What is space made of? It’s complicated …
"Space is big. Really big. You just won't believe how vastly, hugely mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to space."
Douglas Adams was right. And not only is space big, we don't know what it's made of.
Astronomers have worked out that only about 5 per cent of our universe consists of baryons — the particles which make up atoms, which in turn make up molecules, which in turn make up everything we see, touch, smell, and taste.
About 20 per cent is dark matter — a mysterious substance that interacts with our universe only through its gravitational pull — and the rest, a whopping 75 per cent, is dark energy, a cosmic field that permeates everything.
It might sound the stuff of science fiction, but it is the best explanation of the large-scale features of our universe.
Baryons and dark matter tend to clump together due to their gravitational pull, while dark energy pushes everything further apart. And that's causing our universe not only to expand, but to accelerate in that expansion.
We've estimated the rate of acceleration by using the Planck telescope to map the cosmic background radiation left over by the Big Bang. Final data, released by the Planck mission last month, reaffirm that dark matter and dark energy must exist, even if we don't know what they are.
In a few hundreds of billions of years, everything but our nearest neighbouring galaxies will have moved out of reach: no matter how fast future humans might travel, we'll never be able to reach anything else.
A recent study speculated that if an alien civilisation grew to the point where it needed whole galaxies as energy sources, it might have to leave its own galaxy and "mine" other galaxies for stars, reconfiguring the cosmos itself, before it all expands out of reach.
Farfetched? If you're advanced enough to worry about the stars going out, perhaps it's just contingency planning at its finest.
Aliens and dark energy aside, there sure is a lot of space — we know the universe is at least 30 billion light years across, and might even be infinite.
What is in this space?
But almost everything that is out there, is hydrogen.
Hydrogen: mostly hot air?
Hydrogen is the lightest element — it is simply a single electron and a single proton, orbiting each other.
Hydrogen can be found everywhere, from hot, dense stellar nurseries where new stars form, to the cold and tenuous voids between galaxies.
It's the most abundant element in the universe, making up 75 per cent of all its atoms.
"The two most common things in the universe are hydrogen and stupidity," said American writer Harlan Ellison.
But most hydrogen atoms floating in space are so spread out, they're essentially invisible to astronomers. So how can we tell if they're there?
Well, it depends on what type of hydrogen it is: ionic, atomic or molecular.
Ionic hydrogen is formed when a nearby hot, bright star splits (or ionises) a hydrogen atom into protons and electrons.
Atomic hydrogen is created in parts of the universe that are cold enough to allow protons and electrons to recombine.
Molecular hydrogen (H2), which is the same form we find here on Earth, is formed in giant clouds where the hydrogen becomes dense enough that two pairs of atoms stick together.
This is really how astronomers see the periodic table according to astronomer and author Heidi Weissman Kneale
Supplied: Heidi Weissman Kneale (@heidikneale)
We also sometimes find other molecules in these clouds ranging from simple carbon monoxide (CO) to more complicated like ethanol (C2H6O) — that's right, astronomers are searching for beer in space.
Atomic and molecular hydrogen are relatively easy to find because they absorb and emit particular wavelengths of light, giving each a unique telltale signature that astronomers can measure.
Ionised hydrogen is almost totally invisible, except for one amazing feature: it makes things twinkle.
When you look up at the night sky, you might see stars, and also perhaps a few planets. The planets are distinctive because they don't twinkle, while all the stars do. This is because the stars are so far away that all their light travels along a single, very thin beam toward us.
The thin beam is distorted as it travels through our constantly-moving atmosphere, making the light look a little brighter or a little darker moment to moment. Planets are close enough to Earth that their beam of light is too large to be "scintillated" like stars.
Indigenous Australians have used the twinkling of stars to forecast the weather for tens of thousands of years.
The same thing happens with galaxies, which produce radio waves. These waves are scattered by the electrons of the ionised hydrogen in exactly the same way as our own atmosphere scatters light from stars.
Distant galaxies appear to twinkle, and that twinkling tells astronomers that space isn't empty, but filled with incredibly diffuse ionised hydrogen.
Space is broken, I'm calling a quantum mechanic
What about the space between hydrogen atoms? What is a vacuum made of? Is there a smallest possible scale? Is space fundamentally smooth, with infinite resolution, or is it made up of some very, very, very, tiny pixels?
This is where things get really weird.
"I think I can safely say that nobody understands quantum mechanics," theoretical physicist Richard Feynman said.
On the very smallest scales, it appears there is a minimum "resolution" to the universe, called the "Planck length", after the physicist Max Planck.
On these scales, the universe appears to be a strange quantum foam, with "virtual" particles popping in and out of existence, obeying the laws of quantum mechanics.
Despite how counterintuitive this sounds, we can actually measure the effects of quantum mechanics in the laboratory, for instance via the Casimir effect.
If you place two thin, flat plates just nanometres apart, in a vacuum, classical physics (and common sense) would tell you that the space between them is empty.
But the sea of virtual particles predicted by quantum physics is affected by the boundary of the plates: fewer virtual particles can be created in this small confined space than on the outside of the plates.
This causes the plates to get pushed together by the larger number of virtual particles bouncing off the outside. Even though, to all other measurements, both between and outside the plates, there is nothing there.
The Casimir effect: more virtual particles can exist inside the plates than outside, causing the plates to be squeezed together.
Wikimedia commons: Emok
This "vacuum energy" might actually be the key to understanding the dark energy of the universe on its largest scales.
Reconciling the vacuum energy with dark energy is a problem that scientists have been working on for decades, without much success.
String theory, which theorises that all matter and energy might be made up of tiny strings as small as the Planck length, holds hope for some physicists.
Cartoon explaining string theory. Sort of.
Others aren't convinced, since the model doesn't seem to predict anything that can actually be tested.
One thing scientists all agree on, is that empty space is surprisingly complicated.
Dr Natasha Hurley-Walker is an astrophysicist at the International Centre for Radio Astronomy Research at Curtin University. She is also one of the ABC's Top 5 scientists for 2018.