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and now there is a universeis evident proof that you can。 it may be that our universe is merely part of many largeruniverses； some in different dimensions； and that big bangs are going on all the time all overthe place。 or it may be that space and time had some other forms altogether before the bigbang鈥攆orms too alien for us to imagine鈥攁nd that the big bang represents some sort oftransition phase； where the universe went from a form we can鈥檛 understand to one we almostcan。 鈥渢hese are very close to religious questions；鈥潯r。 andrei linde； a cosmologist atstanford； told the new york times in 2001。
the big bang theory isn鈥檛 about the bang itself but about what happened after the bang。
not long after； mind you。 by doing a lot of math and watching carefully what goes on inparticle accelerators； scientists believe they can look back to 10…43seconds after the moment ofcreation； when the universe was still so small that you would have needed a microscope tofind it。 we mustn鈥檛 swoon over every extraordinary number that es before us； but it isperhaps worth latching on to one from time to time just to be reminded of their ungraspableand amazing breadth。 thus 10…43is 0。0000000000000000000000000000000000000000001； orone 10 million trillion trillion trillionths of a second。
**a word on scientific notation： since very large numbers are cumbersome to write and nearly impossible to read； scientistsuse a shorthand involving powers （or multiples） of ten in which； for instance； 10；000；000；000 is written 1010 and 6；500；000bees 6。5 x 106。 the principle is based very simply on multiples of ten： 10 x 10 （or 100） bees 102； 10 x 10 x 10 （or1；000） is 103； and so on； obviously and indefinitely。 the little superscript number signifies the number of zeroes followingthe larger principal number。 negative notations provide latter in print （especially essentially a mirror image； with thesuperscript number indicating the number of spaces to the right of the decimal point （so 10…4 means 0。0001）。 though i salutethe principle； it remains an amazement to me that anyone seeing 〃1。4 x 109 km3鈥櫋ould see at once that that signifies 1。4 most of what we know； or believe we know； about the early moments of the universe isthanks to an idea called inflation theory first propounded in 1979 by a junior particlephysicist； then at stanford； now at mit； named alan guth。 he was thirty…two years old and；by his own admission； had never done anything much before。 he would probably never havehad his great theory except that he happened to attend a lecture on the big bang given bynone other than robert dicke。 the lecture inspired guth to take an interest in cosmology； andin particular in the birth of the universe。
the eventual result was the inflation theory； which holds that a fraction of a moment afterthe dawn of creation； the universe underwent a sudden dramatic expansion。 it inflated鈥攊neffect ran away with itself； doubling in size every 10…34seconds。 the whole episode may havelasted no more than 10…30seconds鈥攖hat鈥檚 one million million million million millionths of asecond鈥攂ut it changed the universe from something you could hold in your hand tosomething at least 10；000；000；000；000；000；000；000；000 times bigger。 inflation theoryexplains the ripples and eddies that make our universe possible。 without it； there would be noclumps of matter and thus no stars； just drifting gas and everlasting darkness。
according to guth鈥檚 theory； at one ten…millionth of a trillionth of a trillionth of a trillionthof a second； gravity emerged。 after another ludicrously brief interval it was joined byelectromagnetism and the strong and weak nuclear forces鈥攖he stuff of physics。 these werejoined an instant later by swarms of elementary particles鈥攖he stuff of stuff。 from nothing atall； suddenly there were swarms of photons； protons； electrons； neutrons； and much else鈥攂etween 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鈥攁t least a hundred billion light…yearsacross； according to the theory； but possibly any size up to infinite鈥攁nd perfectly arrayed forthe creation of stars； galaxies； and other plex systems。
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鈥攊f gravity were fractionally stronger orweaker； if the expansion had proceeded just a little more slowly or swiftly鈥攖hen 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： 鈥渋n 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。鈥潯o which adds guth： 鈥渁lthough the creation of a universe might be very unlikely；tryon emphasized that no one had counted the failed attempts。鈥�
martin rees； britain鈥檚 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： 鈥渋f there is a large stock of clothing； you鈥檙e 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。鈥�
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鈥攕pecifically； in a way that converts seven one…thousandths ofits mass to energy。 lower that value very slightly鈥攆rom 0。007 percent to 0。006 percent；say鈥攁nd no transformation could take place： the universe would consist of hydrogen andnothing else。 raise the value very slightly鈥攖o 0。008 percent鈥攁nd 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鈥斺�渃ritical density鈥潯s the cosmologists鈥櫋erm for it鈥攁nd 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鈥攖hateverything 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鈥檚 notbecause it would take too long to get there鈥攖hough of course it would鈥攂ut 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鈥檛 adequately imagine； in conformance witheinstein鈥檚 theory of relativity （which we will get to in due course）。 for the moment it isenough to know that we are not adrif