- 4113 - NEUTRINOS - where do they come from? Pinpointing cosmic neutrino sources opens up the possibility of using the particles as a new probe of fundamental physics. Researchers have shown that the neutrinos can be used to open cracks in the reigning Standard Model of particle physics and even test quantum descriptions of gravity.
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4113 - NEUTRINOS
- where do they come from?
-
- Physicists finally know where at least some
of these high-energy particles come from, which helps make the neutrinos useful
for exploring fundamental physics. Since
2012, the “IceCube Neutrino Observatory” at the South Pole has detected a dozen
or so cosmic neutrinos each year.
-
- The Sun send us photons of light. And photons are how we know and undersand our
world. But, the Sun and other parts of
the universe sends us neutrinos too.
They are almost massless like photons.
They carry no electric charge, they are neutral, so do not interact with
matter.
-
- Of the 100 trillion neutrinos that pass
through you every second, most come from the sun or Earth’s atmosphere. But a
smattering of the particles, those moving much faster than the rest, traveled
here from powerful sources farther away.
-
- For decades, astrophysicists have sought
the origin of these “cosmic” neutrinos. Now, the IceCube Neutrino Observatory
has finally collected enough of them to reveal telltale patterns in where
they’re coming from.
-
- The new map shows a diffuse haze of cosmic
neutrinos emanating from throughout the Milky Way, but strangely, no individual
sources stand out. “It’s a mystery” .
-
- IceCube study from last fall was the first
to connect “cosmic neutrinos” to an individual source. It showed that a large
chunk of the cosmic neutrinos detected so far by the observatory have come from
the heart of an “active” galaxy called NGC 1068. In the galaxy’s glowing core,
matter spirals into a central supermassive black hole, somehow making cosmic
neutrinos in the process.
-
- Little is known about how the activity
around some supermassive black holes generates these neutrinos, and so far the
evidence points to multiple processes or circumstances.
-
- Abundant as they are, neutrinos usually zip
through Earth without leaving a trace; a
huge detector had to be built to detect enough of them to perceive
patterns in the directions they arrive from. IceCube, built 12 years ago,
consists of kilometer-long strings of detectors bored deep into the Antarctic
ice.
-
- Each year, IceCube detects a dozen or so
cosmic neutrinos with such high energy that they clearly stand out against a
haze of atmospheric and solar neutrinos. More sophisticated analyses can tease
out additional candidate cosmic neutrinos from the rest of the data.
-
- Astrophysicists know that such energetic
neutrinos could only arise when fast-moving atomic nuclei, known as cosmic
rays, collide with material somewhere in space. And very few places in the
universe have magnetic fields strong enough to whip cosmic rays up to
sufficient energies.
-
- Gamma-ray bursts, ultrabright flashes of
light that occur when some stars go supernova or when neutron stars spiral into
each other, were long thought one of the most plausible options. The only real
alternative was active galactic nuclei, or AGNs —galaxies whose central
supermassive black holes spew out particles and radiation as matter falls in.
-
- In 2016, IceCube began sending out alerts
every time they detected a cosmic neutrino, prompting other astronomers to train
telescopes in the direction it came from. They tentatively matched up a cosmic
neutrino with an active galaxy called TXS 0506+056, or TXS for short, that was
emitting flares of X-rays and gamma rays at the same time.
-
- More and more cosmic neutrinos were
collected, and another patch of sky began to stand out against the background
of atmospheric neutrinos. In the middle of this patch is the nearby active
galaxy NGC 1068. IceCube’s recent analysis shows that this correlation almost
certainly equals causation.
-
- There’s less than a 1-in-100,000 chance
that the abundance of neutrinos coming from the direction of NGC 1068 is a
random fluctuation. These two AGNs
appear to be the brightest neutrino sources in the sky, yet, puzzlingly,
they’re very different. TXS is a type of AGN known as a blazar: It shoots a jet
of high-energy radiation directly toward Earth.
-
- Yet we see no such jet pointing our way
from NGC 1068. This suggests that different mechanisms in the heart of active
galaxies could give rise to cosmic neutrinos.
Astronomers
suspect there is some material surrounding the active core in NGC 1068 that
blocks the emission of gamma rays as neutrinos are produced. We know very little about the cores of active
galaxies because they are too complicated.
-
- The cosmic neutrinos originating in the
Milky Way muddle things further. There are no obvious sources of such
high-energy particles in our galaxy, in particular, no active galactic nucleus.
Our galaxy’s core hasn’t been bustling for millions of years.
-
- Astronomers speculate that these neutrinos
come from cosmic rays produced in an earlier, active phase of our galaxy. The accelerators that made these cosmic rays
may have made them millions of years ago.
-
- What stands out in the new image of the sky
is the intense brightness of sources like NGC 1068 and TXS. The Milky Way,
filled with nearby stars and hot gas, outshines all other galaxies when
astronomers look with photons. But when it’s viewed in neutrinos, “the amazing
thing is we can barely see our galaxy,” . “The sky is dominated by
extragalactic sources.”
-
- Neutrinos offer rare clues that a more
complete theory of particles must supersede the 50-year-old set of equations
known as the Standard Model. This model describes elementary particles and
forces with near-perfect precision, but it errs when it comes to neutrinos: It
predicts that the neutral particles are massless, but they aren’t, not quite.
-
- Physicists discovered in 1998 that
neutrinos can shape-shift between their three different types; an electron
neutrino emitted by the sun can turn into a muon neutrino by the time it
reaches Earth. And in order to shape-shift, neutrinos must have mass, the
oscillations only make sense if each neutrino species is a quantum mixture of
three different very tiny masses.
-
- Dozens of experiments have allowed particle
physicists to gradually build up a picture of the oscillation patterns of
various neutrinos, solar, atmospheric, laboratory-made. But cosmic neutrinos
originating from AGNs offer a look at the particles’ oscillatory behavior
across vastly bigger distances and energies.
-
- Cosmic neutrino sources are so far away that
the neutrino oscillations should get blurred out wherever astrophysicists look, they expect to
see a constant fraction of each of the three neutrino types. Any fluctuation in
these fractions would indicate that neutrino oscillation models need
rethinking.
-
- Another possibility is that cosmic neutrinos
interact with dark matter as they travel, as predicted by many dark-sector
models. These models propose that the universe’s invisible matter consists of
multiple types of nonluminous particles. Interactions with these dark matter
particles would scatter neutrinos with specific energies and create a gap in
the spectrum of cosmic neutrinos that we see.
-
- Or, the quantum structure of space-time
itself can drag on the neutrinos, slowing them down. A group based in Italy
recently argued in Nature Astronomy that IceCube data shows hints of this
happening, but other physicists have been skeptical of these claims.
-
- The exact fraction of each neutrino type
depends on the source model, and the most popular models, by chance, predict
that equal numbers of the three neutrino species will arrive on Earth. But
cosmic neutrinos are still so poorly understood that any observed imbalance in
the fractions of the three types could be misinterpreted. The result could be a
consequence of quantum gravity, dark matter or a broken neutrino oscillation
model, or just the still-blurry physics of cosmic neutrino production.
-
- We need to detect many more cosmic
neutrinos. IceCube is being upgraded and expanded to 10 cubic kilometers over
the next few years, and in October, a neutrino detector under Lake Baikal in
Siberia posted its first observation of cosmic neutrinos from TXS.
-
- And deep in the Mediterranean, dozens of
strings of neutrino detectors collectively called “KM3NeT” are being fastened
on the seafloor by a robot submersible to offer a complementary view of the
cosmic-neutrino sky.
-
- What will we learn with these new eyes?
-
-
August 6, 2023 4113
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