It’s a little hard to describe what Priyamvada Natarajan does. On a broad scale, her work involves mapping dark matter in search of the elusive particle that scientists strongly believe exists, but have not discovered yet. As a professor of astronomy and physics at Yale University, Natarajan is able to put it in simpler terms. “It’s like looking at sand dunes, but not knowing what a grain of sand looks like,” she says.
This, of course, is no easy task, particularly when dealing with particles as elusive as dark matter. “We call it dark because it does not emit, absorb or reflect light, and the particles have a very peculiar equation of state, where they do not collide with, but instead graze past each other. The only force it does feel is gravity.”
At this point, the first question that pops to mind would be – why bother? The answer Natarajan offers is that discovering it would allow scientists to “close a gap in the cosmic inventory”. This ‘gap’ also just happens to be a large one. “All the particles that we are made of, only account for about four per cent of the cosmic inventory. Dark matter is one of the dominant constituents of the universe, which piled up in certain parts of the universe due to gravity, and in those regions, galaxies were formed. It is the unseen thing that holds the universe together. And once we fill this gap, we may find new classes of particles. We scientists have an obsession with unification, a grand synthesis. And dark matter is an irritating missing piece.”
Priyamvada Natarajan
Scientists are making progress however, helped along by modern technology. In her book Mapping the Heavens , she refers to the current era as the ‘golden age of cosmology’. “It is because of the sophistication of our theoretical understanding, the sophistication of the instruments that we have, and the computational power that we now have to interpret the data.” Images generated by the Hubble Space Telescope are Natarajan’s biggest aid in her quest for dark matter. The telescope captures high-resolution images of light distortions caused in areas around galaxies with a high-concentration of the particle, which are compared against images of the known shape of the galaxies, to plot the maps of its distribution.
Another instrument that Natarajan is looking forward to is the James Webb Space Telescope, which is set to provide unprecedented images of the low-energy infra-red window of the electromagnetic spectrum, which is not something the Hubble is built to handle. “The early universe was a dusty place, and the UV radiation from the hot, young black holes and stars would get enshrouded by dust re-radiated and scattered into red wavelengths like infra-red, causing these objects to remain obscured. Think of it like the X-Ray. If it wasn’t for the X-Ray, we would never know what our own physical structure was made up of.”
The massive boost in the computational power available to them is also aiding scientists like Natarajan in their work. Compared to when she started out, the computers of today can crunch significantly larger chunks of data much faster. “The desktop computers these days have the power to process the data I gather on gravitational lensing as part of my work on black holes,” she says, adding that other aspects of her work, like the calculations scientists are doing for the proposed Laser Interferometer Space Antenna (LISA) mission by the European Space Agency, are enough to give even supercomputers a few months of work. “In many ways,” she says nonchalantly, “astronomy was the first big data field.”
A scale model of the James Webb Space Telescope
The search for dark matter is still on, and Natarajan is constantly faced with new challenges. Some theories have explained away large quantities of dark matter by presenting modified versions of Newton’s laws, but she is sure that this elusive particle is out there. “It’s a matter of time. The Laser Interferometer Gravitational-Wave Observatory (LIGO) detected a collision between two super-massive black holes last year, but we’d been looking for that for 45 years. Maybe our instruments are not advanced enough, maybe we’re not looking in the right places, but I remain open-minded.”
And so the search for one of the building blocks of the universe continues.
