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The saga of how an odd hypothetical particle became a star dark matter candidate.
Conundrums
pervade physics, and they are, in some ways, what keeps it going. These
perplexing problems encourage a quest for the truth. But, of all the
conundrums, I'd say two of them are unquestionably priority A.
To
begin with, when astronomers gaze up at the sky, they see stars and galaxies
moving away from our planet and from each other in every direction. The
expanding cosmos resembles a bubble inflating up, which is how we know it's
expanding. But something isn't adding up.
There
doesn't appear to be enough matter floating around in space — stars, particles,
planets, and everything else — for it to expand at such a rapid rate. To put it
another way, the universe is expanding much faster than our physics predicts,
and it's getting faster as you read this. This brings us to the second issue.
According
to the best estimates, galaxies are spinning so fast as everything zips about
that the spirals should act like out-of-control merry-go-rounds tossing metal
horses from the ride. There doesn't appear to be enough material in the universe
to hold them all together. Despite this, the Milky Way does not appear to be
migrating apart.
So…
what's going on?
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Physicists
refer to "missing" things pushing the cosmos outward as dark energy,
and parts holding galaxies together as dark matter, presumably in a halo-like form.
They don't interact with visible light or matter, thus they're effectively
invisible. Dark matter and dark energy account for nearly all of the universe's
mass and energy.
"It
may well consist of one or more types of fundamental particle... while part or
all of it might consist of macroscopic lumps of some invisible form of matter,
such as black holes," the authors of a recent review published in the
journal Science Advances wrote.
Dark
matter, whether it exists in black holes or not, is completely elusive.
Scientists have chosen a few suspects from the cosmic lineup in order to unlock
its secrets, and one of the most sought particles is a strange little speck
known as the axion.
The wide-eyed hypothesis of axions
You've
probably heard of the Standard Model, which is essentially the holy grail of
particle physics and is constantly being strengthened. It explains how each and
every particle in the universe functions.
Some
"particle physicists are restless and dissatisfied with the Standard Model
since it has many theoretical inadequacies and leaves many crucial experimental
concerns unsolved," according to the Science Advances assessment. For us,
it immediately leads to a dilemma involving a well-known scientific principle
known as CPT invariance. The physics riddles keep on coming.
CPT
invariance states that when it comes to C (charge), P (parity), and T (time),
the cosmos must be symmetrical (time). As a result, it's also known as CPT
symmetry. It asserts that the cosmos would remain the same if everything had
the opposite charge, was left-handed instead of right-handed, and went through
time backwards instead of forwards.
CPT
symmetry seemed unshakable for a long time. Then there was 1956.
To
cut a long storey short, scientists discovered something that breaks the CPT
symmetry's P component. It's known as the weak force, and it governs events
such as neutrino collisions and the fusing of elements in the sun. Everyone was
taken aback, perplexed, and terrified.
CPT
symmetry is used in nearly every fundamental physics idea.
Researchers
found the weak force, which also violated C symmetry, about a decade later.
Things were starting to come apart. Physicists can only hope and pray that even
if P is broken... and CP is broken... CPT isn't broken. Perhaps the trio is
only required by weak forces to maintain CPT symmetry. Fortunately, this notion
appears to be right. Despite C and CP blips, the weak force follows entire CPT
symmetry for some reason. Phew.
But
there's a problem. You'd think that if weak forces violate CP symmetry, strong
forces would as well, right? They don't, and physicists have no idea why. This
is known as the strong CP problem, and it's here that things start to get
interesting.
The
strong force is observed by neutrons, which are uncharged particles within
atoms. Furthermore, their neutral charge means they violate T symmetry,
allowing for simplification. And, according to the study, "if we uncover
something that violates T symmetry, it must also violate CP symmetry in such a
way that the combination CPT is not violated." But... that's strange. The
strong CP problem prevents neutrons from doing so.
And
so the idea of the axion was born.
Roberto
Peccei and Helen Quinn, physicists, proposed adding a new dimension to the
Standard Model years ago. It involved a field of ultralight particles known as
axions, which solved the strong CP problem and hence relaxed neutron
requirements. According to the report, Axions' method seemed to work so well
that it became the "most common solution to the strong CP problem."
It was nothing short of a miracle.
To
be clear, axions are still hypothetical, however consider the following
scenario. Physicists have introduced a new particle to the Standard Model,
which depicts the universe as specks. What does this signify for the rest of
the world?
The key to dark matter?
Axions
would be "cool," or travelling very slowly through space, according
to the Peccei-Quinn theory. "The presence of [dark matter] is deduced from
its gravitational effects," the researchers write, "and astrophysical
measurements suggest that it is 'cool.'"
"There
are experimental top limitations on how strongly [the axion] interacts with
visible matter," the article adds.
In
other words, axions that assist explain the strong CP problem appear to have
theoretical features that are similar to dark matter. Very well done.
CERN,
the European Council for Nuclear Research, which runs the Large Hadron Collider
and is leading the charge for antimatter research, also emphasises the
importance of antimatter research "One of the most intriguing properties
of axions is that they could be produced in large numbers in a natural way soon
after the Big Bang. This population of axions would still exist today and
possibly make up the universe's dark matter."
So
there you have it. Axions are one of physics' trendiest topics since they
appear to explain so much. Those sought-after pieces, though, are still
hypothetical.
Will we ever find axions?
Scientists
have been looking for axions for 40 years.
According
to the authors of a recent assessment published in Science Advances, the
majority of these searches "primarily utilise the action field interaction
with the electromagnetic fields."
CERN,
for example, created the Axion Search Telescope, an equipment designed to
detect a trace of particles produced in the sun's core. There are tremendous
electric fields inside our star that might conceivably interact with axions —
assuming they're there at all.
However,
the effort has faced a number of significant obstacles thus far. For one thing,
"the particle mass is not theoretically foreseeable," according to
the authors, which means we have no idea what an axion would look like.
Scientists
are still looking for them, assuming a wide range of masses in the process.
However, recent evidence suggests that the particle's energy is likely between
40 and 180 microelectron volts. At around 1 billionth the mass of an electron,
that's unimaginably minuscule.
"The
axion signal is also likely to be very limited... and exceedingly low due to
very weak couplings to Standard Model particles and fields," the team
says. In other words, even though microscopic axions make every effort to alert
us to their presence, we may miss them. Their cues could be so subtle that we
wouldn't perceive them.
Despite
these obstacles, the axion hunt continues. Most scientists say that they must
exist somewhere, but when it comes to adequately describing dark matter, they
appear to be too good to be true.
The
study authors underline that "most experimental approaches assume that
axions compose 100 percent of the dark matter halo," suggesting that there
may be a possibility to "probe into axion physics without relying on such
an assumption."
What
if axions are merely one chapter of dark matter history, despite being the star
of the show?
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