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SUNDAY SCIENCE: BLACK HOLE FLYBY – DAVID KAISER ON THE MYSTERY OF
DARK MATTER  
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David Kaiser
May 30, 2024
London Review of Books
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_ What if dark matter is just ordinary matter locked inside black
holes – from which, after all, light cannot escape. Such massive,
dark objects would trundle around the cosmos, nudging the motion of
visible matter while evading direct detection. _

Hubble views a supermassive black hole burping - twice, NASA

 

For​ more than fifty years, physicists have been stumped by dark
matter. Careful measurement of a range of phenomena, from the motion
of enormous clusters of galaxies to the rate at which individual
galaxies spin, have indicated that all the stuff astronomers can see
– the trillions of stars dotted across the night sky – contributes
just a fraction of the total mass of the universe. The observations
suggest that ‘missing mass’ exerts a gravitational pull on visible
matter, altering the paths of the objects that we can see. The
mysterious matter doesn’t light up on its own; it remains dark. And
there is a lot of it: for every kilogram of matter visible throughout
the cosmos, more than five kilograms of dark matter seem to lurk
unseen.

Dark matter – whatever it is – played an essential role in the
development of the universe. It was thanks to dark matter that pockets
of ordinary matter began to clump into stars and galaxies soon after
the Big Bang; without that added gravitational effect, the rapid
expansion of the universe would have diluted ordinary matter before
such structures could have formed. No dark matter, no stable galaxies;
and without stable galaxies, like our own Milky Way, no humans to
search the sky and wonder.

Physicists’ first solution, when originally confronted with the
puzzle of dark matter, remains the most popular: perhaps some new,
hypothetical elementary particles exist – cousins to the familiar
electrons and quarks – which interact via gravitation but remain
impervious to light. Over the years, various contenders have been
proposed, from WIMPs (weakly interacting massive particles), which
might weigh anything between ten thousand and a million times more
than an electron, to the pipsqueak ‘axions’, which might be
trillions of times lighter than an electron. In fact, both WIMPs and
axions were first posited to address other riddles in particle
physics, but before long, physicists recognised that if such particles
really were skittering around the cosmos, they would behave just like
dark matter.

There’s just one snag: after decades of meticulous experiments, no
clear evidence has turned up that any such particle exists. Buried
deep beneath the Gran Sasso mountains in northern Italy, for example,
the XENON collaboration has been surveilling huge vats of liquid
xenon for about twenty years, looking out for the telltale flashes of
light that should occur when a xenon nucleus is struck by an
incoming WIMP. The researchers have pushed the sensitivity of their
detector to record levels, yet no WIMPs have been found. Meanwhile,
other projects, such as the Axion Dark Matter Experiment (ADMX) based
at the University of Washington, have been trying to catch an axion.
When it traverses a strong magnetic field, this hypothetical particle
should convert into pairs of photons. Yet after thirty years of
dedicated searching, aided by remarkable improvements in magnet and
detector technologies, not an axion in sight.

Others have wondered whether taking account of dark matter requires
that the laws of gravity themselves be altered. For more than a
hundred years, physicists and astronomers have tried to make sense of
the universe by using Einstein’s general theory of relativity. It
explains all the phenomena we associate with gravity – from the fall
of an apple on Earth to the swirling motions of distant galaxies and
beyond – in terms of the warping of space and time. Large clumps of
matter distend spacetime much as a bowling ball warps a trampoline;
this curvature, in turn, bends the paths of nearby objects away from
the straight and narrow. Predictions based on Einstein’s relativity
have withstood every test that physicists and astronomers have been
able to concoct. And when attempts have been made to modify general
relativity to accommodate, say, data on various galaxies’ rates of
spin, these changes have typically introduced inconsistencies between
theory and observations of other phenomena.

A third possibility remains, equal parts audacious and mundane. What
if dark matter is just ordinary matter locked inside black holes –
from which, after all, light cannot escape. Such massive, dark objects
would trundle around the cosmos, nudging the motion of visible matter
while themselves evading direct detection. No need to speculate about
hypothetical particles with exotic properties; no need to wreck the
rules of relativity. The idea, in outline, is not new, but it has
attracted increasing attention in the scientific community over the
past decade.

Einstein himself resisted the notion of black holes, though eventually
physicists came to see them as a robust prediction of relativity.
Among the most important clarifications came from J. Robert
Oppenheimer, who was teaching theoretical physics at Berkeley in the
1930s when he and a graduate student, Hartland Snyder, worked out what
would happen to a star after it exhausted its nuclear fuel. With no
more outward-directed pressure coming from nuclear reactions in its
core, they concluded, a massive star would collapse in on itself. The
dense concentration of matter that remained would severely deform the
surrounding spacetime, trapping even light-rays. Their paper, treated
at the time as a theoretical curiosity, appeared in the _Physical
Review_ on 1 September 1939, just as Nazi tanks rolled into Poland.
Before long, Oppenheimer was swept up in the nascent nuclear weapons
project; neither he nor his student published on black holes again.

Years later, in a paper published in 1966, the Soviet astrophysicist
Yakov Zeldovich wondered whether black holes might have formed soon
after the Big Bang. Zeldovich, a long-time leader of the Soviet
nuclear weapons programme, had a clearer understanding than most of
the way matter behaved under extreme heat and pressure. Thanks to an
intensive journal-translation effort launched in the 1950s – with
secret underwriting by the US Air Force and CIA – Zeldovich’s
article was republished in English in 1967, though it didn’t find
many readers in the West.

Stephen Hawking independently broached the idea in a brief, crisp
paper from 1971 demonstrating, more explicitly than Zeldovich, that
black holes could have formed very early in the history of the
universe. He first noted that ‘ordinary’ black holes, of the sort
that Oppenheimer had considered, would result from the collapse of a
star, and that their mass would have to be roughly equal to the mass
of the Sun. In contrast, primordial black holes – a distinct type
that could have formed immediately after the Big Bang – would bypass
stellar evolution altogether, forming directly from the gravitational
collapse of some local lumpiness in the early distribution of matter.
As Hawking emphasised, such a direct collapse meant that primordial
black holes could form with an enormous range of masses, either much
smaller or much larger than the mass of the Sun. Hawking even
suggested that primordial black holes – having formed long before
the first stars or galaxies – might play the role of dark matter. He
pursued the idea at Cambridge in the 1970s, but few paid much
attention. Black holes of any type still struck most physicists and
astronomers at the time as a speculative curiosity.

By the mid-2010s, they were no longer regarded this way. Astronomers
had collected indirect evidence since the 1970s that enormous black
holes might lurk at the centre of most galaxies. They suspected that
such ‘supermassive’ black holes might have formed from the
collapse of ordinary stars long ago – exactly as Oppenheimer had
described – and then grown bloated over time by gobbling up huge
amounts of matter from their surroundings. But a few years ago the
picture changed dramatically. In February 2016, the
international LIGO-Virgo-KAGRA Collaboration, consisting of more
than a thousand researchers across 133 institutions on five
continents, announced the first successful detection of gravitational
waves. Einstein himself had predicted that objects’ violent motions
should excite tiny ripples in the taut fabric of spacetime. Yet
evidence of these ripples remained elusive for the next hundred years,
until the LIGO-Virgo team first measured them using a pair of
L-shaped detectors with legs four kilometres long. Early in the
morning on 14 September 2015, the detectors in Louisiana and in
Washington state rang in perfect synchrony – once you took into
account the time it takes for a gravitational wave, washing over the
Earth at the speed of light, to cross the distance between them. The
wave’s specific pattern indicated that it had originated in the
cataclysmic collision and merger of two large black holes far beyond
the limits of our own galaxy.

Three years later, another globe-spanning collaboration, the Event
Horizon Telescope team, released the first composite image of the
immediate vicinity of a black hole, a gargantuan entity at the centre
of galaxy M87, more than fifty million light years from Earth. The
swirl of visible matter and radiation revealed the shadow of a black
hole about 6.5 billion times more massive than the Sun.

These dramatic observations renewed physicists’ interest in
primordial black holes. As Hawking had emphasised in the 1970s,
stellar-collapse black holes form with masses comparable to that of
the Sun, whereas primordial black holes could form with a large range
of masses. Both the black holes that had caused the LIGO-Virgo
gravitational waves and the monster black hole in M87 were much more
massive than the Sun; especially in the latter case, astrophysicists
have struggled to put forward any plausible mechanism by which a
solar-mass black hole could have grown so large over the timescale
involved. In the past few months, data from the James Webb Space
Telescope has been used to identify other black holes that seem to be
much too large and much too old to be consistent with known
stellar-formation processes.

These whoppers boost the prospect that primordial black holes might
really exist. But they are awkward candidates for dark matter. If dark
matter consisted of such objects, they should be straying across
astronomers’ lines of sight with some regularity. Since black holes
warp the spacetime around them, their transit across the axis between
an observer and a distant star results in a temporary distortion in
the star’s brightness – an effect known as ‘gravitational
lensing’. But in 2019, astronomers using the Subaru Telescope on
Mauna Kea, Hawaii showed, by observing tens of millions of stars in
the nearby Andromeda galaxy and recording the frequency of lensing
events, that primordial black holes of a mass comparable to or greater
than that of the Moon could account for no more than 1 per cent of all
dark matter. Indeed, the implication was that if primordial black
holes were to account for dark matter, their typical mass could be no
greater than about one ten-billionth that of the Sun.

The lensing survey left open the possibility that dark matter might
consist of smaller primordial black holes. However, they can’t be
too small. The reason relates to the work Hawking did soon after
writing his first papers about primordial black holes, work in which
he made his landmark claim that, because of certain quantum-mechanical
effects, it was possible that black holes emitted radiation after all.

Since the 1930s, quantum physicists had predicted, on the basis of
Werner Heisenberg’s uncertainty principle, that pairs of tiny
particles – particle and antiparticle – must flit in and out of
existence all the time. Having temporarily ‘borrowed’ excess
energy from empty space, such pairs of particles must square their
accounts, recombining with each other and winking back out of
existence on a short timescale set by Heisenberg’s relation. Such
quantum fluctuations, first observed at Columbia University in the
late 1940s, are now routinely measured in precision experiments; the
phenomena match predictions from quantum theory all the way out to
twelve decimal places.

Hawking reasoned that if such a particle pair were to form near a
black hole, one of them might fall into the black hole before the pair
could recombine. Since nothing – not even a wayward quantum particle
– can escape from a black hole, the particle left outside would not
then be able to recombine with its partner to ‘pay back’ the
energy that together they had borrowed. Hawking suggested that instead
the gravitational field of the black hole would supply the balance of
energy, leaving the abandoned particle free to jet off. To a distant
observer, this process would appear as if the black hole itself had
radiated the particle. As Hawking summarised in his book _A Brief
History of Time_, ‘Black holes ain’t so black.’

Hawking​ emission would be much too weak to measure for large-mass
black holes, but it should have a more pronounced effect in the case
of small-mass black holes. What’s more, a tiny black hole should
shrink as it lends energy to the abandoned particles; and the smaller
its mass, the more efficiently it should radiate, shrinking at an ever
faster rate until it has evaporated away completely. The late-stage
emission process should spew out energetic charged particles as well
as high-energy gamma radiation.

Incredibly, some of the most important evidence concerning any
possible relationship between dark matter and small-mass primordial
black holes comes from equipment that is nearly as old as Hawking’s
prediction. Back in September 1977, Nasa launched the Voyager 1
spacecraft to probe the outer solar system and beyond. After it had
beamed back spectacular images of Jupiter and Saturn, the tiny craft
continued on its path further and further away from the Sun. In August
2012, Voyager 1 officially left the solar system. Since then, it has
been beyond the influence of the Sun’s solar wind and magnetic
field, and its onboard particle detectors – humble little 1970s
devices – have been immersed in a flux of interstellar cosmic rays.

If our universe were filled with ultra-low-mass primordial black holes
– enough of them to account for all the dark matter in the cosmos
– there should be a steady thrum of high-energy charged particles
criss-crossing empty space, the late-stage emission products from
black holes undergoing Hawking emission. In that case, Voyager 1
should by now be awash in such particles. Right up until the closing
weeks of 2023 (when a glitch in its onboard computer temporarily
interrupted its communications), it dutifully continued its reports,
each signal taking eighteen hours to reach Earth. The counts of
charged particles it detected remained low enough to rule out the
presence of a large population of ultra-low-mass primordial black
holes. And, basing their calculations on the connection between the
Hawking emission rate and the mass of a black hole, physicists have
used the Voyager data to place a lower bound on the mass of primordial
black holes that could comprise dark matter: no smaller than ten
million billion times less than the mass of the Sun.

These two groups of observations – modern lensing surveys and
particle counts logged on the rickety Voyager space probe – thus
delimit a range of masses within which microscopic primordial black
holes could account for all of dark matter: no larger than one
ten-billionth the mass of the Sun, and no smaller than one
ten-million-billionth the mass of the Sun. To press further, several
research groups (including my own) have proposed turning our local
cosmic neighbourhood into a vast high-precision dark-matter detector.
Astronauts on three Apollo missions – beginning with the Apollo 11
landing in July 1969 – placed special reflectors on the surface of
the Moon. Unpiloted craft from the Soviet Union in the early 1970s
and, in July last year, from India added several more. Within days of
the first installation, astronomers on Earth began directing lasers
from ground-based observatories to the lunar reflectors and carefully
timing the arrival of the return signals. Since 2007, these efforts
have achieved millimetre-level precision in measuring the distance
between the Earth and the Moon. Meanwhile, telemetry with Mars
orbiters and rovers over the past twenty years has enabled astronomers
to routinely measure the Earth-Mars distance to within ten
centimetres. Closer to home, the dozens of satellites in medium-Earth
orbit that comprise the Global Positioning System (GPS) network have
been tracked to within a centimetre, moment by moment for decades.

Given all this data, we may ask: are there any hints that a tiny
primordial black hole, with a mass within the prescribed range for
dark matter, has flown through the inner solar system? A flyby from a
microscopic primordial black hole would set visible objects wobbling,
just a tiny bit at first, but more and more over time and with a
particular pattern. The effect would be subtle, but could be just
large enough to register amid the glut of high-precision tracking data
for nearby satellites, the Moon or Mars.

If dark matter really does consist of tiny primordial black holes,
then such a flyby should have occurred about once every ten years.
Knowing how a visible object’s wobbles from such an event change
over time, we can now sift through decades of data to search for
specific types of anomaly – hints that Mars, say, was just a little
bit off from the location it should have been, if no black holes had
sped by and altered its path. We can pose the same question
prospectively, collaborating with astronomers who already track the
motions of objects in the solar system, keeping a lookout for
unexpected shifts in various objects’ locations over time. Compared
with the decades-long drought from experiments designed to detect
hypothetical dark-matter particles, the task of searching astronomical
data for tiny wiggles in the motion of Mars seems downright concrete.
By combining ageing Cold War infrastructure – space probes,
lasers, GPS – with otherworldly notions of warped spacetime,
researchers around the world may soon discover the nature of the
mysterious matter that shapes our universe.

_DAVID KAISER is a professor of physics and the history of science
at MIT. His most recent book, Quantum Legacies, appeared in 2020._

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Japan’s Push to Make All Research Open Access Is Taking Shape
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Dalmeet Singh Chawla
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Japan will start allocating the ¥10 billion it promised to spend on
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May 30, 2024

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