What is the Universe made of? And similar interesting questions.

Here are some recent news headlines: “Renewed interest in space and the quest for humans to be an interplanetary species”. “AlphaZero beats the world Go champion and the threat of artificial intelligence taking over the world”. “The Hubble telescope has been fixed and continues to map distant galaxies”.

These are exciting times and these headlines definitely spark several interesting follow-up questions. I am posting some of these questions in this post. The goal is to educate myself on these topics and to try and answer some of the questions here to the best of my ability, and in as non-jargony way as possible. I don’t have any formal education in these fields so please keep that in mind. Several of these questions are in fact still open research areas and are being actively worked on by scientists. 

On being interplanetary: Is there life elsewhere in our Solar System? Is there intelligent life in our galaxy, The Milky Way? What is the definition of life?

On artificial intelligence: What is intelligence? What is consciousness? How did humans evolve into the species we are today? Are we living in a simulation?

On mapping the galaxies: How do we know so precisely when the universe started? What is the Big Bang theory? Are there other universes i.e. are we part of a multiverse? I heard that our universe is expanding at an accelerated pace – how do we know that? Can we predict the future of the universe?

And when we take a step back, an overarching question which might help answer the above is ‘what is the universe made of anyway?’ So let’s start there. A good starting point to begin answering this might be ‘atoms’. Everything we see – stars, planets, comets, rocks, plants, animals, the air we breath and the water we drink – is made up of atoms. And we have a Periodic Table which lists all the known atoms and forms the basis for Chemistry. We also know that atoms themselves are made of protons, neutrons and electrons. The number listed in the periodic table for each atom is the number of protons (and typically the electrons) in that atom, and an interesting property is that these are always whole numbers. There is no atom with 1.5 protons or 2.5 electrons. 

Can we further split up protones, neutrons or electrons? Turns out that protons and neutrons are indeed made up of even more fundamental particles called quarks – specifically the Up quark and the Down quark. Quarks combine in triplets – Protons are made of 2 up quarks and 1 down quark, and neutrons are made of 1 up quark and 2 down quarks. Electrons can’t be further split up and are thus considered a fundamental particle. And electrons, up quarks and down quarks are part of what is called the Standard Model of particle physics. Yay, Physics enters the picture.

So to sum up so far, if we were to describe the universe to an alien species, we could say: All living things (bacteria, plants, animals, humans) are made of cells (Biology), which in turn are made up of atoms (Chemistry), which in turn are made up of fundamental particles (Physics). And if we were allowed to only hand them only one brief document to sum up our understanding of ‘what the universe is made of’, we could just hand them the Standard Model of particle physics, and they can build up the rest of Chemistry and Biology from that.

What else is in the Standard Model of particle physics? Apart from the two types of quarks and the electron, the standard model has another elementary particle called an electron neutrino (quick background: electrons have a negative charge, protons have a positive charge and neutrons have neutral charge. While neutrons and protons are stable inside an atom when they are together, a free neutron decays and emits a proton, an electron and another neutral charged particle – which is called the electron neutrino – essentially a baby neutron.)

Ok, so far we have four particles, <the two quarks and the two leptons> (electron and electron neutrino are called leptons – yes a bit of jargon). Turns out that there are another two sets (also called generations) of such four particle quadruples, each generation having heavier mass and more charge. But most of the matter we see around us is made of the first set, and specifically within that, the up quark, down quark, and the electron. The particles in the second set and third set are not stable and disintegrate into the first set very quickly. 

Apart from classifying the elementary particles, the Standard model also describes three of the four known forces in the universe – electromagnetic force, strong force, weak force, gravitational force (not in the Standard model). 

Electromagnetic force is the interaction between electrically charged particles (electrons and protons). Strong force binds the quarks to make protons and neutrons (and that’s why we don’t see quarks in isolation). This force also binds protons and neutrons together to make the atom’s nucleus. Weak force comes into picture during the radioactive decay of the atom. Why is gravitational force not part of the Standard model? We will come back to this later.

There is a little bit more to the Standard model – e.g. photons – but we will come to that and Quantum Physics later.

Observable matter vs non-observable matter: So far we have described what the observable universe – galaxies, stars, planets, living beings – is made of, however, this is only about 15% of the total matter believed to be in the universe! 

I know, I know, this leads to even more questions!

• What do you mean non-observable matter?
  • Non-observable i.e. ‘Dark matter’ is called that because it doesn’t interact with any of the particles in the Standard model. Not with electromagnetic force, or weak or strong forces. The observable matter interacts with our instruments and telescopes so we can actually understand the nature of protons or electrons etc. Dark matter doesn’t seem to so we are in the dark!
• How do you know it exists if you can’t observe it!?
  • Short answer: Gravity! Unexplained gravity in the universe makes us believe that there is dark matter out there. Remember that gravity is still a mystery because we couldn’t include it in the Standard model. But we do know that all matter has gravity. Some evidence of dark matter:
    • In a disk shaped galaxy, stars revolve around the center. The stars at the farthest on the disk are observed to be revolving at a much higher velocity than expected. This indicates that there is a lot more mass in the center of the galaxy holding onto those stars otherwise they would have flown away from the galaxy!
    • Gravitational lensing is when light bends due to gravity (Note: to clarify, we do understand gravity via the theory of general relativity, however, it’s at odds with the Standard model and Quantum physics. More on this topic in a future post!). When we observe a galaxy, the light from galaxies behind the galaxy that we are observing bends more than the observed mass of the galaxy that we are observing, thus indicating there is more mass. (Interesting question: How on earth do we predict the mass of a galaxy!? Future post.)
    • Black holes (can’t be observed but inferred) and neutrinos (remember these don’t interact with any other particles) have been ruled out as the cumulatively they are not sufficient in mass to explain the above two phenomena.
• Ok, so what is it then?
  • The best guess right now is that dark matter is a new kind of elementary particle(s) that we haven’t discovered yet. As soon as we find it, we will add it to the Standard model so that we can complete the Standard model, and explain the rest of the 85% of the matter in the universe!

Ok, to sum up our answer so far: What is the universe made of? 

• Observable matter (15%) is made up of elementary particles as described in the Standard model. 
• Dark matter (85%) is still a mystery. 
• What about the energy in the universe? Remember that E = mc^2 so mass (matter) is equivalent to energy. So we must also include energy in our answer to describe the universe! Let’s save that for another post some day.

Namaste,

Abhi Khune

Abhiram Ganesh Khune