## [1906.10825] A new cosmological probe from supermassive black hole shadows

 Authors: Jing-Zhao Qi, Xin Zhang Abstract: In this letter, we study the prospect for using the low-redshift and high-redshift black hole shadows as a new cosmological standard ruler to measure the cosmological parameters. We show that, using the low-redshift observation of the black hole shadow of M87$^\star$, the Hubble constant can be independently measured at the precision of about 11\%, with the measurement result $H_0=70\pm 8$ km s$^{-1}$ Mpc$^{-1}$, well consistent with the measurement result from the gravitational-wave standard siren observation, but with smaller error. If we assume that in the current stage for single data points the errors are all similar, a naive estimation shows that only about 30 data of the the low-redshift observation of black hole shadows can be used to improve the measurement of $H_0$ to be at the precision of about 2\%. The high-redshift observations of supermassive black hole shadows might accurately measure a parameter combination of $H_0$ and ${\Omega_{\rm m}}$, and we show by a simple simulation that the combination of the black hole shadows with the type Ia supernovae observation would give precise measurements for cosmological parameters. [PDF]  [PS]  [BibTex]  [Bookmark]

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### [1906.10825] A new cosmological probe from supermassive black hole shadows

[These comments can also be viewed as annotations on the paper via Hypothesis.]

There are three main critical points in this paper:
1) The assumption that the black hole shadow observed by the EHT is an event horizon is not correct, as explained in detail in https://arxiv.org/abs/1906.00873. The shadow feature is strongly disturbed by lensing effects and depends on the particular shape and inclination of the accretion disk, which renders the relation used to relate the observed angular size to $H_0$ inapplicable. This casts doubts on the main result of the paper.

2) Even if the real angular size of the horizon would be known, the estimation of peculiar velocities of M87 and the associated errors are given less attention than they need. 50% of the velocity relative to the Virgo centre of mass (which is small, therefore the error on H0 seems small) is assumed, but this is not the relevant quantity: an error on the absolute measured velocity of M87 is likely more around 200-300km/s which would increase the error budget considerably.

As an aside: the discussion of the angular size of black holes at high redshift is not the only relevant factor. While it is true that the size increases at very high z, at the same time the flux decreases dramatically. It is not clear how a measurement at high z could be done even assuming very futuristic technology. It would thus have been interesting to see a discussion of possible astrophysical methods to determine masses of these objects at high redshifts.

[These comments were shared with us by a member of the cosmology community. They do not necessarily reflect the opinion of the Cosmo Comments team.]