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Register a new userTesting the No-Hair Theorem with Event Horizon Telescope Observations of Sagittarius A*
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The advent of the Event Horizon Telescope (EHT), a millimeter-wave very-long baseline interferometric array, has enabled spatially-resolved studies of the sub-horizon-scale structure for a handful of supermassive black holes. Among these, the supermassive black hole at the center of the Milky Way, Sagittarius A* (Sgr A*), presents the largest angular cross section. Thus far, these studies have focused upon measurements of the black hole spin and the validation of low-luminosity accretion models. However, a critical input into the analysis of EHT data is the structure of the black hole spacetime, and thus these observations provide the novel opportunity to test the applicability of the Kerr metric to astrophysical black holes. Here we present the first simulated images of a radiatively inefficient accretion flow (RIAF) around Sgr A* employing a quasi-Kerr metric that contains an independent quadrupole moment in addition to the mass and spin that fully characterize a black hole in general relativity. We show that these images can be significantly different from the images of a RIAF around a Kerr black hole with the same spin and demonstrate the feasibility of testing the no-hair theorem by constraining the quadrupolar deviation from the Kerr metric with existing EHT data. Equally important, we find that the disk inclination and spin orientation angles are robust to the inclusion of additional parameters, providing confidence in previous estimations assuming the Kerr metric based upon EHT observations. However, at present the limits upon potential modifications of the Kerr metric remain weak.
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The black hole in the center of the Milky Way, Sgr A*, has the largest mass-to-distance ratio among all known black holes in the Universe. This property makes Sgr A* the optimal target for testing the gravitational no-hair theorem. In the near future, major developments in instrumentation will provide the tools for high-precision studies of its spacetime via observations of relativistic effects in stellar orbits, in the timing of pulsars, and in horizon-scale images of its accretion flow. We explore here the prospect of measuring the properties of the black-hole spacetime using all these three types of observations. We show that the correlated uncertainties in the measurements of the black-hole spin and quadrupole moment using the orbits of stars and pulsars are nearly orthogonal to those obtained from measuring the shape and size of the shadow the black hole casts on the surrounding emission. Combining these three types of observations will, therefore, allow us to assess and quantify systematic biases and uncertainties in each measurement and lead to a highly accurate, quantitative test of the gravitational no-hair theorem.
Thanks to the release of the extraordinary EHT image of shadow attributed to the M87* supermassive black hole (SMBH), we have a novel window to assess the validity of fundamental physics in the strong-field regime. Motivated by this, we consider Johannsen & Psaltis metric parameterized by mass, spin, and an additional dimensionless hair parameter $epsilon$. This parametric framework in the high rotation regimes provides a well-behaved bed to the strong-gravity test of the no-hair theorem (NHT) using the EHT data. Incorporating the $epsilon$ into the standard Kerr spacetime enrich it in the sense that, depending on setting the positive and negative values for that, we deal with alternative compact objects: deformed Kerr naked singularity and Kerr BH solutions, respectively. Shadows associated with these two possible solutions indicate that the deformation parameter $epsilon$ affects the geometry shape of standard shadow such that it becomes more oblate and prolate with $epsilon<0$ and $epsilon>0$, respectively. By scanning the window associated with three shadow observables oblateness, deviation from circularity, and shadow diameter, we perform a numerical analysis within the range $a_*=0.9mp0.1$ of the dimensionless rotation parameter, to find the constraints on the hair parameter $epsilon$ in both possible solutions. For both possible signs of $epsilon$, we extract a variety of upper bounds that are in interplay with $a_*$. Although by approaching the rotation parameters to the extreme limit, the allowable range of both hair parameters becomes narrower, the hairy Kerr BH solution is a more promising candidate to play the role of the alternative compact object instead of the standard Kerr BH. The lack of tension between hairy Kerr BH with the current observation of the EHT shadow of the M87* SMBH carries this message that there is the possibility of NHT violation.
Black hole event horizons, causally separating the external universe from compact regions of spacetime, are one of the most exotic predictions of General Relativity (GR). Until recently, their compact size has prevented efforts to study them directly. Here we show that recent millimeter and infrared observations of Sagittarius A* (Sgr A*), the supermassive black hole at the center of the Milky Way, all but requires the existence of a horizon. Specifically, we show that these observations limit the luminosity of any putative visible compact emitting region to below 0.4% of Sgr A*s accretion luminosity. Equivalently, this requires the efficiency of converting the gravitational binding energy liberated during accretion into radiation and kinetic outflows to be greater than 99.6%, considerably larger than those implicated in Sgr A*, and therefore inconsistent with the existence of such a visible region. Finally, since we are able to frame this argument entirely in terms of observable quantities, our results apply to all geometric theories of gravity that admit stationary solutions, including the commonly discussed f(R) class of theories.
The Event Horizon Telescope is a millimeter VLBI array that aims to take the first pictures of the black holes in the center of the Milky Way and of the M87 galaxy, with horizon scale resolution. Measurements of the shape and size of the shadows cast by the black holes on the surrounding emission can test the cosmic censorship conjecture and the no-hair theorem and may find evidence for classical effects of the quantum structure of black holes. Observations of coherent structures in the accretion flows may lead to accurate measurements of the spins of the black holes and of other properties of their spacetimes. For Sgr A*, the black hole in the center of the Milky Way, measurements of the precession of stellar orbits and timing monitoring of orbiting pulsars offer complementary avenues to the gravitational tests with the Event Horizon Telescope.
We analyze gravitational-wave data from the first LIGO detection of a binary black-hole merger (GW150914) in search of the ringdown of the remnant black hole. Using observations beginning at the peak of the signal, we find evidence of the fundamental quasinormal mode and at least one overtone, both associated with the dominant angular mode ($ell=m=2$), with $3.6sigma$ confidence. A ringdown model including overtones allows us to measure the final mass and spin magnitude of the remnant exclusively from postinspiral data, obtaining an estimate in agreement with the values inferred from the full signal. The mass and spin values we measure from the ringdown agree with those obtained using solely the fundamental mode at a later time, but have smaller uncertainties. Agreement between the postinspiral measurements of mass and spin and those using the full waveform supports the hypothesis that the GW150914 merger produced a Kerr black hole, as predicted by general relativity, and provides a test of the no-hair theorem at the ${sim}10%$ level. An independent measurement of the frequency of the first overtone yields agreement with the no-hair hypothesis at the ${sim 20}%$ level. As the detector sensitivity improves and the detected population of black hole mergers grows, we can expect that using overtones will provide even stronger tests.
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