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.Holes of that size, perhaps up to 1,000 times bigger, ought tohave lifetimes roughly comparable to the age of the Universe.Near theend of its life as a black hole, such an object would radiate energy at avery high rate, so that in the last tenth of a second, as it exploded, about1030 ergs of energy would be liberated.That is small beer byastronomical standards the Sun radiates 3,000 times as much energyevery second but it is still equivalent to the simultaneous explosion ofabout a million one-megaton nuclear bombs.For holes 1,000 timesbigger, a comparably greater amount of energy could be liberated in thefinal stages, and, if there are large numbers of black holes in ourGalaxy, explosions in this range would certainly be detectable.Onceagain, speculators have been quick to link this idea with the mysterious,recently discovered gamma-ray bursts.Mathematically, there is little tochoose between a black hole that has evaporated to the point where itexplodes and a retarded mini core that suddenly bursts forth as a whitehole.The calculations of Narlikar and Apparao need but littlemodification to be applied to the case of exploding black holes.In its standard form, this theory, as developed by Hawking, is of onlyminor interest in the framework of the violent activity observed on muchgreater scales in exploding galaxies, quasars, and so on.Conceptually,however, the theory is probably one of the most significant advancessince the realization that the Universe expands, and the new lines ofJohn Gribbin ElecBookWhite Holes: Cosmic Gushers in the Universe 143research it has opened up could prove the most fruitful of all in takingour understanding of the Universe a stage farther.Exploding black holescannot, however, explain the presence of the most violent objectsknown in the Universe, unless our detailed understanding of the time-scale of the Universe or of black-hole explosions is wrong.In any case,it does seem that the largest black holes would gain energy fromaccretion faster than they would radiate it, like a tub being filled from apowerful hose more quickly than the water can drain away through asmall leak.We certainly should not rule out the possibility that ourpresent understanding of these phenomena is inaccurate.Some recentspeculations provide examples of how physical speculation withinsufficient recourse to rigorous mathematics can be as seductive and aspotentially misleading as beautiful mathematical models having little orno grounding in physical reality.Depending on the elementary particle physics and quantummechanical element in a study of exploding black holes, and leavingaside the time-scale problem and the question of rapid accretion of verylarge holes, an idea of what the material blasting out from such anexplosion should be like can be built up.The most intriguing possibilityis that instead of being gamma-ray bursts at about 1030 ergs in 0.1 s,the explosions might be bursts of particles of many different kinds,releasing 1035 ergs (100,000 times more energy) in 10 23 s (100 billion-billionths of 0.1 s).The description of an event like this for times afterthat critical 10 23 s is very similar to the cosmological description of theexpansion of the Universe of particles and radiation after the initial tinyfraction of a second.Now, if you follow the Hoyle type of argument topreserve a form of Steady State theory by imagining a Universe which isSteady State on the really big scale but looks like a Big Bang on thescale of all the visible galaxies and quasars, there is room for a neat butJohn Gribbin ElecBookWhite Holes: Cosmic Gushers in the Universe 144implausible trick.In a truly infinite Steady State universe, there is infinitetime and infinite space available for a very big black hole to grow, onecontaining as much mass as the observable Universe in which we live.Speculating wildly and brushing aside quibbles about accretion, thisgrand-scale black hole could explode outward again, after a long enoughwait, producing something very much like the expanding region of spacewe see around us.It s easy to shoot down such naive speculation theaccretion problem, the question of how much mass would actually beleft when the giant hole exploded (since it would have been radiatingenergy for an awesomely long time), and so on.Wilder ideas have beenaired and have gained devotees who publish learned papers about themin respectable journals.Part of the attraction of cosmology is theopportunity for grand speculation.As you can see, some of thisspeculation is too far out to reach the pages of even a science-fictionmagazine.The importance of exploding black holes in the developmentof ideas about cosmic gushers is chiefly that their possible existenceemphasizes that the whole concept of white-hole explosions is notentirely ridiculous.The definitive answer to the question of why a whitehole should gush (or where the gusher gets its material) is not yetavailable, and it would probably justify a Nobel Prize.Meanwhile, somespeculations less far out than those outlined above could fit into ascience- fiction story and provide a plausible way of relating the white-hole creation event to some of the more rigorous calculations ofmathematicians such as Narlikar and Apparao.Tree Trunks and TrousersI first encountered the concept of space-time trousers in 1969 at theJohn Gribbin ElecBookWhite Holes: Cosmic Gushers in the Universe 145Institute of Theoretical Astronomy in Cambridge during a conference onrelativity and related topics.The highlight, for me, of that meeting was adiscussion of white holes.It was sparked by the problem of where thematerial being ejected from quasars and nuclei of objects such as N-galaxies and Seyferts comes from: could it be pouring into our Universethrough holes in space? At that meeting W.Bildich, from Gottingen,offered a neat mathematical model that included treatment of such aneffect, showing that the sudden appearance of matter in our visibleUniverse can be explained within the framework of relativity.In view ofrecent developments in the study of both black and white holes, hisideas seem even more apposite today.Since we cannot build four-dimensional models showing variations ofthe theoretical Universe in both space and time, and since our drawingsare restricted to two-dimensional paper and blackboards, astronomersuse a simplified space-time diagram in which one direction (up the pageor board) represents the flow of time and the other (across the page orboard) represents movement through space.Extending this to representspace by a series of planes at right angles to the paper, and with a littleskill in perspective drawing, it is possible to represent three space andone time dimensions as two space and one time dimensions drawn injust two dimensions.The easy way to picture the Universe in this way islike the trunk of a tree.We are somewhere in the middle, unable to seethe edges of the tree s surface.The flow of time is equivalent to movingupward through the trunk.Imagine what the space-time equivalent of afork in the tree would be.Suppose we sat in galaxy A next door toanother galaxy, B, moving up through the trunk until upon reaching thefork each galaxy moved into a separate arm of the fork [ Pobierz całość w formacie PDF ]