Bang on clusters

THE big bang cosmological model is one of the most successful models of our universe. Its efficacy in predicting the temperature of the cosmic microwave background radiation and the profusion of light elements in the universe (like helium and lithium) remains unsurpassed by any of the competing cosmological models (Nature, Vol 377,September 7, 1995).

Successful as it is, the big bang model has its own share of problems. Outstanding among them is that of the formation of structure in the universe.

The universe is made up of stars which group into galaxies. Galaxies in their turn, get together in clusters. For instance, our sun is a part of the Milky Way galaxy (which has some 100 billion other stars). The Milky Way is part of another system, a cluster of galaxies called the Local Group. The big bang model offers no natural way in which these structures could have been formed.

To create a clearer picture, we can incorporate another class of models called the inflationary universe models into the big bang scenario. According to these models, the universe - after the initial big bang - undergoes a phase of tremendous expansion. But we are still far from having a complete and satisfactory theory of understanding how matter in the universe groups into galaxies and dusters.

Now a group of researchers have ftported a new sample of clusters of pi"es which could shed some light on the formation and evolution of kqescale structure in the universe, Cknters of galaxies are visible as regions of extended x-ray emission.

Thus, one could identify clusters by etamining photographic plates and ksAing for associated galaxies. This, boonever, is not very reliable since there could be an artificial enhancement along the line of sight of the telescope (objects far apart could appear to be closer to each other). Observation of xray emission is far more trustworthy. When th@- clusters collapse under their gravitation, the inter-cluster gas is compressed and heated. The clusters' gravitation pull confines the hot gas and thus x-ray emission can be correlated to the presence of clusters.

Using the Rosat International x-ray Optical Survey (RLXOS), a project aimed at cataloguing some 400 x-ray sources, the team of scientists from England, Germany, Spain and Finland have concluded from their studies of clusters that in the recent past there were few high mass (and thus high luminosity, luminosity being a measure of the energy given out by an object) objects. In cosmology, one can look into the past by looking into the distant. This conclusion is significant because it implies that the dusters - the largest gravitationally bound systems in the universe - are forming now (compared to galaxies which formed long ago).

The x-ray emission from a cluster is also dependent on the temperature and density of the hot gas from which it is coming. Thus x-ray luminosity can be used to study the history of the 'seeds' from which the cluster grew as well as the evolution of the hot gas. Several models have been proposed to under- stand x-ray emission and its correlation with the history of the hot gas; an important finding of the recent study has been to rule out some of these models as being inconsistent with data.

Though the study is a significant step towards understanding the formation and evolution of structure in the universe, several questions remain unanswered. One of the foremost concerns the amount of matter in the universe and its composition. The nature of the matter in the universe - whether it is the well known ordinary baryonic matter or something more exotic like wimps (weakly interacting massive particles) which comprise the dark matter - necessarily influences the formation of structure.

In any case, with better observational facilities (from satellites and the Hubble Space Telescope) as well as more sophisticated computer simulations, we are gradually moving towards determining how the universe became what it is.

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