Horizonless spacetimes as seen by present and next-generation Event Horizon Telescope arrays

Abstract

We study the capabilities of present and future radio very-long-baseline-interferometry arrays to distinguish black holes from horizonless spacetimes. We consider an example of a horizonless spacetime, obtained by overspinning a regular black hole. Its image is distinct from the image of a Kerr spacetime due to a second set of photon rings interior to the shadow. These photon rings cannot be directly resolved by present and even next-generation Event Horizon telescope arrays, but instead imprint themselves in horizon-scale images as excess central brightness relative to that of a black hole. We demonstrate that future arrays can detect such indirect imprints.

Toward very large baseline interferometry observations of black hole structure

“…Other situations in which the interiors of the central objects are transparent have been analyzed in [28][29][30][31]. The image features obtained in these cases are associated with photons that circle the central object but also travel through it, and are thus complementary to the ones analyzed here, as we are assuming the opposite situation in which the central object is optically thick.…”

“…While we find the assumption of optical thickness of the central object to be physically well motivated, we understand the value of adopting an agnostic perspective and analyzing the observability of the features associated with (partially) transparent central objects. The image features analyzed in [28][29][30][31] would appear in situations in which κ þ Γ þ Γ < 1, with a strength proportional to the magnitude of this deficit, and would be added to the ones analyzed here (which are present whenever Γ or Γ ∝ B are nonvanishing).…”

“…This will give us information about the experimental designs that maximize the capabilities of testing these foundational aspects. Most existing analyses in the literature have focused on specific models [26][27][28][29][30][31], while works that have attempted to keep the discussion more general using effective field theory arguments have only estimated the imprint that reflection would have on EHT images, without systematically discussing the associated image features [32]. Our work complements and extends these previous analyses, by providing a comprehensive discussion of any novel image features, as well as their observability, within a modelindependent framework in which specific models can be embedded.…”

Breaking Buchdahl: Ultracompact stars in semiclassical gravity

“…Due to the characteristics of the exterior solution (whose details are in [46]), the crossing time stays finite in the C R → 1 limit. Hence, these objects are appealing from a phenomenological point of view due to their capability of mimicking black holes, while also leading to potentially observable signatures such as gravitational-wave echoes [48,49] or additional rings in shadows [50,51].…”

Scale-invariance at the core of quantum black holes

“…Nevertheless, there is room in current experimental data for exotic compact objects as mimickers of black holes, see for instance gravastars [6,7], various versions of wormholes [8,9], fuzzballs and firewalls [10][11][12], or, more recently, 2:2 holes [13][14][15][16][17]. Potential observable signatures of horizon-like structures with a non-zero reflectivity include gravitational wave echoes associated with time-delayed signals following the main merger ringdown [18][19][20][21][22] and excess emission in the central brightness depression of a black-hole shadow [23][24][25].…”