You will be presented with the full text of a science research paper. Please write an abstract for this paper. Your response should include the abstract and no additional text.

Paper text:
big - bang nucleosynthesis is an observational cornerstone of the hot big - bang cosmology . for more than two decades the predicted abundances of the light elements
d , @xmath0he , @xmath5he and @xmath1li have been used to test the consistency of the hot big - bang model at very early times ( @xmath10 ) @xcite .
the state of affairs in 1995 was summarized by a concordance interval for the baryon density , @xmath11 0.007  0.024 , for which the predicted abundances for all four light elements were consistent with the observational data @xcite .
in addition to testing the standard cosmology , bbn also gave the best determination of the baryon density and was the linchpin in the case for nonbaryonic dark matter .
the big - bang abundance of deuterium is most sensitive to the baryon density @xcite , making it the `` baryometer . ''
however , deuterium is fragile and is destroyed by stars even before they reach the main sequence .
thus , local measurements of its abundance , where about 50% of the material has been through stars , do not directly reflect its primeval abundance .
recently , the situation has changed dramatically , ushering in a new , precision era for bbn @xcite .
burles and tytler measured the deuterium abundance in high - redshift ( @xmath12 ) hydrogen clouds where almost none of the material has been processed through stars , and they have made a strong case for a primeval deuterium abundance , ( d / h)@xmath13 @xcite .    from this 10% measurement of ( d / h@xmath14 , the baryon density can be inferred to around 10% , @xmath15 , or in terms of baryon - to - photon ratio , @xmath16 . with the baryon density in hand
, one can predict the abundances of the other three light elements . then
, @xmath5he and @xmath1li can test the consistency of bbn ; d and @xmath0he can probe stellar processing since bbn ; and @xmath1li can test stellar models .
furthermore , a precise determination of the baryon density can make bbn an even sharper probe of particle physics ( e.g. , the limit to the number of light particle species ) .    to take full advantage of bbn in the precision era
requires accurate predictions .
the uncertainty in the deuterium - inferred baryon density comes in almost equal parts from the ( d / h ) measurement and theoretical error in predicting the deuterium abundance .
the bbn yields depend upon the neutron lifetime and eleven nuclear cross sections ( see table [ table1 ] ) . in 1993 ,
smith , kawano and malaney ( skm ) estimated the theoretical uncertainties by using a conservative approach @xcite . while their work has set the standard for the past five years , it was not without its shortcomings : experiments were not weighted strictly by their precision ; treatment of systematic effects was neither uniform nor explicit ; fits used theoretical rules of thumb . in addition , there have been new measurements@xcite .
after a careful analysis and updating of the microphysics for small but important effects , the theoretical uncertainty in the predicted @xmath5he abundance has been reduced to that in the neutron lifetime , @xmath17 ( 95% cl ) @xcite .
motivated by the primeval deuterium measurement , we decided to refine the error estimates for the other light elements , using the nuclear data themselves and monte - carlo realization to make our error estimates .
this method also allowed us to identify where improvements in the nuclear data would be most useful .
.for each reaction and nuclide , the energies ( in kev , center - of - mass ) at which the sensitivity functions for d and @xmath1li attain half their maximum value ; these intervals indicate the energies relevant for bbn ( @xmath18 ) . [ cols="^,^,^ " , ]
the details of our method are described in a longer paper @xcite ; here we outline the salient points .
the nuclear inputs come in the form of measurements of cross sections , @xmath19 , or equivalently , the astrophysical @xmath20-factor , @xmath21 , where @xmath22 is the coulomb - barrier tunneling probability . from these ,
the needed thermally - averaged reaction rates per particle follow @xmath23 where @xmath24 is the reduced mass .
we use monte - carlo realizations of all the experimental data sets to determine thermal reaction rates and final yields . for each realization , we proceed as follows . for every data point from every data set
we draw a value from a gaussian distribution whose mean is the central value and whose variance is the standard error reported for that point .
we account for correlated normalization error in a data set by similarly drawing a value for the overall normalization . for each reaction , a smooth representation of @xmath25
is obtained by fitting a piecewise spline to the data , with individual points weighted by their standard errors in the usual way . using the spline fits
, we evolve light - element abundances with a standard bbn code . from 25,000 such realizations ,
we produce distributions of the light - element yields and compute means and 95% cl intervals . our results , as a function of the baryon density ,
are shown in fig .
[ fig : etaplot ] .    data points and uncertainties were extracted from a comprehensive review of the experimental literature from approximately 1945 onward , beginning with a careful reading of the original sources .
we excluded a small number of data sets for which insufficient information for our technique was provided .
as always , there is the sticky problem of systematic error , especially for cross sections represented by only a few measurements .
a case in point is the reaction @xmath26 , which produces nearly all of the @xmath27 for @xmath28 .
activation measurements @xcite show an apparent disagreement with prompt - photon measurements ( see fig .
[ fig : he3ag ] and ref .
because these measurements are not in the energy range of relevance for bbn , they have little influence on our results .
( skm omitted activation measurements from their analysis altogether . )
we take them into account by performing a second monte carlo , where the prompt - photon measurements are renormalized by the weighted mean ( and uncertainty ) of the three activation measurements .
this shifts the @xmath1li / h 95% cl interval upward by 11% ( see fig .
[ fig : li7 ] ) .
our method breaks down completely for the process @xmath29 .
this is because of a near - complete lack of data at the energies relevant for bbn .
the approach used for this reaction is a constrained theoretical model that is normalized to high - precision thermal neutron capture cross - section measurements . in particular , we use the most recent evaluation , from endf - b / vi @xcite .
this evaluation was performed around 1970 ( with a minor update in 1989 ) , and it fitted a capture model to data of similar vintage for the neutron - proton system .
no documentation survives , and the uncertainty is difficult to quantify  especially in light of known systematic problems with the likely input data @xcite .
( efforts are underway to construct a new model for this reaction , based upon more modern nuclear models and data @xcite . ) for consistency , we follow skm and assign a 5% @xmath30 uncertainty in the overall normalization ( also consistent with an estimate from the evaluation s
authors @xcite ) , and we use this value for our monte - carlo calculations .    to investigate the role of each reaction independently , we ran the bbn code using the skm rates for all but one reaction , studying that reaction alone with our monte - carlo method .
this produced , for each of the eleven key reactions , a best fit to the cross - section data , 95% cl intervals for the cross sections ( fig .
[ fig : he3ag ] ) , and 95% cl uncertainties for d and @xmath1li yields for each reaction ( see figs .  [
fig : deuterium ] and [ fig : li7 ] ) .
our most important result is apparent : the uncertainty estimate from our method is a factor of two smaller than the skm estimate .
not only have we reduced the theoretical error estimate , but we have also put it on a firmer footing .
our `` most probable '' yields also differ slightly ( less than @xmath30 ) from the corresponding results of skm .
this reflects both differences in weighting the nuclear data , and the inclusion of new data .
we computed `` sensitivity functions '' for the yields of d and @xmath1li for each reaction .
these functions measure the fractional changes in yield caused by a delta - function change in cross section at a given reaction energy ( see fig .
[ fig : he3ag ] and table [ table1 ] ) .
the sensitivity functions quantify where precise cross section measurements are required .
we have reduced the theoretical error estimate for bbn deuterium production by a factor of two , so that the deuterium abundance itself dominates the uncertainty in baryon density .
the deuterium determination of the baryon density is thus sharpened , from 8% to 6% ( at @xmath30 ) , or @xmath4 ( 95% cl ) . in the next five years
, the precision of the primeval deuterium measurement should improve significantly , because the sloan digital sky survey will increase the number of qsos with measured redshifts by a factor of almost 100 , with a similar increase in the number of deuterium systems expected .
further improvement in the theoretical prediction is possible ; the key reactions in this regard are : d(p,@xmath31)@xmath0he ; d(d , p)@xmath0h above 100kev ; d(d , n)@xmath0he above 100kev ; and p(n,@xmath31)d at 30130kev ( see fig .  [
fig : deuterium ] ) . turning
the deuterium determination of the baryon density into a few percent measurement will make possible a beautiful consistency test @xcite : comparison with a similarly accurate measurement of the baryon density from cbr anisotropy .
the deuterium - inferred baryon density leads to a prediction for the big - bang @xmath5he abundance : @xmath32 ( all 95% cl ) . when the primeval @xmath5he abundance is determined to three significant figures
, this will be a powerful consistency test . at the moment , systematic effects dominate the error budget ; in particular , underlying stellar absorption in the most metal - poor hii regions . izotov and thuan s sample @xcite excludes the tainted or suspected - to - be tainted systems , and they find @xmath33 .
this is consistent with the deuterium prediction .
a less homogeneous sample @xcite , which includes some of the tainted systems , indicates a lower value , @xmath34 , which is not consistent with the deuterium prediction .
additional light particle species present around the time of bbn lead to increased @xmath5he production , and an upper limit to the primeval @xmath5he abundance can be used to constrain their existence @xcite . using @xmath33 , the deuterium - determined baryon density , and the prior @xmath35
, we derive the 95% cl limit , @xmath36 .
one should be mindful that systematic error in @xmath37 could change the limit , and that it will become more secure with better @xmath5he measurements .
finally , we turn to @xmath1li , the light element for which the uncertainty in the predicted abundance is largest .
our analysis has reduced the theoretical uncertainty by a factor of two , though a small systematic uncertainty remains . using our full monte carlo with the deuterium observations , we predict ( @xmath1li / h)@xmath38 \times 10^{-10}$ ] .
the abundance derived from old , pop ii halo stars is ( @xmath1li / h)@xmath39\times 10^{-10 } = ( 1.73\pm 0.3)\times 10^{-10}$ ] @xcite ( all at 95% cl ) .
the discrepancy could represent a real inconsistency or merely a depletion of @xmath1li by a factor of around two in these stars ( predicted by some models of stellar evolution @xcite ) . a nuclear solution for the discrepancy is unlikely
 a @xmath40 ( or @xmath41 ) change in the @xmath42d rate would be required , and the unresolved systematics of @xmath26 can only make the problem worse .
there is still much room to improve the bbn @xmath1li prediction ; the key reactions are : @xmath43 ; @xmath44 ; @xmath45 ; and @xmath46 .
perhaps the most rewarding result of this work is that we have verified what david schramm many times proclaimed , `` the predictions of bbn are very robust because the key cross section are measured at the energies where they are needed . ''
in particular , if all eleven critical cross sections were set to zero outside the intervals where they are measured , the final light - element abundances would change by less than 10% of their current theoretical uncertainty .                                      g.  m. hale , d. dodder , e. siciliano , and w. wilson , los alamos national laboratory , endf / b - vi evaluation , mat # 125 , rev .
2 , oct . 1997 ; retrieved from the endf database at the nndc online data service .
motivated by the recent measurement of the primeval abundance of deuterium , we re - examine the nuclear inputs to big - bang nucleosynthesis ( bbn ) . using monte - carlo realization of the nuclear cross - section data to directly estimate the theoretical uncertainties for the yields of d , @xmath0he and @xmath1li , we show that previous estimates were a factor of @xmath2 too large . 
 we sharpen the bbn determination of the baryon density based upon deuterium , @xmath3 ( @xmath4 ) , which leads to a predicted @xmath5he abundance , @xmath6 and a stringent limit to the equivalent number of light neutrino species : @xmath7 ( all at 95% cl ) . the predicted @xmath1li abundance , ( @xmath1li / h)@xmath8 , is higher than that observed in pop ii stars , @xmath9 ( both , 95% cl ) . 
 we identify key reactions and the energies where further work is needed .