The cosmological constant –
an essential part of the complex equations that account for an expanding
universe – may not be all that constant, analysis from the University of
Portsmouth and the National Astronomical Observatories of China suggests. This technically
changes everything. It can lead to major changes in our understanding of the
universe.
More precisely, dark energy,
an undefined force regarded as the embodiment of the cosmological constant in
models describing the expanding universe, may in fact be dynamic: exhibiting factually
varying energy states over time.
If so – and the upcoming
2018 Dark Energy Spectroscopic Survey planned
by the US Department of Energy will be able to settle the matter definitively –
the implications for cosmology will be profound.
The possible detection of
dynamic properties in dark energy is reported in a research
paper in the journal Nature Astronomy.
The research team was led by Gong-Bo Zhao, who holds dual chairs in Portsmouth
and China, and is described by co-author Bob Nichol, director of Portsmouth’s
Institute of Cosmology and Gravitation, without irony, as one of the “brightest
stars” in the field.
The idea of the cosmological
constant was first posited by Albert Einstein, as a way of plugging what he saw
as a problem with his general theory of relativity. His calculations predicted
that the universe must be either contracting or expanding, while Einstein
himself was convinced it was static. He at last introduced the constant remove
this size-changing from his results.
Later he came to see the
cosmological constant as his greatest mistake.
However, a later generation of
cosmologists embraced the idea, not as a way of preventing the universe
expanding, but in order to explain why it did, and continues to do so.
The amount of matter in the
universe should exert a gravitational force sufficient to slow down the
expansion that began with the Big Bang. However, calculations arising from
observations of supernovae (the “standard candles” of the cosmos) show no such
effect. Indeed, not only in the universe still growing, but the rate at which
it is doing so is actually increasing.
Clearly, the observations
are missing something. In order to explain the results, cosmologists suggest
the existence of an unknown, gravitationally repulsive force that is uniform
throughout the universe: dark energy.
The standard current model
of the cosmos is known as the Lambda
Cold Dark Matter model, in which dark energy is represented by the “lambda”
bit. In the equations, dark energy is assigned an “energy of state” of a
constant minus one, which means it contributes a fixed amount of expansionary
pressure throughout the universe.
Dark energy is not an
insignificant component of the model. It is calculated to comprise some 70% of
everything in the universe, with dark matter accounting for most of the rest.
“Baryonic” matter – planets, stars and all the rest of the obvious bits – accounts
for just 5%.
The Lambda Cold Dark Matter
model works, with predictions matching observations fairly closely, but the
research by Zhao and his colleagues may well represent a significant challenge.
Over the past two years his
team have been measuring baryonic acoustic oscillations – periodic fluctuations
in the overall density of visible matter – across multiple epochs.
The results are curious. The
scientists found that accounting for the changes in the density of visible
matter required compensating changes in the state of dark energy, at times
taking it above the minus-one value.
The results imply that dark
energy is dynamical, and therefore not constant. In their paper, Zhao and his
colleagues are quick to state that at present there is not enough data in
existence to favour a dynamical dark energy model over the standard Lambda Cold
Dark Matter model.
However, the paper
concludes, the new calculations revealing dark energy as an inconstant
candidate for the cosmological constant work “if the current best-fit dark
energy happened to be the true model.”
It is a question that will
likely soon be resolved one way or another. The Dark Energy Spectroscopic
Survey, which will be conducted by the Mayall 4-m telescope at Kitt Peak in
Arizona, will vastly increase the amount of information on dark energy
available.
Zhao and his colleagues are
hopeful of being able to either confirm or modify their calculations within
just a couple of years.
Via Cosmos Magazine