October 23, 2013
Dark energy accelerated Expansion of the universe.
Latest findings show that the cosmological constant, an important element of the theory on dark energy, may have a different value than we thought it had. This could have big consequences on theory on universe expansion accelerated by the dark matter, something we don't fully understand as it is.
According to current theory, the visible matter accounts for only 4.9% of the total mass-energy of the universe. The visible, or ordinary matter, is basically all that we know – molecules, atoms, subatomic particles. The rest of the universe is made of dark matter and dark energy, accounting for 26.8% and 68.3% of the mass, respectively.
Dark matter and dark energy are yet to be proven beyond any doubt. They are called "dark" because we can't see them, yet they are currently the best explanation for galaxies' rotation (dark matter) and universe expansion (dark energy). Our universe does not simply expand, it does so at accelerated rate. This is relatively recent discovery, from the 1990s, but to make the issue at hand clearer, we'll go further back in time.
While developing his theory of general relativity, Albert Einstein introduced something called cosmological constant which was supposed to counter the effect of the gravity – cosmological constant is a repulsive force. Einstein was strongly convinced that the universe is static. If it was expanding, it means it had a beginning at some point and if God made the universe, as Einstein thought when he published the theory of general relativity in 1916, the universe can't have neither the beginning not the end. It was static and everlasting. If there wasn't any kind of repulsive force, the universe would be shrinking. Later, when he was proven wrong, Einstein declared the cosmological constant the biggest blunder in his life.
He was wrong because it was proven that the universe was expanding, which meant that it had the beginning, The Big Bang, and according to the theory at the time, gravity was too feeble to stop the expansion that started with explosion of sorts. There was no room for cosmological constant anymore.
Fast forward to 1990s. When it was discovered that the universe is expanding at an accelerated rate, cosmological constant was reintroduced to explain the expansion of the universe in new models, but it works only if the equation of state equals -1.* A couple of recent studies raise doubts when it comes to the accuracy of the described theory because w's value was calculated at −1.186. The latest of these studies, Pan-STARRS (Panoramic Survey Telescope and Rapid Response Systems) used measurements from other projects and combined it with their own observations of type Ia supernovae, star explosions used to measure astronomical distances.
"If has w this value, it means that the simplest model to explain dark energy is not true," said Armin Rest, Space Telescope Science Institute, USA, the lead author of the study. He points out that the results are still preliminary and no conclusions should be drawn yet. "I don't think we can say now that we've really found a discrepancy. We still have to look if this is due to some issues with any of these projects."
The earlier mentioned Ia supernova marks the end of the white dwarf's life. It happens when the star reaches its mass limit, which is similar for all white dwarfs. This means that the explosion from two or more different white dwarf stars has the same intrinsic brightness. This is crucial when measuring large distances (forget our galaxy, think galaxies millions, even billions of light-years away) because the apparent brightness can be compared to intrinsic brightness of the explosion. With other stars, we can't say how big the star was before the supernova, so very bright supernova might mean that the star was huge, but it might not have been as huge as some more distant star that because of the larger distance ended in less bright explosion.
With white dwarfs we can deduce exactly how far the explosion occurred. The Pan-STARRS telescope PS1 in Hawaii observed 146 of these supernovae between 2006 and 2009. With additional spectroscopic observations of supernovae, they observed how much the light's wavelength has been stretched by the expansion of the universe. Finally, the researchers combined their data with Planck satellite observations of cosmic microwave background radiation and calculated w's value.
"It's generally accepted that telescope calibration, supernova physics and galaxy properties are big sources of uncertainties, so everyone's trying to figure these out in different ways," says Daniel Scolnic at Johns Hopkins University, Baltimore, USA. In science, it is difficult to overthrow the established theory based on a few findings that still don't prove anything. With careers built on current dark energy theory, many experts agree that the new results are interesting, but are far from proving the theory wrong. "The Pan-STARRS paper presents a very thorough, careful analysis and a solid result, but it doesn't qualitatively change our view of the cosmological parameters," says Joshua Frieman, an astrophysicist at Fermilab, Batavia, USA.
Some others, like Nobel Prize winner for discovery of dark energy, Adam Riess, is more open-minded about what these discoveries could mean. "This paper is now the third survey of distant supernovae that's coming to this conclusion. We can't just say this survey or that survey screwed up. It could be something fundamental to one of these measurements. Or it could be that dark energy is more interesting in a way that actually we hope. I expect in the next year or two this will probably either become definitive, or go away."
Not knowing how something like dark energy came to be is difficult. Cosmological constant explains it mathematically, but that's about it. With new studies suggesting that this constant possibly varies over time, something called quintessence, things are even more difficult to understand. Even if the new observations show the w value to be other than -1, it is still a step forward. If you would like to find out more about dark energy and cosmological constant, click here.