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resting an inpletenessof understanding in the universe at large。
when we last met edwin hubble； he had determined that nearly all the galaxies in our fieldof view are flying away from us； and that the speed and distance of this retreat are neatlyproportional： the farther away the galaxy； the faster it is moving。 hubble realized that thiscould be expressed with a simple equation； ho = v/d （where ho is the constant； v is therecessional velocity of a flying galaxy； andd its distance away from us）。 ho has been knownever since as the hubble constant and the whole as hubble鈥檚 law。 using his formula； hubblecalculated that the universe was about two billion years old； which was a little awkwardbecause even by the late 1920s it was fairly obvious that many things within the universe鈥攏ot least earth itself鈥攚ere probably older than that。 refining this figure has been an ongoingpreoccupation of cosmology。
almost the only thing constant about the hubble constant has been the amount ofdisagreement over what value to give it。 in 1956； astronomers discovered that cepheidvariables were more variable than they had thought； they came in two varieties； not one。 thisallowed them to rework their calculations and e up with a new age for the universe offrom 7 to 20 billion years鈥攏ot terribly precise； but at least old enough； at last； to embrace theformation of the earth。
in the years that followed there erupted a long…running dispute between allan sandage； heirto hubble at mount wilson； and g茅rard de vaucouleurs； a french…born astronomer based atthe university of texas。 sandage； after years of careful calculations； arrived at a value for thehubble constant of 50； giving the universe an age of 20 billion years。 de vaucouleurs wasequally certain that the hubble constant was 100。
2this would mean that the universe wasonly half the size and age that sandage believed鈥攖en billion years。 matters took a furtherlurch into uncertainty when in 1994 a team from the carnegie observatories in california；using measures from the hubble space telescope； suggested that the universe could be as littleas eight billion years old鈥攁n age even they conceded was younger than some of the starswithin the universe。 in february 2003； a team from nasa and the goddard space flightcenter in maryland； using a new； far…reaching type of satellite called the wilkinsonmicrowave anistropy probe； announced with some confidence that the age of the universe is13。7 billion years； give or take a hundred million years or so。 there matters rest； at least forthe moment。
the difficulty in making final determinations is that there are often acres of room forinterpretation。 imagine standing in a field at night and trying to decide how far away twodistant electric lights are。 using fairly straightforward tools of astronomy you can easilyenough determine that the bulbs are of equal brightness and that one is； say； 50 percent moredistant than the other。 but what you can鈥檛 be certain of is whether the nearer light is； let ussay； a 58…watt bulb that is 122 feet away or a 61…watt light that is 119 feet； 8 inches away。 ontop of that you must make allowances for distortions caused by variations in the earth鈥檚atmosphere； by intergalactic dust； contaminating light from foreground stars； and many otherfactors。 the upshot is that your putations are necessarily based on a series of nestedassumptions； any of which could be a source of contention。 there is also the problem thataccess to telescopes is always at a premium and historically measuring red shifts has beennotably costly in telescope time。 it could take all night to get a single exposure。 inconsequence； astronomers have sometimes been pelled （or willing） to base conclusionson notably scanty evidence。 in cosmology； as the journalist geoffrey carr has suggested； wehave 鈥渁 mountain of theory built on a molehill of evidence。鈥潯r as martin rees has put it：
鈥渙ur present satisfaction ＇with our state of understanding＇ may reflect the paucity of the datarather than the excellence of the theory。鈥�
this uncertainty applies； incidentally； to relatively nearby things as much as to the distantedges of the universe。 as donald goldsmith notes； when astronomers say that the galaxy m87is 60 million light…years away； what they really mean （鈥渂ut do not often stress to the generalpublic鈥潱s that it is somewhere between 40 million and 90 million light…years away鈥攏ot2you are of course entitled to wonder what is meant exactly by 〃a constant of 50〃 or 〃a constant of 100。〃 theanswer lies in astronomical units of measure。 except conversationally； astronomers dont use light…years。 theyuse a distance called the parsec （a contraction of parallax and second）； based on a universal measure called thestellar parallax and equivalent to 3。26 light…years。 really big measures； like the size of a universe； are measuredin megaparsecs： a million parsecs。 the constant is expressed in terms of kilometers per second per megaparsec。
thus when astronomers refer to a hubble constant of 50； what they really mean is 〃50 kilometers per second permegaparsec。〃 for most of us that is of course an utterly meaningless measure； but then with astronomicalmeasures most distances are so huge as to be utterly meaningless。
quite the same thing。 for the universe at large； matters are naturally magnified。 bearing allthat in mind； the best bets these days for the age of the universe seem to be fixed on a range ofabout 12 billion to 13。5 billion years； but we remain a long way from unanimity。
one interesting recently suggested theory is that the universe is not nearly as big as wethought； that when we peer into the distance some of the galaxies we see may simply bereflections； ghost images created by rebounded light。
the fact is； there is a great deal； even at quite a fundamental level； that we don鈥檛 know鈥攏otleast what the universe is made of。 when scientists calculate the amount of matter needed tohold things together； they always e up desperately short。 it appears that at least 90 percentof the universe； and perhaps as much as 99 percent； is posed of fritz zwicky鈥檚 鈥渄arkmatter鈥濃�攕tuff that is by its nature invisible to us。 it is slightly gall