How do you observe that the Universe is expanding? 

In 2011, the Nobel Prize in Physics was awarded to Saul Perlmutter, Brian Schmidt and Adam Riess for discovering that the Universe is expanding at an accelerating rate. We’d known for a while that the Universe has been expanding ever since its birth - but we didn’t know whether the expansion was slowing down, staying the same, or speeding up. 

So, how exactly do you discover something like this? 

Perlmutter, Schmidt, and Riess did it by observing a special type of supernovae: Type Ia supernovae. Supernovae are the explosive deaths of large stars, and they usually occur when a star runs out of fuel and collapses under its own weight, generating a shockwave that blasts its material out into space. However, this only happens when a star is big enough - the initial star has to have a critical, threshold mass, called the Chandrasekhar limit. Our sun, for example, won’t go supernova because the Chandrasekhar limit happens to be around 1.4 solar masses. When it runs out of fuel, our sun will instead gently blow off its outer layers and quietly become a dense core of carbon and oxygen, called a white dwarf. 

But here’s the kicker: not all white dwarfs stay white dwarfs. 

Some white dwarfs exist as one half of a binary system, where two stars orbit each other in a perpetual celestial dance. In some situations, the white dwarfs can actually “steal” matter from their partner star, siphoning it off and guzzling it up to grow more and more massive. Eventually, when their mass hits the Chandrasekhar limit, the white dwarf is ripped apart in a supernova. 

Image Credit

This happens in binary systems all across the Universe, and because these white dwarfs all go supernova at exactly the same mass, this means we know exactly how bright the supernovae are. When they’re observed through telescopes, some look brighter and some look fainter depending on their distance - but because we know their actual intrinsic brightness, we can work out how far away they really are. (You could do this yourself using a more earthly standard candle.) For this reason, Type Ia supernovae are called “standard candles”. 

In their observations, Perlmutter, Schmidt, and Riess realised that far away supernovae were more redshifted than the supernovae close by. “Redshift” is essentially a measure of how much the Universe has expanded since the light left the supernovae, so by comparing the distance and the redshift of the supernovae, they could create an “expansion history” of the Universe. 

This showed pretty clearly that the universe isn’t just expanding, it’s accelerating - i..e, everything’s flying apart more quickly than it was yesterday, or a century, or a billion years ago. Why? Dark energy. 

The Universe’s Baby Photo 

The space between stars and galaxies looks completely black, but if you were to point a satellite dish towards any dark point in space, you would actually see a staticky glow a bit like the white noise that sometimes crackles across your TV screen. But this static is coming from all across the universe: no matter where you pointed your satellite dish, you’d record the same signal. But there are no astronomical objects that could create such a uniform signal - so what causes it? 

Let’s quickly review a fundamental idea: everything in the universe emits radiation. You, me, trees, zebras, toasters, tacos, Uranus, stars - they all emit a continuous spectrum of thermal radiation with a peak that depends on their temperatures. Cool things mainly emit radiation with long wavelengths and low energies, while hot things mainly emit radiation with short wavelengths and high energy. 

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The uniform signal we observe everywhere in the sky is radiation with a wavelength of about 1 millimetre, which corresponds to a temperature of 2.7 Kelvin - very close to absolute zero. It’s called the cosmic microwave background (CMB) because it’s actually the leftover radiation from the Big Bang. 

When the Universe was born over 14 billion years ago, it was so hot and pressurised that atoms couldn’t form yet - instead, it spent 300,000 years as a soup of photons and hot plasma, which was made up of protons, neutrons, and electrons: the building blocks of atoms. The photons couldn’t get very far because they kept interacting with the pesky electrons floating around everywhere, and so the early Universe was trapped in a perpetual fog. But as the Universe expanded and cooled, atoms could finally form, and the photons escaped. 

The cosmic microwave background is a record of these early photons at the moment of their jailbreak. The Universe has since expanded massively, so these photons don’t appear as hot and energetic as they once were: they’ve been stretched out or red-shifted to much longer wavelengths and lower temperatures, to almost exactly 2.7 Kelvin (-270 degrees Celsius). They’re everywhere in the universe today - 400 of them in every cubic centimetre - and they’re like the Universe’s baby photos. 

By studying them and the tiny fluctuations within them, we can learn about the Universe’s infancy and how stars and galaxies began to form.

Our Galaxy is Being Invaded

If you shake off the bright lights of the city and head out into the country, you’ll see the bright band of the Milky Way stretching across the sky. The Milky Way is the galaxy we live in, our home metropolis populated by more than 100 billion stars, and it’s part of the Local Group: an imaginatively-named, gravitationally-bound group of galaxies, hanging out close to each other in space. We’ve known about most of our galactic neighbours for decades, like the Large and Small Magellanic Clouds and the Andromeda galaxy - but in 1994, astronomers realised that there was another one lurking right next door.

Image Credit: R. Ibata, R. Wyse & R. Sword

They were studying the stars at the centre of the Milky Way, where there’s a large bulge of high concentrations of stars, when they realised that some of these stars weren’t moving as expected. This odd group of stars were all moving together at the same speed, and it was soon realised that they didn’t belong to the Milky Way at all - they’re part of a dwarf galaxy chilling out on the other side of our galaxy’s central bulge. Dubbed the Sagittarius Dwarf Spheroidal Galaxy, it’s just 50,000 light years from the centre of the Milky Way, which isn’t very far when you consider that our galaxy is 100,000 light years in diameter.

But here’s the interesting part: our neighbour doesn’t just keep to itself. It’s a satellite galaxy of the Milky Way, orbiting us every billion years, and it can get pretty friendly, actually plunging through the plane of the Milky Way as it orbits. It’s orbited us at least 10 times in the past, and is going to pass through us again in the next 100 million years. In the first image above, you can see the blue spiral of the Milky Way with the orbit of the dwarf galaxy traced out in red.

Eventually, the tidal forces from the much bigger Milky Way will probably tear the Sagittarius Dwarf Spheroidal Galaxy apart, and we’ll absorb all of its stars into our own galaxy in an act of immensely cool galactic cannibalism.

Extraterrestrial volcanoes 

Everyone remembers making their first bicarb-and-vinegar volcano back in school, because there’s a special kind of happiness that comes from watching froth pour down the sides of your badly-painted volcano and slosh onto your classroom floor. For most people, volcanoes - simulated and real - are an endless source of fascination and sometimes fear…but did you know that Earth isn’t the only place in the solar system with active volcanoes? 

Lighting up Jupiter with Io 

Io, one of Jupiter’s largest moons, is the most volcanically active world in the solar system. It’s caught in a never-ending game of tug of war between Jupiter and two of its neighbouring moons, Europa and Ganymede, which literally stretch and compress Io with with their huge and uneven gravitational forces. The actual solid rock that Io is made of can bulge out by more than 100 metres and then back again, as if Jupiter is using Io as a stress ball. 

All this friction generates a whole lot of energy inside of Io, which creates heat that drives volcanoes on the surface. Voyager 1 and 2 first spotted these volcanoes back when they sped past in 1979, noticing that not only do the volcanoes spew out molten rock like Earth’s volcanoes, but some eruptions also blast out sulfur and sulfur dioxide up to 500km above the surface. 

What’s super interesting is that some of these particles become caught up in Jupiter’s magnetic fields and flow down to the poles, where they interact with the gases there and create brilliant aurorae.

Frozen explosions of Triton 

When Voyager 2 flew past Neptune’s largest moon, Triton, it captured images of geysers erupting from the surface. Plumes of nitrogen gas and fine particles of dust were blasted up to 8 km into the air, before falling back down to the surface like a weird, nitrogen snow - the dark streaks on the image below. 

But Triton’s volcanism isn’t due to immense tidal forces like Io: instead, it’s thought to be caused by solar heating. Triton’s surface is composed of a frozen, transparent layer of nitrogen atop a darker substrate below, and when solar radiation hits the surface, it becomes trapped by the darker layer. It’s kind of like the greenhouse effect, but in a solid object. This heats up subsurface nitrogen and causes it to vaporise, and eventually there’s so much pressure that it erupts out through the crust above. 

Fountains on Enceladus 

In 2005, the Cassini spacecraft flew right through a plume that had erupted from a cryovolcano on Enceladus, a moon of Saturn. Over 100 cryovolcanoes were discovered at Enceladus’s south pole, which blast out jets of ice-water and simple organic molecules. 

Like Io, the volcanic activity on Enceladus is probably caused by the powerful gravity of Saturn and its other moons. Enceladus gets stretched and compressed, heating up the interior and creating a subsurface ocean of liquid water. Sometimes, slushy ice and other materials get shot up through an opening in the surface and out into space. Some of this material falls back to the surface, and the rest is captured by Saturn’s massive E ring and orbits around the planet, so essentially the ring fed by volcanoes. 

Our Explosive Solar System 

Other bodies in the solar system are suspected to have active cryovolcanic activity too, like Ceres, Pluto and its moon Charon, and Saturn’s moon Titan - stay tuned for more explosive updates!

Can a gamma-ray burst cause a mass extinction? 

440 million years ago in the late Ordovician period, a mass extinction wiped out 85% of marine species. Most research indicates that glaciation was to blame, since it looks like the extinction was associated with a cooling climate and sea level decline, but a handful of scientists have a different idea: What if it was caused by a gamma-ray burst? 

A gamma-ray burst (GRB) is an intense flash of high-energy radiation thought to result from the collapse of a massive star. If one occurred close enough to Earth (within a radius of about 3,000 light years), the radiation could severely damage our biosphere. One study estimates that a GRB could occur within this radius twice every billion years. 

Gamma-ray bursts would be harmful because they would expose the Earth to highly energetic photons. When these photons collide with our atmosphere, they break apart molecules there, including the molecular oxygen that makes up the ozone layer - and destroying ozone allows damaging UV radiation in. The energetic photons would also produce nitrogen dioxide in the atmosphere, blocking out sunlight and causing global cooling, which would be lethal to photosynthesising organisms. 

Image Credit: Wikimedia

The damage a GRB can do is fairly consistent with what we know about the Ordovician extinction, more notably the fact that species who lived in environments more exposed to UV radiation seemed to suffer more severely. The surviving fauna of the extinction seem to have come from deeper water, where UV radiation can’t penetrate effectively or from higher latitudes. 

But currently, we don’t have any evidence that a nearby gamma-ray burst actually happened - so maybe a GRB alone might not be sufficient to completely explain the extinction. 

(By the way, recent studies have shown that gamma-ray bursts just aren’t that common in our galaxy anymore, because most of the massive stars that would collapse and produce GRBs have already done so - so don’t get too worried about GRBs causing a mass extinction in our lifetime!)  

Want to hear more about gamma-ray bursts and their threat to Earth? Have a listen to this UniverseToday podcast. 

What happened to the dinosaurs, anyway?

Long ago, dinosaurs roamed the Earth: sweeping through the skies on great leathery wings, hunting in forests with gnashing teeth, and thundering in herds across the plains. But about 65 million years ago, their world was brought to a sudden, crashing end. Have you ever wondered what caused it?

Fossilised remains tell us endless stories about the creatures they were, the world they lived in, and the world that killed them. They tell us that in the past 570 million years, the Earth has been devastated by five major mass extinction events .

During these events, 99% of all species that have ever existed were wiped out, but it didn’t happen spontaneously: evidence indicates they’re caused by cosmic impacts smashing into our planet.

What are cosmic impacts?

The asteroids and comets orbiting our sun are leftovers from when our solar system formed out of a cloud of dusty gas: bits that didn’t end up forming into planets. Earth is battered with a steady shower of them, and although most just burn up in the atmosphere, once in a while, we get one big enough to devastate our biosphere.

How much damage can an impact cause?

When you think of an asteroid, your mind probably conjures a picture of a burning rock hurtling down through the atmosphere, slamming into the Earth’s surface and creating a massive crater. Earthquakes and tsunamis may also feature in in this vision, since two-thirds of cosmic impacts land in the ocean (because our planet is so covered with them). The impact that killed the dinosaurs struck a shallow sea near Chicxulub, Mexico, and it created tsunamis up to 300m high. We’ve found deposits of marine fossils in Florida, Texas and Haiti that tell us the tsunami rushed 300km inland.

The location of the K-T impact crater, Mexico. Source: Atlasobscura

An impact like this can cause wildfires, which would create huge amounts of smoke, as well as put out chemicals that could case acid rain lasting years.

But surprisingly, the biggest threat to life is plain old dust. When they smash into the surface, impacts can hurl massive amounts of dust into the atmosphere, causing long-lasting dust storms that block the sun and darken the world. Without light and warmth, photosynthesising organisms like plants and algae - which rely on the sun for energy - would suffer, and so would all the organisms that depend on them.

The asteroid that wiped out the dinosaurs was a devastating event, causing the extinction of three-quarters of the animals living on Earth at the time.

But many species survived, including most mammals, birds, turtles, crocodiles, lizards, snakes, and amphibians - and from these lucky creatures, our world today evolved.

More cool stuff:

• Check out this dramatic but super interesting video about researchers “recreating” asteroid impacts
• Worried about a new mass extinction? See how we might stop a killer asteroid

The new kid on the block has arrived! Just an hour ago, the Juno spacecraft braved intense radiation and successfully slid into orbit around Jupiter, ready to study the mysteries of the biggest planet of our solar system.

Welcome to Jupiter, little guy - and congrats to the HARDCORE NASA TEAM who guided Juno INCREDIBLY PRECISELY into orbit around a world over 800 million kilometres away.

Wanna know more about what Juno’s up to?