The Ghost of the Universe: Why 95% of the universe is invisible

The Ghost of the Universe:

Why 95% of the universe is invisible

By- Saachi Khemchandani

Introduction

Have you ever solved a mystery? Or tried to uncover something you don’t even know about? Scientists have been trying to solve a similar mystery since the 1930s. And that is the mystery of dark matter and missing mass. Now to further investigate this astrophysics puzzle, we first need to understand how scientists came to know about its existence in the first place.

Back in the 1930s, the astrophysicists got their first clue. Fritz Zwicky discovered that visible matter in some clusters of galaxies wasn’t enough to hold the cluster together . The amount of extra mass needed was so large that Zwicky believed a new form of “ invisible matter” was the reason behind it. He further proposed the name “ dark matter” for it.

Fig 1 : Dark matter

https://www.esa.int/Science_Exploration/Space_Science/What_are_dark_matter_and_dark_energy

What really is dark matter?

At first the scientists began wondering if dark matter was just normal matter that is too faint to see and may be present as planets like dwarfs which have cooled down and dimmed. However, modern technology revealed a strange truth. Dark matter is a new and different kind of matter that has yet to be fully identified, and has completely different behaviour than normal mass. Another interesting fact to be taken into account is that normal “ visible” matter makes up only about 5% of the content of the universe.

The next question that comes into our mind is how do we know it exists if it is completely invisible?

The answer to this question is dark matter’s gravitational effect on visible matter. Dark matter’s gravity can seemingly bend light from distant galaxies, creating an effect called gravitational lensing.

Use of Gravitational Lensing for the discovery

Gravitational lensing occurs when a massive celestial body causes a sufficient curvature of spacetime for the path of light around it to be visibly bent. In 2006 when scientists observed the bullet cluster it was considered the best direct evidence for dark matter. The cluster was formed when two galaxy clusters collided.

During this massive collision, the hot gas from one cluster interacted with hot gas from the other. After this collision, NASA made mainly two observations-

1. Hot, X-ray emitting gas made of normal matter.

2. Portions of dark matter revealed by using gravitational lensing.

It was observed most of the mass in these 2 clusters was dark matter and was distributed differently as compared to the hot gas.

Fig 2: Hubble gravitational lens

https://science.nasa.gov/mission/hubble/science/science-behind-the-discoveries/hubble-gravitational-lenses/

The next task that was taken up was to discover where dark matter was present in the universe so as to gain a better understanding about the organisation of the universe.

Organisation of dark matter

Dark matter is majorly present within the galaxies along with the stars, planets and gas. It is said to contribute around 85% of the total mass of galaxies.

Another form in which dark matter is present are dark halos. A dark halo is an inferred halo of dark matter that permeates and surrounds galaxies as well as clusters of galaxies. In the Milky way, the dark halo appears to extend out to at least 300,000 light years.

Fig 3: One dark matter halo

https://astrobites.org/2020/07/15/dm-profile-hold-the-subhaloes/

And the most interesting thing is that millions to trillions of particles of dark matter are continuously flowing through your body without you even realising!

How does dark matter exist

By now we know that dark matter exists in the form of particles that have a gravitational effect on normal mass and do not reflect light. However scientists have still not been able to find these different particles that compose dark matter. There have been many theories about these such as WIMPS(weakly interacting massive particles), axions, sterile neutrinos and primordial black holes. Out of all these WIMPS is considered the leading candidate for dark matter.

Some theorists have even proposed modifications to our laws of gravity, however this idea is less favoured than the others.

Dark matter or dark energy?

The two largest components of the universe are dark matter and dark energy. Ironically, they form most of the universe, and yet are the components we know the least about. Both of these together determine the fate of the universe. While dark matter provides the gravitational binding mass that holds galaxies together, dark energy drives the accelerated expansion of the universe as a whole. While dark energy forms about 68% of the universe dark matter forms 27% of the universe.

Dark energy behaves as an expansive property inherent to spacetime, whereas dark matter acts locally to pull the visible matter into structures.

Fig 4: Contents of the universe

https://secretsofuniverse.in/blog/dark-matter-dark-energy-illusions

Conclusion

At present it is clear that there is much more to the universe than the visible matter we see around us. What we observe is only about 5% of the total contents of this universe. While we have seen the effects of dark matter and assumed it exists in the form of “halos” throughout the universe. As of now dark matter is one of the biggest mysteries of astrophysics.

References