Cosmologists have long suspected that conditions in the early universe shortly after the Big Bang may have led to the formation of black holes. These “primordial black holes” have a much broader mass range than those formed later in the universe by the death of stars, with some even condensed to the size of a single atom.
No primordial black holes have been observed so far. If they exist, they could explain at least some of the “dark matter” in the universe: matter that does not appear to interact with normal matter through electromagnetism, but affects the gravitational dynamics of galaxies and other objects in the universe.
We may now have a new way to detect primordial black holes, albeit in a very limited form.
This method uses gravitational waves.
This image shows the merger of two black holes (detected by LIGO on December 26, 2016) and the gravitational waves propagating outward as the black holes spiral toward each other. Image credit: LIGO/T. Pyle
Gravitational waves were first discovered in 2015 by the LIGO gravitational wave observatory. They are “ripples” in spacetime caused by dramatic events in the universe – most commonly the collision of giant stellar-mass objects such as black holes and neutron stars. The LIGO-Virgo-KAGRA (LKV) program has found about 90 confirmed gravitational wave sources since 2015.
In a research note published this month, Harvard astrophysicist Avi Loeb investigated whether the LKV detectors can capture the signature of primordial black holes – particularly those that speed past at nearly the speed of light – or other similar objects moving at high speeds.
“All gravitational wave sources discovered so far are mergers of astrophysical objects with stellar mass, such as black holes or neutron stars, at cosmological distances,” Loeb wrote in a Medium post in August. But these are not the only possible sources.
“Imagine a relativistic object moving at nearly the speed of light at a distance from LIGO comparable to the radius of the Earth. At closest approach, such an object would generate a gravitational signal,” one that depends strongly on its mass and the speed at which it is moving, Loeb says.
With LKV's current capabilities, the detectors could detect any object with a mass of 100 megatons (the mass of a smaller asteroid several hundred meters in diameter) moving at nearly the speed of light, but only if they approach the detectors to within half the diameter of the Earth.
In other words, the LKV detectors would have detected if an object of this mass had passed the Earth or in the immediate vicinity of its surface in the decade since 2015, provided it had been traveling at very high speed.
If an asteroid of this mass were to hit Earth at this speed, we would obviously know it by the devastating impact. Therefore, this ability is of particular interest for compact objects such as primordial black holes with diameters the size of an atom or smaller, which could fly past or even penetrate the Earth unnoticed.
The LKV detectors have not seen any such object.
This is not a surprising result, considering that this is a very limited detection capability. It does not provide information about objects further than 6,000 kilometers from the Earth's surface, nor does it detect slower-moving objects.
Future gravitational wave detectors such as ESA's LISA detector, expected to launch in the next decade, will extend this range, although not significantly.
However, if you're looking for answers to some of the universe's toughest questions, it's worth looking wherever you can. This special stone has not been left untouched.
Read the research note in RNASS here.
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