Nomen non est omen Why it is too soon to identify ultra-compact objects as black holes Sebastian Murk1 2 3

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Nomen non est omen:

Why it is too soon to identify ultra-compact objects as black holes

Sebastian Murk

1, 2, 3, ∗

Okinawa Institute of Science and Technology, 1919-1 Tancha, Onna-son, Okinawa 904-0495, Japan

School of Mathematical and Physical Sciences, Macquarie University, Sydney, New South Wales 2109, Australia

Sydney Quantum Academy, Sydney, New South Wales 2006, Australia

Black holes play a pivotal role in the foundations of physics, but there is an alarming discrepancy between

what is considered to be a black hole in observational astronomy and theoretical studies. Despite claims to the

contrary, we argue that identifying the observed astrophysical black hole candidates as genuine black holes is not

justiﬁed based on the currently available observational data, and elaborate on the necessary evidence required to

support such a remarkable claim. In addition, we investigate whether the predictions of semiclassical gravity are

equally compatible with competing theoretical models, and ﬁnd that semiclassical arguments favor horizonless

conﬁgurations.

I. INTRODUCTION

Black holes are arguably the most celebrated prediction of

general relativity. Initially regarded merely as mathematical

curiosities, steady advances in the precision of astronomical

observations have gradually shifted our perception of black

holes from purely mathematical objects to potentially real phys-

ical entities. The last few decades in particular have produced

strong evidence for the existence of dark massive compact

objects commonly referred to as “astrophysical black hole can-

didates” (or simply “astrophysical black holes”). Prominent

examples include the tracking of stellar orbits [

], detec-

tion of gravitational waves by laser interferometry [

], and

imaging of the photosphere/shadow by very-long-baseline-

interferometry [

]. Nonetheless, the true physical nature of

these objects remains unknown [

–

]: contemporary mod-

els describe them either as horizonless ultra-compact objects

(UCOs) or genuine black holes with some type of horizon.

If black holes exist, we must come to terms with the conse-

quences of their existence, such as the nontrivial causal struc-

tures they inevitably introduce into the spacetime at large [

–

]. Somewhat surprisingly, despite the profound impact on

our understanding of fundamental physics, the not-so-subtle

differences between black holes and horizonless UCOs are

often ignored in contemporary scientiﬁc endeavors. It is fre-

quently asserted (see Refs.

and

for recent claims) that the

presently available results provide “direct evidence” that the

observed astrophysical black hole candidates are indeed black

holes as opposed to alternative descriptions of dark massive

compact objects. The aim of this short report is to outline the

physical implications of these claims.

The remainder of this article is organized as follows: in

Sec. II, we review the classiﬁcation scheme for different types

of UCOs, highlight their differences, and clarify what empir-

ical evidence is required to identify the observed astrophys-

ical black holes candidates as black holes. In Sec. III, we

investigate whether the predictions of semiclassical gravity

are equally compatible with competing theoretical models. In

∗sebastian.murk@oist.jp

Sec. IV, we brieﬂy summarize the observational state of affairs

and examine whether any of the characteristic properties of

distinct theoretical models can be identiﬁed with the currently

available astrophysical data. Lastly, we discuss the conse-

quences of our ﬁndings and comment on promising avenues

for future observations and theoretical considerations (Sec. V).

II. NAVIGATING THE ULTRA-COMPACT OBJECT ZOO

A. Black holes and their horizons

To assess the validity of the claim that the observed objects

are black holes, we must have a clear understanding of what

makes a black hole a black hole, and, in particular, what dis-

tinguishes a black hole from alternative descriptions of dark

massive compact objects such as horizonless UCOs. While

there is no unanimously agreed upon deﬁnition of a black hole

[

], trapping of light is a commonly accepted feature that

naturally emerges in all conventionally relevant contexts. The

notion of a trapped spacetime domain can be expressed math-

ematically using the concept of a closed trapped surface pio-

neered by Sir Roger Penrose [

]. The outermost boundary

of a trapped spacetime region, from which not even light can

escape, is popularly referred to as its horizon. A well-known

example is the event horizon; it is the boundary of the black

holes predicted by general relativity (so-called mathematical

black holes, abbrev. MBHs). Schwarzschild and Kerr black

holes are typical examples, and astrophysical observations are

typically modeled based on the Schwarzschild/Kerr paradigm.

However, it is worth pointing out that the event horizon,

while mathematically convenient, is a highly idealized global

deﬁnition of the boundary of a black hole. As such, it re-

quires knowing the entirety of spacetime, either from initial

data or by direct construction [

]. As pointed out in Ref.

this implies that “one needs to know the entire history of the

Universe, all the way into the inﬁnite future, and all the way

down to any spacelike singularity, to decide whether or not

an event horizon exists”. Consequently, it is impossible for

us as quasilocal observers (who do not have access to global

topological information) to determine the existence, presence,

or absence of an event horizon: they are not physically ob-

arXiv:2210.03750v4 [gr-qc] 14 Dec 2023

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Nomennonestomen:Whyitistoosoontoidentifyultra-compactobjectsasblackholesSebastianMurk1,2,3,∗1OkinawaInstituteofScienceandTechnology,1919-1Tancha,Onna-son,Okinawa904-0495,Japan2SchoolofMathematicalandPhysicalSciences,MacquarieUniversity,Sydney,NewSouthWales2109,Australia3SydneyQuantumAcademy,Sydney,N...

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Nomen non est omen Why it is too soon to identify ultra-compact objects as black holes Sebastian Murk1 2 3

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