x Abu Dhabi, UAETuesday 23 January 2018

Astrophysicist maps universe using gamma rays

Dark matter and a blazing bright Milky Way are all in a day's work for Julie McEnery, writes John Henzell.

In this artist's concept, dense knots of dust in otherwise normal galaxies dim the light of a gamma ray burst in the centre. The dust absorbs most of a burst's visible light, but not gamma rays. Aurore Simonnet / Nasa / Swift
In this artist's concept, dense knots of dust in otherwise normal galaxies dim the light of a gamma ray burst in the centre. The dust absorbs most of a burst's visible light, but not gamma rays. Aurore Simonnet / Nasa / Swift

There is something indescribably apt that on the evening Abu Dhabi residents were invited to gaze at the stars with an expert on what's out there, cloudy skies meant little could be seen.

For the expert, Nasa astrophysicist Julie McEnery, that would have seemed entirely appropriate. Given that this is a woman who not only understands E=MC2 but uses it in her day-to-day life, the kind of things she is interested in can't be seen with a telescope.

Or at least not with a telescope that the rest of us would recognise.

For the past four years, McEnery and her colleagues have been creating a map of the universe by using the Fermi space telescope, which is in orbit around the earth, to plot gamma ray bursts.

As she puts it, the Fermi puts her one up on Superman - "he only had X-ray vision" - but it also provides an advantage over previous generations of astrophysicists by getting a more complete picture of exactly what's out there.

"With gamma rays, there is so much more to the universe than we can see with our eyes," she explained.

"The gamma ray sky is constantly changing. Each day, we find something new."

When she says "something new", it's somewhat of an understatement for one recent discovery: the biggest feature ever found in our galaxy.

The feature - one leading theory is it might be a remnant of an eruption from a supersized black hole at the centre of the galaxy - spans 50,000 light years.

First noticed by astronomers at the Harvard-Smithsonian Center for Astrophysics using images from the Fermi space telescope, the two huge bubble-shaped features extend above and below the centre of the disc-like galaxy.

To put that into perspective, our galaxy is between 100,000 and 120,000 light years across and the circumference of the Earth is 0.13 light seconds.

"We didn't know about it until we looked through the Fermi," she adds.

The feature was only detected because the Fermi space telescope has been scanning and rescanning the sky every three hours since its launch in June 2008.

As it detected gamma ray bursts - it recorded its thousandth in September, well ahead of pre-launch predictions - Fermi also discovered scores of pulsars shining only in gamma-rays. These remnants of exploding stars produce tens to hundreds of intense gamma-ray pulses every second.

That's a vindication for the millions of dollars it cost to build the Fermi space telescope and then launch it into orbit around the Earth, above the atmosphere that usually stops gamma rays from reaching terrestrial sensors.

It also demonstrates how visible light is just a tiny fraction of the spectrum of electromagnetic radiation that can be detected. Trying to work out what's happening in the universe by only observing visible light, as astronomers have had to do for nearly the entire history of the science, is a bit like buying the full cable television service from OSN and only watching the Cartoon Channel.

McEnery describes gamma ray observation as "the exciting part of astrophysics" because they are often created when stars explode.

"You don't produce gamma rays from gently shining stars - we have gamma ray bursts from exploding stars," she added.

"These are the brightest explosions we know of - close to the Big Bang."

Whereas the Milky Way is a faint glow visible on moonless nights on earth, she said it looks completely different to the sensors of the Fermi space telescope.

"If you see the sky with gamma ray vision, the Milky Way is blazing bright.

"We can figure out where the [gamma ray burst] came from and how much energy it has."

The space telescope is a combination of complexity and simplicity. Just to design it required an understanding of the theory of relativity.

"This is one of the places where Einstein comes to the rescue. E=MC2 actually matters here," she says.

"It's an equation telling us that we can take pure energy like gamma rays and convert it to mass, which is the M of the equation."

They then had to build the largest silicon detector the world had ever seen, with enough silicon to build a million digital cameras.

"It's powered by the equivalent of a hair dryer - 650 watts produced entirely by solar energy," McEnery adds.

"The information is sent back using the equivalent of a dial-up [internet] connection."

And then they had to blast it into space, where it has to dodge the flotsam of space junk that is also in low-Earth orbit.

That latter challenge is not an academic one. In April, the Fermi space telescope found itself on a collision course with an old Soviet-era satellite.

"It was moving at 12 kilometres per second and it was predicted to be within 200 metres of us. It was expected the two craft would occupy the same place in space for 20 milliseconds," she says.

Fortunately for Nasa, the space telescope had been fitted with a propulsion system intended to manoeuvre it to fall into the ocean when it reached the end of its useful life. It was used instead to move it out of the path of the Soviet satellite.

"In the game of chicken, we moved out of the way," she adds. "This is the first time we've ever used the propulsion system."

In the meantime, researchers' information about the universe is growing incrementally with each three-hour scan of the sky. Nasa and its academic partners are hoping for even more impressive results.

"Another thing the Fermi can do is in measuring dark matter. The universe is largely made up of material that we can't see or interact with," McEnery adds.

"One common explanation for what it might be is a kind of particle that doesn't interact very much. You can see the gravitational effect but very little else.

"It's possible the Fermi might be able to directly detect dark matter in our galaxy or anywhere where dark matter could be. At the moment, we have tentative results and some evidence to see dark matter annihilation at the centre of our galaxy. If this result gets confirmed, we'll be identifying the nature of dark matter."

If there's a Nobel Prize to be based on the Fermi's data, she explains, this is likely to be the area of research for which it is awarded.

McEnery had been in Abu Dhabi to give a presentation about the Fermi space telescope in the week before the supposed December 21 Mayan apocalypse and there was a certain depressing inevitability that she would be asked about the likelihood that the world would end.

After sidestepping the question with the sort of aplomb to which most politicians could only aspire - "It's been discussed by others but it's not closely related to the research I do" - she did say there was the prospect of the Earth being zapped by a burst of gamma rays and "it would indeed be quite bad".

This was an instant wake-up call for an audience of which some (by which I mean, of course, me) had struggled to grapple with casually cited terms like foamy space-time (in which subatomic turbulence means energy decays into particles and antiparticles that then annihilate in a way that can curve space time), collapsars (a type of high-energy supernova when a star collapses) and string theory (an attempt to reconcile quantum mechanics and Einstein's theory of general relativity).

And especially when she mentioned that one theory postulated for the demise of the dinosaurs had been a gamma ray blast.

"At the minimum, you'd destroy electronics on the satellites in orbit because they're not protected by the atmosphere," she said.

"The side of the Earth facing the gamma ray burst would be inundated with energy very rapidly and it would cause serious issues for anyone on that side and, in a few hours, the rest of us."

And when is this going to happen? Well, there was no answer for that.

John Henzell is a senior features writer for The National.