The accelerating expansion of the universe due to a mysterious quantity called “dark energy” may not be real, according to research claiming it might simply be an artefact caused by the physical structure of the cosmos.
The findings, reported in the Monthly Notices of the Royal Astronomical Society, claims the fit of Type Ia supernovae to a model universe with no dark energy appears to be slightly better than the fit using the standard dark energy model.
The study’s lead author David Wiltshire, from the University of Canterbury in New Zealand, says existing dark energy models are based on a homogenous universe in which matter is evenly distributed.
“The real universe has a far more complicated structure, comprising galaxies, galaxy clusters, and superclusters arranged in a cosmic web of giant sheets and filaments surrounding vast near-empty voids”, says Wiltshire.
Current models of the universe require dark energy to explain the observed acceleration in the rate at which the universe is expanding.
Scientists base this conclusion on measurements of the distances to Type 1a supernovae in distant galaxies, which appear to be farther away than they would be if the universe’s expansion was not accelerating.
Type 1a supernovae are powerful explosions bright enough to briefly outshine an entire galaxy. They’re caused by the thermonuclear destruction of a type of star known as a white dwarf – the stellar corpse of a Sun-like star.
All Type 1a supernovae are thought to explode at around the same mass – a figure known in astrophysics as the Chandrasekhar limit – which equates to about 1.44 times the mass of the Sun.
Because they all explode at about the same mass, they also explode with about the same level of luminosity.
This allows astronomers to use them as standard candles to measure cosmic distances across the universe – in the same way you can determine how far away a row of street lights is along a road by how bright each one appears from where you’re standing.
On a galactic scale, gravity appears to be stronger than scientists can account for, using the normal matter of the universe, the material in the standard model of particle physics, which makes up all the stars, planets, buildings, and people.
To explain their observations, scientists invented “dark matter”, a mysterious substance which seems to only interact gravitationally with normal matter.
To explain science’s observations of how galaxies move, there must be about five times as much dark matter as normal matter.
It’s called dark because whatever it is, it cannot emit light. Scientists can only see its effects gravitationally on normal matter.
On the even larger cosmic scales of an expanding universe, gravity appears to be weaker than expected in a universe containing only normal matter and dark matter.
And so, scientists invented a new force, called “dark energy”, a sort of anti-gravitational force causing an acceleration in the expansion of the universe out from the big bang 13.8 billion years ago.
Dark energy isn’t noticeable on small scales, but becomes the dominating force of the universe on the largest cosmic scales: almost four times greater than the gravity of normal and dark matter combined.
The idea of dark energy isn’t new. Albert Einstein first came up with it to explain a problem he was having when he applied his famous 1915 equations of general relativity theory to the whole universe.
Like other scientists at the time, Einstein believed the universe was in a steady unchanging state. Yet, when applied to cosmology, his equations showed the universe wanted to expand or contract as matter interacts with the fabric of spacetime: matter tells spacetime how to curve, and spacetime tells matter how to move.
To resolve the problem, Einstein introduced a dark energy force in 1917 which he called the “cosmological constant”.
It was a mathematical invention, a fudge factor designed to solve the discrepancies between general relativity theory and the best observational evidence of the day, thus bringing the universe back into a steady state.
Years later, when astronomer Edwin Hubble discovered that galaxies appeared to be moving away from each other, and the rate at which they were moving was proportional to their distance, Einstein realised his mistake, describing the cosmological constant as the biggest blunder of his life.
However, the idea has never really gone away, and keeps reappearing to explain strange observations.
In the mid 1990s two teams of scientists, one led by Brian Schmidt and Adam Riess, and the other by Saul Perlmutter, independently measured distances to Type 1a supernovae in the distant universe, finding that they appeared to be further way than they should be if the universe’s rate of expansion was constant.
The observations led to the hypothesis that some kind of dark energy anti-gravitational force has caused the expansion of the universe to accelerate over the past six billion years.
Wiltshire and his colleagues now challenge that reasoning.
“But these observations are based on an old model of expansion that has not changed since the 1920s”, he says.
In 1922, Russian physicist Alexander Friedmann used Einstein’s field equations to develop a physical cosmology governing the expansion of space in homogeneous and isotropic models of the universe.
“Friedmann’s equation assumes an expansion identical to that of a featureless soup, with no complicating structure”, says Wiltshire.
This has become the basis of the standard Lambda Cold Dark Matter cosmology used to describe the universe.
“In reality, today’s universe is not homogeneous”, says Wiltshire.
The earliest snapshot of the universe – called cosmic microwave background radiation – displays only slight temperature variations caused by differences in densities present 370,000 years after the Big Bang.
However, gravitational instabilities led those tiny density variations to evolve into the stars, galaxies, and clusters of galaxies, which made up the large scale structure of the universe today.
“The universe has become a vast cosmic web dominated in volume by empty voids, surrounded by sheets of galaxies and threaded by wispy filaments”, says Wiltshire.
Rather than comparing the supernova observations to the standard Lambda Cold Dark Matter cosmological model, Wiltshire and colleagues used a different model, called ‘timescape cosmology’.
Timescape cosmology has no dark energy. Instead, it includes variations in the effects of gravity caused by the lumpiness in the structure in the universe.
Clocks carried by observers in galaxies differ from the clock that best describes average expansion once variations within the universe (known as “inhomogeneity” in the trade) becomes significant.
Whether or not one infers accelerating expansion then depends crucially on the clock used.
“Timescape cosmology gives a slightly better fit to the largest supernova data catalogue than Lambda Cold Dark Matter cosmology,” says Wiltshire.
He admits the statistical evidence is not yet strong enough to definitively rule in favour of one model over the other, and adds that future missions such as the European Space Agency’s Euclid spacecraft will have the power to distinguish between differing cosmology models.
Another problem involves science’s understanding of Type 1a supernovae. They are not actually perfect standard candles, despite being treated as such in calculations.
Since timescape cosmology uses a different equation for average expansion, it gives scientists a new way to test for changes in the properties of supernovae over distance.
Regardless of which model ultimately fits better, better understanding of this will increase the confidence with which scientists can use them as precise distance indicators.
Answering questions like these will help scientists determine whether dark energy is real or not – an important step in determining the ultimate fate of the universe.