Other Ways to Look at the Big Bang
The “Big Bang” is the popular name given for our current understanding of the origin and early evolution of the Universe. It encompasses a mathematical description of how space and time evolve, constrained by our understanding of the basic physics of matter and our observations of how the universe looks.
But for those working in the field of cosmology, there is more than one “Big Bang” model—any number of possible assumptions exist on which mathematical models can be based. Thus the work of the cosmologist is to test these alternative models, to see which ones are consistent with observations, and to suggest new observations or measurements that, in turn, may be able to provide definitive tests for the different Big Bang models. Some of this work is being carried out at the Vatican Observatory by cosmologist William Stoeger, S.J., in collaboration with cosmologists around the world.
The simplest, and thus in many ways most elegant, variations of the Big Bang model are based on assumptions about the mathematical shape of the universe first developed by four scientists—Alexander Friedmann, Georges Lemaître, Howard Percy Robertson, and Arthur Geoffrey Walker—in the early 20th century. In these models, referred to as “FLRW” for the scientists, the universe is presumed to be perfectly smooth (homogeneous) on the largest scales. Stoeger and Marcelo Araújo (Universidade Federal do Rio de Janeiro, Brazil) have been working for several years to develop a framework for observationally testing cosmological models that do not presume large-scale homogeneity and thus go beyond an FLRW interpretation of cosmological data. Using this approach one can determine whether or not the universe is indeed close to FLRW on the largest scales, instead of simply assuming that it is.
Mathematical models for the evolution of the universe after the Big Bang must always simplify the complexities of nature. What Stoeger and Araújo have been doing is to gradually add more and more complexities into previous models, bringing them a little closer to reality while seeing just how important those added wrinkles are to our understanding of how the universe evolved.
Over the past year, the researchers have been extending and improving their approach developed in previous studies. They looked into what happens if, while still assuming the universe is the same in every direction (“spherically symmetric”), it is filled with vacuum energy—the most likely type of “dark energy”—in a uniform way. This would explain the apparent acceleration of cosmic expansion, for which there is increasing strong evidence. They then constrained these models to match the data for that part of the universe that can be causally connected to our local neighborhood. Apart from being interesting in its own right—showing how that kind of data can control how the models behave—this work also illustrates how such data controls the evolution of a universe that follows the FLRW assumptions; and how to treat the general equations that control such a universe even in the non-FLRW case.
About this study: This work is reported in Araújo et al., Phys. Rev. D., 78, Issue 6, id. 063513.