The Universe is accelerating, says a team of researchers led by Masamune Oguri, Kavli IPMU and Naohisa Inada at Nara National College of Technology, courtesy the data acquired by observing distant quasars. This is supplementary to the studies of distant supernovae, which also showed that the Universe’s expansion is accelerating, and for which the 2011 Nobel Prize in Physics was given. This study with quasars again shows that dark energy is definitely present, but we still don’t know what it might be.
Larger Data and more inferences
The data is derived from the Sloan Digital Sky Survey (SDSS), the huge collaborative experiment responsible for tracking about 100,000 quasars for nearly 10 years, with nearly 50 new quasars discovered in the last few years. Quasars are bright objects, believed to be formed, or at least fuelled, by the accretion of gas and dust by a supermassive black hole. The infalling material glows due to the enormous heat produced and can thus be detected from very far away. This makes them ideal for mapping the gravitational lensing occurring in the Universe.
Prof. Oguri, heading the study, says:
In 2011, the Nobel Prize in Physics was awarded to the discovery of the accelerated expansion of the universe using observations of distant supernovae. A caution is that this method using supernovae is built on several assumptions… Our new result using gravitational lensing not only provides additional strong evidence for the accelerated cosmic expansion, but also is useful for accurate measurements of the expansion speed, which is essential for investigating the nature of dark energy.
The Science of Gravitational Lensing
Gravitational lensing refers to the bending of light due to the presence of matter in the path of light, as explained by Einstein’s General Theory of Relativity. This process creates (at least) two identical images of one object, separated by a gap, thus the name ‘Cosmic Mirage’, referring to the similar process by which mirages on Earth are created.
The farther away the quasar, the greater its chances to be gravitationally lensed. Accelerated expansion of the Universe increases the distance of the quasar from us and thus the images also seem to separate (refer to figure above). This can be used to deduce how fast the quasars are receding from us. By plotting the velocity graph (velocity versus distance curve), we can see the deviation from the straight line expected from Hubble’s law, if the Universe was expanding at a constant rate. Sure enough, there is deviation and all the deviations fall on a curve, showing that it’s not just a mere statistical fluctuation or measurement error. The Universe is indeed accelerating! And this suggests that the estimates for dark energy are also not very off.
Dark Energy in Einstein’s Theory
Einstein’s theory of General Relativity allows for an expanding Universe without any extraneous assumptions. However, this expansion should have been at uniform speed. But it seems that the expansion rate is increasing. To get this prediction from Einstein’s equations, scientists tweak it a bit, adding a ‘cosmological constant’ term. This adds a bit of energy per unit volume of the Universe, contributing a lot to the entire energy of the Universe. By adjusting the sign of this extra term, the universe can be made to be accelerating.
In fact, it can be shown that we are exiting a phase dominated by matter, where the major contribution to energy comes from matter, and entering a phase dominated by the cosmological constant. This inevitably leads to accelerated expansion. We might not be able to see any galaxies in another 5-10 billion years, if the accelerated expansion of the Universe continues unabated.
The SDSS data also shows that treating dark energy as the cosmological constant is not such a wrong thing to do.
The future of the SDSS project is Planck and the SuMIRe projects. Both aim to study the distribution of cosmic dark energy and all this in the not-too-distant exciting future!