This episode is brought to you by the Music for Scientists album,
available now on all streaming services. To start listening, check out the link in the description.
{♫Intro♫}
Astronomers are trying to answer all sorts of very big questions about the nature of the universe,
but there are two puzzles that loom larger than the rest: dark matter and dark energy.
Dark matter is a strange form of matter that exerts gravity but doesn’t directly interact
with light. Scientists have lots of evidence that it’s there, but no idea what it actually is.
Dark energy is far more mysterious. It’s a vague term for an unknown force that seems
to be pushing the universe apart. Scientists basically know nothing about it, which is kind
of unfortunate since calculations suggest it might make up 70% of everything in nature.
Or… maybe it doesn’t exist at all.
That’s the idea advanced in a new study published last week in The
Astrophysical Journal, whose authors imagine a universe without dark energy.
instead, they say, its influence could be explained by dark matter.
The existence of dark energy was proposed to account for a surprising observation made in 1998.
In work that would later win the Nobel Prize,
two teams of astronomers used the Hubble Space Telescope to determine that the
universe isn’t just expanding -- its expansion is actually speeding up.
Which didn’t really make sense: after all, if every atom in the cosmos is pulling on every
other atom using gravity, then expansion should be slowing down or even reversing.
What could be strong enough to overwhelm the gravitational attraction of the entire universe?
Astronomers didn’t know then, and they don’t know now.
It got a catchy name -- we called it dark energy -- and that’s basically where things stand today.
Well, the authors of this new study are essentially saying, “OK, look,
we already have one mysterious substance in the universe: dark matter. Why can’t that be enough?”
They imagine that dark matter exerts
a previously unknown force that works kind of like magnetic repulsion.
Like gravity, this new force would get weaker with increasing distance.
But it would be different in that its power would increase
as two particles of dark matter were moving faster relative to each other.
Now that sounds absolutely wild, but it’s basically how the Lorentz force in
electromagnetism works, but it would only be exerted between atoms of dark matter.
So, as the universe expands, dark matter particles would start to move
away from one another, and that velocity would cause them to repel each other.
That repulsion would speed them up, causing them to repel even more, and accelerating
the speed of the expansion—exactly what astronomers have observed.
This is an attractive idea because it replaces two giant mysteries with one
slightly more complicated mystery.
But, like many things in theoretical physics,
it’s just an elegant bit of mathematics until we can find some actual evidence for it.
And since this is a brand new hypothesis,
we’re sure scientists will want to debate the idea a whole lot more.
But now on to something we can actually see -- or at least, observe.
Jupiter is blessed with a diverse set of auroras that are among the brightest in the solar system.
And according to a paper published recently in the Journal of Geophysical Research Space
Physics, there’s a new type we hadn’t seen before.
In the new paper, a team of scientists describes this aurora as a ringlike
structure that encircles the planet’s pole and expands outward up to a thousand kilometers.
The observations that led to this discovery were made by NASA’s Juno spacecraft,
which has been in orbit around Jupiter since 2016.
Juno has had a huge impact on aurora observations.
In the past, these were mostly done using the Hubble Space Telescope,
but its location in Earth orbit meant that it could only ever see part of Jupiter.
Juno instead passes over Jupiter’s poles on every orbit, giving it a unique view
that was critical to revealing these new auroras.
Like most of Jupiter’s auroras, the newly spotted rings shine
with ultraviolet light due to interactions with the hydrogen that makes up the planet.
And while they appear in Jupiter’s atmosphere, their true source is much farther away.
Scientists think these rings occur due to interactions that take place at the
boundary of the planet’s magnetic field, more than a hundred times Jupiter’s radius away.
There, Jupiter’s magnetic field encounters the solar wind, a stream of electrically charged
particles that emanates from the Sun’s surface and permeates the solar system.
Researchers aren’t totally sure how this interaction leads to these new auroras,
but they’ve got a couple of ideas.
They could be directly due to magnetic reconnection,
a process in which magnetic field lines bend, snap, and then rejoin in a new shape.
After a reconnection event, charged particles race along the new field lines
and into the planet’s atmosphere, causing some types of aurora.
Another possibility is a Kelvin-Helmholtz instability,
which occurs when the solar wind interacts with plasma trapped in Jupiter’s magnetic field.
Differences in velocities between these plasmas causes them to swirl,
twisting up the magnetic field lines and leading to a different kind of reconnection.
Either way, these auroras will help teach space physicists more about the violent
interaction between the Sun’s solar wind and the most powerful planetary magnetic field.
And since these phenomena may be involved in Earth’s aurora as well,
Juno may help us understand one of our own planet’s most spectacular sights.
Today we talked about how scientists are presenting and testing different
ideas about what the universe is made of -- is it dark energy? Dark matter?
Discarding wrong ideas until you find one that works is a cornerstone of science
and one that’s celebrated in the song The Idea from the album Music for Scientists.
The song also has a music video that’s a fusion of art and science,
combining traditional painting with machine learning to create unique visuals.
You can listen to the album, and check out the music video, at the link in the description below.
And thanks for the support!
{♫Outro♫}