Our galaxy, the Milky Way, is on a collision course.
But don’t hold your breath.
In roughly 4.5 billion years, the Milky Way and its nearest galactic neighbor, Andromeda, will merge. These galaxies — like all the others — each have a supermassive black hole at their center. As gravity pulls them closer and closer together, the black holes will almost certainly begin orbiting each other.
These objects contain so much matter (the Milky Way’s black hole is roughly four million times more massive than the Sun) that their dance will send a spiraling ripple through the fabric of spacetime.
Cosmologists call those ripples “gravitational waves.” They can roll through the universe for a very, very long time.
In a study published Thursday in the academic journal Science, a team of researchers report how they used our galaxy as a gargantuan scientific instrument to detect ancient gravitational waves that have been rolling through the universe since the first galaxies began to merge billions and billions of years ago.
“These ripples in space-time all merge together… and ultimately produce this extremely long-wavelength background of gravitational waves,” cosmologist Aditya Parthasarathy, a co-author of the new study, told IE.
This is the first study to use gamma rays — electromagnetic radiation in its most energetic form — to take measurements of the gravitational wave background.
If gravity had its way, the sheer mass of a star would cause it to collapse right away. And that's exactly what happens to stars of a certain mass once they start creating atoms so heavy that fusing them together doesn't create enough energy to push back against the patient, crushing pressure of gravity. As soon as it gets the upper hand, the star's own gravity sends it into a dramatically violent collapse-explosion that produces either a small black hole or a very dense bundle of matter called a neutron star.
Neutron stars spin very fast, and some of them shoot out a chaotic beam of radiation that includes everything from low-energy radio waves to highly energetic gamma-ray photons.
Here’s why that’s important for studying billions-of-years-old gravitational waves: the beam doesn’t have to point along the neutron star’s axis of rotation. More often than not, it sweeps the sky like the spinning light on top of an old-fashioned fire truck. That means that Earth-bound astronomers observing from far away can only "see" the light when it’s pointed more or less straight at Earth. From that perspective, the object looks like a rapid pulse of light, which is why that kind of neutron star is called a pulsar.
“You can imagine a pulsar like a lighthouse out in the galaxy,” Parthasarathy says.
“Certain kinds of pulsars, called millisecond pulsars, rotate really, really fast and very, very precisely,” Parthasarathy says. “We can know, based on a very simple model of the pulsar’s rotation, exactly when to expect the pulse.”
These rapid flashes of light are an invaluable source of information for cosmologists, especially when they flash hundreds of times per second. Researchers have found more than 400 of these millisecond pulsars scattered across the Milky Way. Roughly one-third of them emit the gamma rays that were useful to Parthasarathy and his fellow researchers
“Most of those 130 [pulsars] aren't bright enough to do this kind of analysis, so we get down to 30,” astrophysicist Matthew Kerr, another co-author, tells IE.
For almost 14 years, the Fermi Gamma-ray Space Telescope has orbited Earth at an altitude roughly a hundred miles higher than the International Space Station, quietly collecting evidence of gamma rays for researchers across the world.
“When the gamma-ray comes into our detector, we don't focus it or anything like that. It hits an atom nucleus somewhere and basically explodes,” Kerr says. “It actually just makes a particle shower in our detector, and we recreate the original direction of the particle.”
“We realized a couple of years ago that if we were to go and look for these gravitational waves with our gamma-ray data, we'd actually be in the same ballpark as [the network of radio telescopes on Earth that monitor pulsars by detecting lower-frequency radio waves]. This was kind of a surprise to us because we hadn't really thought about doing this before,” he says.
It’s time to zoom out. Way out. Nearly all the pulsars ever detected — and absolutely all of those that act as celestial beacons for astronomers — are located within our galaxy, the Milky Way. The pulses of light arrive at Earth after journeys of thousands of years. That’s because even light, which travels very fast, takes time to move across great distances.
But in the scheme of things, thousands of lightyears isn’t so far away, and thousands of years in the past isn’t so long ago.
Billions of years ago, before the Earth formed, entire galaxies were already swirling around. Every now and then, gravity would pull two of these galaxies toward each other.
“We know that at the heart of massive galaxies is a supermassive black hole,” Parthasarathy says. “In the early Universe, massive galaxies merged, which means that [their] supermassive black holes also merged.”
This is thought to have happened quite often over the course of cosmic history. But these are not straight-on collisions. Instead, the black holes orbit each other for some time before eventually falling into each other’s gravity. Probably. The details of how this endgame unfolds (cosmologists call it the “final parsec problem”) aren’t yet understood. Results from research like this could clear up the confusion.
Supermassive black holes are so massive— often containing something like hundreds of millions or billions of times the amount of matter in the Sun — that they orbit each other from a great distance. It can take a period of many years for a pair to make a full revolution around each other.
That amount of mass moving through space does something so difficult to imagine that it was literally Albert Einstein who, just over a century ago, first predicted that such a thing might happen.
“Each time a pair of supermassive black holes merge, they create ripples in spacetime,” Parthasarathy says.
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