Chiara Mingarelli is a PhD student in the Gravitational Wave Group at the University of Birmingham. She is part of a team testing Einstein’s theory of gravity by attempting to detect and measure gravitational radiation. Chiara is also heavily involved in astrophysics outreach, speaking at a number of science events and festivals.
We think we live in a finite universe where we don’t have infinite densities floating around.
On becoming interested in science
I got interested in science when I was very young. My house was in the country, in Canada, where I’m originally from, and the skies were very dark. So at night I would go out with my friends and we would play hide and seek and I would have a lot of opportunity to look up at the stars and always wondered what was out there and if I ever would be lucky enough to be one of the people who could find new things and be a new kind of explorer and figure out what the secrets of the universe were, yeah.
On current research
So, right now I study gravitational waves from supermassive black holes, and black holes were one of the key things when I was much younger that really inspired me to go into science. They seemed to be these, you know, cosmic hoovers that were going around sucking up things, and for a certain amount of time I was frightened that the Earth would be sucked into one and I really wanted to know exactly what they were, how they worked and what their role was in the universe. So I’m really fortunate now to have been able to continue my study to the point where I think I understand what they are, what they do, and what their role in the universe is right now!
On black holes
So black holes are formed from stars that are about 20 times the mass of the sun, and at the end of their lives they implode on themselves and this implosion in smaller stars stops at about a neutron star, whereas in a black hole this reaction keeps going and the neutrons get compressed and compressed and compressed down to a point that we can only mathematically model as being a singularity, and that’s a point of infinite density.
Now we live in – we think we live in – a finite universe where we don’t have infinite densities floating around in infinite masses, but this is our mathematical way of describing it. So a black hole then has so much mass, and is so heavy, that it’s this big dip in spacetime. So, if you can imagine the surface of the universe being like this big rubber sheet, a black hole would be something so heavy that it deforms the rubber sheet. And light particles, or planets, or other stars, just following along, minding their own business, following along the curvature of spacetime, now, if you get to this black hole you start to follow the curvature down into its gravitational well and because the escape velocity of something that heavy is equal to the speed of light, you have to go a little bit faster than the speed of light to get out. And nothing can travel faster than the speed of light and that’s why black holes look black, because nothing can get back out of the well. So they just seem to be these large vacuum systems, right, where things go in but don’t come out again. And that’s true that things can’t escape from black holes, well, things like stars.
But some interesting developments have happened in that field as well. There’s something called Hawking radiation, and that’s when you’re at the event horizon of the black hole. So that’s where you’re at a point where on one side you can’t escape the gravitational pull of the black hole and the other side you can. So it’s like an imaginary line and on this side it’s black on this side it’s not. So, the universe is a really funny place. There’s particles that are created and annihilated all the time. So it’s like a bank: you can go and borrow the money as long as you pay it back right away. So you have something like an electron and an anti-electron, which is a positron. So you can borrow zero dollars, or zero pounds, from the bank. And this means you can have a pair of particles where one’s the particle and the other one is an anti-particle and together they make nothing. But you can separate them so you have, like, plus one and minus one and together you have zero.
Now in space, you have all of these fluctuations happening everywhere, where you have plus one and minus one and zero, and plus one and minus one, and they keep recombining. Now, this also happens on this boundary of a black hole, but what happens is you have this bank loan that happens and you get plus one and minus one but they can’t recombine because one is on the wrong side of the event horizon. So one is in the black hole and the other one leaves the black hole, and this we can model. And this is what’s called Hawking radiation. So the black holes have a kind of halo around them from all of these bad debts, as you could call them, that circle the black hole. They are very difficult to see but it’s one of the very interesting predictions of how black holes could eventually evaporate, because you have to pay the bank back. So what happens is that the black hole loses mass throughout his process, where we have all these unpaid debts of these interactions that happen on the event horizon. And this is how some baby black holes, ones that have very small mass, can just evaporate into nothingness.
On the existence of gravitational waves
So pulsar timing arrays right now are the only ways to detect very low frequency gravitational waves. So, because the distance from us to a pulsar is so long, it acts like a really long arm in a gravitational wave detector like the ones on Earth, and those are called LIGOs: Laser Interferometer Gravitational-wave Observatories.
Hopefully my research will culminate in the detection of gravitational waves and for pulsar timing arrays we can detect gravitational waves that have a period of one over the total time of observation of the pulsar. So, for example, if we’re monitoring a pulsar for ten years we are sensitive to gravitational waves with a period of ten years, so that’s a frequency of around three nanohertz, and that’s very long frequencies!
So these gravitational waves come from supermassive black holes when they’ve just started merging. So when they just get close to each other and start going around, but they’re very, very early on in their merger phase. Gravitational waves have not been directly detected yet. We know that they exist because Hulse and Taylor, two scientists in the 80s, found a double neutron star system and one of the neutron stars was a pulsar and they monitored the system over many years and there were two hypotheses: one that gravitational waves existed and the other that they didn’t!
So one was essentially a Newtonian idea, no gravitational waves, and the other was the Einsteinian theory, so there are gravitational waves. Now should gravitational waves exist, we would expect there to be a characteristic shrinking of the orbit because you’re losing energy due to this gravitational radiation and so you’d expect the orbit to shrink a little as they merge, whereas if you believe in Newton’s theory and think that Einstein was wrong, they’re not emitting gravitational waves, then they should just really stay in the same kind of orbit.
So they monitored this system over around ten years and they found that the gravitational wave prediction model was the best model and predicted the results to within half a percent. And so Hulse and Taylor then won the Nobel Prize in 1993 for the first indirect detection of gravitational waves. But because their effect, their direct effect on things on the Earth is so small, it’s so difficult to measure. We still haven’t been able to directly detect them yet but the race is on.
So there’s not only pulsar timing arrays that can detect gravitational waves directly, there’s also the Laser Interferometer Gravitational-wave Observatories called LIGO, and there’s two of those in the US and there’s one called the VIRGO and that one’s in Italy, and they can detect higher frequency gravitational waves. So they can detect gravitational waves from neutron star mergers which are in our galaxy or a close by galaxy. And they can detect gravitational waves to around 1KHz, whereas pulsar timing arrays can detect these very long wavelength gravitational waves, so in the nanohertz regime. So they’re completely complimentary experiments: they have different sources and they’re sensitive to different frequencies of gravitational waves.
On the importance of the research
It’s difficult to say immediately what the effect on human life or everyday life would be should we detect gravitational waves. One thing that’s really interesting from all of the technology that we need to build for gravitational waves is that we’re improving precision engineering, we are at the cutting edge of science with the laser interferometers. When we look for these signals we use supercomputers, parallel processing and we have big clusters that crunch all of this data and so we’re advancing a lot of different fields of science just by looking for these waves and letting scientists that are curious in these things do what they love.
So the effects on everyday life I’m not sure would be immediate but it’s one of the big question marks in the general theory of relativity. It’s one of the only predictions that haven’t been, ah, completely confirmed yet, or we don’t think that it’s very wrong, or wrong enough, that another theory would be correct, but in that sense it’s another way of testing Einstein’s theory of gravity. There are other theories out there that make similar predictions in weak gravitational fields but their predictions diverge in stronger gravitational fields like those around merging neutron star binaries or black holes, and we hope to be able to distinguish between these theories so we can either prove that Einstein was right, or he was right to 0.52%, or that another theory matches it better and maybe we should consider modelling our data in a different way. So it’s really the last big check in Einstein’s tick box for this gravitational wave detection.