You may have heard that the universe is expanding, but new research suggests that it could also be oscillating back and forth. This gentle swaying won't knock you over, though, as each oscillation takes place on a time scale of over a billion years. The oscillations are like the ringing of an enormous, universe-sized glass. 

These findings, published in the Astronomical Journal, indicate that instead of the universe expanding constantly, it gently wobbles in and out. It is also suggested that the oscillations are dampening over time. So while the first undulation after the Big Bang was dramatic, the wobbles that we're supposedly experiencing now are comparatively mild.

The first evidence that the universe was expanding was the discovery of the cosmic microwave background (CMB) in 1964. The existence of the CMB suggests that the universe expanded from a single point and is therefore evidence of the Big Bang. Then, in 1998, scientists found that the universe wasn't just expanding but also accelerating – the model of the universe that can be seen in the diagram below. The acceleration suggested that there was matter in the universe that we couldn't see, and that there was a lot of it. This prompted the theory of dark matter. 

^{The standard view of the expanding universe. NASA.}

The first figure is a NASA diagram representing the evolution of our universe, starting with the Big Bang on the left, all the way to the current day on the right. The universe is represented as a bell shape to convey that it is expanding in size. Immediately after the Big Bang there is a very rapid expansion in a short space of time. After this, the universe continues to expand but at a slower acceleration. 

This isn't the end of the story though, as the new theory predicts that the universe oscillates over time. The discovery was actually made accidently by a duo of scientists from the University of Southern Mississippi (USM), Harry Ringermacher and Lawrence Mead. While they were attempting to plot a graph describing the scale of the universe against its age in a novel way, their results unexpectedly showed the presence of these oscillations. 

 “Analyzing this new plot to locate the transition time of the universe, we found there was more than one such time – in fact multiple oscillations with a frequency of about 7 cycles over the lifetime of the universe. It is space itself that has been speeding up its expansion followed by slowing down 7 times since creation,” said Ringermacher.

^{The modified diagram of a universe that rings as it expands. Superimposed onto NASA image.}  

"The new finding suggests that the universe has slowed down and speeded up, not just once, but 7 times in the last 13.8 billion years, on average emulating dark matter in the process," said Mead. "The ringing has been decaying and is now very small – much like striking a crystal glass and hearing it ring down."

These aren't going to be oscillations that you can feel – they won't cause tidal waves or knock down buildings. Instead, they are gentle ripples in the fabric of space-time. The model also greatly exaggerates the amplitude (height) of the ripples. If the height of the undulations were drawn to scale then they wouldn't be observable on this diagram. However, the period of the oscillations is unchanged. 

[Via The Southern University of Mississippi, The Astronomical Journal: Paper 1, Paper 2]