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This episode is brought to you by the Music for Scientists album! Stream the album on major music services here: https://streamlink.to/music-for-scientists. Check out “The Idea” music video here: https://www.youtube.com/watch?v=tUyT94aGmbc.

Dark Energy is what we call the mysterious force that seems to be pushing the universe apart. By some calculations, it makes up 70% of everything in nature. Or...maybe it doesn’t exist at all! Plus, Juno’s observations give us new information about Jupiter’s magnificent magnetic light shows!

Hosted by: Hank Green

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Sources
https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2020JA028971
https://www.swri.org/press-release/swri-scientists-discover-new-auroral-feature-jupiter
https://www.missionjuno.swri.edu/orbit?show=hs_orbit_story_polar-orbit
https://www.nasa.gov/content/goddard/science-of-magnetic-reconnection
https://astronomy.swin.edu.au/cosmos/d/Dark+Matter
https://astronomy.swin.edu.au/cosmos/D/Dark+Energy
https://iopscience.iop.org/article/10.3847/1538-4357/abe5a2
https://arxiv.org/pdf/2102.07792.pdf
https://www.science.ku.dk/english/press/news/2021/new-study-sews-doubt-about-the-composition-of-70-percent-of-our-universe/
https://www.nasa.gov/mission_pages/hubble/news/hubble-nobel.html
https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy

Image Sources:
https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy
https://www.swri.org/press-release/swri-scientists-discover-new-auroral-feature-jupiter
https://upload.wikimedia.org/wikipedia/commons/d/d5/Jovian_magnetosphere_%28view_from_the_north_pole%29.png
https://www.missionjuno.swri.edu/orbit?show=hs_orbit_story_polar-orbit
https://hubblesite.org/contents/media/videos/2016/24/866-Video.html?news=true
https://www.nasa.gov/mission_pages/sunearth/news/gallery/cluster_magnetosphere.jpg.html#.YG9n2OhKhhE
https://solarsystem.nasa.gov/resources/2486/hubbles-new-portrait-of-jupiter/?category=planets_jupiter
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♫}.