The universe is comprised of a vast expanse of galaxies, stars, planets, moons, asteroids, comets, gas, dust and more. But what are all these things actually made of at the most fundamental level? To understand the answer, we must look deep into the building blocks of matter.
Atoms
All physical things are made up of atoms. Atoms are the basic units of matter that consist of a nucleus containing protons and neutrons, surrounded by electrons. The different types of atoms are defined by the number of protons they contain. For example, carbon atoms have 6 protons, oxygen atoms have 8 protons, and so on.
Over 100 different types of atoms exist in nature, with many more created artificially in laboratories. The diversity of atoms allows for the wide variety of molecules and materials we observe in the universe.
Subatomic Particles
Within the atom, protons and neutrons can be further broken down into even smaller particles called quarks. There are six types of quarks which combine in different ways to make up protons and neutrons. Quarks are held together by gluons to form protons and neutrons. Besides quarks and gluons, there are also ephemeral particles inside atoms called electrons, muons, tau particles, and neutrinos.
So the solid objects we interact with daily like rocks, water, metals and more are actually made up of swarms of tiny quarks, electrons, and other subatomic particles interacting with each other.
Matter vs Antimatter
For every type of subatomic particle, there is an corresponding antimatter particle that has the same mass but opposite charge. For example, the antimatter counterpart of an electron is called a positron. When a particle collides with its respective antimatter particle, they annihilate each other and release energy.
In the early moments after the Big Bang, the universe contained equal amounts of matter and antimatter. But as the universe expanded and cooled, matter began to dominate. Why this imbalance occurred is one of the great unsolved mysteries in physics. Today, pretty much all the antimatter has disappeared, with only tiny trace amounts remaining.
Dark Matter
Normal matter as we know it – things like stars, planets, dust, gas, living creatures, – makes up only about 4-5% of the total mass-energy content of the universe. The rest is dark matter and dark energy.
Evidence for Dark Matter
| Observation | Explanation |
|---|---|
| Galaxy rotation curves – The outer regions of galaxies rotate faster than expected based on visible mass alone | The gravitational pull of additional invisible “dark” matter affects the rotations |
| Gravitational lensing – Light from distant objects is bent more than expected by foreground galaxy clusters | The extra lensing is caused by the gravitational pull of dark matter in the clusters |
| Cosmic microwave background – Small fluctuations indicate there is more gravity than normal matter accounts for | The excess gravity is likely caused by dark matter |
| Galaxy cluster collisions – Galaxies pass through each other after a collision instead of scattering apart as expected | Gravitational effects of dark matter in the clusters keeps the galaxies bound together |
Despite not being visible, dark matter makes up about 25% of the universe. The leading hypothesis is that dark matter consists of a new as-yet-undiscovered subatomic particle that feels gravity but does not interact electromagnetically with normal matter. Finding direct evidence for this dark matter particle is one of the great challenges in physics today.
Dark Energy
Dark energy is even more mysterious than dark matter. It makes up about 70% of the universe’s mass-energy. The leading hypothesis is that dark energy is associated with the inherent energy contained in empty space across the universe. This energy exerts a negative pressure that causes space to expand at an accelerating rate.
The discovery of dark energy in the late 1990s shocked scientists, as there was no expectation that empty space should contain so much intrinsic energy. The nature of dark energy remains one of the biggest unanswered questions in cosmology.
Fundamental Forces
The Four Forces
There are four known fundamental forces governing interactions between particles:
| Force | Relative Strength | Range | Acts Between |
|---|---|---|---|
| Gravity | 1 | Infinite | Massive particles |
| Electromagnetism | 10^36 | Infinite | Charged particles |
| Weak Force | 10^13 | Short (less than 10^-17 m) | Quarks, leptons |
| Strong Force | 10^38 | Short (less than 10^-15 m) | Quarks, gluons |
These fundamental forces govern everything from radioactivity, to fusion in stars, to the structure of atoms and molecules. Understanding the origins and connections between the four forces is a major goal of theoretical physics.
Theory of Everything
The holy grail of physics is to find a single overarching theory that describes all fundamental interactions in the universe – a Theory of Everything. Currently, gravity and the quantum forces are described by incompatible theories. A unified theory would need to merge quantum mechanics and general relativity into a coherent framework.
String theory is one promising approach toward a Theory of Everything. It posits that subatomic particles are actually tiny vibrating strings. By giving particles an inherent extension, string theory can describe all forces under the same mathematical formalism. However, concrete experimental tests of string theory remain elusive. The search continues for the right insight that can unlock a full understanding the universe’s fundamental forces.
Conclusion
From massive clusters of galaxies down to infinitesimal quarks, the known universe is made up of various particles interacting through fundamental forces. Normal matter comprising atoms makes up only a small fraction of the universe’s content. Mysterious dark matter and dark energy account for 95% of the total mass-energy budget. Dark matter provides extra gravitational glue, while dark energy drives the accelerating expansion of space.
Unlocking the origins and nature of dark matter and dark energy are major priorities in physics and astronomy. On the smallest scales, physicists continue working toward a Theory of Everything that can unify the four fundamental forces. While there are still many unanswered questions, scientists have made incredible progress in understanding the basic constituents and forces that built our universe. The quest continues to find the ultimate building blocks and unified framework describing our vast and intricate cosmos.