When will the universe end?
Is it possible to know when - and how - our universe will die?
It all depends on unravelling the mystery of the intriguing materials that make up 95 per cent of the universe, which we now describe as dark matter and dark energy.
World-renowned US theoretical physicist Professor Brian Greene says there's a "very good chance" scientists will have identified dark matter within the next 20 years.
"My guess is it's going to be an exotic particle of some sort. We have ideas but no evidence yet one way or another," he says.
"And the grander mystery is to figure out what the dark energy is.
"This is energy that fills space and which was learned about in the late 1990s, but nobody knows what it is or where it comes from."
The Large Hadron Collider in Switzerland, the world's largest and most powerful particle accelerator, is one tool that might help scientists gain fresh insights.
"But there are a number of dark matter experiments around the world today that are basically just trying to capture a dark matter particle that's wafting through space and goes by the Earth, as opposed to trying to produce it in a collision, which is what the collider does," Greene says.
"And there is a chance that those experiments will turn up, finally, some positive evidence.
"We have been going for a long time and are yet to find anything, but in 20 years, one way or another, I think we will learn whether that approach is effective.
"My guess is that we are going to find something."
Greene considers dark energy to be a more difficult problem.
If scientists could fully understand what it was, they could work out what the fate of the universe would be.
"If the dark energy is weakening over time, it's possible that the universe will expand for a while, reach a maximum size and then collapse back on itself.
"On the other hand, if the dark energy is growing over time in strength, then it yields a far less comfortable future - a future in which we imagine that the expansion will speed up so violently that even matter itself will get caught up in the expansion and be ripped apart.
"So those are the two alternatives: Do things end in a big crunch or do things end in a big rip?"
Nanoparticle body patrols
Picture an army of microscopic, man-made particles marching around our bodies, picking out potential problems.
This is something Greene believes could be reality by 2037.
"I think there's a real good chance that in 20 years, our bodies will be under constant surveillance by nanoparticles in the bloodstream," he said.
"In the best of all variations of this, any time some disease starts to take hold, the nanoparticle patrol will be right there to stem it before it spreads."
One particular area of exploration into the idea of using nanoparticles in the body is in cancer cells.
In mice studies, scientists have investigated particles that, when injected into the body, attach only to cancer cells that have specific antibodies they are attracted to.
These nanoparticles could be heated by shining light on the skin surface until they reached 50C - enough to kill the cells the particles the cells were touching.
Already by the turn of this decade, scientists at the Massachusetts Institute of Technology (MIT) had designed nanoparticles targeting the walls of the arteries around the heart.
They bind specifically to the proteins that stick out from the inner lining of these blood vessels only when they are damaged.
Each nanoparticle was built to target a specific part of the body and to release drugs in a controlled manner over a given period of time. They were so small that millions of them could be injected into the bloodstream without harming healthy tissues.
Once the nanoparticles took up position in the diseased arteries, they were programmed to release small quantities of drugs over weeks or months to help cardiovascular patients to recover without having to expose other parts of the body to much higher doses of potentially toxic drugs.
But Greene envisages nanoparticles that constantly swam around in our bodies, running a 24/7 diagnostic check.
"What I read about suggests that's a reasonable possibility, which would change the way medicine is administered.
"We would already be under surveillance, as opposed to having to go through the check-up."
The recent discovery of Proxima Centauri b, an exo-planet orbiting within the habitable zone of the closest star to the sun, predictably brought up again two questions long facing scientists.
How many Earth-like planets are there in the universe?
Are any of those planets home to intelligent life?
Whether it will be intelligent or not, Greene says it's not outlandish to think the next two decades may yield the first confirmed evidence of extra-terrestrial life.
"As we all know, the number of exo-planets we have discovered has grown enormously, even in just the past few years, and with the ability to measure the atmospheric chemical makeup of these exo-planets, we may be able to see the telltale signatures of biological impact on the atmosphere.
"It's even possible that we'll find evidence for some kind of microbial life even on Mars. This really has not been ruled out."
Even more exciting but more complicated was the prospect of discovering a form of life radically different from ours.
"This is not impossible, either, and that will show that life doesn't have to take a unique biological chemical form, which would change our perspective on everything."
Just how we might find other life is hard to say.
But Greene thinks it unlikely that it will be using radio telescopes such as the gigantic Square Kilometre Array being built in Australia and South Africa.
"The universe is such a big place that for us to tune in and capture radio signals that happen to be sent our way - while exciting and something we definitely need to look for - a lot of pieces need to fall into place for that."
Not only would this require extraterrestrial life - which no one has yet confirmed the existence of - but would also require that life form to have the intelligence and technology to transmit such signals.
"I don't think bacteria are likely to be sending out radio signals that we can capture," he said.
"But I love the idea that we are looking for life.
"Do I think it's possible to find it in the next 20 years? Absolutely. But the uncertainty is enormous."
Man on Mars
Former President Barack Obama touted taking the ideas in films like The Martian and Red Planet from science fiction to science reality as the "next chapter of America's story in space".
The ultimate colonisation of Mars is also a goal of SpaceX, the company founded by billionaire Elon Musk and headquartered in California. Musk has said he hopes to send humans to the red planet's surface within 20 years.
Greene is convinced the Obama administration's vision - sending humans to Mars by the 2030s and returning them safely to Earth, with the ambition to one day remain there for an extended time - was a likelihood in the next two decades.
"No doubt. I think there's enough will and interest that I do think we'll have a presence on Mars in the next 20 years," he said.
"But scientifically, that's a dubious proposition as the technical details and the danger and cost of sending humans to Mars is unclear. As is whether it gives enough scientific benefit to make it all worth it."
The Martian environment is extremely inhospitable. It has Antarctic temperatures, no water, little oxygen and gravity, and deadly amounts of carbon dioxide - not to mention that the Sun's UV radiation would fry exposed skin.
But first, colonists would have to get there, making a 225 million kilometre, eight-month journey in a craft yet to be designed.
Just breaking free of Earth's gravity requires an escape velocity of 40,000km/h, or 11km a second, but our planet's orbit would mean the spacecraft would be speeding around the Sun at 150,000km/h.
Although unmanned craft have arrived at Mars at a speed of 20,000km/h and used a heat shield for initial deceleration, a solution to landing humans safely on Martian soil is still not obvious.
"But certainly from the standpoint of excitement, there's nothing that can beat moving on to the final frontier," Greene said.
"So I think there will be widespread support to do that and I think it's likely we will."
The age of AI
Artificial Intelligence (AI) is a cute thing to consider: Think Rosie from The Jetsons, or the burger-flipping robot that this month completed its first day on the job at a restaurant in California.
But the certainty of a future of daily interaction with intelligent machines can also be terrifying, especially when we ponder the kind of AI being designed by Cambridge and Microsoft engineers - potentially advanced enough to write its own code.
It's a short step from the creepy concepts explored with Skynet in Terminator or Kubrick's 2001: A Space Odyssey.
Then there's the emerging topic of robot anxiety.
Recent research by consultants McKinsey & Co found nearly half of jobs in the United States could be automated by technology that already exists.
Whatever its form, Greene says it's very likely, in the next 20 years, that we'll have truly functional artificial intelligence as part of our daily lives. "That's both exciting to some and frightening to others," he said.
"My personal view is that there's nothing particularly special about intelligence that resides in the wet, smooshy, grey thing inside a human skull.
"And if we are able to generate a version of intelligence that lives within a different environment, a mechanical, a silicon, or any kind of version of a computer-based intelligence, to me that's an exciting step forward.
"I do think that within the next 20 years we are going to make incredible strides toward that. And we may encounter the interesting situation of having interactions with artificial intelligence."
● Dr Brian Greene will present A Time Traveller's Tale at Auckland's Bruce Mason
Centre at 7.30pm Sunday March 26. Tickets are available