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By Root 616 0

evidence for black holes comes from the center of our own galaxy. Infrared telescopes have allowed us to glance into this cluster of stars for some time, despite the intense light surrounding it. One can even identify single stars and follow their trajectories in this region. It turns out that they revolve around the galactic center in much the same way that planets do around the sun.11 One deduces the gravitational force acting on them from the orbital size and velocity. The force, in turn, reveals the mass concentrated in the center, and it is enormous: millions of solar masses. But then, no central object such as a star can be identified: The mass is concentrated within too small a region. This gargantuan mass is contained in a region about the size of a single star. If it were a highly compressed star, it should radiate fiercely; but that would make it visible from afar. There is no explanation other than an immense black hole as the center of the galaxy, a tough drillmaster eating its own light and forcing an army of stars, the whole galaxy, to surround it in an elegant march.

Usually, black holes are very heavy, because enough collapsing matter must be present to generate their strong gravitational force. If just a relatively small amount of matter collapses, of about the size of our sun, most of it is ejected before a black hole can form. Since the effects of quantum gravity in a large universe—as in the context of dark energy—are expected to be small, they are also weak in the neighborhood of heavy black holes. They can be large in their center, near the singularity of highest density, but this region is buried deep in the black hole, cut off from the exterior and from observations. While a black hole does not have a surface like a planet or a star, it is surrounded by a bizarre cover called the horizon. Its properties will be seen in more detail in the next chapter; for now we just keep in mind that this veil, for a massive black hole, is far away from the singularity, preventing access to strong quantum gravity effects. For possible observations, we must content ourselves with weak effects in the cover’s neighborhood.

Around the cover, there are no strong quantum effects of gravity, but those of matter do arise. The structure of space-time changes dramatically, for here we are at the boundary between the exterior, where falling into the black hole could still be prevented by sufficiently strong forces, and the interior, where this is no longer possible. Once the horizon is traversed, there is no holding back; in the classical world one is irresistibly drawn in to be shattered at the singularity. In the interior, the gravitational force is overwhelming and cannot be compensated for by any other force.

In this behavior, space-time differs from our well-known, familiar cosmic neighborhood and resembles, as it did during inflation, a trembling, shaking ground. As in the cosmology of an accelerated universe, matter is created from the vacuum: quantum fluctuations lead to a particle number differing from zero or, more intuitively, the different balances of forces within and outside the cover can tear apart particle-antiparticle pairs. One member of the pair falls into the black hole; the other one can escape and possibly hit a detector much later on. This phenomenon, discovered by Stephen Hawking in 1973, is known as the Hawking radiation of black holes.

The rate by which radiation disappears from the neighborhood of a black hole is very small for massive black holes, much less even than the weak background of cosmic microwaves. Large black holes will not be discovered by this phenomenon because their Hawking radiation simply drowns in the microwave background. For black holes already identified, the radiation is clearly too weak to show these quantum effects. Smaller black holes do have more intense radiation: For them, the cover lies closer to the central singularity and force differences in their neighborhood are more pronounced. With a sufficiently strong force, the radiation should be more intense than the cosmic