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orces—the stuff of physics。 these werejoined an instant later by swarms of elementary particles—the stuff of stuff。 from nothing atall； suddenly there were swarms of photons； protons； electrons； neutrons； and much else—between 1079and 1089of each； according to the standard big bang theory。
such quantities are of course ungraspable。 it is enough to know that in a single crackinginstant we were endowed with a universe that was vast—at least a hundred billion light…yearsacross； according to the theory； but possibly any size up to infinite—and perfectly arrayed forthe creation of stars； galaxies； and other pleystems。
what is extraordinary from our point of view is how well it turned out for us。 if theuniverse had formed just a tiny bit differently—if gravity were fractionally stronger orweaker； if the expansion had proceeded just a little more slowly or swiftly—then there mightnever have been stable elements to make you and me and the ground we stand on。 had gravitybeen a trifle stronger； the universe itself might have collapsed like a badly erected tent；without precisely the right values to give it the right dimensions and density and ponentparts。 had it been weaker； however； nothing would have coalesced。 the universe would haveremained forever a dull； scattered void。
this is one reason that some experts believe there may have been many other big bangs；perhaps trillions and trillions of them； spread through the mighty span of eternity； and that thereason we exist in this particular one is that this is one we could exist in。 as edward p。 tryonof columbia university once put it： “in answer to the question of why it happened； i offer themodest proposal that our universe is simply one of those things which happen from time tobillion cubic kilometers； and no less a wonder that they would choose the former over the in a book designed for the generalreader； where the example was found）。 on the assumption that many general readers are as unmathematical as i am； i will usethem sparingly； though they are occasionally unavoidable； not least in a chapter dealing with things on a cosmic scale。
time。” to which adds guth： “although the creation of a universe might be very unlikely；tryon emphasized that no one had counted the failed attempts。”
martin rees； britain’s astronomer royal； believes that there are many universes； possibly aninfinite number； each with different attributes； in different binations； and that we simplylive in one that bines things in the way that allows us to exist。 he makes an analogy witha very large clothing store： “if there is a large stock of clothing； you’re not surprised to find asuit that fits。 if there are many universes； each governed by a differing set of numbers； therewill be one where there is a particular set of numbers suitable to life。 we are in that one。”
rees maintains that six numbers in particular govern our universe； and that if any of thesevalues were changed even very slightly things could not be as they are。 for example； for theuniverse to exist as it does requires that hydrogen be converted to helium in a precise butparatively stately manner—specifically； in a way that converts seven one…thousandths ofits mass to energy。 lower that value very slightly—from 0。007 percent to 0。006 percent；say—and no transformation could take place： the universe would consist of hydrogen andnothing else。 raise the value very slightly—to 0。008 percent—and bonding would be sowildly prolific that the hydrogen would long since have been exhausted。 in either case； withthe slightest tweaking of the numbers the universe as we know and need it would not be here。
i should say that everything is just right so far。 in the long term； gravity may turn out to be alittle too strong； and one day it may halt the expansion of the universe and bring it collapsingin upon itself； till it crushes itself down into another singularity； possibly to start the wholeprocess over again。 on the other hand it may be too weak and the universe will keep racingaway forever until everything is so far apart that there is no chance of material interactions； sothat the universe bees a place that is inert and dead； but very roomy。 the third option isthat gravity is just right—“critical density” is the cosmologists’ term for it—and that it willhold the universe together at just the right dimensions to allow things to go on indefinitely。
cosmologists in their lighter moments sometimes call this the goldilocks effect—thateverything is just right。 （for the record； these three possible universes are known respectivelyas closed； open； and flat。）now the question that has occurred to all of us at some point is： what would happen if youtraveled out to the edge of the universe and； as it were； put your head through the curtains？
where would your head be if it were no longer in the universe？ what would you find beyond？
the answer； disappointingly； is that you can never get to the edge of the universe。 that’s notbecause it would take too long to get there—though of course it would—but because even ifyou traveled outward and outward in a straight line； indefinitely and pugnaciously； you wouldnever arrive at an outer boundary。 instead； you would e back to where you began （atwhich point； presumably； you would rather lose heart in the exercise and give up）。 the reasonfor this is that the universe bends； in a way we can’t adequately imagine； in conformance witheinstein’s theory of relativity （which we will get to in due course）。 for the moment it isenough to know that we are not adrift in some large； ever…expanding bubble。 rather； spacecurves； in a way that allows it to be boundless but finite。 space cannot even properly be saidto be expanding because； as the physicist and nobel laureate steven weinberg notes； “solar systems and galaxies are not expanding； and space itself is not expanding。” rather； thegalaxies are rushing apart。 it is all something of a challenge to intuition。 or as the biologist j。
b。 s。 haldane once famously observed： “the universe is not only queerer than we suppose； itis queerer than we can suppose。”
the analogy that is usually given for explaining the curvature of space is to try t