## About this Research Topic

Einstein's theory of general relativity has predicted the most interesting cosmological object, later named black hole by John Wheeler. At the same time, black holes are known to be the simplest cosmological objects, described only by their mass (M), charge (Q), and angular momentum (J).

Black holes are centers of strong gravity such that even light cannot escape from their gravitational attraction. These objects commonly possess an event horizon at which the roles of space and time generally get switched around, leading to a dynamic spacetime even inside the event horizon of a static black hole, such as the Schwarzschild black hole. In addition to its event horizon, this type of object admits another unexpected feature which is known as spacetime singularity. This is located at the center of the black hole and it is believed that whatever falls into the event horizon eventually vanishes in the singularity. Such spacetime singularity is not very well accepted in the physics community and in some sense, it is an undesired consequence of Einstein's theory. Historically, there have been attempts to obtain a theory of gravity without singularity. Some of these efforts led to the concept of a regular black hole, where the supporting matter field is chosen to avoid a singularity at the center of the black hole.

While constructing regular black holes through the method of cut and paste was known long ago, the first regular black hole which is the direct solution of Einstein's field equation with a regular energy-momentum tensor was introduced by Bardeen, in 1968. Ayón-Beato and Garcı́a (2000) showed that Bardeen's regular black hole is powered by a particular model of nonlinear electrodynamics. Ever since regular black holes have appeared in different contexts and their physical properties have also been investigated. In spite of the development of the subject in the last few decades, several questions remain unanswered, including:

1. How realistic are models of a regular black holes?

2. How can regular black holes be distinguished through observations?

3. What are the effects of rotation in the regularity of a black hole?

4. Is the singularity of the well-known black holes such as the Schwarzschild and the Reissner-Nordstrom black holes real or can it be removed through a modified theory of gravity?

This Research Topic aims to address the questions above as well as any other related matter, such as regular black holes in higher and lower dimensions as well as in modified theories of gravity.

We welcome all article types: Brief Research Report, Hypothesis & Theory, Mini Review, Original Research, Perspective and Reviews.

**Keywords**:
Regular black holes, Einstein's gravity, Thermodynamics, Modified Theory of Gravity, Geodesics

**Important Note**:
All contributions to this Research Topic must be within the scope of the section and journal to which they are submitted, as defined in their mission statements. Frontiers reserves the right to guide an out-of-scope manuscript to a more suitable section or journal at any stage of peer review.

Black holes are centers of strong gravity such that even light cannot escape from their gravitational attraction. These objects commonly possess an event horizon at which the roles of space and time generally get switched around, leading to a dynamic spacetime even inside the event horizon of a static black hole, such as the Schwarzschild black hole. In addition to its event horizon, this type of object admits another unexpected feature which is known as spacetime singularity. This is located at the center of the black hole and it is believed that whatever falls into the event horizon eventually vanishes in the singularity. Such spacetime singularity is not very well accepted in the physics community and in some sense, it is an undesired consequence of Einstein's theory. Historically, there have been attempts to obtain a theory of gravity without singularity. Some of these efforts led to the concept of a regular black hole, where the supporting matter field is chosen to avoid a singularity at the center of the black hole.

While constructing regular black holes through the method of cut and paste was known long ago, the first regular black hole which is the direct solution of Einstein's field equation with a regular energy-momentum tensor was introduced by Bardeen, in 1968. Ayón-Beato and Garcı́a (2000) showed that Bardeen's regular black hole is powered by a particular model of nonlinear electrodynamics. Ever since regular black holes have appeared in different contexts and their physical properties have also been investigated. In spite of the development of the subject in the last few decades, several questions remain unanswered, including:

1. How realistic are models of a regular black holes?

2. How can regular black holes be distinguished through observations?

3. What are the effects of rotation in the regularity of a black hole?

4. Is the singularity of the well-known black holes such as the Schwarzschild and the Reissner-Nordstrom black holes real or can it be removed through a modified theory of gravity?

This Research Topic aims to address the questions above as well as any other related matter, such as regular black holes in higher and lower dimensions as well as in modified theories of gravity.

We welcome all article types: Brief Research Report, Hypothesis & Theory, Mini Review, Original Research, Perspective and Reviews.

**Keywords**:
Regular black holes, Einstein's gravity, Thermodynamics, Modified Theory of Gravity, Geodesics

**Important Note**:
All contributions to this Research Topic must be within the scope of the section and journal to which they are submitted, as defined in their mission statements. Frontiers reserves the right to guide an out-of-scope manuscript to a more suitable section or journal at any stage of peer review.