A new study challenges the cosmological model and suggests that the universe is not expanding at an accelerated rate.
The standard model of cosmology assumes that the universe is isotropic with no preferred direction and no preferred frame of reference; that is, we are not special and our position in the universe is not from a privileged vantage point. Within this framework, observational data led us to the conclusion that 70% of the universe is expanding at an accelerated rate, and this accelerating force is due to an unknown form of energy known as ‘dark energy’. This so-called ‘dark energy’ is now thought to be due to quantum fluctuations of the vacuum energy.
However, a new study by a team of European scientists explored these ideas further. They wanted to see what would happen when they measure the deceleration parameter – the measurement of cosmic acceleration – from our own ‘special’ frame of reference.
The expansion of the universe is measured in terms of the Hubble constant, which is currently measured by two different methods. One method looks at the early universe through the observation of the Cosmic Microwave Background (CMB) and the other method looks at the local universe through the light emitted by galaxies, Cepheid variables and/or Type 1a supernovae. It was the latter method that led to the conclusion that the universe was expanding at an accelerating rate, resulting in astrophysicists Adam Reiss, Brian Paul Schmidt and Saul Perlmutter receiving the 2011 Nobel Prize in Physics.
However, in each case, the measurements are taken in the framework of the cosmological model which assumes that the universe is isotropic and homogeneous. This assumption is contradicted by the inhomogeneous distribution of galaxies and the lack of correlations on large angular scales, with the only confirmation coming from studies of the early universe through observed temperature fluctuations in the CMB radiation. It has therefore been suggested that this isotropic and homogeneous universe only exists at the larger scales, although this has yet to be confirmed.
The team therefore decided to see what happens when they remove this assumption from their analysis and measure the expansion in our own ‘heliocentric’ frame of reference.
“In the absence of any evidence of convergence to the CMB rest frame, this assumption is unjustified since it is very possible that the observed bulk flow stretches out to much larger scales.”
– Jacques Colin, Roya Mohayaee, Mohammed Rameez and Subir Sarkar
Utilising the latest extended sample size of supernovae data from the Joint Lightcurve Analysis catalogue, they were able to extract the redshifts for 740 Type 1a supernovae. To convert from a heliocentric frame of reference to a CMB frame of reference, the observed redshifts are generally corrected for ‘peculiar’ velocities – that is, velocities relative to a standard frame of rest. Therefore, to obtain the redshifts in our local frame of reference – the heliocentric frame – these corrections had to be undone.
Intriguingly, their results showed that the acceleration is a relatively local effect with a significant dipole component directed along the direction we are moving with respect to the CMB. This dipole component, in alignment with the CMB dipole moment, rejects the assumption of isotropy. It could therefore be that the cosmic acceleration inferred from supernovae observations is not due to dark energy and instead due to us being tilted observers located in a bulk flow.