ScienceProblem: IntegratedSachsWolfeEffect

PrimaryActor:

Research astronomer

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ScienceGoal:

To measure the Integrated Sachs-Wolfe Effect, to determine the late-time evolution of the gravitational potential produced by the cosmological density field and thereby constrain cosmological models.

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DataSets:

A large galaxy catalogue with (spectroscopic or photometric) redshift information, to trace the low-redshift matter distribution, and a map of the cosmological background radiation (CBR) in the same region of sky. To address this ScienceProblem seriously will require the next generation of datasets of both types: e.g. galaxy catalogues from SDSS (and UKIDSS to extend the photometric redshift range) or, ultimately, VISTA, plus CBR datasets from (as a feasibility test) MAP or (for a proper study) Planck.

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ProblemDescription:

The cosmological background radiation is, loosely speaking, the heat left over after the Big Bang. It is very nearly isotropic (i.e. of equal intensity across the whole sky), but those low-amplitude anistropies which exist can reveal a lot about the physical conditions of the early Universe. In addition to these 'primary anisotropies' there are also a range of 'secondary anisotropies' which are caused by a variety of physical processes operating on the CBR photons since the 'epoch of last scattering', the time (about 250,000 years after the Big Bang) at which matter and radiation decoupled.

The mechanism causing one of these secondary anisotropies is called the Integrated Sachs-Wolfe (ISW) effect, and it results when the CBR photons travel through a time-varying gravitational potential: if the gravitational potential varies along the path that the photon travels from surface of last scattering to the observer, then it experiences a net change in energy as it passes through the under-/over-dense regions along that path.

In a flat, matter-dominated Universe, no secondary anisotropy is generated via the ISW, so the detection of an ISW signal provides evidence of exotic physics of some sort. Recently, interest in the ISW effect has grown, because the so-called 'Quintessence' models (e.g. Caldwell et al. 1998), proposed to explain the apparent existence of an effective cosmological constant now, generate an appreciable ISW effect, because the quintessence field comes to dominate the universe's energy budget at late times.

Since the ISW effect results from the passage of CBR photons through cosmological density perturbations, it can be detected through the cross-correlation of a CBR map and any catalogue of objects (e.g. galaxies) tracing the cosmological density field projected onto the same patch of sky. The small amplitude of this cross-correlation signal in most realistic cosmological models (e.g. Peiris and Spergel 2000) means that a large area of sky is likely to be required to detect the ISW effect, while, to use it to trace the late-time evolution of the gravitationl potential would require that this cross-correlation be peformed for a galaxy catalogue split into a series of redshift slices, thereby increasing the size of catalogue needed still further.

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CurrentSolution:

No detection of the ISW effect has been made to date. Attempts have been made to detect it - notably that of Banday et al. (1996), who cross-correlated the COBE-DMR 4-year maps with a range of tracers of the extragalactic mass distribution - but to date the available CBR maps and galaxy catalogues have not been sufficiently good for the non-detection of the ISW to be a strong constraint on cosmology.

The first of the new generation galaxy catalogues is that of the Sloan Digital Sky Survey, while the first of the new generation of CBR experiments is MAP, so their cross-correlation will be the first serious attempt to detect the ISW effect, but the analysis of Peiris and Spergel (2000) suggests that this is unlikely to yield a significant detection, so that event is likely to have to await the advent of Planck and the full SDSS or VISTA, well into the VO era.

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VOSolution:

The VO solution to attempting to measure the Integrated Sachs Wolfe Effect will initially comprise the cross-correlation of the CBR emission component map from Planck with the photometric redshift galaxy catalogue resulting from the complete Sloan Digital Sky Survey.

The Planck maps themselves will not be very large (GB in size, no more) so that this analysis is likely to involve the uploading of the relevant Planck map onto a computer system which can access a copy of the SDSS database (or some suitably compressed representation thereof). This problem, therefore, is one of a class (another example is EvolutionOfBias ) whose essence is the ability to perform computational intensive operations on large arrays generated from major survey databases, so the requirement on the VO is that it can provide significant computational resources in close network proximity to major data resources, together with allowing the user to upload code to run on those resources.

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KeyReferences:

Banday A.J., Gorski K.M., Bennett C.L., Hinshaw G., Kogut A., Smoot G.F., 1996, ApJ, 468, L85

Caldwell R.R., Dave R., Steinhardt P.J., 1998, Phys Rev Lett, 80, 1582

Peiris H.V., Spergel D.N., 2000, ApJ, 540, 605

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GoodStyle: Please add comments below. This area should be used for refinement of the above document. If you want to ask questions or start a dialogue with the author, please use (or create) a topic in the Science Problems Forum. For other ScienceProblems, refer to the ScienceProblemList.
Author: Once the refinements here and comments in the forum die down, perhaps you could rewrite the problem, incorporating the comments and refinements.

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A VO driver rather than for AG similar - requirements to the EvolutionOfBias case

-- NicholasWalton - 17 Apr 2002

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-- BobMann - 09 Feb 2002