757 research outputs found
Black hole shadow with a cosmological constant for cosmological observers
We investigate the effect of the cosmological constant on the angular size of
a black hole shadow. It is known that the accelerated expansion which is
created by the cosmological constant changes the angular size of the black hole
shadow for static observers. However, the shadow size must be calculated for
the appropriate cosmological observes. We calculate the angular size of the
shadow measured by cosmological comoving observers by projecting the shadow
angle to this observer rest frame. We show that the shadow size tends to zero
as the observer approaches the cosmological horizon. We estimate the angular
size of the shadow for a typical supermassive black hole, e.g M87. It is found
that the angular size of the shadow for cosmological observers and static
observers is approximately the same at these scales of mass and distance. We
present a catalog of supermassive black holes and calculate the effect of the
cosmological constant on their shadow size and find that the effect could be
for distant known sources like the Phoenix Cluster supermassive
black hole.Comment: 8 pages, 2 figures, a few typos in table numbers and text correcte
A Broad Evaluation of the Tor English Content Ecosystem
Tor is among most well-known dark net in the world. It has noble uses,
including as a platform for free speech and information dissemination under the
guise of true anonymity, but may be culturally better known as a conduit for
criminal activity and as a platform to market illicit goods and data. Past
studies on the content of Tor support this notion, but were carried out by
targeting popular domains likely to contain illicit content. A survey of past
studies may thus not yield a complete evaluation of the content and use of Tor.
This work addresses this gap by presenting a broad evaluation of the content of
the English Tor ecosystem. We perform a comprehensive crawl of the Tor dark web
and, through topic and network analysis, characterize the types of information
and services hosted across a broad swath of Tor domains and their hyperlink
relational structure. We recover nine domain types defined by the information
or service they host and, among other findings, unveil how some types of
domains intentionally silo themselves from the rest of Tor. We also present
measurements that (regrettably) suggest how marketplaces of illegal drugs and
services do emerge as the dominant type of Tor domain. Our study is the product
of crawling over 1 million pages from 20,000 Tor seed addresses, yielding a
collection of over 150,000 Tor pages. We make a dataset of the intend to make
the domain structure publicly available as a dataset at
https://github.com/wsu-wacs/TorEnglishContent.Comment: 11 page
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