THE WONDERS OF RADIO ASTRONOMY
Radio can unmask many hidden properties about the universe, including merging galaxies and Fast Radio Bursts. Professor Lisa Harvey-Smith speaks to All About Space about the mysterious majesty of the field
What can be learnt from radio astronomy that isn’t revealed in other spectrums of light?
Until about 400 years ago, we were, as human beings, completely blinkered. We didn’t have telescopes. We just had our own eyes and we just had the eyes that have evolved over millions of years of animal evolution on planet Earth. For that reason we can only see a tiny, narrow part of the spectrum. It was only much more recently that we started taking off the blinkers, certainly in the early 20th century. We started to realise there were other types of light that we could gather. And one of those invisible colours is radio waves.
We started to use those radio waves, not just for communication, but to passively receive information from space. Radio waves let you see the invisible. And they also let you find out so many layers of colours that we can’t see. For example, in the middle of our galaxy – the Milky Way – there’s so much dense, dusty gas that is completely black. All of that is hidden from view until we use infrared radiation, or if we use radio waves, and that’s what those different colours can teach us about how hot the gas is, about what it’s made of, what chemicals are up there in space and also the types of physical conditions in the stars.
A lot of the scientific goals of ASKAP are centred around galaxies, whether it’s their evolution, formation or population. What links them?
There are lots of forces in our universe.
Some hold together the middle of atoms, like electromagnetism. They hold together all the sort of physical matter that we are made of, a table, a chair, the planet. But gravity holds together things on these larger scales, so gravity is the master of the universe. It’s quite a weak force but it’s a pulling force. We live in a single galaxy called the Milky Way with 200 billion stars and tonnes and tonnes of gas. But we also live in a region with about 50 other galaxies, and they are our sort of galactic family, called the ‘Local Group’, and gravity glues all that together.
How important is the study of galaxies to understanding how the universe was born?
Galaxies are kind of the basic unit of the universe, if you like. There are smaller things. There are atoms, I would say, as an analogy of the universe. [Galaxies] are made of stars, just like atoms are made of electrons, protons and neutrons. There are smaller objects, but galaxies are kind of the fundamental unit, and each galaxy tells us a different story. It’s a snapshot of a very, very long life of billions of years. Most galaxies are 12 or 13 billion years old. Some are younger. But many of them have a very complicated life story. They started off with a particular colour, a particular shape and a particular size, and they’ve evolved through time. Many of them have collided, many of them have merged with other galaxies and many have had huge things smashed into them, completely transforming their colour, shape, mass and size. The fact that they’re all rushing apart from each other due to expansion has taught us about the Big Bang and the fact that the universe started all in one place.
Do you see a correlation in galaxies’ appearances as you look further back in time?
Yeah. Well, a lot of the galaxies started to appear
quite soon after the Big Bang. We don’t know exactly how long. That will be the job for the new Square Kilometre Array (SKA) telescope to see when the very first stars and galaxies were born. But we think it’s less than a billion years after the Big Bang, which isn’t too long, given that we’re nearly 14 billion years now on the clock.
You’ve talked about the eventual collision between the Milky Way and the Andromeda Galaxy. Assuming humanity is still alive on Earth at this point, what kind of effects would we experience from that kind of galactic collision?
This is absolutely fascinating. I could go on all day about it. I explained this in my book, When Galaxies Collide, but the Milky Way galaxy and the Andromeda Galaxy – the nearest spiral galaxy to us – are currently rushing together at 400,000 kilometres (250,000 miles) per second. And we should collide in about 3.8 billion years.
Now when two objects like two cars collide: smash! You get an immediate rebound. You get a lot of damage. But galaxies are mostly empty space – like space is full of space. The stars are very small, and there are billions of them, but there’s a lot of gaps between them. When the two galaxies collide, they should actually just pass through each other like two ghosts.
There is other stuff in the galaxy though, like the gas that fills the spiral arms. Now that gas will – due to gravity – sort of stick together, and it will kind of crush the gas and cause that gas to get very dense, or squashed. That can trigger star formation, so there will be a whole new generation of stars created, kind of like a firecracker of events. Suddenly there will be probably thousands, if not millions, of new bright stars, just like the brightest ones that we see in the sky, like Sirius and Rigel. They’ll be big, bright, white supergiant stars. It’s going to be very exciting. Will we be here on Earth? Not quite, because the Sun is getting a bit hotter every year. Just a tiny bit, and in a billion years the Earth will be uninhabitable. But maybe humans will have moved to another planet or another moon in the Solar System. And maybe life will be there somewhere looking up into the sky and watching the trillion stars of the new ‘Milkdromeda’ galaxy twinkling in the night sky.
How do you come to this conclusion? Is this based on observations of other galaxies or is this the result of computer simulations?
Both. We look for galaxies that are currently colliding, and you can see loads of them. If you use the Hubble Space Telescope and you look at one piece of sky, you can see thousands of galaxies, and some of them will be smashing together. One beautiful example is the Antennae Galaxies. That is just spectacular and you can see the buckling of the spiral galaxies as they are merging together. You can see how it will evolve.
But also simulations, as you say. With computers we can program a computer to do a ‘let’s pretend’. Let’s pretend two galaxies collide. There are 200
billion stars in this galaxy; there are 300 billion stars in the other galaxy. Type the laws of gravity in and it simulates what will happen. It’s an incredibly powerful tool for astronomers to then determine what is likely to happen in the future.
Doesn’t this idea of the Andromeda and the Milky Way galaxies moving towards each other contradict Hubble’s law in the sense that galaxies are supposed to be moving away from each other?
Yeah. It’s confusing to a lot of people, and understandably so. Hubble’s law, yes – the universe is expanding. And if you look at a galaxy farther away, it’s moving away from us faster. Hubble’s law can tell us the age of the universe. It’s perfection. But there are other factors in there too that we don’t always mention in that, which is the fact that gravity works on small scales. When I say small scales – I mean within trillions of light years or so – two galaxies can actually have a gravitational effect on each other. Whereas for galaxies on the other side of the universe, that effect is almost zero. Therefore gravity is irrelevant in terms of the expansion of the universe.
The ASKAP project has had a hand in the recent news regarding Fast Radio Bursts, and more specifically FRB 180924. What are Fast Radio Bursts and why are they such a mystery?
They are really cool. They’re huge bursts of radiation that are as strong as a star exploding as a supernova. They’re incredibly short-lived. They last less than a millisecond, while other big explosions will last a little while. You imagine a bomb going off or if somebody had dropped something, you can kind of hear the noise for a little while. But these things are just a tiny, tiny flash. [They are] very, very compact and very fast. They happen and then they go away, and they never come back, largely. Recently [astronomers] have started finding ones that repeat, so that’s very interesting.
The answer to ‘what are they?’ We literally don’t know. We have no idea. They might be something related to stars and black holes interacting. They might be related to stars colliding, but we just don’t know.
With the recently discovered FRB 180924, it has now had an observed period of activity – lasting around 16 days – and it’s even had its location pinpointed to a galaxy. How important is this information for trying to understand these weird cosmic signals?
It’s incredibly important. We’ve been trying to pinpoint them to galaxies since they were discovered, and that’s only really been done once or twice now. We can guess how far away they are, and we do that by looking at the colours of light from these flashes.
We know that they are really, really far away.
But these are the first times, with the last couple of Fast Radio Bursts, where it’s been able to pinpoint exactly where they are in space. That’s really important because that tells us how bright it is. That tells us what kind of process it can be and rules out some of the weaker kinds of explosion that we might think it could be.
Is it kind of like a gravitational wave detection where you receive a detection and then you need to focus all the other available global telescopes on to the source?
That’s right. Certainly when the first gravitational wave detection came through from LIGO. I was working at the Australian Telescope National Facility at the time, and I got an email at 02:00 or 03:00 or something. I didn’t pick it up because I was asleep, but I was getting calls saying, “Can we get the telescope onto this object?” I think by 06:00 or 07:00 we got there and we’re searching the sky for radio signals from this gravitational-wave event. LIGO is doing incredibly well now, so there’s lots more of that science going on.
Is there plenty more for radio astronomy to get involved in with that line of research?
Absolutely. We know what gravitational waves are caused by; they are caused by two masses accelerating, and it could be anything. If they’re visible to our detectors they are likely to be very heavy and compact stars like pulsars, neutron stars and black holes. By looking at the radio signals to see if there’s any emission when, say, stars collide or black holes collide, we can tell what types of objects they are.
When you were researching your book, was there a particular topic within astronomy or astrophysics that you think will see some major revolutions in the future?
Yes. I think our understanding of dark energy is something that will shift very dramatically. This idea that the whole universe is filled with this so-called ‘dark energy’, which is kind of like an antigravity, or vacuum energy, but what it means is that it’s pushing the universe to expand faster and faster as time goes on. We don’t know if dark energy is uniform throughout the universe. We don’t know if it’s real. We don’t know if it’s made of particles, or if it’s like a field or a force field. We don’t know anything about it apart from what we can observe, which is that in the past the universe wasn’t expanding as fast as it is today.
That is an amazing mystery. It affects everything in cosmology – about the history of the universe – and it affects where we’re going to in the future and whether the universe will expand forever.