Paradigm of cosmic expansion

Study performed at USP’s Institute of Astronomy, Geophysics and Atmospheric Sciences shows that the Universe’s rate of expansion may be diminishing.

By Mônica Pileggi

Agência FAPESP – The Universe is expanding, but not necessarily as fast as indicated by the Lambda-CDM (Cold Dark Matter) model, the one most accepted by specialists. This is the idea put forth by a study realized at the Universidade de São Paulo’s Institute of Astronomy, Geophysics and Atmospheric Sciences (IAG-USP).

According to Antônio Cândido de Camargo Guimarães, author of the study published in the periodical Classical and Quantum Gravity, there was an accelerated expansion phase in recent times. “But today we are not so certain of this situation. It’s possible that the acceleration is already diminishing,” he told Agência FAPESP.

The study, which is part of the project entitled  "Investigating the distribution of dark matter through its effects as gravitational lensing" supervised by IAG Professor José Ademir Sales de Lima, received FAPESP funding under its Post-Doctoral Fellowships program.

Guimarães tells that about ten years ago, accelerated expansion of the Universe became the general consensus throughout the scientific community based on observations of 1a supernova explosions whose brilliance was less than expected. In order to describe this rapid expansion, scientists adopted the Lambda-CDM. This cosmological model is based on the existence of “dark matter,” which makes up 70% of the Universe.

“Dark matter is a very speculative physical substance. There are some hypotheses and ideas, but we don’t know what its nature is,” commented the astronomer.

In his study, Guimarães said that the idea was to describe the expansion in a manner independent of the dark energy models. To do so, he used a cosmographic approach. This method is based on the description of cosmic expansion as a sum of terms in function of supernova redshift (the measure of the velocity of separation), which is used to describe the intensity of a star’s light (indicating distance).

The supernovas were divided into three groups: old, recent, and very recent. Through these cosmographic analyses, the researcher observed that the more recent the supernova events were, the more likely the current slowing of the Universe was.

“The Lambda-CDM model says that acceleration always tends to increase. It is interesting, as our work questions this paradigm, which uses a particular form for dark energy to describe cosmic expansion,” said Guimarães.

The article Could the cosmic acceleration be transient? A cosmographic evaluation (doi:10.1088/0264-9381/28/12/125026), by Antônio Cândido de Camargo Guimarães and José Ademir Sales de Lima, may be read in Classical and Quantum Gravity at iopscience.iop.org/0264-9381/28/12/125026.