Daksha would be an invaluable tool to study flares from the Sun, X-rays from certain pulsating neutron stars, gamma-ray lightnings in the Earth's atmosphere, and so on.

If the dark matter of the universe is in the form of tiny black holes, an Indian astronomical mission has the best chance of unveiling it.

Everything in the celestial world visible to telescopes, such as stars, planets, and warm gas, makes up but one-sixth mass of the universe. The rest is in the form of some mysterious stuff that has no interactions with light, revealing its existence only by its pull on visible matter through the force of gravity. Known as “dark matter”, it is this invisible mass that shapes the structure and evolution of the cosmos.

And what is dark matter built from? Speculations abound. Theoretical physicists have offered that it is composed of closely packed particles that behave like waves (much as light does) or loosely packed particles that resemble fine flour, or lumped agglomerates of particles that resemble asteroids or planets. In all these cases, the species of particle is as yet undiscovered. A different and intriguing possibility is that dark matter is a population of black holes.

Black holes are mostly formed when a heavy star collapses due to its own gravity, but such specimens cannot make up much of the dark matter. Since dark matter is known to have been around since long before the very first stars were born, its constituent black holes must have emerged in other ways. They may be forged, for instance, via the crunching of matter in the violent conditions of the universe’s infancy when it was just 10-32 seconds old. Moreover, the familiar black holes formed through stellar deaths are, without exception, heavier than a few solar masses. But the “primordial black holes” (PBHs) of dark matter may be lighter, with the heft of even a minor asteroid. They cannot, though, be any lighter than 10-16 solar masses, as otherwise, they would have evaporated away by now through Hawking radiation.

So, how would you find these funny light black holes? By catching them in the act of bending starlight. Through this effect of general relativity, a star in the background may be briefly magnified in a telltale way. By looking for the fingerprints of this “gravitational microlensing” over the past few decades, telescopes have ruled out the possibility of PBHs making up all of the dark matter over a wide range of masses — from about 10-11 to 10 solar masses. That leaves open a wide “PBH mass window” of 10-16 to 10-11 solar masses, where tiny black holes could comprise all the dark matter. How tiny? In this range of masses, black hole sizes range from 0.01 nanometres to a micrometre, that is, a thousandth of the dimensions of an atom to the width of a bacterial cell wall!

For such a microscopic black hole to bend light and give a gravitational microlensing signal, it had better be larger than the wavelength of the light, for otherwise, the light wave would simply jump over it. This means it cannot deflect visible light — but can bend X-rays. And one source of X-rays shown to be promising for microlensing efforts is a class of objects called gamma-ray bursts (GRBs). These are hyper-energetic fountains of photons of all wavelengths lasting between milliseconds and half a minute. They are thought to be triggered by powerful supernovae or the mergers of ultra-dense solar mass bodies called neutron stars.

What Daksha Can Do

This is where the IIT Bombay-led project Daksha comes in. Named after the mythological son of creator-god Brahma, Daksha is a pioneering project proposed by astronomers across India. Two satellites would be placed in orbit on opposite sides of the Earth, monitoring most of the sky at any given time for GRBs. This double-satellite setup would be a 10-fold improvement over the sky coverage provided by existing missions.

Perhaps its significance is best illustrated by what happened in 2017. In a now-historic event dubbed GW170817, two neutron stars merged and emitted intense gravitational waves that were marked at the LIGO and Virgo experiments, a detection that was the first of its kind. The emission of gravity waves from such mergers are believed to be accompanied by copious discharge of electromagnetic energy. Sure enough, two seconds later, the Gamma Ray Burst Monitor instrument on the Fermi spacecraft, in orbit around the Earth, registered a signal. At this moment, other satellites in orbit — ISRO’s AstroSat and NASA’s Swift, which could also have seen a gamma-ray flash. That would have helped astronomers immediately localise the source with precision. But alas, AstroSat and Swift were just then on the other side of Earth, so their view of the GW170817 source was completely eclipsed by our planet.

Several neutron star mergers have been detected since 2017, but only in gravitational waves; no electromagnetic counterpart. The lesson from this episode rang clear. If a gravitational wave event is on, and you want the most rapid response to confirm and pinpoint an electromagnetic counterpart, be ready with not one but two sentries strategically placed in orbit. That is just what Daksha’s all-seeing twin satellites would do.

In addition to this advantage, Daksha would also carry on board three of the most sensitive detectors to catch X-rays and gamma-rays in space, spread over as large an area as practicable. It is estimated to turn up several thousands of GRBs, and tens of electromagnetic counterparts to binary neutron star mergers. It is worth emphasising that Daksha would not point at any particular source, but is rather a broad surveyor. “It is like having two ears open to pick up any sound from anywhere,” explained Varun Bhalerao, associate professor of astrophysics at IIT Bombay, at a recent workshop on the mission that he co-organised at the Indian Institute of Science, Bengaluru. For good measure, he has a second analogy up his sleeve: Daksha is a cricket fielder covering the entire field as opposed to a bowler focussing on a finite target.

What does all this have to do with the hunt for dark matter? Bear in mind that two satellites staring at one GRB give two slightly different lines of sight. Now imagine that, just when a GRB flash turns on for a few dozen seconds, there is an atom-size black hole somewhere between us and the GRB. (We are talking about a distance of a billion light years. Despite the colossal scale, the odds of a celestial body lying within the narrow view to a GRB are slim—yet not so slim that it is beyond the reach of human search.) Due to gravitational lensing, the GRB will be magnified, but due to the difference in lines of sight, either satellite will register a different magnification. A crude everyday comparison to this “parallax microlensing” technique would be the following. Close an eye and put out a thumb to partially shield the open eye from a bright lightbulb, and then open the first eye. You will find that it perceives a different brightness. Such a difference in magnification in the two satellites is an unmistakable sign of an object in the intervening line of sight; precise measurements could proclaim its properties such as its mass. This way, the PBH mass window would begin to get probed for the first time.

There is a slight catch, though. As Inter-University Centre for Astronomy and Astrophysics (IUCAA) astrophysicists Priyanka Gawade and Surhud More and IIT-B’s Varun Bhalerao show in their 2023 paper on Daksha’s microlensing capabilities, separating the satellites by about the diameter of the Earth will not quite suffice for catching PBHs in the act. The chances of finding at least one PBH transiting the view to a GRB would, at best, be about 50 per cent to 80 per cent. Those are pretty good odds, but one can ask for better. Practically, one could set the two satellites wider apart. In the analogy above, the brightness of the lightbulb seen by your two eyes would be yet more different if they were farther separated. Therefore, to say something conclusively about the make-up of dark matter by tiny PBHs, it would be ideal to have the two satellites separated by the Earth-Moon distance. This could be achieved either by having one satellite of Daksha orbiting the Moon while the other orbits the Earth, or by both satellites on a highly elliptical Earth-based orbit that also contains the Moon. While such a proposal sounds ambitious, it is doable, and a well-motivated design that may be taken up in the future.

Gravitational microlensing at Daksha exemplifies the principle: “If you build it, they will come”. An experiment may be devised with certain primary goals, but it can nonetheless birth impactful science that is tangential, or even dissimilar, to them. In addition to monitoring GRBs and neutron star merger electromagnetic counterparts, Daksha would be an invaluable tool to study flares from the Sun, bursts of highly magnetised neutron stars, X-rays from certain pulsating neutron stars, electromagnetic counterparts of enigmatic radio sources called fast radio bursts, remnants of past supernovae, gamma-ray lightnings in the Earth’s atmosphere, and so on. And as we saw here, with no extra cost at all — save for the data-analysing efforts of curious scientists — Daksha can make an emphatic statement about the nature and origins of our universe.

(Advanced Astronomy)