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---------------------- 2253 - Expanding Universe, how can it be flat?
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- The human mind is always drawn towards the unknown. It is incredibly frightening, but curiosity burns the will to explore beyond.
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- Traveling across the universe isn’t just traveling through space, but through time as well. Our universe is expected to be 13.772 billion years old. Scientists today estimate it to be 28.5 Giga-Parsecs in width. To get a grasp of how huge “gig parsecs” actually is, let us get to the basics of astronomical units.
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- To understand how large a parsec is we first need to know what an astronomical unit (AU) is. 1 AU is essentially the average distance between the Sun and the Earth , which is approximately 93,000,000 miles. A parsec is defined as the longest length of a triangle whose shortest side is 1 AU and which subtends an angle of one arc second at the opposite vertex. Or, the height of a isosceles triangle that has one arc second degrees at its vertex. Or, a triangle with an height of 3.26 lightyears. 1 parsec = 3.26 lightyears. Or, how big is a lightyear?
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- A lightyear is the distance light travels in one year, which is about 5,900,000,000,000 miles. How big is our universe if it is 28.5 gigaparsecs in width , which is times 93 billion light years, which is: 548,000,000,000,000,000,000,000 miles Ok, that did not help much either. Let’s just say the Universe is really big!
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- Maybe this will help?; The length of DNA of all humans on Earth combined together, when stretched, will be about 0.119 billion light years long. Ok, that didn’t help either.
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- The speed of light is the absolute maximum speed possible. So, if the universe is 13.7 billion years old, how can the width of the universe be 93 billion light years? If an object was created just after the big bang and it moved with the speed of light, it should by now have traveled a maximum distance of 13.7 billion light years. So where did all the matter come from beyond this limited Universe radius of 13.7 billion lightyears?
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- Hubble’s Law states that objects observed in deep space are found to be red-shifted. Redshift, as the name implies, is the shifting of the wavelength of light to the red end of the spectrum. This is the same as an increase in the wavelength of light. But why does the wavelength expand? There can be two possible reasons, a gravitational field or the expansion of space.
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- When we say, the universe is expanding, it’s not just that objects are moving away from one another, but the fabric of space between objects is itself expanding! And, the expansion is accelerating! It is assumed that dark matter and dark energy are responsible for this accelerating expansion. This extra distance due to the expansion of space needs to be taken into account when estimating of the width of the universe growing over time.
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- The edge of the universe is clearly not at some 13 billion light years from Earth. It is estimated to be much beyond that! Does the universe have an edge? Maybe, we can surely explore what it could be like if there was an edge. There is no way to accurately prove it or disprove it. Instead, what we can do is speculate.
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- What is the farthest astronomical object humans can see? At about 2.53 million light years from Earth, stands the Andromeda galaxy. It’s technically the farthest object visible to a naked human eye in the night sky. Almost the entire night sky you observe is just the stars in the Milky Way Galaxy. Andromeda is another galaxy in our neighborhood.
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- Let us go over a thousand times farther, to about 16 billion light years from Earth. This marks the current “cosmic event horizon” of the universe. This distance is the upper limit of light ever reaching us if it originated at that distance today.
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- This implies, beyond 16 billion light years, the expansion of space is faster than the speed of light itself. Obviously light will never be able to reach us as it will have to travel through a relatively expanding space, expanding faster than it is traveling.. This also signifies that any event that may have happened today, beyond the cosmic event horizon will simply be not ever observable by us.
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- At about 32 billion light years from Earth is where you’ll find a galaxy named GN-z11, the most distant galaxy ever observed. This distance is calculated by taking into account the expansion of space as well. What we essentially observe is the structure from 13.4 billion years in the past.
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- Understand that light was emitted by these structures in the past when they were well within the cosmic event horizon and hence we are able to see them. What we see in the telescope is what they were then. We can only guess where they are or what they are today. We are looking at ghosts from a distant past.
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- Clearly our observations are limited by the light, which is electromagnetic waves, that makes it to the Earth traveling through expanding space.
What is the origin of these photons of light? Going back to about 377,000 years after Big Bang has occurred is the epoch of 'recombination', marked by the origin of the very first neutral hydrogen atoms forming from the sub atomic particles , protons and electrons.
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- When these neutral atoms form, their electrons are in an excited state and hence immediately lose energy and go to a more stable state. What happens then to that lost energy? That lost energy is emitted as photons. This era is also called “photon decoupling”, where photons originating from matter were freely able to move in space unbound by the atoms.
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- Today, 13 billion years later, we humans observe these primordial photons as the “cosmic microwave background radiation“. This radiation is uniform and it is everywhere! There is no way one can determine from where it originated which’s different than the radiations originating from a galaxy or other cosmic sources.
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- Moving forward, at about 46 billion light years from Earth, we reach what is called, ‘The Surface of Last Scattering’. This is what you will observe irrespective of the direction you look. The surface of last scattering is like a uniform sphere of fog from which the first ever photons of cosmic microwave background radiation came from after recombination and photon decoupling. Contrary to the name, it is the origin of the very first scattering of light.
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- Is this the edge of the universe? This is the farthest point in space and also the farthest in time that we can observe. That essentially forms the edge of the “Observable Universe”. What lies even beyond that is what is known as the “Total Universe”.
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- This observable edge marks the limit of what we can observe but not what we can imagine. Let us now go even beyond the edge of the observable universe. What can possibly even exist there? All this time, we have only explored what existed at a point in space in the distant past. A more unimaginable question would be, what could possibly be existing there today?
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- The edge of the observable universe also marks what is called the “particle horizon“, the maximum distance one can see into the past. Everything we have seen so far was from the perspective of keeping Earth at the center and scaling time into the past with increasing distance.
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- There is an inevitable question to ask at this point. Is the universe infinite?
To answer it, we will have to refer to the work of Albert Einstein. One fact to state here is that our universe is surprisingly flat. From Einstein’s theory of relativity, it can be estimated directly that a flat universe must be infinite, and hence there is no edge or end to the universe?
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- What I mean by 'flat' is not exactly what you are imagining right now. It is time to understand the difference between topology and geometry. The best and the most famous way to explain it would be through parallel lines. A flat geometry has parallel lines intact even when the topology, or the way they are organized in space, changes.
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- Take out your piece of paper with two parallel lines on it. Wrap one end around to meet the other, making a cylinder. Carefully observe the parallel lines. They remain parallel because cylinders are “flat” in the vernacular of a flat geometry.
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- The universe has a flat geometry but an unknown and unpredictable topology. The existence of uniform cosmic microwave background radiation itself signifies this very fact. We cannot be sure with the indefiniteness of the universe due to various inconsistencies in measurements.
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- A flat universe comes with its own problems contradicting the original Big Bang theory. If the universe is flat now, it should have been flat from the beginning but that doesn’t seem to be the case. This can be resolved by using the concept of inflation which again comes with its own speculations. We do not know and can not know but we can imagine.
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- Like a balloon that inflates to an infinite size on the surface everything would appear to be flat. How to explain all of this? We have so many other theories like the “multiverse” and the ‘string theory’ to explain the universe in which we exist.
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- We can’t be sure yet if the universe is finite or infinite and if there even exists an end to it. Perhaps, this certainty in uncertainty is what makes astronomy truly astonishing.
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- One thing’s for sure, whatever lies beyond the edge of the universe is for the Almighty to know and for humanity to never find out. Mankind will continue to explore this deep abyss of nothingness hoping that someday the truth behind everything will be found. Good luck with that. Even science ends up with faith.
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- February 1, 2019
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-------------------------- Saturday, February 2, 2019 --------------------------
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