By Anna Christodoulou

31 October 2017 - 13:03

Person walking away from the camera on a dark road in the mist
'Dark energy, like all forms of energy, cannot be seen but it can be understood by its effects' Photo  ©

Julien Laurent, used under license and adapted from the original

How do we begin to visualise dark matter, and how does it differ from dark energy? Dark matter enthusiast Anna Christodoulou takes us on a tour of the unseen world this Halloween.

Should we be afraid of dark matter and dark energy?

No – despite its Halloween-y name, being afraid of dark matter is like being afraid of ordinary matter, like the protons, neutrons and electrons that make up our visible world. I say our visible world because dark matter makes up approximately 25 per cent of the universe.

Since dark matter is so abundant – five times more than ordinary matter – why can’t we see it?

An object needs to interact with light to be visible. If it doesn’t emit, reflect or absorb light, we cannot see it. But it doesn’t mean it’s not there. We cannot see dark matter using electromagnetic force, but we know it’s there thanks to gravity.

Astronomers who studied the rotation of galaxies back in the 1930s were surprised to find that the velocity of the galaxies can't be supported by their visible matter. The only solution to that mystery is that more matter exists, matter that we cannot see.

What are scientists looking for when they study dark matter?

Studying dark matter and trying to detect the mysterious particles they are made of is one of the most exciting adventures you can have as a scientist. It is like playing Sherlock Holmes in a mystery plotted by the universe.

Dark matter hunters build large detectors to try to see what is yet unseen. To detect dark matter interactions, researchers need a massive target to increase the number of the atoms a dark matter particle might interact with. They also need to 'cut' the background noise. That means preventing other interactions which could be mistaken for dark matter signal, like cosmic rays. This mystery is a truly complicated one and we are waiting to see what fruit it will bear.·

What’s the difference between dark matter and dark energy?

In the standard model of cosmology, ordinary matter makes up 4.9 per cent of the universe, dark matter 26.8 per cent and dark energy 68.35 per cent. Some scientists are looking into dark matter and dark energy together, but most approach them as two separate areas.

Dark matter explains why galaxies rotate the way they do, and why they are not torn apart by their rotation velocity. Dark energy explains a different phenomenon: the accelerating expansion of our universe.

In 1998 the Hubble Space Telescope made an astonishing discovery. Observations of very distant supernovas showed that the universe used to expand in a slower rate. This was big news, as at the time physicists thought that gravity pulling everything together would slow down the expansion. Nobody expected that the expansion was accelerating.

Dark energy is the suggested solution to this puzzle. It could be a property of space, it could be a force fighting gravity’s effect or it could be some unknown energy filling the space. Even more enigmatic than dark matter, dark energy holds the key to our understanding of the cosmos.

How might new discoveries about dark matter and dark energy affect people’s lives, now or in the future?

In 1897, when the physicist JJ Thomson discovered the electron, he didn’t know what to make of it. His quote 'the electron, may it never be of any use to anybody' is now legendary, and nobody in this planet can imagine life without electricity.

While we don't know what dark matter is, it's difficult to say what its direct applications might be in our everyday life. But the effects of dark matter research have reached us in other ways already. 

According to the World Health organisation, the widespread use of lead (e.g. lead-acid batteries in vehicles) and improper lead waste management has contaminated the environment and causes significant health problems in many parts of the world. Dark matter research uses lead to protect detection experiments from background noise, and it has developed lead detection techniques to prevent lead exposure in the research plants. The technology developed is useful not only to protect researchers in the plants, but to also measure lead levels in the environment and protect exposed communities.

How can we visualise dark matter and dark energy?

Artists have tried to visualise matter particles as tiny, black balls. Being able to visualise them this way is helpful for learning and understanding.

But there is no simple way to visualise dark matter or dark energy. When talking about dark matter, we are talking about mysterious, unseen particles. We are not able to literally see ordinary particles either, because they are very small. We think of a particle as a positive nucleus with an electron cloud around it. And this conception doesn’t include the motion of the particles in the nucleus or reflect entirely our understanding of the way electrons move inside the atom.

From a broader perspective, part of dark matter could be dark objects like black holes or stellar remnants that do not emit any light. Another way to visualise how dark matter holds the galaxies together comes from our expectation that it forms large filaments that connect galaxies to each other.

Dark energy, like all forms of energy, cannot be seen but it can be understood by its effects.

We cannot see kinetic energy, but we know it exists because we see things moving. We cannot see potential energy, but we feel it in the effort needed to raise a heavy object. Similarly, we cannot see dark energy, but we understand it’s there because we know the expansion of the universe is accelerating.

Take part in the annual science communication competition FameLab 2018 and find out more about Dark Matter Day.

Anna Christoloudou is a Physicist, FameLab alumni and SEPnet/ Ogden Physics Outreach Officer of Royal Holloway, University of London.  

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