hola pas si vite ! pas si vite !
Les observations de M87* permettent de mettre pas mal de théories alternatives aux TN à la poubelle, mais il existe encore des objets théoriques, issus de l'imagination d'astrophysiciens pour qui l'idée d'une singularité est une hérésie, et qui tiennent encore (un petit peu) la route : les gravastars, les étoiles à bosons (!)...
Un futur papier de l'équipe de l'EHT doit traiter de ces bestioles et comparer leur propriétés supposées avec les données recueillies par la manip
Il en est déjà question dans leur papier n°5 :
First M87 Event Horizon Telescope Results.
V. Physical Origin of the Asymmetric Ring
7.4. Alternatives to Kerr Black Holes
Although our working hypothesis has been that M87 contains a Kerr black hole, it is interesting to consider whether or not the data is also consistent with alternative models for the central object. These alternatives can be grouped into three main categories: (i) black holes within general relativity that include additional fields; (ii) black hole solutions from alternative theories of gravity or incorporating quantum effects; (iii) black hole "mimickers," i.e., compact objects, both within general relativity or in alternative theories, whose properties could be fine-tuned to resemble those of black holes.
The first category includes, for example, black holes in Einstein–Maxwell–dilaton-axion gravity (e.g., García et al. 1995; Mizuno et al. 2018), black holes with electromagnetic or Newman–Unti-Tamburino (NUT) charges (e.g., Grenzebach et al. 2014), regular black holes in nonlinear electrodynamics (e.g., Abdujabbarov et al. 2016), black hole metrics affected by a cosmological constant (e.g., Dymnikova 1992) or a dark matter halo (e.g., Hou et al. 2018), and black holes with scalar wigs (e.g., Barranco et al. 2017) or hair (e.g., Herdeiro & Radu 2014). While the shadows of this class of compact objects are expected to be similar to Kerr and therefore cannot be ruled out immediately by current observations (Mizuno et al. 2018), the most extreme examples of black holes surrounded by massive scalar field configurations should produce additional lobes in the shadow or disconnected dark regions (Cunha et al. 2015). As these features are not found in the EHT2017 image, these alternatives are not viable models for M87.
The second category comprises black hole solutions with classical modifications to general relativity, as well as effects coming from approaches to quantum gravity (see, e.g., Moffat 2015; Dastan et al. 2016; Younsi et al. 2016; Amir et al. 2018; Eiroa & Sendra 2018; Giddings & Psaltis 2018). These alternatives have shadows that are qualitatively very similar to those of Kerr black holes and are not distinguishable with present EHT capabilities. However, higher-frequency observations, together with the degree of polarization of the emitted radiation or the variability of the accretion flow, can be used to assess their viability.
Finally, the third category comprises compact objects such as spherically symmetric naked singularities (e.g., Joshi et al. 2014), superspinars (Kerr with , which are axisymmetric spacetime with naked singularities), and regular horizonless objects, either with or without a surface. Examples of regular surfaceless objects are: boson stars (Kaup 1968), traversable wormholes, and clumps of self-interacting dark matter (Saxton et al. 2016), while examples of black hole mimickers with a surface are gravastars (Mazur & Mottola 2004) and collapsed polymers (Brustein & Medved 2017), to cite only a few. Because the exotic genesis of these black hole mimickers is essentially unknown, their physical properties are essentially unconstrained, thus making the distinction from black holes rather challenging (see, however, Chirenti & Rezzolla 2007, 2016). Nevertheless, some conclusions can drawn already. For instance, the shadow of a superspinar is very different from that of a black hole (Bambi & Freese 2009), and the EHT2017 observations rule out any superspinar model for M87. Similarly, for certain parameter ranges, the shadows of spherically symmetric naked singularities have been found to consist of a filled disk with no dark region120 in the center (Shaikh et al. 2019); clearly, this class of models is ruled out. In the same vein, because the shadows of wormholes can exhibit large deviations from those of black holes (see, e.g., Bambi 2013; Nedkova et al. 2013; Shaikh 2018), a large portion of the corresponding space of parameters can be constrained with the present observations.
A comparison of EHT2017 data with the boson star model, as a representative horizonless and surfaceless black hole mimicker, and a gravastar model as a representative horizonless black hole mimicker, will be presented in Olivares et al. (2019a). Both models produce images with ring-like features similar to those observed by EHT2017, which are consistent with the results of Broderick & Narayan (2006), who also consider black hole alternatives with a surface. The boson star generically requires masses that are substantially different from that expected for M87 (H. Olivares et al. 2019b, in preparation), while the gravastar has accretion variability that is considerably different from that onto a black hole.
In summary, because each of the many exotic alternatives to Kerr black holes can span an enormous space of parameters that is only poorly constrained, the comparisons carried out here must be considered preliminary. Nevertheless, they show that the EHT2017 observations are not consistent with several of the alternatives to Kerr black holes, and that some of those models that produce similar images show rather different dynamics in the accretion flow and in its variability. Future observations and more detailed theoretical modeling, combined with multiwavelength campaigns and polarimetric measurements, will further constrain alternatives to Kerr black holes.