Greetings from Palmia Observatory
Well this week has been a busy, over scheduled week of two online conferences, which both occurred at the same time of the day.
When I explained to Astronomer Assistant Ruby that I planned to switch back and forth between the two events, and make at least some sense of it all, she seemed a bit skeptical!
|Astronomer Assistant Ruby is a little skeptical (Source: Palmia Observatory)|
Anyway the plan is to comment now on the 13th annual Laser Interferometer Space Antenna (LISA) Symposium, which was held September 1-3, 2020. Next week's post I plan to make some comments on the Human To Mars Summit, which was also held online from August 31 to September 3, 2020.
|Front web page for LISA XIII Symposium (Source: lisasymposium13.lisamission.org)|
LISA is an ESA mission planned for the 2030 time frame and will extend the range of observed gravitational waves to the 0.0001 to 1 Hz range, much lower than possible with LIGO. The gravitational wave spectrum is shown below in a slide from Mansi Kasliwal's presentation. This extended frequency ranges bring more astrophysical objects, like binary objects such as white dwarfs and other objects that will continue spiraling around each other for many years while continuously emitting gravitational waves.
|LISA to observe Gravitational Waves at lower frequencies (Source: Mansi Kasliwal, LISA VIII)|
So, how does LISA detect gravitational waves? It detects the stretching of space between freely falling test masses just like at LIGO, except the arms are much longer. Remember that the LISA system will consist of three orbiting satellites where the path between the three make something like an equilateral triangle. This combination of satellites operates as an optical interferometer, just like LIGO, with exception that the length of the arms is now 2.5 million km, not just a mere 4 km. In the diagram below, from Jeffrey Livas's presentation, you can see two of the satellites, each with their own isolated, free floating and falling test mass, and the optical paths to the other satellite.
|LISA arm length of 2.5 million km, compared to LIGO 4 km (Source: Jeffrey Livas, LISA XIII)|
Just in case you are into telescope optical design, this next slide shows some of the LISA telescope optical specifications. Just imagining that the telescope has a 20 micro-radian field of view is enough to give me a headache. Just to think that this field of view is maintained over the distance of 2.5 million km between orbiting satellites that are being perturbed by other planetary gravitational fields, etc., is just amazing that it can be done. Well, this work has been going on for at least 13 years and still a lot more work is necessary to get this LISA system ready for launch.
|Key LISA Telescope Optical Requirements (Source: Jeffrey Livas, LISA XIII)|
In addition to very tight telescope pointing accuracy requirements the noise floor for measuring the stretching of space between the two test masses is also very daunting. This next slide, by Daniel Vetrugno, shows some of the noise sources that interfere with making gravitational wave measurements. Recall for our visit to LIGO in Louisiana, we know that motion of the ground for Earth based measurements and isolation from such motion is a key design requirement and the mechanical and seismic filtering is a big part of the installation. In space, there is no ground motion noise, but the satellites do move around a bit. But look at the other sources of noise. I had not recognized that the test masses, which are floating inside each satellite, must not be allowed to contact the walls of their confinement chamber. It turns out that because of high energy cosmic rays that can pass right through the satellite can also interact with the test masses and cause them to become charged. The electrostatic force from these charges must be neutralized or the test mass will collide the chamber walls. Also as the satellite adjusts its course, ever so slightly, the test mass must be controlled so that is always operating as if it were in free fall.
|Key sources of noise in LISA system (Source: Daniel Vetrugno, LISA XIII)|
Notice too that even though the satellites are in outer space, there are still enough residual gas molecules that the occasional molecule striking the test mass can impart a random acceleration that also must be neutralized. It is pretty amazing to think that a randomly moving gas molecule can bump into the floating test mass and cause enough acceleration to disturb the interferometer performance with an arm length of 2.5 million km.
|Even sparse molecules in outer space introduce Brownian noise (Source: Daniel Vetrugno, LISA XIII)|
So, after all of these noise sources and telescope pointing issues are taken into account the expected gravitational wave signal sensitivity can be pretty good as seen in this chart below. You can see the sensitivity in the 0.1 to 0.001 Hz GW frequency range is very good.
|Expected gravitational strain sensitivity for LISA (Source: www.elisascience.org)|
Ok, so assuming that all of these system requirements can be met, how will LISA contribute to this next era of Multi-messenger Astronomy? The main new measurement capability occurs not by measuring what happens when black holes collide and merge over time scales of a few seconds, but the long term observation of binary systems that continuously emit low frequency gravitational waves over periods of many years and that the associated electromagnetic spectrum from those systems can be measured at the same time. It can also happen that the binary stars will end up as a Type 1a supernova. Just consider that white dwarf binaries are located throughout all the Milky Way, including the satellite galaxies, like the SMC and LMC, as outlined in this slide by Valeriya Koral.
|Binary white dwarf pairs are located everywhere (Source: Valeriya Koral, LISA XIII)|
There are already apparently catalogs of tens of thousands of detectable white dwarf binaries and they are located all over the visible sky. When LISA is operational, we can get direct measurement of their distances. Knowing their distance and other characteristic measurement of their optical properties helps with better understanding of the ongoing astrophysics.
|Multi-messenger astronomer with binary white dwarfs (Source: Valeriya Koral, LISA XIII)|
There are thousands of binary pairs and they all generate a gravitational wave signal. It is not yet clear to me how the gravitational wave from one particular binary pair is going to be extracted from this cacophony of all the other binary sources? The received signal could be the summation of maybe thousands of sinewaves of different frequencies and amplitudes. It seems you want to be able to identify the direction in space of one particular sinewave associated with the one particular binary pair. Measurement of the light curve phase could help constrain and identify the correct gravitational signal.
In this next slide, Valeriya explains how the gravitational wave signal can exist over many observable years, during which we should be able to measure the slowly declining orbital period during which the binary pair slowly gets closer and closer together. You can also see in the right panel, how optical measurements of binary pairs now would be able to generate gravitational waves that are louder than the projected minimum sensitivity of LISA. Great!
|Predicted change in binary white dwarf period over many years (Source: Valeriya Koral, LISA XIII)|
One of my friends, Searching for Gravity Waves, Dr. Gary, is doing research on how measurement of both the electromagnetic signals and gravitational wave signals can be used to constrain features of general relativity. For instance, you can compare the arrival times of the phase of the optical eclipse timing from a binary pair and the arrival of the gravitational waves generated by the orbiting pair of stars. The best case would be for a binary pair that we look at edge on. Both signals should arrive at the same time according to GR. In this next slide, from Dr. Gary's presentation at LISA XIII, you see an example of how both signals are received from the one binary pair.
|Comparing phase difference between arrival of GW and EM (Source: Gary Lamotte, LISA XIII)|
In this next slide, you can see a simulated light curve from a binary pair of white dwarfs and the associated gravitational wave signal. The GW signal helps determine the distance to the pair independently of other measurements. The phase difference, if any, can be used to test various theories of quantum gravity, such as if the mass of the graviton is other than zero. Thanks for that, Gary!
|Comparing eclipsing binary EM phase with GW phase (Source: Gary Lamotte, LISA XIII)|
Ok, so that is my brief summary of some of the LISA XIII Symposium. Next week, hopefully, I will report some of the findings from the Humans to Mars Summit, and we will see if the Astronomer Assistant Ruby was right to be skeptical that listening to two conferences at the same time would make any sense or not!
Until next time, here from our burrow, stay safe, as we recover more of our freedom,