The Indian Express (Delhi Edition)
New 3-D map of the universe hints at nature of dark energy
THAT THE universe is expanding has been known for close to a century, thanks to the observations by the American astronomer Edwin Hubble in 1929. More recently, in the late 1990s, scientists found that this expansion was happening at an accelerated rate — that is, not only was the universe expanding, it was expanding at an increasingly faster pace.
This discovery, honoured with the Nobel Prize in Physics (for Saul Perlmutter, Brian P Schmidt, and Adam G Riess) in 2011, forced scientiststohypothesise‘dark’energy.thereasoning was this: If the rate of expansion did not increase, it could be explained as a continuing after-effect of the expansion caused by the Big Bang. That would keep open the possibility of gravity prevailing at some point to either pull things back or to keep the universe in a stable state. But the accelerated rate of expansion meant some other ingredient — an invisible energy — was at work too. Since scientists did not have any clue about what this could be, they called it “dark energy”.
Since then, scientists have not got any closer to shedding new light on the hypothesis — even though dark energy must be accounting for nearly 70% of the universe if its observed behaviour is to be explained. The results from an ongoing experiment involving more than 900 researchers around the world have now offered the first glimmer of hope.
The DESI experiment
The results, announced on April 4, have come from observations of the Dark Energy Spectroscopic Instrument (Desi),auniqueequipmentwith5,000robotic ‘eyes’, each one of which can separately capture and process light coming from a galaxy. This gives DESI, mounted on the Nicholas W Mayall 4-meter Telescope at the Kitt Peak National Observatory in Arizona, US, the ability to observe 5,000 galaxies at the same time.
DESI has been operating for three years, and is scheduled to continue for at least another two years. Data from the first year of DESI observations — during which light from 6 million galaxies, some of which existed as far back as 11 billion years ago, was captured — have now been used to create the most comprehensive three-dimensional evolutionary map of the universe till date.
“We say it is a three-dimensional map because we have been able to measure the distances between these galaxies to a very high level of precision. Some of these galaxies existed billions of years ago at great distances from us. Lights originating from those galaxies are reaching us only now. These 6 million galaxies together produce a very good evolutionary picture of the universe,” Shadab Alam of Tata Institute of Fundamental Research (TIFR), Mumbai, who is part of the DESI collaboration, said.
The precise distances to these galaxies that scientists have been able to calculate has led to the mapping of the distribution and movement of these galaxies over time, by comparing the data with similar data for some of the galaxies obtained through other experiments.
This has, in turn, allowed scientists to work out the expansion rate of the universe through different times in history. Using the first year’s observational data, the DESI collaboration has calculated that the speed of expansion of the universe is increasing at the rate of 68.5 km per second after every 3.26 million light years of expansion.
Through these precise measurements, the scientists have found that some of the calculated values are not consistent with current well-established theoretical models, which otherwise describe the universe very well.
Dark energy intensity
These theoretical models suggest that the energy density of dark energy, or the amount of dark energy contained in any volume of space, remains constant even under expansion. So, while the space itself expands, the energy density in the expanded space does not go down. In these models, a change in energy density would make the universe unstable.
The results of DESI, however, suggest that there are changes in energy density. “In fact, the whole purpose of the DESI collaboration is to look for possible changes in the energy density of dark energy,” Alam said.
“In the first year’s data, there is just this slight hint that energy density may not be constant. Energy density of dark energy is seen to increase as well as decrease. But we are being extremely cautious as of now. The confidence level is around two and a half sigma, translates to about 95% confidence — not enough for a scientific discovery of this magnitude and implication. We need to have a six sigma confidence level, a near certainty,” he said.
But the initial hints have excited the scientific community. If change in energy density is confirmed, it could lead to a complete unravelling of our current understanding of the universe. It would be the first glimpse into the nature of dark energy, and could lead to entirely new physics.
“Right now, we know nothing about the nature of dark energy. Some scientists have speculatedthatitmightbeanewinvisiblefield, like... a magnetic, or a gravitational field. There is also speculation about dark energy being a new particle. All these people must be looking at the data from DESI very keenly,” Alam said.
Thedesicollaborationhasannouncedthat it will begin analysing data from subsequent years of observations almost immediately.