Constraining fundamental physics with the event horizon telescope

Abstract

We show how Event Horizon Telescope (EHT) observations of the supermassive object at the center of M87 can constrain deviations from General Relativity (GR) in a relatively model-independent way. We focus on the class of theories whose deviations from GR modify black holes into alternative compact objects whose properties approach those of an ordinary black hole sufficiently far from the would-be event horizon. We examine this class for two reasons: (i) they tend to reproduce black-hole expectations for astrophysical accretion disks (and so do not undermine the evidence linking black holes to active galactic nuclei); (ii) they lend themselves to a robust effective-field-theory treatment that expands in powers of ℓ/r, where ℓ is the fundamental length scale that sets the distance over which deviations from GR are significant and r is a measure of distance from the would-be horizon. At leading order the observational impact of these types of theories arise as modifications to the transmission and reflection coefficients of modes as they approach the horizon. We show how EHT observations can constrain this reflection coefficient, assuming only that the deviations from GR are small enough to be treated perturbatively. Our preliminary analysis indicates that such reflection coefficients can already be constrained to be less than of order 10% (corresponding to ℓ ≲ 100 μ m), and so can rule out some benchmark cases used when seeking black-hole echoes. The precise bounds depend on the black hole spin, as well as on detailed properties of the reflection coefficient (such as its dependence on angular direction).

Quantum Black Holes in the Sky

“…While this review article focuses on the gravitational wave echoes, as arguably the most concrete and promising signature of quantum black holes, other possible observable signatures can be (and should be) explored. For example, interactions of photons or neutrinos with near-horizon quantum structure could lead to signatures in radio images in Event Horizon Telescope observations [172], or ultra high energy neutrinos in Ice Cube observatory [173], respectively. However, these signals will be suppressed if Boltzmann reflectivity is assumed, as they have hω kT H .…”

Influence through mixing: hotspots as benchmarks for basic black-hole behaviour

“…The second class of black hole models to which our hotspot setup can be relevant are those for which probe scales, λ, and the UV cutoff length, , are both much smaller than the horizon size, but where an effective description -whether of conventional [55][56][57][58][59] or more exotic [60][61][62][63][64][65][66][67][68][69][70] physics -is envisaged to apply sufficiently near the event horizon (see figure 3). The beginnings of an EFT treatment of this kind of physics are developed in [71,72], and involves an effective 3-dimensional action defined on a membrane that shrink-wraps the world-tube a distance from the black hole event horizon. EFT methods underline that the microscopic length is a regulator scale and so drops out of all physical predictions (as regulators always do), and this makes the EFT framework particularly useful for understanding the physical significance 1 of the length-scales involved in these types of models.…”

“…In particular, the relevant physical scale involves both couplings and the intrinsic UV length scale, and so for weak coupling is often smaller than are the physical length scales of any micro-physics that may be involved[71,72].…”

The Shadow of Supertranslated Schwarzschild Black Hole

“…Theoretically, it was developed based on the works of Synge [12] and Bardeen [13]. By extracting information of a black hole from its shadow, one can tell the difference of the spacetime geometry for different black holes, and provide a way to explore properties of the black hole, such as reflection coefficients on the horizon [14], quantum structure [15,16] and naked sin-gularity [17], or test general relativity [18][19][20] and the No-hair theorem [21][22][23].…”