I study and detect gravitational waves using millisecond pulsars. A very exciting turning point in my research occurred on 29 June 2023 when we (NANOGrav) announced the first detection of nanohertz gravitational waves. (Also see Haverford's news story.) To tell you that story let’s talk first about millisecond pulsars first and then talk about gravitational waves.
Millisecond pulsars are dead stars with about the mass of our sun, collapsed down to about the size of a small city (10km across), and spinning about as fast as your kitchen blender. The fastest pulsar known, PSR J1748-2446ad, spins on its axis 716 times each second. (Although stay tuned on that account…)
Pulsars are very accurate clocks, meaning they are predictable at the same level that atomic clocks are predictable. I use this feature of pulsars extensively. In particular, you can think of pulsars as a collection of clocks distributed throughout the galaxy. Anything that could disturb time, such as a perturbation in space-time, will affect the clocks. Pulsars are so predictable that we may be able to detect such a disturbance by observing pulsars.
Where would a space-time perturbation come from? It turns out that the Universe is filled with gravitational waves (traveling space-time disturbances) as both NANOGrav and LIGO have shown! This radiation is created, for example, anytime two black holes orbit around each other, and finally coalesce into a massive black hole.
Two current Haverford students Riley Starling '24 and William Hatfield '24 are doing their senior theses on what the new NANOGrav mean. They are interested in how many black holes are actually represented by the NANOGrav background detection. Could it be just one? Or is that ruled out by the data?
Much of my research is concerned with increasing the sensitivity of the Pulsar Timing Array (PTA), which is a collection of millisecond pulsars that are precise enough clocks that we use them to detect gravitational radiation. One of the largest sources of noise in PTAs is the delay caused by all the ionized hydrogen in space. It would be fine if it were a constant delay, but it changes as the pulsar moves relative to the earth, so the pulsar signal is going through variable amounts of hydrogen. The amount of the delay is dependent upon the radio frequency of the pulse (it’s proportion to 1/frequency ), so by observing at multiple frequencies we can (allegedly) calculate the amount of the delay. However, this doesn't working as well as we’d like. The scheme seems to get us to microsecond precision and we would like to get to nanosecond precision in the arrival times of the pulse.
So we observe in x-rays! Why does this help? Well x-rays are such high frequency that the (1/frequency) term is basically zero, and there is essentially no delay (at least on nanosecond timescales.) In the summer of 2017 the team I’m a part of launched an x-ray telescope called NICER (https://www.nasa.gov/nicer) and installed it on the International Space Station. We completed our mission of achieving timing precision of one part in 10^14 using this instrument, and even detected red noise in one of our pulsars.
Now we are moving on to further studies with NICER, for example Nate Ruhl '22, Noah Schwab '22, and Romana Hladky '22 developed a method for doing space navigation using an x-ray telescope (available in Advances in AAS/AIAA Guidance, Navigation, and Control 2022 when it becomes available). Seamus Flannery '23 extended the method to include gas planets not just rocky planets, and Jacob Kohn '25 mined the RXTE database to find additional serendipitous crossing events. Currently Kaia Reenock '26 and Allen Gift '25 are figuring out how to do the calculations that will allow to navigate in the space around the moon, so-called cislunar space.
For a review of this project please see the following: http://iopscience.iop.org/article/10.1088/0034-4885/78/12/124901/meta
Course web pages:
Waves and Optics was deemed one of Haverford's "cool" classes. Check us out: https://blogs.haverford.edu/haverblog/2018/11/16/cool-classes-waves-and-optics/
Pictures of my research group follow!
(Here's us on Haverford's twitter feed: https://twitter.com/haverfordedu/status/1052607755717103617
Summer 2019 at the International Pulsar Timing Array Meeting in Pune, India
Summer 2018 at the International Pulsar Timing Array Meeting in Albuquerque and Socorro, NM