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:
recently , there has been a great deal of activity on attempts of using grbs as cosmological probes  @xcite . in the original proposal  @xcite , it has been suggested that the magnitude versus redshift plot could be extended to redshifts up to @xmath2 , via correlations found between the isotropic equivalent luminosity , @xmath3 , and two grb observables , namely the time lag ( @xmath4 )  @xcite and variability ( @xmath5 )  @xcite .
the isotropic equivalent luminosity is the inferred luminosity ( energy emitted per unit of time ) of a grb if all its energy is radiated isotropically , the time lag measures the time offset between high- and low - energy arriving grb photons , while the variability is a measure of the complexity of the grb light curve . using these correlations , one can infer two estimates of the absolute isotropic equivalent luminosity , which are combined through a weighted average . knowing the absolute isotropic equivalent luminosity together with the observed fluence yields an estimate of the luminosity distance to the grb .    unfortunately , these correlations are affected by a large statistical ( or intrinsic ) scatter .
this statistical spread affects not only the cosmological precision via its direct statistical contribution to the distance modulus uncertainty , @xmath6 , but also through the calibration uncertainty given that the suitable grb sample with known redshift is rather small . in what follows
we show that a relatively small sample of grbs with low redshifts is sufficient to greatly reduce the systematic uncertainty thanks to a more robust and precise calibration .
more recently , a new correlation has been suggested  @xcite , which is subjected to a much smaller statistical scatter .
the so - called ghirlanda relation is a correlation between the peak energy of the gamma - ray spectrum , @xmath7 ( in the @xmath8 plot ) , and the collimation - corrected energy emitted in gamma - rays , @xmath9 .
this collimation - corrected energy is a measure of the energy released by a grb taking into account that it is beamed into a narrow jet .
unlike the @xmath10 and @xmath11 relations , the ghirlanda relation is not affected by large statistical uncertainties , however , it depends on poorly constrained quantities related to the properties of the medium around the burst . indeed , to infer @xmath9 one must estimate the angular opening of the jet , which can be performed assuming a density profile for the medium around the burst ( or circum - burst medium for short ) , where a fraction @xmath12 of the fireball kinetic energy is emitted in the prompt gamma - ray phase , and where one has measured the jet break time , @xmath13 @xcite .
assuming that the circum - burst medium has a constant density , the simplest possible assumption , requires one additional parameter .
this constant density has been measured for a few bursts  @xcite , and it exhibits a wide variation from burst to burst .
another difficulty involving grbs is that they tend to occur at rather large distances , which makes it impossible to calibrate any relationship between the relevant variables in a way that is independent from the cosmological model .
the method that is usually employed consist in fitting both , the cosmological _ and _ the calibration parameters , and then use statistical techniques to remove the undesired parameters . in here , we follow a different procedure @xcite .
we consider that the luminosity distance for @xmath14 was previously measured using type ia supernovae , and divide the grbs sample in two sets ; the low redshift sample , with @xmath14 , and the high redshift one , with @xmath15 .
since the luminosity distance of grbs in the range @xmath14 is already known , one can calibrate the luminosity estimators independently of the cosmological parameters and use the high redshift sample as a probe to dark energy and dark matter models .
this method also allows us to verify whether the larger redshift range probed by grbs can compensate for the larger uncertainty associated with the distance estimates thus obtained .
we have analyzed the use of these correlations in order to study of gcg , a model that unifies the dark energy and dark matter in a single fluid @xcite through the equation of state @xmath16 , where @xmath17 and @xmath18 are positive constants .
the case @xmath19 describes the the chaplygin gas , that arises in different theoretical scenarios .
if the curvature is fixed , there are only two free variables , @xmath17 and @xmath18 , although it is more convenient to use the quantity @xmath20 instead of @xmath17 .
thus , we consider two free parameters , @xmath18 and @xmath21 . a great deal of effort has been recently devoted to constrain the gcg model parameters  @xcite , which include , for instance , supernovae  @xcite , cosmic microwave background radiation  @xcite , gravitational lensing  @xcite and cosmic topology  @xcite .
in addition to the gcg model , we also study the more conventional flat xcdm model .
likewise the gcg model , the xcdm model is also described by two free parameters , the dark energy equation of state , @xmath22 , and the fraction of non - relativistic matter , @xmath23 .
testing these models is particularly relevant since they are degenerate for redshifts @xmath24  @xcite .
let us describe here the approach based on the correlation between the isotropic luminosity , and the variability and time lag .
the @xmath10 and @xmath11 correlations are written as @xmath25 the parameters @xmath26 and @xmath27 are found through fitting of these relationships to the data points , that is , via calibration of the luminosity estimators .
given that the grb sample with measured redshifts is rather small , at present the calibration is rather poor . to test
what improvements one expects to achieve in the future , we assess the gain in calibrating these relationships with larger samples .
three mock , yet realistic , samples were generated , using the method detailed in ref .
@xcite ; these mock data sets were used to calibrate the luminosity estimators .    we find that a calibration performed with @xmath28 grbs greatly improves the previous results , decreasing @xmath6 by close to half , yielding @xmath29 .
however , by increasing the calibration sample to @xmath30 grbs the resulting improvement is just marginal , suggesting that very large calibration samples are not required .
it is worth noting that a sample of about @xmath28 grbs may be available in the near future thanks to the _ swift _ satellite .
we also find that despite the large statistical scatter , due to improvements in calibration , the uncertainty for this estimator becomes quite close to that obtained with the ghirlanda relation , @xmath31 ( c.f .
however , it is also evident that , due to this large statistical uncertainty , one can not expect to significantly reduce the observational uncertainty of the variability and time lag method beyond about @xmath32 . in a sense , one arrives at a minimum possible uncertainty plateau , beyond which any further improvement seems impossible .    ) and 400 high - redshift ( @xmath15 ) grbs , while the dashed line show the 68% cl constraints for a sample made up of 500 high - redshift grbs only .
on the left figure , the @xmath33 and @xmath34 relations have been used , while on the right one the ghirlanda relation was employed .
the degeneracy of the @xmath18 parameter is quite evident.[gcg ] ]    next , we examine how grbs fare when used to constrain both models under consideration .
the methodology is essentially the same of that used for type ia supernovae @xcite .
one starts by defining a fiducial cosmological model , described by the parameters @xmath35 , and then use the @xmath36 function , defined as @xmath37 ^ 2~,\ ] ] where @xmath38 is the dimensionless luminosity distance , to build confidence regions in parameter space .
the dimensionless luminosity distance of the grbs can be estimated using @xmath10 and @xmath11 .
somewhat against our expectation , we found that grbs are not very suited to study the gcg model .
the @xmath21 parameter can be constrained , however no limit can be imposed on @xmath18 , as shown in figure [ gcg ] .
larger samples of grbs decrease the area of the allowed parameter space , however there is no significant improvement on the constraints imposed on either parameter . we also find that using some low - redshift grbs or the ghirlanda relation does not alter these conclusions significantly .
the results for the xcdm model are , however , more promising .
we find that grbs are sensitive essentially to @xmath23 , and very weakly sensitive to @xmath39 .
the reason for this is the redshift range probed by grbs .
we have verified that when using a sample that includes 100 grbs with @xmath14 , the constraints on the xcdm model are substantially better , as depicted in fig .
it should be pointed out that this redshift dependence is not found for the gcg fiducial model .
these results were found using the minimal @xmath40 .
this uncertainty is essentially due to the statistical component , and hence it can not be reduced by better calibration or data , only through larger grb samples .    ) and 400 high - redshift ( @xmath15 ) grbs , while the dashed lines show the 68% cl constraints for a sample made up of 500 high - redshift grbs only . on the left figure ,
the @xmath33 and @xmath34 relations have been used , while on the right one the ghirlanda relation was employed .
we discuss here the use of the ghirlanda relation , which is known to be intrinsically more precise .
a drawback of this relation is its dependence on more parameters , and on how the circum - burst medium is modeled  @xcite .
the calibration testing procedure was not repeated , as the main sources of uncertainty in the ghirlanda relation are the poorly constrained values of the peak energy , jet break time and circum - burst density  @xcite .    as before ,
we find that the characteristic feature of grbs of having rather high redshifts , makes them somewhat unsuitable to study dark energy models , even the gcg one . despite the increased precision , the allowed parameter range for the gcg model
is not greatly improved when one uses the ghirlanda relation . as for the xcdm model
, one finds that results are better if one uses the ghirlanda relations , but not significantly in what concerns the dark energy component .
however , this independence on the the nature and amount of the dark energy component means that grbs can provide a estimate of @xmath23 alone , something which is not possible when using type ia supernovae .
thus , while an improvement in calibration should not greatly alter the above conclusions , it should be noted that data quality and statistics will greatly improve in the future thanks to _ swift _ and _ hete 2 _ experiments .
thus , as one expects significant improvements on the determinations of the peak energy , jet break time and circum - burst density , it is reasonable to assume that the distance modulus uncertainty for the ghirlanda relation will decrease .
]    the main conclusion of our study is that although grbs are poor dark energy probes , for @xmath15 , their luminosity distance is quite sensitive to the dominating energy density component . for the xcdm model ,
this is dark matter , and we find that the amount of dark matter can be remarkably constrained . for the gcg model , on the other hand , it turns out that what arises is a combination of the @xmath21 and @xmath18 parameters , and the data can not lift the degeneracy on @xmath18 .
actually , if @xmath41 , the hubble function for the gcg becomes @xmath42 and one can easily observe that the allowed parameter region predicted from grbs does follow the line @xmath43 .
this feature is also encountered in various phenomenological studies of the gcg , the only exception being on data from large scale structure formation  @xcite .
also , the transition into an accelerated expansion phase in a gcg universe is faster , and at a lower redshift , than for the xcdm model .
this explains why using some @xmath14 grbs improves the results for the latter model , while it does not have any impact on the former .
the sensitivity on the non - relativistic matter density in xcdm models means that grbs may have a complementary role to play with respect to type ia supernovae . to test the impact of such a joint use
, we built joint confidence regions for grbs and type ia supernovae constraints that may be imposed by the _
snap _ satellite .
it was found that grbs may play an important role in the near future if a large grb data set is built before the promised scientific bounty of _ snap _ becomes available ( see figure [ sngrb ] ) .
however , grbs should only marginally improve the constraints imposed by _
snap _ , unless the uncertainty is reduced by at least a factor of two by then .
it must be realized that these results did not take into account several other systematic sources of error , namely selection and gravitational lensing effects .
while for a tentative study , such as the one considered in this contribution , these potential sources of uncertainty may be neglected , a more careful assessment must be performed if one aims to impose robust constraints on cosmological models .
furthermore , it is relevant to point out that the used correlations are purely phenomenological and lack , so far , a theoretical explanation .
it is interesting that a new correlation has been recently proposed  @xcite which does not require any assumptions with regards to the circum - burst medium or the gamma - ray production efficiency .
we are currently in the process of assessing the potential of such a relationship and testing whether marginalization methods , such as those used for type ia supernovae , can be advantageously employed  @xcite .
we discuss the prospects of using gamma ray bursts ( grbs ) as high - redshift distance estimators , and consider their use in the study of two dark energy models , the generalized chaplygin gas ( gcg ) , a model for the unification of dark energy and dark matter , and the xcdm model , a model where a generic dark energy fluid like component is described by the equation of state , @xmath0 . 
 we find that this test yields rather disappointing results for the gcg model , being mainly sensitive to the total amount of matter present in the universe in the case of the xcdm model . 
 we also find that , within the framework of the xcdm model , a large sample of grbs ( @xmath1 ) may turn out to be quite useful to improve the forthcoming type ia supernovae data .     
 address = instituto superior tcnico , departamento de fsica + av . 
 rovisco pais , 1049 - 001 + lisboa , portugal + e - mail : orfeu@cosmos.ist.utl.pt ; paptms@ist.utl.pt