Updated: Aug 19, 2021
Towards the end of nineteenth century, scientists believed that they were close to unravelling the complete mystery about the universe. They thought that the universe exists in a continuous medium called the ‘ETHER’.
Just as sound waves travel in an existential medium like air, water etc., light waves also travel in ether. But the only thing needed to prove the existence of ether was careful measurements of the elastic properties of the ether (since it was considered a continuous medium).
In the excitement to prove this, the Jefferson lab at Harvard University was built entirely without iron nails to not interfere with the magnetic measurements. But unfortunately, they forgot about the iron content in the bricks which was used to make the Lab. By the end of the century, tests that gave a foundation to prove the idea wrong began to appear. It was expected that light rays travelling through ether would not be same in all direction. A light ray would be travelling in the fixed speed while moving in the direction same as the ether. On the other hand, light ray would be travelling slower when it travels in the direction opposite to that of ether. Yet, a series of experiments failed to prove this idea. The most accurate experiment was carried out by Albert Michelson and Edward Morley at the Case School of Applied Sciences at Cleveland, Ohio in 1887.
They compared the speed of light in two beams at right angles to each other. As the earth rotates on its axis and orbits the sun, the apparatus moves through the ether with varying speeds and directions. But they found no daily or yearly differences between the two beams of light. It was as if light travelled at the same speed irrespective of its directions. This formed the base of Albert Einstein's theory to ultimately crush the ether theory. Einstein’s paper of 1905 and general theory of relativity, which states that laws of nature must be same for all freely moving observers, ultimately crushed the primitive ether theory. There were many tests and experiments that proved to favor Einstein’s theory. Einstein’s general theory of relativity transformed space and time from a passive background in which events take place to active participants in the dynamics of the universe.
It was previously thought that our universe is static, meaning its shape and characteristics in real time are stationary. Einstein’s equations seemed to predict that space is not static, but it is either expanding or contracting. Einstein fudged up his equations and added the cosmological constant that seemed to make the state of universe static. This was one of the great missed opportunities of theoretical physics. If Einstein stuck to his original equations, he could have predicted that the universe is either expanding or contracting. This paved the way for Edwin Hubble who discovered that the universe was expanding.
He looked at the stars of neighboring galaxies and measured their distances through their relative light spectrum. Later, he discovered with the help of Doppler effect that all the galaxies where red shifted. Doppler effect is the relationship between speed and wavelength which can be observed through everyday experiences. Listen to the music being played in an open ground in a car moving through a constant speed. As you approach near the ground where the music is being played, the music will sound with a higher pitch than it is originally. But as you move away from the ground, the music will sound with a lower pitch. The higher pitch corresponds to sound waves with a shorter wavelength (the distance between one wave crest and the next) and a higher frequency (the number of waves per second) and vice versa. This is because, as the car moves closer to the music source, you will be closer when the music source will emit the next wave crest, hence lessening the distance between wave crests. Similarly, as you move away the wavelength increase and pitch you perceive is lower. As per Hubble’s situation, the galaxies which move away from earth is red shifted and the galaxies which move closer to earth is blue shifted (blue light wavelength is lesser than red light). This way Edwin Hubble through his telescope proved universe is expanding as all galaxies and stars he observed are red shifted. These telescope observations revealed that the farther other galaxies are from us, that faster they are moving away from us. The universe is expanding, with the distance between any two galaxies steadily increasing with time. This discovery removed the need for cosmological constant in the equation. Einstein called cosmological constant as the greatest mistake of his life.
So, if the galaxies are moving faster with increase in the distance from us, this also proves that our universe is expanding with constant acceleration. This also forms the basis of Big bang theory the theory that describes the beginning of our universe. You can read about it in our blogs. Now let us jump on the main topic which we are here to discuss, The non-observable universe. If the universe with expanding with acceleration, then the speed of a celestial object that is very distant from earth would be enormous (maybe near or more than the speed of light). Since we do not know the boundaries of this universe, it may be right to think of the celestial objects that are so much distant that their speed will be greater than light. If the speed of the object or group of objects or galaxies are faster or near than light from the perspective of our earth, then the light emitted from those objects will not be able to reach us as the wavelength of that light would be stretched so much due to doppler effect that it will not be visible to us or may not be able to even reach us!! This forms the basis of the Non- Observable Universe.
It is predicted that only 4 percentage of our universe is visible to us while the rest 96 percentage is still unseen and undiscovered. This means that the Universe is at least 10^(1030) times the size of our observable Universe! And good luck living long enough to even write that number down. This proves that we Homo Sapiens have just started on the quest of solving the ultimate question of our universe- Its origin and fate. There is still a lot to be discovered and a lot of stones unturned on this marvelous journey. So join us in uncovering every fact and theory discovered till this very date!!
Sources- Stephen Hawking’s universe in a nutshell
Image credit- https://www.quora.com/How-big-is-the-observable-universe http://a-levelphysicstutor.com/wav-doppler.php http://www.extremetech.com/extreme/238279-oxford-scientists-call-into-question-the-ideathat-the-universes-expansion-is-acceleratin