 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to be talking about gravitational wave astronomy. This is a relatively new field even though it was predicted gravitational waves to exist by Einstein in the General Theory of Relativity. It has only been within the last decade that they have been detected. So what are gravitational waves? This was a prediction of general relativity. Essentially any accelerating object with mass will produce gravitational waves. Just as accelerating objects with electrical charges will produce electromagnetic waves – light that we see. So gravitational waves are another one of those based on the gravitational force. However, the gravitational force is weak. It is by far the weakest of the four fundamental forces of nature that we know of. And gravitational waves are really hard to detect. The earliest detection was in a binary pulsar. This was not proved gravitational waves, but it was found that the two stars orbiting each other were losing energy by the exact amount that was expected by general relativity. So we'd have two pulsars here and we could detect their pulses, and they would remember that pulses as they slowed down would lose energy, and we could calculate that as to their orbits and the orbit was slowing by the exact amount that was described by general relativity. It wasn't until 2015 that we got the first direct detection of gravitational waves from two colliding black holes. So how can we directly detect gravitational waves? Well, in order to detect it we need a strong source of gravitational waves. Anything moving with mass, you or me, are emitting gravitational waves – the sun, the moon, earth, all the objects that are moving around the solar system. They're all emitting gravitational waves. But with a very low mass, those waves are undetectable – they're much too weak. So we need things with big mass. Well, one thing is a black hole or a neutron star. These are high mass objects, and that will then give us an object that is much larger. Now their coalescing means that they're accelerating. They are zipping around each other very fast, and at that instant, as they are coalescing together to form a black hole, they give off very strong gravitational waves, which actually can be detected. Now they are detected by a gravitational wave observatory known as LIGO. LIGO stands for Laser Interferometer Gravitational Wave Observatory. Now it's not just one observatory, it actually has multiple sites. And there are two of these in the United States that are used, one in Louisiana and one in Washington State. Why? Because that eliminates local effects. We are looking for very tiny variations in the measurements of these lasers as they go down these very long paths and are bounced back and forth. So the distance needs to be measured to one ten thousandth the diameter of a proton. So you can imagine that somebody shutting a door or a truck rumbling by could easily give you a false detection. However if you have multiple sites, then you would only see it at one of them and not at both. So if you see something at both that matches the same thing, then you could conclude that that is a detection of gravitational waves. And after in 2015 we had our very first detection. Let's take a look at that detection here. So our very first detection of gravitational waves was known as GW150914. It's gravitational wave event 2015, September the 14th. That's when it was detected. It was not announced until it had been confirmed nearly six months later in February. Now the observations were consistent with the merger of two black holes, one of twenty times the mass of the sun and the other of thirty-six times the mass of our sun. And here's the observations from Hanford and Washington state and here from Livingston and Louisiana so we could match the two and find out that it was the same event that we were detecting. This was a merger that occurred one point three billion years ago. Gravitational waves travel very quick but when it's a large distance such as one point three billion light years away it takes them that long to get here to Earth. So we have to wait that time from the detection. So they were traveling through the universe for all that time and reached Earth in September 14th of 2015 giving us our first detection. Now that's not the only detection. We've actually had several more detections over the past few years and we actually have dozens of detections of gravitational waves now. Now let's look at some of those more detections. Again the observations from 2015 to 2020 and a new set of observations is scheduled to start in 2023. We have dozens like fifty or so that have now been detected. So trying to locate them is a little difficult. Again we only have two detectors there. Now the advanced LIGO which uses another other sets including one over in Italy allows us to then narrow down these a little bit more. But here we see them as stretched regions across the sky because we are limited by only the two observatories that were working at the time. Now as we look at others of these, again we have many black hole mergers. We also have neutron star mergers as well that have occurred. So again it's about fifty or so that as of this recording have been detected. Now what is the future for gravitational wave observatories? Well this again just like looking at things in other wavelengths of light it is getting a whole new view on the universe. As things improve we'll be able to detect weaker and weaker sources of gravitational waves. So the next generation we're looking at is LESA, the Laser Interferometer Space Antenna which is planned for launch in 2037 and will be three spacecraft in a triangle that are two and a half million kilometers on a side. So this will be able to detect mergers from supermassive black holes. We need this much longer baseline out here to be able to detect the gravitational waves from these supermassive black holes at the centers of galaxies. So continuing to use LIGO and looking at adding in LESA here in another decade or so will again is helping us to open up the universe to a new method of discovery much as radio waves did in the 1930s, 1940s, 1950s as that really got started opened up a whole new view on the universe and that's what we're getting here with gravitational waves that will again are being one of the next big things to study and another way of looking at objects in the universe. So let's go ahead and finish up with our summary and what we've looked at is that gravitational waves are produced by the motion of any massive objects but of course it's the high mass objects moving quickly that will produce the strongest gravitational waves. They were first detected in 2015 by LIGO and there have been dozens of detections since that time. We are now able to detect gravitational waves from neutron star mergers as well as black hole mergers. So continued studies of these will give us another insight into mergers of these very massive objects. So that concludes this lecture on gravitational wave astronomy. We'll be back again next time for another topic in astronomy. So until then, have a great day everyone and I will see you in class.