In the flash of stellar explosions seen halfway back to the big bang= , two groups of astronomers have read clues to the future of the universe. With= the orbiting Hubble Space Telescope and ground-based observatories, they have= analyzed light from these remote cataclysms to estimate their distances a= nd determine how fast the stars were rushing away from Earth billions of yea= rs ago when they exploded. Their goal is to learn how the universe's expansion r= ate has changed over time--whether it has been slowed by gravity, or perhaps boos= ted by large-scale repulsive forces. The groups, longtime rivals, have been work= ing independently, but their results agree: The universe's expansion rate has= slowed so little that gravity will never be able to stop it.
The new results imply that the universe contains far less mass than = many theorists had hoped: less than 80%25 of the amount that would be needed t= o slow its expansion to a halt, and perhaps far less than that. The results even= leave open the possibility that a so-called cosmological constant--a hypothetic= al property of empty space that might generate repulsive forces--is at work,= giving the universe an expansive antigravity boost. "The results are very exciti= ng and the method is very promising," says Neta Bahcall of Princeton University.=
Bahcall points out that the small numbers of stellar explosions, or supernovae, analyzed by the groups mean that the conclusions are not defi= nitive. But the agreement between the two results, coming on the heels of other h= ints of a low-density universe, has many cosmologists taking them seriously. And = because the supernova technique directly measures how the makeup of the universe = is affecting its evolution, says astrophysicist Michael Bolte at the Univers= ity of California, Santa Cruz, "I think this is the surest way to make some of t= hese measurements."
Both groups stress that they need to analyze more supernovae to redu= ce the uncertainty in their results, reported in two just-completed papers. One = of the papers, by the supernova Cosmology Project led by Saul Perlmutter of Lawr= ence Berkeley National Laboratory and the University of California, Berkeley, = is in press at Nature. The other, by the High-Z Supernova Search Team led by Br= ian Schmidt of Mount Stromlo and Siding Spring Observatory in Australia, was = under review at Astrophysical Journal Letters as Science went to press but is p= ublicly available on the Los Alamos National Lab electronic preprint server (http://xxx.lanl.gov).
If the results hold up as the groups add more supernovae to their sa= mples, they could have a major impact on how theorists picture the universe's fi= rst few moments. Already, as word of these developments makes its way through the= astrophysics community, the findings are adding to a growing sense that t= he simplest version of the reigning cosmic creation theory, known as inflati= on, may not work. Inflation traces key features of the universe to a burst of exponential growth in the first fraction of a second after the big bang, = and its simplest version predicts a universe that contains just enough matter for= gravity to stop the big-bang expansion after an infinite time--a mass den= sity that would make the large-scale geometry of space-time "flat."
"What we are finding is that matter cannot be the only source of a f= lat universe," says Peter Garnavich of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, lead author of the High-Z Super= nova Team's paper. The results still leave an opening for some theories in whi= ch matter plus its equivalent in energy, supplied by the cosmological consta= nt, add up to a flat universe. But that picture is far less palatable to most astronomers. "If I were a theorist, I'd be getting worried at this stage,= " says Alexei Filippenko of Berkeley, a co-author on the Garnavich team's paper.=
Candles in the dark
Both groups are looking for clues to the fate of the universe by ext= ending a simple line, which plots the distance of far-off objects against their velocity as they are swept from Earth by cosmic expansion. Nearby, within= a few hundred million light-years, astronomers already know that the line is st= raight. The recession velocity of galaxies increases steadily with distance from = Earth, implying that space itself is expanding at the same rate everywhere. But = objects seen at greater distances, billions of light-years away, emitted their li= ght much earlier in cosmic history. The line should subtly bend at great dist= ances, and the bending should reveal how gravity or a cosmological constant has = changed the expansion rate over time.
Measuring how fast an object in the distant universe is flying away = from Earth is straightforward: Just determine the "redshift" of its light, a stretching of its wavelengths analogous to the drop in pitch of a recedin= g train's whistle. Measuring distance is another matter, requiring objects = that can be seen far out in the universe and have a roughly constant intrinsic= brightness, so that their apparent brightness can be taken as a distance indicator. That's where the exploding stars called type Ia supernovae ent= er the picture.
Type Ia's are thought to be white dwarf stars that suck material fro= m a companion star until they blow up as a star-sized hydrogen bomb. "They're= very bright, so you can see them across the universe," says Michael Turner of = the University of Chicago. And because white dwarfs all have about the same m= ass, all type Ia's should have roughly the same intrinsic brightness, fuming t= hem into appealing "standard candles."
Both teams now have identified dozens of type-Ia supernovae in the d= istant universe using an efficient discovery technique developed by the Perlmutt= er group. Researchers compare survey images of the same regions of sky, made= weeks apart. A computer "subtracts" one image from the other, and any new point= of light in the hundreds of galaxies in each image pair jumps out. Then the = teams go to large ground-based observatories like the 10-meter Keck Telescope i= n Hawaii or, lately, to the Hubble Space Telescope. There they confirm that= the bright spot really is a new type Ia, measure its redshift, and record its= light curve as it brightens to a peak and then declines over the following mont= hs.
Collecting those measurements is just the beginning. Because type Ia= 's don't reach exactly the same peak brightness, "they certainly are not ide= al standard candles," says Mario Hamuy of the University of Arizona, a co-au= thor of the Garnavich paper. Fortunately, he adds, "we can correct for the variat= ions." Mark Phillips of the Cerro Tololo Inter-American Observatory in Chile, co= -author Adam Riess of Berkeley, Hamuy, and others have shown that the "light curv= e" declines more slowly for intrinsically brighter supernovae. By studying a= bout 30 nearby supernovae, Hamuy and Riess tightened up the relationship so that observers can use it to correct each new supernova's brightness.
Both groups have used these data to calibrate their supernovae. They= also have had to beware of other factors that might prevent the explosions fro= m serving as perfect standard candles--for example, the dimming of their li= ght by interstellar dust between a supernova and Earth. "You spend a lot of time= making sure you get [the corrections] right," says Perlmutter of the Berkeley La= b.
Last year, Perlmutter and his colleagues in the Supernova Cosmology = Project finally worked through the corrections for a handful of supernovae observ= ed from the ground. The supernovae had redshifts of up to about 0.4--a few billio= n light-years away. When the researchers plotted brightness against distanc= e, the line had a slope that was broadly consistent with a flat universe, contai= ning a full complement of matter (Science, 4 April, p. 37). But the game changed= just after those results became public, when both teams were granted observing= time on the Hubble.
"The Hubble is a much more precise instrument than ground-based tele= scopes for measuring these light curves," explains Gerson Goldhaber of Berkeley = and the Berkeley Lab, a co-author of the Nature paper. Hubble observations of a supernova over its months-long fade aren't plagued by moonlight scattered= in the atmosphere. And Hubble's high resolution makes it much better at separati= ng the light of a supernova from the shine of its host galaxy.
Universe without end
When the Supernova Cosmology Project added just one Hubble supernova= to its sample, at a redshift of 0.83 (a distance of roughly 7 billion light-year= s), the future of the universe began to look different. The data are now most consistent, says Perlmutter, with a universe containing far less than the= critical density of matter. If the universe is flat, matter may account f= or only 40%25 to 80%25 of the critical density, with the cosmological constant ma= king up the rest. If the universe lacks any cosmological constant, the supernovae imp= ly that the mass density of the universe, known as omega-matter, is still lower, = and the universe is destined to expand forever.
Those conclusions match those of Garnavich and his colleagues, who a= nalyzed Hubble observations of three type Ia's--the most distant at a redshift of= 0.97--and one seen only from the ground. Their analysis suggests still lo= wer mass densities for a flat universe (10%25 to 70%25), but the error bars f= or the two sets of results easily overlap. "The fact that we're coming out with [omega-matter] numbers that are pretty consistent and low is very excitin= g," says Filippenko of Berkeley.
Even so, Perlmutter says, kernels of doubt remain. Although the spec= trum from his most distant event is "strikingly similar" to those of nearby supernovae, he says, the mix of galactic environments may have been quite= different at those remote times. Such differences might have an effect on= the relationship between the light curve and the peak brightness. And then th= ere are the small numbers of supernovae behind the conclusions. Four Hubble event= s, says Berkeley's Marc Davis, who was not involved in the work, "are not enough = to define the solution to a long-standing cosmological puzzle."
But Davis adds "I think it's fairly clear this is going to be the wa= y to do it." Chicago's Turner agrees: "For me, the exciting thing is what's to co= me. We have a very promising standard candle, and we have two groups [using] it;= this is the tip of the iceberg." The teams say they are seeking more Hubble ti= me and now are analyzing many more ground-based observations.
And it escapes no one's attention that these first conclusions fall = broadly in line with an increasing number of other observational hints that the c= osmic mass density may be low. One of the most recent appeared in the 20 August= Astrophys. J. Lett., where Princeton's Bahcall and two colleagues showed = that massive clusters of galaxies have changed little over recent cosmic histo= ry, implying that large-scale gravitational forces are feeble and pointing to= a matter density of just 40%25 of the critical value. Other hints of a low-= density universe emerge from computer simulations of how different mass densities= would affect the formation of giant clusters of galaxies, and from searches for= invisible dark matter in our cosmic neighborhood. "If you look at the observational data, they all suggest a low density," says Bahcall.
Inflation can be modified to cope with a low-mass universe, says And= rei Linde, a theorist at Stanford University who helped develop the theory. B= ut "at some point you can't patch a theory too much before it gets too ugly to a= ccept," says Bolte of Santa Cruz. "That's what's going to come under fire, I thin= k: whether inflation is the correct model or not for the early universe."
With those debates still to come, along with plenty more supernovae,= "it's early times, my friend," says Princeton University's Jim Peebles. "You sh= ouldn't start paying off your bets."
Article Dated 12-DEC-97
COPYRIGHT 1997 American Association for the Advancement of Science