Our map of the Milky Way is wrong!

We've been looking at our own galaxy for centuries, but it turns out that we've been completely misled. Our galaxy is way bigger than we originally thought. Hey Space Cats, I'm Dr Maggie Lu, let's talk about how the Milky Way is wrong, how we missed this, and what it changes for science. So up until the early 1920s, most astronomers thought that the Milky Way was our entire universe. It wasn't until 1923 that Edwin Hubble showed that the Milky Way was just one of many galaxies. Now fast forward 90 years to 2013, the European Space Agency's Gaia mission launched to build the most accurate 3D map of the Milky Way ever made. For over a decade, Gaia mapped the positions, the movements, and the colours of about 2 billion stars, revealing our galaxy's structure, including its spiral arms, that central bar, and that outer halo. Now thanks to Gaia, we thought our local cosmic neighbourhood looked like this. But a brand new study just suggests that the Milky Way actually looks more like this. So how could we get this so wrong? Well it turns out that mapping a galaxy from the inside out is incredibly difficult. You see, to map the galaxy, Gaia uses a technique known as parallax. It measures the distance to stars by calculating the slight apparent shift of nearby stars against distant background objects like other stars as the Earth orbits the Sun. The closest stars will appear to move more dramatically against a distant background than stars that are further away. Now this is great for nearby stars, but with severe dust extinction, stars become too faint to detect. And this is a problem because we live in the galactic disk and we're looking at stars through the spiral arms. In order to see through that fog, astronomers have to stop looking at optical light and start listening to the radio waves. Instead of stars, we map giant glowing clouds of hydrogen gas called H1 and H2 regions and cosmic masars, which are essentially naturally occurring space lasers that are powered by deep space chemistry. Now because these clouds are the nurseries where massive new stars are born, they trace the shape of our galaxy's spiral arms perfectly. And unlike starlight, radio waves are largely unaffected by absorption in the interstellar medium, so it easily penetrates the dust lanes. But radio telescopes don't just give us a simple distance photo. Instead, they measure speed. They track how fast these gas clouds are moving towards or away from us. And they do this by looking at how their radio frequencies stretch and squish. Now we call this the Doppler effect. To turn these speeds into an actual map with x and y coordinates, we have to rely on a mathematical model of how fast the galaxy is rotating at different distances from the centre. And now that is the Achilles heel of galactic cartography. If our rotational models are even slightly off, our whole map gets warped. So to sidestep this problem, a new study ditches the rotation models altogether and uses a purely geometric approach with x-ray light echoes from distant cosmic explosions. When a gamma-ray burst, a GRB, flares in a highly distant galaxy, its extremely powerful x-ray emission travels across the universe, billions of light years straight at us. But before it hits us, before it reaches the earth, that light has to pass right through the dusty spiral arms of our Milky Way. Most of the light shoots straight through, but some of it will hit dust grains and bounce off them at different angles. Because that scattered light had to take a detour, it travels at a slightly longer path and arrives at our telescopes just a fraction of a second after that direct flash of the GRB. To our space telescopes like NASA's Chandra and ESA's XMM-Newton, this manifests as a stunningly perfect concentric expanding rings of light. They're literally x-ray echoes in space. Now because we know the speed of light is an absolute constant, measuring the exact size of these expanding rings and time delay of these light echoes gives us pure raw geometry and it tells us exactly how far away those dust clouds are, with zero guesswork about how fast our galaxy is spinning. By digging through the archives, the research team analysed data from three separate gamma-ray bursts. One of them was GRB 221009A, the brightest cosmic explosion ever recorded by humanity. And the results were stunning. They found that the two most outer spiral arms of our galaxy, the outer arm and the outer Scutum Centaurus arm, are approximately 10% further away from our galactic centre than our standard velocity models had predicted. Now you may be thinking maybe they just hit a of isolated random clumps of dust, but the team checked for that. They verified that these dust layers span a massive continuous thickness of about 3,500 light years across. We aren't looking at isolated patches of dust, we are looking at the true massive structure of the outer galactic disc. So why does a 10% extension actually matter? Well because in galactic physics, size is fundamentally linked to mass and gravity. If the Milky Way's outer disc is larger and more spread out, we have to completely rewrite our models of how its mass is distributed. And this is where things get a bit crazy. When you combine this physically larger disc with how fast our outer stars are orbiting, the math reveals something unexpected. The Milky Way likely contains far less dark matter than we previously thought. The dark matter halo seems to be highly concentrated in those inner regions of the galaxy, falling sharply off at the edges. We are living in a much lighter, much fluffier galaxy. And a lighter Milky Way fundamentally reshapes our understanding of evolution. It alters the gravitational interaction in our local neighbourhood, rewriting the exact timeline and physics of our inevitable multi-billion year collision course with our neighbour the Andromeda galaxy. Now replicating this study isn't easy. It relies on incredibly rare events, gamma ray bursts. Gamma ray bursts are completely unpredictable and to map the galaxy this way, the burst has to happen at a very low galactic latitude, meaning its light has to pass directly through the dusty plane of our own disc. Over the last 25 years, astronomers have only found a handful of GRBs that are perfect enough to do this. But the future of galactic cartography is very bright. Next generation X-ray telescopes with massive upgrades in sensitivity are on the horizon. Instead of waiting for rare, monstrous explosions, they'll be able to capture faint X-ray echoes from much more common everyday cosmic events. Things like ESA's new Athena telescope will be scanning the night sky, capturing thousands of these faint glowing rings of light and slowly assembling them into a geometrically perfect 3D hologram of our home galaxy. It's wild to think that until 2014, most models predicted our Milky Way only had two major spiral arms before we confirmed that actually it has four. We are still learning. We are still rewriting this map of our home. But anyway, that is all I have time for this week. Thank you to my YouTube perks members for supporting this video. If you enjoyed it, please don't forget to leave me a like, share and subscribe. Soaring past my veiling thoughts to explore to the stars and more