New Delhi, Jun 22: Dark matter pulls the universe and dark energy pushes, both mysteries that endure. And the discovery that a majority of the universe is made up of “stuff” that makes gravity push rather than pull was a gamechanger, says Nobel laureate Brian Schmidt.
The US-born Australian astronomer along with Adam Riess and Saul Perlmutter from the US discovered the “stuff”, later termed dark energy, in 1998. The three won the Nobel Prize for Physics in 2011.
Explaining the significance of their discovery that changed the understanding of how the universe functions, Schmidt told PTI, “Dark energy is really saying (that) there is energy tied to space itself.”
“If we didn’t have dark energy, the universe would be curved and the universe wouldn’t accelerate — and that changes how cosmic objects, such as galaxies, looks. It really makes a difference,” the astronomer, who was visiting Ashoka University for the Lodha Genius Programme, added.
The term dark energy is intentionally similar to dark matter.
Dark matter refers to particles in the universe that hold galaxies and other structures in space (the cosmos) together. It is said to have peculiar properties, such as being invisible, as it does not interact with light.
However, while “dark matter and atoms (that make up ordinary matter) are pulling the universe, dark energy is pushing the universe. There’s a balance at any given time of who’s winning the war — dark energy has won the war, it seems now and is pushing the universe apart”, Schmidt explained.
That’s because dark energy had a density set at the time of the Big Bang, said the 58-year-old former president of the Australian National University and currently a distinguished professor of astronomy.
The Big Bang, believed to have given birth to the universe, happened some 13.8 billion years ago. Dark matter is among the particles formed immediately after the event, gravity exerted from which is said to produce a slowing effect on the universe’s evolution.
“And (dark energy) stayed at that density. But as the universe expanded, and the density of atoms and dark matter dropped over time, the two crossed about 6.5 billion years back — and that crossing meant the dark energy could take over and accelerate the universe,” Schmidt said.
Work on the discovery that the universe is expanding at an accelerating rate and that dark energy is the driving force began in 1994.
Schmidt and colleagues intended to look at distant objects and measure how fast the universe was expanding in the past, and then look at nearer objects to see how it slowed down over time.
“And if we measured the universe slowing down really quickly, then we’d know that the universe was heavy and you’re gonna get a Gnab Gib — the Big Bang in reverse. But if the universe was slowing down slowly, then we’d know the universe is light and it’s gonna exist forever. So that’s what we were going to do.”
Three and a half years later came the answer.
“What we saw was the universe was expanding slower in the past and it sped up. So instead of slowing down, it’s actually the other way — it’s speeding up,” the Nobel laureate said.
In 1917, physicist Albert Einstein first imagined dark energy as a concept — only he did not think of it in those exact words but instead accounted for it in his equations of general relativity as a ‘lambda’ term.
Einstein is said to have considered the lambda term irrelevant, even denouncing it as his “greatest blunder”.
“When we made our discovery of the acceleration (of the universe), it was the only sensible way of making it happen. So that thing (the lambda term), that he (Einstein) brought in 1917 and then later discarded as being irrelevant, that seems to (be validated from) what we discovered,” Schmidt continued.
“In 1998, cosmology was shaken at its foundations as two research teams presented their findings…,” states the press release dated October 4, 2011, announcing the recipients for the Nobel Prize in Physics for 2011.
The 1998 model has since been scrutinised through experiments, mainly aimed at understanding the nature of dark energy — is it constant or does it vary?
“We put in some extra knobs in the model of 1998, where we allow dark energy to change over time. The models with the most recent data seem to prefer a dark energy that changes,” Schmidt said.
But he is sceptical.
“I’m not saying they’re wrong. I’m saying I need better data to be convinced they’re right.” He said he is also glad that someone else is working on it.
Schmidt leads the ‘SkyMapper Telescope Project’ for which he conducted a survey of the southern sky as seen from Australia, focussed on looking at the “oldest, first stars in the galaxy”.
“We could see essentially what the chemistry of the universe was back really close to the Big Bang — because if a star was formed right after the Big Bang, it’s made up of the stuff that was in the universe at the time.
“And so, we found the most chemically pure stars that have ever been discovered, ones that were almost certainly not formed from the remnants of the Big Bang, but from a single exploding star after the Big Bang. That just gives us a sense of what the first stars look like,” said Schmidt, who has published his findings in several journals, including Nature.
Schmidt, who addressed high schoolers and others on science as a potential career at the university, advised them to get the skills that seem useful for life by working on something that interests them.
Not knowing what to do in life and the fact that he enjoyed astronomy made Schmidt pursue the field.
“In learning astronomy, I’d learned math, I’d learned physics, I’d learned computing, I’d learned some engineering. And (while) I didn’t think it was likely that I would get a job to be an astronomer, I knew math, engineering, physics, and computing liable to give me a good job doing something. And of course, I did end up being an astronomer,” he said.
“You don’t really know how all of this is going to come together in your life, but if you work on something you’re interested in, with a set of skills that seem useful for life, then don’t overthink your life, don’t overplan your life,” Schmidt said. (Agencies)
