agreed that the signal of merging is unclear and there are astrophysical contenders for the pre-merging signal.

 

[PAllen]

One may say the LIGO interpretation was conservative in that they only considered explanations involving known forms of matter and energy, modeled via GR, finding a perfect fit. They did not attempt to prove that no exotic matter models (within GR) or small modifications to GR, could produce an indistinguishable signal. Note that both stable wormholes and gravastars require large amounts of exotic matter (if GR is assumed) for which there is currently no evidence or any well founded reason to believe exist.

 

[Dale]

The Editors have invited me to start this new thread, now it’s agreed that the signal of merging is unclear and there are astrophysical contenders for the pre-merging signal

I am not sure why you chose to reference that unpublished work:
https://arxiv.org/abs/1605.04767v1

When you discussed this with the mentors you referenced a published paper:
http://arxiv.org/abs/1602.07309

 

[PAllen]

From the published paper, the following provides summary completely consistent with my earlier post:

"Horizonless compact objects require exotic matter con-
figurations and almost inevitably possesses a stable light
ring at r < 3M [27]. The latter might be associated
with various instabilities, including fragmentation and
collapse [27] and the ergoregion instability [44–47] when
the object rotates sufficiently fast. While our results
are generic, the viability of a BH mimicker depends on
the specific model, especially on its compactness and
spin [48].
The recent GW detection by aLIGO [1] enormously
strengthens the evidence for stellar-mass BHs, whose ex-
istence is already supported by various indirect observa-
tions in the electromagnetic band (cf. e.g. Refs. [49, 50]).
While BHs remain the most convincing Occam’s razor
hypothesis, it is important to bear in mind the elusive
nature of an event horizon and the challenges associated
with its direct detection."

 

[Dale]

I think that the paper was not over-hasty. The signal detected is certainly consistent with black holes, and does provide novel confirmatory evidence. Furthermore, this evidence was predicted in advance and the experiment was built to detect just such evidence.

It is certainly possible to take any experiment, in isolation, and find some alternative explanation. Which is why experiments are compared in the context of all the available information. The alternatives violate the energy conditions, so their priors are low.