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codex (= cosmic dynamics experiment ) is a concept for an extremely stable , high - resolution optical spectrograph for the european extremely large telescope ( e - elt ) .
the science case for codex encompasses a large range of topics , including the search for exo - planets down to earth - like masses , primordial nucleosynthesis and the possible variation of fundamental constants . however , its prime science driver is the exploration of the universal expansion history by detecting and measuring the cosmological redshift drift using qso absorption lines .
this is also one of the 9 ` prominent ' science cases chosen by the e - elt science working group to be among the highlights of the entire e - elt science case .
a description of the codex project as a whole was given by @xcite .
the recognition that an ultra - stable , high - efficiency , high - resolution optical spectrograph would not only be an extremely valuable instrument for the e - elt but also for the vlt led to the development of the espresso concept (= echelle spectrograph for precision super stable observations , see l.  pasquini s contribution to these proceedings ) .
again , there are a large number of applications for such an instrument , as several other contributions to these proceedings have highlighted .
hence , there is also a very strong science case for espresso , including e.g.  detailed studies of the intergalactic medium and of stellar abundances .
however , apart from these scientific drivers , espresso will also fulfil another role : codex will represent a major development in high - resolution optical spectrographs compared to existing instruments such as uves and harps . in order to achieve its science goals codex will have to deliver an exceptional radial velocity accuracy and stability , i.e.  @xmath0  cm  s@xmath1 over a timescale of @xmath2  yr .
although the basic design concepts are already in place , several of the sub - systems needed to achieve the codex requirements do not currently exist .
however , they will be implemented in espresso for the first time .
hence , in many respects espresso will be a codex precursor instrument that will allow us to test and gain experience with the novel aspects of these instruments . here
we will discuss espresso in the context of its codex precursor role , both in a technical sense as well as with respect to the main codex science driver , which we briefly describe next .
the discovery from type ia sne that the universal hubble expansion appears to have begun accelerating at a relatively recent epoch , and its profound implications for fundamental physics have sparked an intense interest in the observational exploration of the universe s expansion history .
several methods for measuring the hubble parameter @xmath3 already exist but all of them are either geometric in nature or use the dynamics of localised density perturbations .
the simplest , cleanest and most direct method of determining the expansion history , however , is to observe the dynamics of the global robertson - walker metric itself .
one way to achieve this is by measuring the so - called redshift drift , i.e.  the tiny , systematic drift as a function of time in the redshifts of cosmologically distant sources ( fig.[vdot ] ) .
this effect is directly caused by the de- or acceleration of the universal expansion and can hence be used to determine its history .
a measurement of this effect would be able to provide evidence of the acceleration that is entirely independent of the snia results or any other cosmological observations , and that does not require any cosmological or astrophysical assumptions at all .
it would also provide @xmath3 measurements over a redshift range inaccessible by other methods .
recently , @xcite found that a @xmath4-m telescope would indeed have the photon collecting power to detect the redshift drift by monitoring the redshifts of qso absorption lines over a timescale of @xmath2 years ( fig .
[ vdot ] ) , providing a strong motivation for a codex - like instrument at the e - elt .
the uves and harps experiences have allowed us to identify a number of properties that a spectrograph must feature in order to deliver exceptional radial velocity accuracy and long - term stability , including : simultaneous wavelength calibration , a fully passive instrument with zero human access , located inside a vacuum tank which is itself located inside a nested environment in an underground facility that allows progressively more precise temperature and pressure control , and the flux - weighted timing of observations with sub - second precision .
temperature control of the ccd will be particularly important , while high system throughput will also be of the essence .
some of these concepts are already established .
however , two of the most critical aspects are also those requiring the most r&d : light scrambling and wavelength calibration .    at a resolution of @xmath5 a typical pointing accuracy of @xmath6  arcsec corresponds to an error of @xmath7  m  s@xmath1 necessitating a scrambling gain of @xmath8 in order to reach @xmath0  cm  s@xmath1 accuracy . hence , in addition to any fibre we will require a dedicated scrambling device to ensure that a photon s position on the ccd only depends on its wavelength but not on its position on the entrance aperture .
current wavelength calibration sources such as thar lamps and i@xmath9 cells are sub - optimal in several respects , their non - uniformity and lack of long - term stability being among the concerns . however , a new concept for wavelength calibration has recently emerged . a ` laser frequency comb '
system provides a series of uniformly spaced , very narrow lines whose absolute positions are known a priori with a relative precision of @xmath10@xmath11 ( see a.  manescau s contribution ; @xcite ) .
neither of the two systems above currently exist , but they would be developed for espresso .
being able to test and validate them ` on the sky ' would provide valuable experience and input for further improvements .
the scientific goals of codex are sufficiently removed from current observational reality to require validation and demonstration of feasibility of all aspects of data handling and analysis .
this includes data acquisition strategies ( e.g.  minimum and maximum viable exposure times ) , the tracking of ccd distortions , and the accuracy of flat - fielding , sky subtraction and scattered light corrections .
the extraction of the cosmological signal from the data will also require testing .
issues include how to deal with qso variability and the accuracy of the conversion to the cosmological reference frame .
in addition espresso will allow us to determine the currently unknown intrinsic widths of the narrowest metal absorption lines in order to reassess their usefulness for the drift experiment , and to look for any sources of astrophysical noise so far overlooked .
data on qsos collected with espresso for other scientific purposes would allow us to address all of the above issues .
we estimate that @xmath12 hours of observations of the brightest known qsos would provide an end - to - end system verification , from data acquisition to signal extraction , at the level of @xmath13@xmath14  cm  s@xmath1 .
since espresso will have characteristics similar to those of codex the question arises whether espresso can be used to make a start on the redshift drift experiment .
the idea is that since espresso would be available several years before codex data appropriately collected with espresso could serve as a ` zeroth ' epoch measurement , effectively extending the time baseline of the experiment for a few years , thereby improving the final result without delaying it .
[ ol_om ] shows the comparison of the cosmological constraints in the @xmath16-@xmath17 plane expected from a drift experiment using codex only ( coloured ellipses ) and codex + espresso ( solid contour ) , where we have assumed that espresso would take up operation @xmath18 years before codex .
evidently the extension of the time baseline by @xmath18 years is not enough to offset the lack of the vlt s photon collecting power compared to the e - elt : the improvement of the constraints is only quite modest , with the lower limit on @xmath16 increasing by @xmath2 per cent .
codex and espresso are concepts for ultra - stable , high - resolution spectrographs at the e - elt and vlt , respectively . 
 both instruments are well motivated by distinct sets of science drivers . 
 however , espresso will also be a stepping stone towards codex both in a scientific as well as in a technical sense . 
 here we discuss this role of espresso with respect to one of the most exciting codex science cases , i.e. the dynamical determination of the cosmic expansion history .