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Black hole

A black hole is an object having relativity with a strong gravitational field it is so strong that nothing can escape it — not even light.

It is defined to be a region of spacetime where escape to the outside universe is impossible. The boundary of black hole region is a surface called the event horizon. This surface is a set of points in spacetime from where nothing can move from inside the event horizon to the outside.

The existence of black holes in the universe is also supported by astronomical observation, particularly X-ray emission, X-ray binaries and active galactic nuclei. It has also been hypothesized that black holes radiate energy due to quantum mechanical effects known as Hawking radiation.

History

The concept of black hole was put forward by the English geologist John Michell in a 1784 to the Royal Society.

“If the semi-diameter of a sphere of the same density as the Sun were to exceed that of the Sun in the proportion of 500 to 1, a body falling from an infinite height towards it would have acquired at its surface greater velocity than that of light, and consequently supposing light to be attracted by the same force in proportion to its vis inertiae (inertial mass), with other bodies, all light emitted from such a body would be made to return towards it by its own proper gravity.”

Formation

General relativity says that black holes can exist, and even they will be formed in nature whenever a sufficient amount of mass is packed in a given region of space, through a process called gravitational collapse. It is because as the mass inside the given region of space increases, its gravity becomes stronger and deforms the space around it, and nothing can escape the gravity; at this point an event horizon is formed, and matter and energy must inevitably collapse to a density resulting high density which creates an event horizon around it, and a black hole.

Observation

No object beyond the event horizon of a black hole can ever escape, including light. The black holes can be detected from observation of phenomena such as gravitational lensing, galactic jets, and stars that appear to be in orbit around space and very near to the black hole where there is no visible matter. The internal viscosity of the disk makes it extremely hot, and emit large amounts of X-ray and ultraviolet radiation and can convert about 10% of the rest mass energy of an object into radiation. Other observed effects are narrow jets of particles at relativistic speeds heading along the disk’s axis.

Have we found them?

Now there is a great deal of indirect astronomical observational evidence for black holes in two mass ranges: stellar mass black holes with masses of a typical star and super massive black holes with masses ranging from on the order of 105 to 1010 solar masses. Additionally, there is some evidence that these black holes may be responsible for the emission from ultra luminous X-ray sources (ULXs).

From observations in the 1980s of motions of stars around the galactic centre, it is now believed that such supermassive black holes exist in the centre of most galaxies, including our own Milky Way. The orbits of stars within a few AU of Sagittarius A* rule out any object other than a black hole at the centre of the Milky Way assuming the current standard laws of physics are correct. Sagittarius A* is now generally agreed to be the location of a supermassive black hole at the centre of the Milky Way galaxy.

In 2002, the Hubble Telescope identified evidence for the black holes stemmed from the orbital velocity of the stars in the globular clusters; however, a group of neutron stars could cause similar observations.

Recent discoveries

In 2004, the lead scientist said that there are from two to five times as many supermassive black holes as previously predicted.

In June 2004 astronomers found a super-massive black hole, Q0906+6930, at the centre of a distant galaxy about 12.7 billion light years away.

In November 2004 a team of astronomers discovered the first intermediate-mass black hole in our Galaxy. This medium black hole of 1,300 solar masses is within a cluster of seven stars. This observation may add support to the idea that supermassive black holes grow by absorbing nearby smaller black holes and stars.

In February 2005, a blue giant star SDSS J090745.0+24507 was found to be leaving the Milky Way at twice the escape velocity .The high velocity of this star supports the hypothesis of a super-massive black hole in the centre of the galaxy.

The formation of micro black holes on Earth in particle accelerators was reported, but not yet confirmed. So far there are no observed candidates for primordial black holes.

Features and theories

Black holes require the general relativistic concept of a curved spacetime: their most striking properties rely on a distortion of the geometry of the space surrounding them

Gravitational field

The gravitational field outside a black hole is identical to the field of any other spherically symmetric object of the same mass. The objects can orbit around black holes indefinitely without getting any closer. The strange properties of spacetime only become noticeable closer to the black hole.

Rotating black holes

According to theory, of a black hole that is not spinning is spherical, and its singularity is a single point. If the black hole carries angular momentum it begins to drag space-time surrounding the event horizon in an effect known as frame-dragging. This spinning area surrounding the event horizon is called the ergosphere and has an ellipsoidal shape. Since the ergosphere is located outside the event horizon, objects can exist within the ergosphere without falling into the hole.

The singularity inside a rotating black hole is a ring. It is possible for an observer to avoid hitting this singularity, for example, by proceeding along the black hole spin axis; however, it is still not possible to escape the black hole’s event horizon.