What if I told you that all the objects we observe in the night sky, all the stars, planets and galaxies
only make up 5% of the total universe.
The other 95% is something we can't observe and weren't even aware of 100 years ago. I
am of course talking about dark matter and dark energy, the two greatest mysteries in cosmology at the moment.
In this video,
we will be tackling dark matter to see how we discovered this mysterious part of the universe and what it means for the future.
The search for dark matter began with the Swiss American astrophysicist Fritz Zwicky who observed something very strange in the Coma galaxy cluster.
This cluster consists of 1000 individual galaxies that all circle around a central point much like bees around a beehive.
However, Zwicky noticed that the galaxies within the cluster were moving way too fast for the amount of matter in the galaxies.
Newton tells us that any object that has mass has gravity and larger the mass, the stronger the gravity.
But for the Coma problem Zwicky measured, the mass required to keep the cluster together was 400 times what was actually observed.
He proposed that an unseen mass was to blame, which he called dark matter, but his work was largely ignored at the time.
It wasn't until 1980 when Vera Rubin an American astronomer provided comprehensive evidence for dark matter's existence.
She was observing the rotation of stars around the galaxy Andromeda, the closest galaxy to Earth.
Before jumping into what she found, we must first consider the orbital speeds of celestial objects.
The orbital speeds of planets follow a simple rule. In the solar system, the near objects orbit faster than the far objects,
so Mercury orbits a lot faster than Neptune.
Just like with the orbital speeds of planets, the orbital speed of stars around the galactic center should follow the same pattern. However, they don't.
Rubin noticed that the orbital speed of stars at the edge of Andromeda was higher than the stars towards the center.
When plotting the orbital radius against orbital speed, you would expect a downwards curve as the speed decreases for more distant objects.
But for the Andromeda galaxy this was the exact opposite.
The graph she plotted over many years of collected data was constantly increasing. But how can this be true?
The outer regions of the galaxy would simply fly off into space if this was the case.
The only possible conclusion is that some stronger force is holding them in place, one that we can't see and
this is where dark matter comes in.
Dark matter is a material that doesn't reflect, absorb or interact with the electromagnetic spectrum at all.
This means that it doesn't absorb or reflect heat or emit any sort of rays such as x-rays or gamma rays. The
only interaction it has with normal matter is through gravity and this is the only way that Rubin's data can work. If
there is some unknown particle that is exerting more gravity than is observed from the normal matter,
then it would explain how the stars on the outer edges of the galaxy remain in orbit.
It would also explain how clusters and superclusters of galaxies form
so easily. If normal matter was the only source of gravitational attraction,
then it would have taken far longer for matter to clump together and form the stars, nebulae and galaxies that we see today.
Through computer-generated simulations that incorporate dark matter, a model of the universe's formation becomes much easier to determine. In
fact, if we didn't have dark matter, we wouldn't even be here.
It allowed everything in the universe to form the way it did: our Sun, our planet and our galaxy.
Without dark matter, we simply wouldn't exist.
So, how do we measure how much dark matter is in the universe? Well it was using gravitational lensing.
Einstein's theory of relativity
predicted that light was affected by gravity and could be bent and distorted by it.
This becomes obvious when looking at light around black holes.
Their extreme gravitational pull bends light to such an extent that it becomes severely distorted.
This phenomena is also seen in galaxies.
Light from distant galaxies is bent around closer galaxies such that the distant galaxies are even duplicated.
The light has bent around both sides of this galaxy, to produce two identical images of the more distant galaxies. If
there was only normal matter in the universe, then this light would not have been bent to such an extent.
So depending on how distorted the distant galaxy appears, will show how much dark matter lies in that individual galaxy.
From these measurements, we were able to work out that the universe is made up of 26 percent dark matter,
five times the amount of normal matter in the universe.
This is a staggering result. Something
we can't see, can't touch and barely understand, is 5 times more common in the universe than the stuff that we can see. And
this begs the question. What exactly is dark matter? Let's get into some of the theories.
The first theory is that dark matter is MACHOs or Massive Compact Halo Objects.
These are any objects that do not emit enough energy to be detected and are not bound to any star system.
These objects include black holes, neutron stars, white dwarfs and brown dwarfs which freely move throughout the galaxy.
Because we can't see these objects, they could be the hidden mass that has not been considered. The
problem with this theory is that there simply wouldn't be enough of these objects to create the enormous hidden mass.
So after they were shown to not account for large amounts of dark matter, they were discarded as a candidate.
The second theory is that dark matter is just another particle of matter that we haven't discovered yet,
one that could fit into the standard model of particle physics.
There are plenty of hypotheses of which theoretical particle dark matter could be made of,
some of these being axions, sterile neutrinos and Weakly Interacting Massive Particles also known as WIMPs.
I know, great name.
Axions are the first possibility. Right now, neutrinos are the smallest particles in our current particle model.
However, axions are theoretically even smaller than neutrinos.
Therefore to create the enormous gravity of dark matter, there must be an almost infinite number of these small particles.
So they should be easy to detect. This is not the case.
The axion dark matter experiment or
ADMX is essentially a magnet that can turn any trapped axions into microwave signals that can be observed.
Although theoretically our home galaxy should be saturated with axions, so far, none have been found.
This leads into the second possibility for a new particle of dark matter known as Weakly Interacting Massive Particles or WIMPs.
These particles are large, slow moving and do not interact with many other particles because they are electromagnetically neutral.
While these particles have not been predicted by the standard model of particle physics, they are theoretically possible.
However, just like axions, WIMPs have never been detected by scientists.
An even crazier theory is that dark matter is gravity leaking from other universes.
These universes exist in different spatial dimensions to our own, so their radiation energy is trapped in their own space-time.
The matter in the other universes may be only meters from our own, but the only way they could possibly
interact is through gravity and this is what we observe as dark matter.
So while we can't prove what dark matter is now, we are one step closer to working out the mysteries of the universe, just from discovering it.
Dark matter is the next great puzzle in cosmology and if solved, could reveal to us the secrets of our universe.
And
if you enjoyed the video, be sure to leave a like and comment on what you think dark matter is.
Be sure to subscribe to the channel for my next video on dark energy and the fate of the universe.
Until next time, goodbye.
