Help, our mass is missing

The universe is not what you think it is. We look out into space and imagine it’s full of worlds like ours, but most of what’s out there is completely different and entirely baffling.

First, the obvious. Those lights you see in the night sky are mostly stars – billions and billions of stars. Our galaxy alone contains about 300 billion of them, and with an estimate of more than 170 billion galaxies in the universe, that makes the total number over 50×1021. That’s 50 followed by 21 zeroes, which is an exceptionally big number.

Hubble Ultra Deep Field image of more than 10,000 galaxies in a tiny patch of sky (click to embiggen - it's worth it)
Hubble Ultra Deep Field image, taken in 2004, showing more than 10,000 galaxies looking back about 13 billion years in time, all within a patch of sky less than 1/10 the diameter of the full Moon and that looks empty when seen from Earth (Image: NASA)

The point here is that they far outweigh any planets that may be out there. In our solar system, all the planets, dwarf planets (hello Pluto!), asteroids, comets, etc. make up only about 0.14% of the mass – the remaining 99.86% is the Sun.

But although stars are pretty much all we can see, there’s plenty more out there that we can’t see. And recently, two separate teams of Australian researchers have shed some light on what this missing mass may be.

In what must surely be inspiration to all science students out there, undergraduate Amelia Fraser-McKelvie from Monash University found a lot of the missing mass in wispy filaments that stretch between galaxies. These galactic filaments have a temperature of about 1 million degrees Celsius, which means they’re made out of ionised plasma: atomic nuclei and electrons moving around so fast – and with such a low density – that they can’t stick together to form atoms.

The spider-web-like structure of the universe, with filaments stretching between galaxies (click to embiggen and see the full universal majesty)
What the structure of the universe really looks like, a 3-dimensional spider web of filaments stretching between galaxies (Image by NASA, ESA, and E. Hallman, University of Colorado, Boulder, via Wikimedia Commons)
But although their density is very low, because these filaments join the gaps between galaxies they’re actually very big. Their combined mass could be up to 30 times the mass of all the stars in the universe.

Even so, they’re still ordinary matter, what we call baryonic matter, made out of protons, neutrons and electrons. But there’s still a lot more out there, that seems to be made from something else entirely.

By looking at the structure of the universe shortly after the Big Bang – which we can see in the cosmic microwave background radiation that was produced when the universe was only 379,000 years old –  we can calculate how much ordinary or baryonic matter there actually was, and what proportion of the total mass of the universe it makes up.

The result? Ordinary matter, the stuff that ourselves, stars and galactic filaments are made from, is only 4.5% of the universe.

What’s the rest? Well, there’s the famous dark matter. It’s called “dark” simply because it cannot be seen with any electromagnetic radiation – not visible light, infrared, X-rays, nothing. We can only detect it by the effect its gravity has on the stuff we can see.

But wait, there’s more. As well as dark matter, there’s something called dark energy. Dark energy is seriously strange, and we have no idea what it may be. Its effect on the universe is to contribute to its continuing expansion. Not only to make the universe keep expanding, but to make the expansion accelerate.

How do we know it’s there? Well, a project called WiggleZ, based at Swinburne University in Victoria, looked at ripples in the cosmic microwave background radiation (which we mentioned earlier) and tracked how the expansion of the universe turned those ripples into gaps between galaxies (which we also mentioned).

Their measurements confirmed that the universe has indeed been expanding under the influence of dark energy, which seems to be equivalent to 10-29 grams in every cubic centimetre of space. Again, this is a very small amount, but because it fills the entire universe, it adds up to quite a bit. Nearly three quarters of the universe, in fact.

Pie chart showing the mass of the universe is 74% dark energy, 22% dark matter, 3.6% in intergalactic gas and only 0.4% in stars, etc.
Pie chart showing how the total mass or energy of the universe is distributed (Image by PeteSF at en.wikipedia, from Wikimedia Commons)
So going back to where we started, stars are less than 0.4% of the universe, and planets like ours are less than 0.14% of that. It’s amazing that we scan the skies for worlds like ours, thinking they’re so important, but 96% of the universe we can’t even see.

And yet… There is still so much on Earth that we don’t understand or haven’t discovered yet. And everyone we know lives here, so that’s got to count for something.

There may be a lot more going on out there in the big, mysterious universe, but our planet is still pretty special. Even if in the scheme of things it’s just a fraction of a pinch of a smidge of a sparrow fart.

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