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east 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’t 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’satmosphere； 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 “a mountain of theory built on a molehill of evidence。” or as martin rees has put it：
“our present satisfaction ＇with our state of understanding＇ may reflect the paucity of the datarather than the excellence of the theory。”
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 （“but do not often stress to the generalpublic”） is that it is somewhere between 40 million and 90 million light…years away—not2you 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’t know—notleast 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’s “darkmatter”—stuff that is by its nature invisible to us。 it is slightly galling to think that we live ina universe that； for the most part； we can’t even see； but there you are。 at least the names forthe two main possible culprits are entertaining： they are said to be either wimps （for weaklyinteracting massive particles； which is to say specks of invisible matter left over from the bigbang） or machos （for massive pact halo objects—really just another name for blackholes； brown dwarfs； and other very dim stars）。
particle physicists have tended to favor the particle explanation of wimps； astrophysiciststhe stellar explanation of machos。 for a time machos had the upper hand； but not nearlyenough of them were found； so sentiment swung back toward wimps but with the problemthat no wimp has ever been found。 because they are weakly interacting； they are （assumingthey even exist） very hard to detect。 cosmic rays would cause too much interference。 soscientists must go deep underground。 one kilometer underground cosmic bombardmentswould be one millionth what they would be on the surface。 but even when all these are addedin； “two…thirds of the universe is still missing from the balance sheet；” as one mentatorhas put it。 for the moment we might very well call them dunnos （for dark unknownnonreflective nondetectable objects somewhere）。
recent evidence suggests that not only are the galaxies of the universe racing away fromus； but that they are doing so at a rate that is accelerating。 this is counter to all expectations。 itappears that the universe may not only be filled with dark matter； but with dark energy。
scientists sometimes also call it vacuum energy or； more exotically； quintessence。 whatever itis； it seems to be driving an expansion that no one can altogether account for。 the theory isthat empty space isn’t so empty at all—that there are particles of matter and antimatterpopping into existence and popping out again—and that these are pushing the universeoutward at an accelerating rate。 improbably enough； the one thing that resolves all this iseinstein’s cosmological constant—the little piece of math he dropped into the general theoryof relativity to stop the universe’s presumed expansion； and called “the biggest blunder of mylife。” it now appears that he may have gotten things right after all。
the upshot of all this is that we live in a universe whose age we can’t quite pute；surrounded by stars whose distances we don’t altogether know； filled with matter we can’tidentify； operating in conformance with physical laws whose properties we don’t trulyunderstand。
and on that