The 2011 Nobel Prize in physics, awarded just a few weeks ago, went to research on the light from Type 1a supernovae, which shows that the universe is expanding at an accelerating rate. The well-known problem resulting from these observations is that this expansion seems to be occurring even faster than all known forms of energy could allow. While there is no shortage of proposed explanations – from dark energy to modified theories of gravity – it’s less common that someone questions the interpretation of the supernovae data itself.

In a new study, that’s what Arto Annila, Physics Professor at the University of Helsinki, is doing. The basis of his argument, which is published in a recent issue of the Monthly Notices of the Royal Astronomical Society, lies in the ever-changing way that light travels through an ever-evolving universe.

“The standard model of big bang cosmology (the Lambda-CMD model) is a mathematical model, but not a physical portrayal of the evolving universe,” Annila told PhysOrg.com. “Thus the Lambda-CMD model yields the luminosity distance at a given redshift as a function of the model parameters, such as the cosmological constant, but not as a function of the physical process where quanta released from a supernova explosion disperse into the expanding universe.

“When the supernova exploded, its energy as photons began to disperse in the universe, which has, by the time we observe the flash, become larger and hence also more dilute,” he said. “Accordingly, the observed intensity of light has fallen inversely proportional to the squared luminosity distance and directly proportional to the redshifted frequency. Due to these two factors, brightness vs. redshift is not one straight line on a log-log plot, but a curve.”

As a result, Annila argues that the supernovae data does not imply that the universe is undergoing an accelerating expansion.

The principle of least time

As Annila explains, when a ray of light travels from a distant star to an observer’s telescope, it travels along the path that takes the least amount of time. This well-known physics principle is called Fermat’s principle or the principle of least time. Importantly, the quickest path is not always the straight path. Deviations from a straight path occur when light propagates through media of varying energy densities, such as when light bends due to refraction as it travels through a glass prism. 

The principle of least time is a specific form of the more generally stated principle of least action. According to this principle, light, like all forms of energy in motion, always travels on the path that maximizes its dispersal of energy. We see this concept when the light from a light bulb (or star) emanates outward in all available directions.

Mathematically, the principle of least action has two different forms. Physicists almost always use the form that involves the so-called Lagrangian integrand, but Annila explains that this form can only determine paths within stationary surroundings. Since the expanding universe is an evolving system, he suggests that the original but less popular form, which was produced by the French mathematician Maupertuis, can more accurately determine the path of light from the distant supernovae.

Using Maupertuis’ form of the principle of least action, Annila has calculated that the brightness of light from Type 1a supernovae after traveling many millions of light-years to Earth agrees well with observations of the known amount of energy in the universe, and doesn’t require dark energy or any other additional driving force.

(via A second look at supernovae light: Universe’s expansion may be understood without dark energy)