The Dynamic Universe
The universe didn’t always look the way it does now. Long before stars, galaxies, or planets existed, it was just a mix of energy and particles, slowly taking shape. Over time, humans have tried to make sense of it all. From ancient civilizations tracking the movements of stars to Einstein’s ideas about space and time, our understanding has grown bit by bit. But even now, there’s so much we don’t know. Let’s begin.
The closest star to Earth is the Sun 8 light-minutes away, and the next one Alpha Centauri 4 light-years away. The average distance to the bright stars we see in the sky is 100 light-years. The Earth orbits the Sun at 30 km per second or 108,000 km/h, which is equivalent to flying from London to New York in just 3 minutes. Earth is also accelerating towards the Sun at 9.8 m/s² and at the same time it is flying at 108,000 km/h in a direction perpendicular to the direction of acceleration, with the net effect, luckily for us, that it stays in a permanent stable orbit around the Sun.
We here on planet Earth are travelling around the Sun at 28 km per second, around the centre of the Milky Way at 220 km per second, and around the centre of ‘The Local Group’ of galaxies at 600 km per second. A galactic year is the time it takes our Solar system to complete one orbit around the centre of the galaxy, which is approximately 250 million terrestrial years, therefore humans emerged on Earth less than 1 galactic day ago. The Milky Way belongs to a collection of around 30 galaxies, spanning 10 million light-years, known as 'The Local Group’. These galaxies orbit a shared center of gravity. Our neighboring galaxy, Andromeda, is heading directly toward us at roughly 500,000 kilometers per hour. This means the Milky Way and Andromeda are on a collision course and will likely merge in about 3 billion years.
Two types of waves traverse the Universe, enabling astronomers to observe distant phenomena: electromagnetic waves and gravitational waves. Both propagate at the speed of light. The speed of light is 299,792,458 metres per second. The metre is officially defined as the length of the path travelled by light in vacuum during a time interval of 1/299,792,458ᵗʰ of a second. The speed of light is absolute for all observers in any frame of reference, but time and space are relative. As an object approaches the speed of light, time slows down relative to an outside observer. For a photon traveling at the speed of light, from its perspective, time does not pass, no matter how long it is traveling from an external observer’s frame of reference. This is Special Theory pf Relativity. According to Einstein’s special relativity, the closer an object travels to the speed of light, the shorter the distance will appear to it. For a traveller going at 99% of light-speed, the 4.4 light-years distance to Alpha Centauri would appear to shrink to 0.6 light-years.
Then there is Einstein’s General Theory of Relativity. In Einstein’s theory of general relativity, gravity is not a force in the traditional sense but rather the curvature of spacetime caused by mass and energy. The gravitational force is by far the weakest of the four fundamental forces; however, it has two distinctive properties: it can operate over long distances and it affects any particle, regardless of its type, as long as it possesses mass or energy.
The force of gravity on the surface of Mars is approximately 40% of that on Earth, which explains why the highest mountain on Mars, Olympus Mons, can be 3 times as high as Mount Everest on Earth. Earth is gaining weight due to natural space debris such as meteoroids and cosmic dust constantly falling on our planet, but at the same time it loses weight due to atmospheric gas escaping into space, the combined effect being a net loss of around 50 million kg per year.
Any rocky object in the cosmos would have to be 600 km in diameter to be spherical due to the force of its own gravity, or just 400 km if made of ice.
Massive objects bend spacetime, and other objects move along these curves, which we perceive as gravitational attraction. Spacetime bends, curves, and stretches around the matter or energy it contains. Spacetime is the fabric of the Universe in the context of Einstein’s theory of general relativity. It bends by the energy it contains whatever its form. Mass is energy.
A star the size of our Sun, produces new elements through nuclear fusion from helium all the way to carbon and oxygen. A much larger star can carry on producing elements up to iron, after which it would die in a supernova explosion that would produce elements heavier than iron. The core then gets converted into Black Hole or Neutron Star.
A black hole is a region of spacetime where light can’t escape. Einstein’s theory of gravity, also known as General Relativity, has deep limits. It breaks down before we can reach the beginning of spacetime, or the bottom of a black hole. The curvature of space-time at the event horizon of a black hole is such that there are no paths that lead away from the black hole and therefore matter and light can only move inward towards a point in its centre where all the mass of the black hole is concentrated. The surface area of a black hole’s event horizon always increases when additional matter falls into it. For a non-rotating black hole, the radius of the event horizon is proportional to its mass.
Sagittarius A* is the supermassive black hole at the center of the Milky Way galaxy. It is located about 26,500 light-years from Earth, and it has a mass of approximately 4.3 million times the mass of the Sun
Neutron stars, the smallest and densest stars and the most rigid objects known, are subject to extreme gravitational pressure that causes electrons to merge with protons to form neutrons, which in the absence of electromagnetic repulsion pack up in a relatively small space. Hyperons are composed of 3 quarks, similar to protons and neutrons, but at least one of the quarks is a strange quark or another heavier quark. Hyperons are unstable and decay rapidly into lighter particles, but they might exist in a stable form within the core of neutron stars.
Otherwise, Stars of the mass of sun will implode into a White Dwarf. Sirius B is the closest white dwarf star to Earth, it is mainly made of carbon and oxygen and it orbits Sirius A, the brightest star in our night sky which is twice as massive as our Sun.
Meanwhile, none of the red dwarfs formed in the universe have yet left the main sequence phase, which is when they fuse hydrogen into helium because their lifetimes exceed the current age of the universe.
For us, the visible Universe is a dynamic sphere centered on Earth, composed of past events reaching us at this very moment. If we could see right now the light emitted from a galaxy 13.8 billion years ago, due to the expansion of the universe, that galaxy would currently be 46 billion light years away, on the edge of the observable universe.
Only about 40% of the total ordinary matter in the universe is found within galaxies. This includes stars, planets, gas clouds, dust, and other galactic structures. The remaining 60% is dispersed in intergalactic space in the form of a diffuse, hot gas between galaxies.
About 70% of the universe is Dark Energy. The amount of dark energy per square meter in the Universe is constant, however, due to the expansion of the Universe, the total amount of dark energy is increasing, leading to an ever-accelerating cosmic expansion. The accelerating expansion of the universe was only discovered in 1998.
The mysterious dark matter is thought to make up about 4/5 of the mass of the Universe. It is frustrating that we still don’t know what it is. Its exact nature remains one of the most profound mysteries in astrophysics.
