I think gravitational wave astronomy is one of the most exciting breakthroughs we’re tracking on Centauri Dreams. The detection of black hole and neutron star mergers has been a reminder of the tough elasticity of spacetime itself, its interplay with massive objects that are accelerating. Ripples in the fabric of spacetime move outward from events of stupendous energy, which could include neutron star mergers with black holes or other neutron stars. Earth-based observing projects like LIGO (Laser Interferometer Gravitational-Wave Observatory), the European Virgo and KAGRA (Kamioka Gravitational Wave Detector) in Japan continue to track such mergers.
But there is another aspect of gravitational wave work that I’m only now becoming familiar with. It’s background noise. Just as ham radio operators deal with QRN, which is the natural hum and crackle of thunderstorms and solar events, so the gravitational wave astronomer has to filter out what is being called the astrophysical gravitational wave background, or AGWB, as the inevitable acronym would have it. Astronomers also have to consider GW signals associated with events in the early universe, stochastic background ‘static’ that could have originated, for example, in cosmic inflation or the creation of cosmic strings.
The AGWB is the background noise of countless astrophysical events, a ‘hum’ from all sources emitting gravitational waves in the universe. Recent work has been showing that this collective signal, primarily from black hole and binary neutron star mergers, is detectable by the technologies we’ll be deploying in the 2030s in the European Space Agency’s Laser Interferometer Space Antenna (LISA) mission. And it’s clear that for gravitational wave astronomy to proceed, we need to remove the AGWB to uncover underlying signals.
New work now makes the case that, surprisingly, we also have to reckon with the background noise of binary white dwarfs, although I see in the literature that scientists were delving into this as early as 2001 (citation below). In two recent papers, Dutch astronomers have developed models demonstrating that the background noise of white dwarfs would actually be stronger than that produced by black holes. Gijs Nelemans (Radboud University (Nijmegen, the Netherlands), who is working with the software and guidance mechanisms for the LISA mission, is a co-author on two papers on the subject. He sees white dwarf background noise as a way of studying stellar evolution on a galactic scale:
“With telescopes you can only study white dwarfs in our own Milky Way, but with LISA we can listen to white dwarfs from other galaxies. Moreover, in addition to the background noise of black holes and the noise of white dwarfs, perhaps other exotic processes from the early universe can be detected.”
Image: Dutch astronomer Gijs Nelemans. Credit: TechGelderland.
Nelemans has been developing the models described in the two recent papers with students Seppe Staelens and Sophie Hofman. Their work is significant given that until now, the LISA mission had not factored in a noisy white dwarf background problem. In a paper published in Astronomy & Astrophysics, the authors point out:
Given the amplitude of the WD component… it is expected that it can be very well measured by LISA. Furthermore, the relative amplitudes show that, if LISA detects an AGWB signal in the mHz regime, it is likely dominated by the WDs. This means that it is likely hard to make statements about the BH (and NS) population based on a measurement of the AGWB unless there is a way to disentangle the two, or to detect the high-frequency component of the AGWB above 40 mHz.
And in terms of the study of white dwarfs, the paper adds:
This offers an opportunity to study the WD binary population to much larger distances, while hampering the detection of the BH AGWB with missions such as LISA. The WD signal reaches a peak around 10 mHz and at higher frequencies the BH AGWB will become the dominant signal. The detectability of this transition by LISA and other mHz missions ought to be studied in detail.
Image: The LISA mission consists of a constellation of three identical spacecraft, flying in formation. They will orbit the Sun trailing the Earth, forming an equilateral triangle in space. Each side of the triangle will be 2.5 million km long (more than six times the Earth-Moon distance), and the spacecraft will exchange laser beams over this distance. This illustration shows two black holes merging and creating ripples in the fabric of spacetime. Some galaxies are visible in the background. In the foreground, the shape of a triangle is traced by shining red lines. It is meant to represent the position of the three LISA spacecraft and the laser beams that will travel between them. Credit: ESA.
This is indeed a unique kind of probe, because we’re talking about studying white dwarf evolution at high redshift in ways beyond the range of optical astronomy. Realize that only a small selection of gravitational wave sources can be detected with our current technologies. Millions of binaries in the Milky Way will simply merge into the stochastic foreground, a signal that is highly anisotropic (i.e., not uniform in all directions) while unresolved binary sources outside the galaxy produce a background signal that is profoundly isotropic, one that “encodes the combined information about the different source populations,” to quote the Hofman & Nelemans paper.
So we learn that filtering out white dwarf background mergers will be a major part of LISA’s investigations, but that the WD background is also a source of new information. LISA is to be the first dedicated space-based gravitational wave detector, involving three spacecraft in an equilateral triangle 2.5 million kilometers long in a heliocentric orbit. The European Space Agency hopes to launch LISA in 2035 on an Ariane 6.
The papers are Hofman & Nelemans, “On the uncertainty of the white dwarf astrophysical gravitational wave background,” accepted at Astronomy & Astrophysics (preprint); and Staelens & Nelemans, “Likelihood of white dwarf binaries to dominate the astrophysical gravitational wave background in the mHz band,” Astronomy & Astrophysics Vol. 683, A139 (March 2024). Full text. The 2001 paper is “Low-frequency gravitational waves from cosmological compact binaries,” Monthly Notices of the Royal Astronomical Society Vol. 324, Issue 4 (July 2001), pp. 797-810 (abstract).
1. Are the mHz frequencies the result of millions of interfering lower-frequency gravity waves?
2. Is the triangle arrangement of the LISA to provide some directionality information to the signal from a single event such as a BH merger?
3. If a large mass object was traveling at fractional c velocities, could it be detected by the “wake” of its moving space curvature?
My error. mHz = milli Hz not mega Hz
From what I know, LIGO and LISA can only detect mergers because only those are powerful enough gravitational radiation energy source to cause a space time ripple that we can detect. These cause a ripple of expansion and contraction oscillation of space time.
1. Frequency and period are inverses: s^-1 and s. 1 mHz implies a period of about 15 minutes. That’s short enough that you are only likely to see mHz frequencies for binary compact objects of some kind. Planets are another matter. For example, the gravitational wave frequency due to Earth’s orbit around the sun is about 30 ns. Of course there are trillions of likely planets in our own galaxy. It adds up to a lot of extremely low frequency “noise”.
2. Yes, that’s one important feature. That’s why LIGO needs 2 or 3 observatories on Earth to get an accurate direction for sources.
3. Linear (unaccelerated) motion generates no gravitational waves. You may be thinking of micro-lensing events by “wandering” rogue planets and brown dwarfs which are routinely observed.
Great! Laser interferometry looking for the gravity waves from white dwarf mergers in space. They are thought to make a supernova, neutron star or a bigger white dwarf. StackEschange source. Maybe LISA will help find evidence for that
I wonder if LISA would be able to detect type Ia supernova and correlate them with visual observations.
While replying to Alex, I remembered that I had an chart in my files that showed pretty clearly the potential gravitational wave sources. It plots amplitude vs frequency. They are many charts that also do the same, and have been updated with new predictions, but I’ve always preferred this one. It’s at the bottom of an old article by Sean Carroll. I did an image search and it popped up as the first result.
https://www.preposterousuniverse.com/blog/2016/02/11/gravitational-waves-at-last/
The chart in question from your excellent link shows the increasing last few waves occurring in about 1/100th of a second, i.e. at less than 10 mHz. I don’t understand why that translates to ~15-minute periods.
I have seen a YouTube video of a merging black hole binary that shows the waves in milliseconds.
Regarding linear motion of masses. No doubt microlensing is the best method to observe them, but it requires that the telescope is looking in the right direction. An all-sky telescope able to detect light changes from occlusions and microlensing and integrated to show motion would be needed to detect large moving masses against the stellar background.
However, while the repeating, high-frequency waves of binaries are detected by gravity telescopes it isn’t clear to me why a moving mass that is bending the curvature of space isn’t creating a transient long wave – a ripple that could be detected – in principle. After all, we detect irregularities in the surface gravity of the Earth by orbiting satellites that pass over areas of higher and lower gravity. Therefore a passing mass should perturb another object as a transient. I accept that detecting such a small, transient signal over a far longer time would be extremely hard and far beyond our technology, but I don’t understand the argument that it wouldn’t create a gravity wave.
@Ron
I am having a very off-day. I used a unit conversion to show that you are correct. 10 mHz has a period of 100s. [ I kept thinking milliHz was another name for kHz]
I need to lay down and let my brain recover. :-(
Is this a more general problem in science — one scientist’s noise is another scientist’s data? I recall reading several years ago about some scientists who were investigating if mini-black holes might be passing through the Earth. They tried to use seismograph data for this, but found they couldn’t because the data they wanted was filtered out as noise by conventional seismographs.
Gamma ray bursts and pulsars were discovered by the US military in the 1960s but were shelved by them as they weren’t Soviet missiles:
https://en.wikipedia.org/wiki/History_of_gamma-ray_burst_research
https://pubs.aip.org/aip/acp/article-abstract/983/1/642/620966/An-Independent-1967-Discovery-of-Pulsars?redirectedFrom=fulltext
FYI: Today just happens to be the 61st anniversary of the first Vela satellites launch.
https://en.wikipedia.org/wiki/Vela_(satellite)
The military’s efforts to detect underwater nuclear detonations has also led to findings in marine research:
https://www.bbc.com/future/article/20230921-scientists-built-a-listening-network-to-detect-nuclear-bomb-tests-they-found-blue-whales-instead
Who knows what else the military has found that they aren’t sharing with the rest of us for various reasons.
Tangential:
I watched a very different sci-fi B-movie Our Man in L.A. about a dealer involved in alien artifacts and trying to recover an intact UFO (probe) while being stymied by a secretive company/agency. Looks like a very cheap production, but the dialog is quite good and the plot is fairly original.
Something intrigues me: the gravitational wave distorts space and time of our universe so does that mean that the measurements of LISA, however accurate they are, will be systematically distorted; so that we can never know with precision the exact geographic point of the source of the wave? Are corrections taken into account in the devices a bit like in GPS ?
The space-time is distorted by a sine wave. Suppose a universe without a g. wave. in which one places a point A and further away a point B. (A is near the source and B further). Thus, one has a distance [AB] and a duration T to travel it in a straight line.
Now, let’s inject a g-wave in this universe: when the wave “go through” on point A and that it is for example at the peak point of the sinusoid, the distance and time [AB] is no longer the same as in a universe without g-wave; there is a distortion. Of where my question on the measurements because at these very low frequency because if the amplitude of the wave is very small but if there well is a small difference.
in some way we can not know the reality (still need to define it) what I find funny is that one discovery often hides another as if the universe does not reveal itself so easily. My story is sexy:)
BTW I’m happy, I found for 1€ a beautiful illustrated edition of 1982, “Cosmos” by C.SAGAN. The books of vulgarisation of that time when I was in high school were of a great level !
Fred
Might white dwarf stars interfere with gravitational wave signals?
https://phys.org/news/2024-10-dutch-students-space-mission-noisy.html
Would this be something fun for this instrument to chew on? https://phys.org/news/2024-10-physicists-neutron-stars-shrouded-clouds.html https://journals.aps.org/prx/pdf/10.1103/PhysRevX.14.041015
The paper predicts a very dense cloud of axions surrounds a neutron star. Unfortunately, they give this density as “potentially” exceeding 1E+22 GeV/cm^3. I’m starting to wonder if these people really should be allowed to use the metric system… unravelling that, I get 1E+3g/kg * 1 kg/(3E+8)^2J * 1.6E-19 J/eV * 1E+9/G * 1E+22GeV = 0.05 g/mL … denser than styrofoam. Can this triangle of light in space pick up whether a pair of colliding neutron stars is surrounded by a cloud of invisible styrofoam?
This is a presentation of LISA where we talk about gold cubes and space-time geometry. Technically, it would be interesting to understand the metrology of LISA…
https://www.esa.int/Science_Exploration/Space_Science/LISA/Capturing_the_ripples_of_spacetime_LISA_gets_go-ahead
Gravity and electromagnetic waves all travel at the same (light)speed; going past a point in space, the longer the wave, the fewer the waves that go past: MHz MegaHertz ⇨ a million waves go past in a second. 1Hz 1 Hertz ⇨ 1 wave goes past in a second. 1mHz 1milliHertz ⇨ one thousandth ¹/₁₀₀₀ of a wave goes past in one second; a thousand seconds (16.6⋅ minutes) for one wave to go past.