THEORIES OF THE UNIVERSE PROP0SED BY FAMOUS SCIENTISTS
The Big Bang Theory
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Proposed by Georges Lemaitre, a Belgian priest, in 1927
The Big Bang theory is a theory of the creation of the Universe, which states the Universe started from a singularity and expanded over 13.8 billion years until present.
At first, the Universe was extremely hot and dense. Light could not travel through the Universe since "free electrons" (electrons that had not yet combine with nuclei) would've scattered the light, preventing the light from shining through (this is why we cannot see the light 13.8 billion years ago from the Big Bang which would allow us to look back in time). After about 380 000 years light finally began to shine through (this is where we can finally detect what happened) since free electrons began combining with nuclei creating atoms. As the Universe began cooling, the energy began turning into matter, such as hydrogen. After about 400 million years the first stars began to appear as the Universe continued to expand. As you can see from the timeline to the right, dark energy began to cause the expansion rate of the Universe to speed up, which Edwin Hubble observed when he discovered galaxies were moving away from us (Earth).
Much of what we know about the Big Bang comes from theories since we do not yet have the tools to understand what came before the Big Bang or just after it. However, astronomers are able to see the cosmic background radiation, the thermal radiation left over from the Big Bang. This is considered as evidence supporting the Big Bang.
In addition, Lemaitre argued that the Universe expanded from a point, which he called the "primeval atom" or the "cosmic egg". He believed that if matter was moving away from us everywhere (as explained in Edwin Hubble's paper, which proposed galaxies that were red-shifted were moving away from us), then it would be logical that something caused the matter to move away, and if we could look back in time we could see a point where all the matter was all compact together. Today, scientists know there is no real "centre" of the Universe depending on where you are. No matter where you are in the Universe, everything will seem to be moving away from you. This known as the cosmological principle.
Panspermia Theory
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First know mention: proposed in the 5th-century writings of Anaxagoras, a Greek philosopher
The panspermia theory suggests that life on Earth (and possibly other planets), originated from microorganisms present in outer space. It is hypothesized, that life, which exists throughout the Universe, is distributed through collisions between protoplanets, or young planets, and comets, asteroids, meteoroids and other celestial bodies.
Extremophiles, organisms that can survive in extremely harsh conditions (e.g. xerophiles are organisms which can grow in extremely dry environments, commonly found in deserts), are theorized to be able to endure the harsh conditions of space travel. Since collisions between early planets were more common than they are now, in the beginning of the Universe extremophiles could have been ejected into space when two planets, or a planet and another celestial body, collided. The extremophiles, floating through space (in a dormant state) on debris from their old planet, could eventually collide on a young planet. If conditions were ideal, like they were on Earth, this would allow the extremophiles to become active ("wake up") on their new host planet, allowing the process of evolution to begin. This theory does not address how life was created, but how it could have arrived and spread throughout the Universe. Evidence of the panspermia theory was found in 2012 when a team of scientists discovered the Polonnaruwa Meteorite which contained diatom (a group of algae) fossils. Again in 2015, scientists discovered remains of biotic life in 4.1 billion-year-old rocks from Western Australia. The scientists found a heavier isotope of carbon than the more regular isotope of carbon, carbon-12. This carbon isotope is found in remnants of life, such as decayed organisms. These discoveries all provide evidence supporting the panspermia theory. In the future, scientists on Earth could transport extremophiles on satellites to other planets too, in order to kickstart life on another planet.
Dark Energy and Dark Matter
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Dark matter attracts, dark energy repels
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Dark energy was hypothesized by Albert Einstein in 1917
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Dark matter was proposed by Jacobus Kapteyn in 1922
In 1917, Einstein was applying his theory of general relativity when he realized his equations did not work in a static Universe (scientists at the time did not know the Universe was expanding). Therefore, in order to make his calculations work, Einstein added a hypothetical repulsive force called the cosmological constant. This "cosmological constant" is now referred to as dark energy.
Dark matter is hypothesized to make up 21% of the Universe. Dark matter cannot be seen because it emits no light and absorbs no light since it does not interact with electromagnetic radiation. Since dark matter cannot be observed, scientists only know it exists because they can analyze the effects dark matter has on the structure of the Universe and visible matter, such as planets.
In the 1990s, some astronomers believed that the gravitational force from all the matter in the Universe should have slowed down the Universe's expansion. At different observatories, two teams of astronomers were trying to discover supernovae called Type 1a's, in the depths of the Universe. Since the luminosity of these supernovae was known, this would allow the scientists to be able to determine the distance of these supernovae. By comparing the distance to the redshift of the supernovae, scientists could figure out how fast the Universe was expanding. However, the supernovae were all dimmer than expected. This meant the supernovae were all moving further away from us faster than expected. Scientists realized something invisible must be working against the gravity of all the matter in the Universe to cause the acceleration of the Universe's expansion. Going back to Einstein's theory of general relativity, they realized his cosmological constant, the repulsive force, which Einstein only hypothesized in order for his equations to work in a static Universe, was actually correct. They had just proven Einstein's "hypothesis" (which he had only written just so his equation could work). Einstein's cosmological constant was renamed "dark energy" since like dark matter, we know it's there because of its effects - we just can't see it. Dark energy makes up about 70% of the Universe. Dark energy is so powerful that it negates the effects of the gravitational force between long-distance objects since its pushing everything away, and at a faster rate every day. This causes our observable Universe to become smaller and smaller as distant galaxies are pushed past the cosmic horizon (the maximum distance light can travel to us - Earth).
Evidence for dark matter was found in 1933 when Fritz Zwicky was studying a cluster of galaxies. In this cluster, two galaxies were orbiting each other. The faster galaxies orbit each other, the stronger the gravitational force must be between them. To have a stronger gravitational force, more mass must be needed. Based on the speed of the galaxies' orbital paths, Zwicky could calculate what the mass of each galaxy had to be for them to be able to orbit each other. However, when he calculated the total mass of both galaxies, he found that there was not enough mass there for the galaxies to be able to orbit each other at such high speeds without flying apart. So how were they being held together? Therefore, he realized that dark matter must be there holding the galaxies together - he just couldn't see it.
In 1906, Hermann Minkowski proposed that in order to accurately describe what we see, both quantities of space and time must be combined together.
"Space-time is a mathematical model which joins space and time into a single idea called a continuum. This four-dimensional (space has three dimensions and time has one dimension) continuum is known as Minkowski space." Space-time | physics. (n.d.). Retrieved January 5, 2016. Minkowski published his theory on space-time directly after Albert Einstein published his theory of relativity in order to help explain Einstein's theory. The theory of space-time states that space-time is like an invisible landscape we cannot see, however, we can feel its contours. Space-time is the fabric of the physical universe, and though we cannot see this invisible landscape, we see its effect through the force of gravity.
Though the theory of space-time is widely accepted, the problem is, that no one knows what it exactly is. Einstein imagined a flat surface warped by the mass of celestial bodies, such as stars, to cause gravity. Basically, if you stretched a piece of fabric (representing the fabric of space-time) really tightly, and you put an orange (representing a star) in the centre, you would cause a depression in the fabric. "Planets", in this example marbles, would roll into the depression caused by the "star" and would begin to circle into the depression. If the marbles moved fast enough they could orbit the "star" endlessly. This is (in Einstein's mind) how space-time and gravity worked. The only evidence we have so far to prove space-time exists is that the mathematics works, and the laws of gravity seem to also function properly within the laws of space-time.
Space-Time Continuum
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Proposed by Hermann Minkowski, Einstein's former college teacher, in 1906
A diatom fossil. This photograph was taken through the use of a scanning electron microscope.
Sourced from: Speigel, L. (n.d.). UK Scientists: Aliens May Have Sent Space Seeds To Create Life On Earth. Retrieved January 16, 2016.
A timeline of the expansion of the Universe.
In this video, Brian Greene uses a cool experiment (at around 5:02) to explain how gravity affects the fabric of space-time.