Tag: gravitational lensing

Can gravitational waves be waylaid by gravity?

Yesterday, I learnt the answer is ‘yes’. Gravitational waves can be gravitationally lensed. It seems obvious once you think about it, but not something that strikes you (assuming you’re not a physicist) right away.

When physicists solve problems relating to the spacetime continuum, they imagine it as a four-dimensional manifold: three of space and one of time. Objects exist in the bulk of this manifold and visualisations like the one below are what two-dimensional slices of the continuum look like. This unified picture of space and time was a significant advancement in the history of physics.

While Hendrik Lorentz and Hermann Minkowski first noticed this feature in the early 20th century, they did so only to rationalise empirical data. Albert Einstein was the first physicist to fully figure out the why of it, through his theories of relativity.

A common way to visualise the curvature of spacetime around a massive object, in this case Earth. Credit: NASA

Specifically, according to the general theory, massive objects bend the spacetime continuum around themselves. Because light passes through the continuum, its path bends along the continuum when passing near massive bodies. Seen head-on, a massive object – like a black hole – appears to encircle a light-source in its background in a ring of light. This is because the black hole’s mass has caused spacetime to curve around the black hole, creating a cosmic mirage of the light emitted by the object in its background (see video below) as seen by the observer. By focusing light flowing in different directions around it towards one point, the black hole has effectively behaved like a lens.

So much is true of light, which is a form of electromagnetic radiation. And just the way electrically charged particles emit such radiation when they accelerate, massive particles emit gravitational waves when they accelerate. These gravitational waves are said to carry gravitational energy.

Gravitational energy is effectively the potential energy of a body due to its mass. Put another way, a more massive object would pull a smaller body in its vicinity towards itself faster than a less massive object would. The difference between these abilities is quantified as a difference between the objects’ gravitational energies.

Credit: ALMA (NRAO/ESO/NAOJ)/Luis Calçada (ESO)

Such energy is released through the spacetime continuum when the mass of a massive object changes. For example, when two binary black holes combine to form a larger one, the larger one usually has less mass than the masses of the two lighter ones together. The difference arises because some of the mass has been converted into gravitational energy. In another example, when a massive object accelerates, it distorts its gravitational field; these distortions propagate outwards through the continuum as gravitational energy.

Scientists and engineers have constructed instruments on Earth to detect gravitational energy in the form of gravitational waves. When an object releases gravitational energy into the spacetime continuum, the energy ripples through the continuum the way a stone dropped in water instigates ripples on the surface. And just the way the ripples alternatively stretch and compress the water, gravitational waves alternatively stretch and compress the continuum as they move through it (at the speed of light).

Instruments like the twin Laser Interferometer Gravitational-wave Observatories (LIGO) are designed to pick up on these passing distortions while blocking out all others. That is, when LIGO records a distortion passing through the parts of the continuum where its detectors are located, scientists will know it has just detected a gravitational wave. Because the frequency of a wave is directly proportional to its energy, scientists can use the properties of the gravitational wave as measured by LIGO to deduce the properties of its original source.

(As you might have guessed, even a cat running across the room emits gravitational waves. However, the frequency of these waves is so very low that it is almost impossible to build instruments to measure them, nor are we likely to find such an exercise useful.)

I learnt today that it is also possible for instruments like LIGO to be able to detect the gravitational lensing of gravitational waves. When an object like a black hole warps the spacetime continuum around it, it lenses light – and it is easy to see how it would lens gravitational waves as well. The lensing effect is the result not of the black hole’s ‘direct’ interaction with light as much as its distortion of the continuum. Ergo, anything that traverses the continuum, including gravitational waves, is bound to be lensed by the black hole.

The human body evolved eyes to receive information encoded in visible light, so we can directly see lensed visible-light. However, we don’t possess any organs that would allow us to do the same thing with gravitational waves. Instead, we will need to use existing instruments, like LIGO, to detect these particular distortions. How do we do that?

When two black holes are rapidly revolving around each other, getting closer and closer, they shed more and more of their potential energy as gravitational waves. In effect, the frequency of these waves is quickly increasing together with their amplitude, and LIGO registers this as a chirp (see video below). Once the two black holes have merged, both frequency and amplitude drop to zero (because a solitary spinning black hole does not emit gravitational waves).

In the event of a lensing, however, LIGO will effectively detect two sets of gravitational waves. One set will arrive at LIGO straight from the source. The second set – originally sent off in a different direction – will become lensed towards LIGO. And because the lensed wave will effectively have travelled a longer distance, it will arrive a short while after the direct wave.

The distance scale here is grossly exaggerated for effect

However, LIGO will not register two chirps; in fact, it will register no chirps at all. Instead, the direct wave and the lensed wave will interfere with each other inside the instrument to produce a characteristically mixed signal. By the laws of wave mechanics, this signal will have increasing frequency, as in the chirp, but uneven amplitude. If it were sonified, the signal’s sound would climb in pitch but have irregular volume.

A statistical analysis published in early 2018 (in a preprint paper) claimed that LIGO should be able to detect gravitationally lensed gravitational waves at the rate of about once per year (and the proposed Einstein Telescope, at about 80 per year!). A peer-reviewed paper published in January 2019 suggested that LIGO’s design specs allow it to detect lensing effects due to a black hole weighing 10-100,000-times as much as the Sun.

Just like ‘direct’ gravitational waves give away some information about their sources, lensed gravitational waves should also give something away about the objects that deflected them. So if we become able to use LIGO, and/or other gravitational wave detectors of the future, to detect gravitationally lensed gravitational waves, we will have the potential to learn even more about the universe’s inhabitants than gravitational-wave astronomy currently allows us to.

Thanks to inputs from Madhusudhan Raman, @ntavish, @alsogoesbyV and @vaa3.

Why do we need dark matter?

The first thing that goes wrong whenever a new discovery is reported, an old one is invalidated, or some vaguely important scientific result is announced has often to do with misrepresentation in the mainstream media. Right now, we’re in the aftermath of one such event: the October 30 announcement of results from a very sensitive dark matter detector. The detector, called the Large Underground Xenon Experiment (LUX), is installed in the Black Hills of South Dakota and operated by the Sanford Underground Research Facility.

Often the case is that what gets scientists excited may not get the layman excited, too, unless the media wants it to. So also with the announcement of results from LUX:

  • The detector hasn’t found dark matter
  • It hasn’t found a particular particle that some scientists thought could be dark matter in a particular energy range
  • It hasn’t ruled out that some other particles could be dark matter.

Unfortunately, as Matt Strassler noted, the BBC gave its report on the announcement a very misleading headline. We’re nowhere near figuring out what dark matter is as much as we’re figuring out what dark matter isn’t. Both these aspects are important because once we know dark matter isn’t something, we can fix our theories and start looking for something else. As for what dark matter is… here goes.

What is dark matter?

Dark matter is a kind of matter that is thought to occupy a little more than 80 per cent of this universe.

Why is it called ‘dark matter’?

This kind of matter’s name has to do with a property that scientists believe it should have: it does not absorb or emit light, remaining (optically) dark to our search for it.

What is dark matter made of?

We don’t know. Scientists think it could be composed of strange particles. Some other scientists think it could be composed of known particles that are for some reason behaving differently. At the moment, the leading candidate is a particle called the WIMP (weakly interacting massive particle), just like particles called electrons are an indicator of there being an electric field or particles called Higgs bosons are an indicator of there being a Higgs field. A WIMP gets its name because it doesn’t interact with other matter particles except through the gravitational force.

We don’t know how heavy or light WIMPs are or even what each WIMP’s mass could be. So, using different detectors, scientists are combing through different mass-ranges. And by ‘combing’, what they’re doing is using extremely sensitive instruments hidden thousands of feet under rocky terrain (or obiting the planet in a satellite) in an environment so clean that even undesired particles cannot interact with the detector (to some extent). In this state, the detector remains on ‘full alert’ to note the faintest interactions its components have with certain particles in the atmosphere – such as WIMPs.

The LUX detector team, in its October 30 announcement, ruled out that WIMPs existed in the ~10 GeV/c2 mass range (because of a silence of its components trying to pick up some particles in that range). This is important because results from some other detectors around the world suggested that a WIMP could be found in this range.

Can we trust LUX’s result?

Pretty much but not entirely – like the case with most measurements in particle physics experiments. Physicists announcing these results are only saying they aren’t likely to be any other entities masquerading as what they’re looking for. It’s a chance, and never really 100 per cent. But you’ve got to draw the line at some point. Even if there’s always going to be a 0.000…01 per cent chance of something happening, the quantity of observations and the quality of the detector should give you an idea about when to move on.

Where are the other detectors looking for dark matter?

Some are in orbit, some are underground. Check out FermiLAT, Alpha Magnetic Spectrometer,Payload for Antimatter Exploration and Light-nuclei Astrophysics, XENON100, CDMS, Large Hadron Collider, CoGeNT, etc.

So how was BBC wrong with its headline?

We’re not nearing the final phase of the search for dark matter. We’re only starting to consider the possibility that WIMPs might not be the dark matter particle candidates we should be looking for. Time to look at other candidates like axions. Of course, it wasn’t just BBC. CBS and Popular Science got it wrong, too, together with a sprinkling of other news websites.

Why do we need dark matter?

We haven’t been able to directly detect it, we think it has certain (unverified) properties to explain why it evades detection, we don’t know what it’s made of, and we don’t really know where to look if we think we know what it’s made of. Why then do we still cling to the idea of there being dark matter in the universe, that too in amounts overwhelming ‘normal’ matter by almost five times?

Answer: Because it’s the simplest explanation we can come up with to explain certain anomalous phenomena that existing theories of physics can’t.

Phenomenon #1

When the universe was created in a Big Bang, matter was released into it and sound waves propagated through it as ripples. The early universe was very, very hot, and electrons hadn’t yet condensed and become bound with the matter. They freely scattered radiation, whose intensity was also affected by the sound waves around it.

About 380,000 years after the Bang, the universe cooled and electrons became bound to matter. After this event, some radiation pervading throughout the universe was left behind like residue, observable to this day. When scientists used their knowledge of these events and their properties to work backwards to the time of the Bang, they found that the amount of matter that should’ve carried all that sound didn’t match up with what we could account for today.

They attributed the rest to what they called dark matter.

Phenomenon #2

Another way this mass deficiency manifests is in the observation of gravitational lensing. When light from a distant object passes near a massive object, such as a galaxy or a cluster of galaxies, their gravitational pull bends the light around them. When this bent beam reaches an observer on Earth, the image it carries will appear larger because it will have undergone angular magnification. If these clusters didn’t contain dark matter, physicists would observer much weaker lensing than they actually do.

Phenomenon #3

That’s not all. The stars in a galaxy rotate around the galactic centre, where most of its mass is located. According to theory, the velocity of the stars in a galaxy should drop off the farther they get from the centre. However, observations have revealed that, instead of dropping off, the velocity is actually almost constant even as one gets farther from the centre. So, something is also pulling the outermost stars inward, holding them together and keeping them from flying outward and away from the galaxy. This inward force astrophysicists think could be the gravitational force due to dark matter.

So… what next?

LUX was a very high sensitivity dark matter detector, the most sensitive in existence actually. However, its sensitivity is attuned to look for low-mass WIMPs, and its first results rule out anything in the 5-20 GeV/c2 range. WIMPs of a higher mass are still a possibility, and, who knows, might be found at detectors that work with the CERN collider.

Moreover, agreement between various detectors about the mass of WIMPs has also been iffy. For example, detectors like CDMS and CoGeNT have hinted that a ~10 GeV/c2 WIMP should exist. LUX has only now ruled this out; the XENON100 detector, on the other hand, has been around since 2008 and has been unable to find WIMPs in this mass-range altogether, and it’s more sensitive than CDMS or CoGeNT.

What’s next is some waiting and letting the LUX carry on with its surveys. In fact, the LUX has its peak sensitivity at 33 GeV/c2. Maybe there’s something there. Another thing to keep in mind is that we’ve only just started looking for dark matter particles. Remember how long it took us to figure out ‘normal’ matter particles? Perhaps future higher sensitive detectors (like XENON1T and LUX-ZEPLIN) have something for us.

(This post first appeared at The Copernican on November 3, 2013.)