pread 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 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； 鈥渟olar systems and galaxies are not expanding； and space itself is not expanding。鈥潯ather； thegalaxies are rushing apart。 it is all something of a challenge to intuition。 or as the biologist j。
b。 s。 haldane once famously observed： 鈥渢he universe is not only queerer than we suppose； itis queerer than we can suppose。鈥�
the analogy that is usually given for explaining the curvature of space is to try to imaginesomeone from a universe of flat surfaces； who had never seen a sphere； being brought toearth。 no matter how far he roamed across the planet鈥檚 surface； he would never find an edge。
he might eventually return to the spot where he had started； and would of course be utterlyconfounded to explain how that had happened。 well； we are in the same position in space asour puzzled flatlander； only we are flummoxed by a higher dimension。
just as there is no place where you can find the edge of the universe； so there is no placewhere you can stand at the center and say： 鈥渢his is where it all began。 this is the centermostpoint of it all。鈥潯e are all at the center of it all。 actually； we don鈥檛 know that for sure； wecan鈥檛 prove it mathematically。 scientists just assume that we can鈥檛 really be the center of theuniverse鈥攖hink what that would imply鈥攂ut that the phenomenon must be the same for allobservers in all places。 still； we don鈥檛 actually know。
for us； the universe goes only as far as light has traveled in the billions of years since theuniverse was formed。 this visible universe鈥攖he universe we know and can talk about鈥攊s amillion million million million （that鈥檚 1；000；000；000；000；000；000；000；000） miles across。 butaccording to most theories the universe at large鈥攖he meta…universe； as it is sometimescalled鈥攊s vastly roomier still。 according to rees； the number of light…years to the edge ofthis larger； unseen universe would be written not 鈥渨ith ten zeroes； not even with a hundred；but with millions。鈥潯n short； there鈥檚 more space than you can imagine already without going tothe trouble of trying to envision some additional beyond。
for a long time the big bang theory had one gaping hole that troubled a lot of people鈥攏amely that it couldn鈥檛 begin to explain how we got here。 although 98 percent of all thematter that exists was created with the big bang； that matter consisted exclusively of lightgases： the helium； hydrogen； and lithium that we mentioned earlier。 not one particle of theheavy stuff so vital to our own being鈥攃arbon； nitrogen； oxygen； and all the rest鈥攅mergedfrom the gaseous brew of creation。 but鈥攁nd here鈥檚 the troubling point鈥攖o forge these heavyelements； you need the kind of heat and energy of a big bang。 yet there has been only onebig bang and it didn鈥檛 produce them。 so where did they e from？
interestingly； the man who found the answer to that question was a cosmologist whoheartily despised the big bang as a theory and coined the term 鈥渂ig bang鈥潯arcastically； as away of mocking it。 we鈥檒l get to him shortly； but before we turn to the question of how we gothere； it might be worth taking a few minutes to consider just where exactly 鈥渉ere鈥潯s。
锛凤挤w。xi锛s锛╱otxt。锛ｏ集锛�
2 WELE TO THE SOLAR SYSTEMAS
銆傚皬％璇碸t*xt…澶ｅ爞
tronomers these days can do the most amazing things。 if someone struck a matchon the moon； they could spot the flare。 from the tiniest throbs and wobbles of distant starsthey can infer the size and character and even potential habitability of planets much tooremote to be seen鈥攑lanets so distant that it would take us half a million years in a spaceshipto get there。 with their radio telescopes they can capture wisps of radiation so preposterouslyfaint that the total amount of energy collected from outside the solar system by all of themtogether since collecting began （in 1951