The universe really is expanding and it's not an illusion as a competing theory suggests, say astronomers using NASA's Hubble Space Telescope.

"We are using the new camera on Hubble like a policeman's radar gun to catch the universe speeding," Adam Riess, a professor of physics and astronomy at Johns Hopkins University in Baltimore, said in a NASA release.

Riess said his team was able to look at more stars than ever before in both visible and near-infrared wavelengths of light. He says this allowed him to eliminate errors introduced in previous work, which compared measurements from different telescopes.

The new measurements are thanks to the Wide Field Camera 3, recently installed on the space telescope.

Riess's work gives new evidence that the universe is in fact expanding at an increasing rate because it is filled with a dark energy that works in opposition to gravity. Dark energy is a type of energy that astronomers believe permeates all space.

NASA says Riess's findings rule out an alternate theory of the nature of dark energy. That alternate theory is based on the notion that an enormous bubble of relatively empty space eight billion light-years across surrounds our galactic neighbourhood.

It suggests that because the bubble has a lower density, it expands faster than the more massive universe around it. 

"To an observer inside the bubble, it would appear that a dark-energy-like force was pushing the entire universe apart," according to NASA's explanation of what it says is a now debunked theory. "The bubble hypothesis requires that the universe‚Äôs expansion rate be much slower than astronomers have calculated, about 60 to 65 kilometres per second per megaparsec." 

According to the space agency, Riess's more precise measurements show the expansion rate is 73.8 kilometres per second per megaparsec.  A megaparsec is a unit of measure for distances in intergalactic space.

"It looks more like it's dark energy that's pressing the gas pedal," said Riess.

The Hubble Space Telescope has provided unprecedented views of the universe from an altitude of 569 kilometres since it was launched in 1990.

Riess's results appear in the April 1 issue of The Astrophysical Journal.