Physicists have been fairly certain that the Universe isn’t only getting bigger ever since the late 1990s, it also appears to be expanding at an ever increasing rate.
Currently thought to be responsible for this accelerating growth is a mysterious force called dark energy. But a new study raises the possibility that what seems to be a type of energy could be an illusion caused by the changing structure of the Universe.
Physicists from Loránd University in Hungary and the Institute for Astronomy at the University of Hawaii are now wondering if approximations in Einstein’s equations introduced “serious side effects” that gave the illusion of a vast, unknown force pushing space apart.
If it exists, dark energy would make up about 68 percent of the energy in the observable Universe, but at just 10-27 kilograms per square metre, it would be incredibly hard to spot in the laboratory.
Dark energy also helps explain things like the overall shape of the Universe and the patterns of matter we see rippling through space.
The thing is, right now it is little more than an empty box without any other properties to describe the nature of its existence.
As such, it’s currently assumed to be a fundamental part of empty space known as the cosmological constant, represented by the Greek letter lambda (Λ).
Back in the early 20th century, Einstein proposed the cosmological constant as a kind of fudge-factor to explain why all the mass scattered through the Universe wasn’t pulling back together under the attraction of its own gravity.
It’s now well known that the Universe appeared to grow at a slower rate in its youth than today, making the cosmological constant useful again as a way to explain this increase in speed.
Put together with another hypothetical ‘black box’ factor – the dark matter, which could comprise of a further 27 percent of the known Universe. We get the Lambda Cold Dark Matter (ΛCDM) model to explain how the Universe evolved.
In this latest study, the researchers argue these approximations have ignored potentially significant influences of large scale structures within the Universe.
“Einstein’s equations of general relativity that describe the expansion of the Universe are so complex mathematically that for a hundred years no solutions accounting for the effect of cosmic structures have been found,” said Laszlo Dobos from Eötvös Loránd University.
If it was possible to step outside of the Universe for a moment and look down upon it, there would be threads of galaxies called super clusters lining what look like relatively empty spaces.
The ΛCDM model assumes a uniform expansion that gets faster progressively, thanks to the increasing push of dark energy overcoming the pull of dark and normal matter distributed throughout space.
According to the physicists involved in this new research, the large scale structures – ‘bubbles’ of seemingly empty space and the galaxies that surrounds them – would create zones where expansion occurs at different rates, almost like mini-universes.
By modelling the effect of gravity, mathematically, on millions of particles representing dark matter, the team managed to recreate the bunching up of matter in the early Universe in such a way that it looked like the large scale galaxy structures.
While the Universe in their model still expands, the differences in how these bubbles expand averages out to an overall acceleration.
Dobos explains: “Our findings rely on a mathematical conjecture which permits the differential expansion of space, consistent with general relativity, and they show how the formation of complex structures of matter affects the expansion.“
“These issues were previously swept under the rug but taking them into account can explain the acceleration without the need for dark energy.”
The model makes its own necessary assumptions, but if it stands up to scrutiny it could explain why the Universe’s expansion seems to be accelerating, all without the need for negative pressure.
While the idea is new by itself, the search for ways to get around the need for a mysterious type of energy has produced a number of creative solutions in recent years.
A study published in Science suggested dark energy could be explained as a kind of deficit, as if the Universe was ‘leaking energy’ at some point in its evolution.
Although it breaks one of the big rules of physics (energy can’t be lost or created) it would also take care of the nagging question mark over what 68 percent of the Universe is made out of.
Dark energy is a tough nut to crack, no doubt, so it might take thinking outside the box – if not outside the whole Universe –in order for a solution to be found.