The dark side of the universe – a primer

Over the past 40 years astronomers have realised that everything we can see – all the stars, planets and galaxies – make up less than 5% of the entire universe. What is the rest? The short answer is, we have no idea.

What we do know is there are two gaping holes in our understanding of our universe. As a placeholder, physicists call them dark matter and dark energy.

In a nutshell, dark matter is the invisible stuff which we can only detect from the way its immense gravity moves stars and galaxies.

Dark energy, on the other hand, is the mysterious something causing the universe to expand with ever increasing speed.

We don’t know if dark matter and dark energy are related – in fact they’re probably two completely different phenomena, both called “dark” just because we can’t see them.

Dark matter

How was it discovered?

Since the 1930s astronomers knew that the way galaxies spin did not make sense. The stars at the edges of galaxies were moving much faster than expected – so fast they should have been flung off the cosmic merry-go-round and out into deep space.

But these strange motions could be explained if there was a bunch of extra matter in and around the galaxies – matter that we can’t see. It’s this “dark matter” that holds galaxies together.

Since then, many other observations beyond the scale of whirling galaxies, from the choreography of galaxy clusters, to the collision of nebulae, all suggested the same thing.

Although some physicists have entertained other theories, such as modifications to gravity, by now most are pretty sure dark matter exists. It’s the only explanation that suits all the data.

What do we know?

We know dark matter doesn’t emit light (nor does it absorb or reflect it), so it can’t be made of rogue planets or clouds of normal matter. We know it’s “cold” (which in physics-speak means it moves slowly compared with the speed of light). We know it has gravity. We also know it doesn’t interact very strongly with anything, even itself – otherwise the dark matter would collapse into flat structures such as galaxies, rather than the spherical haloes we detect.

Oh, and it makes up about 27% of the universe.

What could dark matter be?

The bottom line is it is probably some new kind of particle (or a whole family of particles) that we have never detected before. Dark matter particles could be all around you, and floating through your body right this second.

This means the answer to this grand cosmological puzzle, affecting the universe on scales of mllions of light years, could lie in the physics of tiny particles, much smaller than an atom.

Over the past 30 years physicists have sifted through dozens of different dark matter candidates. The prime suspect at the moment is a kind of particle called a weakly interacting massive particle, or WIMP. This is a kind of heavy particles that feel only the weak force.

One of the goals of CERN’s Large Hadron Collider is to look for WIMPs (the same way it found the Higgs Boson in 2013) – the elusive dark matter particles might be created when protons are smashed together at near the speed of light.

Can we detect dark matter?

Besides CERN, there are more than 30 experiments around the world devoted to finding dark matter.

Some of these are dedicated telescopes searching for the signature of particles created when two particles of dark matter annihilate.

Others are giant vats of liquid xenon watching for a telltale flash when a dark matter particle nudges an atomic nucleus. None has yet made a convincing detection of a dark matter particle, although some of the experiments have ruled out various possibilities of what dark matter might be.

It remains a possibility that dark matter may never be directly detectable – especially if it turns out to be a particle that does not even feel the weak force.

The dark force and dark photons

Some physicists have proposed that dark matter particles can interact with one another via a new force of nature – called, yes, the dark force and transmitted by dark photons (aka dark radiation).

There may even be different kinds of dark matter, some of which feels the dark force, and some do not.

Dark energy

How was dark energy discovered?

In the early 20th century, physicists including Albert Einstein imagined the universe as static and unchanging. But in 1929 American astronomer Edwin Hubble observed the motions of exploding stars and discovered the universe was expanding. In fact the universe must have had a beginning – a moment of creation called the big bang.

We can imagine the big bang a bit like an explosion. But after that initial burst, physicists thought the expansion should begin to slow down over time, as gravity acted to pull everything back to a single point again.

The question was whether the universe would ever stop expanding and reverse direction, falling back into a “big crunch”.

Then, in 1998, things got a bit more complicated.

Using the same method as Edwin Hubble (and with the telescope named after him) astronomers found that the expansion of the universe was not slowing down, but instead was accelerating. Galaxies are flying away from each other faster and faster each year.

It was a strange and unexpected result. A bit like if you were driving on a flat highway, took your foot off the accelerator – and then your car began to speed up!

Yet the data were convincing. Physicists realised this expansion must be driven by some sort of energy, and they called it “dark energy”.

What we know

We know that dark energy affects the universe as a whole. We know it acts a bit like a negative gravity pushing galaxies away from one another.

We also know that dark energy did not kick in until a few billion years ago. (For the first half of its life, the expansion of the universe was slowing down due to gravity pulling everything together.)

This makes physicists think dark energy is somehow tied up with space itself. This means its density in space is always the same, but as the universe expands (that is as more space is created), the amount of dark energy also increases.

This would explain why the amount of dark energy was insignificant when the universe was small.

What could it be?

The answer to the mystery of dark energy might also lie in the minuscule quantum realm.

In quantum theory, “empty space” is not empty at all, but filled with a soup of particles continually popping into and out of existence. As weird as it sounds, physicists have actually measured the force created by these so-called “virtual particles” in the lab.

The problem is, when physicists try to calculate how much energy these virtual particles contribute to each cubic metre of empty space, they come out with a number that’s a factor of 10¹²⁰ too large when compared to the density of dark energy (as measured from the accelerated expansion of the universe). That’s a 1 with 120 zeroes after it, a ludicrous answer called “the worst theoretical prediction in the history of physics”.

Quintessence

Some physicists think dark energy could be akin to a fifth force of nature, pervading all of space. They call it “quintessence”, after the fifth element predicted by the Greek philosophers. As opposed to the cosmological constant, the quintessence is imagined to change over time – it was once attractive, but is now repulsive.

The big rip and phantom dark energy

In some theories, the quintessence can continue to grow stronger (in which case it’s called phantom dark energy).

This could destroy the universe.

If the expansion of the universe continues to accelerate, eventually reach the speed of light – first galaxies and stars would be cut off from one another, then eventually the space between the sun and the Earth would expand faster than the speed of light and individual atoms would be torn asunder as the space within them expanded at faster than the speed of light. This is the big rip.

A new gravity?

Dark energy might not be a new force, it might just be a sign that, at very large scales, gravity does not behave as Einstein’s theory of general relatively describes.

The ΛCDM (lambda cold dark matter) model

This is the name for the astrophysicists’ current best picture of the way the cosmos is screwed together.

Λ (or lambda) stands for dark energy, while cold dark matter describes the consensus that dark matter must be made up of some kind of slow moving, previously unknown particle.

In this picture, dark matter makes up 27% of the mass-energy of the universe, dark energy makes up about 68%, and ordinary matter – that of the stars and galaxies and our own flesh and blood  – makes up less than 5%.