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How Big Is The Universe?

  • Mallika Rangaiah
  • Sep 09, 2021
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Curiosity is the weapon that has led us to this stage, where spanning throughout history, humans have constantly taken up numerous techniques and strategies for helping them in responding to the queries of how far or how big an area, a city, a nation, the world and even the entire universe is. 


From the past till now, we’ve got explorers across generations who have delved into the colossal spread of the universe, making us witness fresh discoveries with the fresh and advanced methods put forth by the evolving world. 

It was in 1515 when Copernicus proposed that the Earth was not the centre of the Solar System, although his fear of criticism led him to not publish his theory until 1543. Even after the confirmation of this theory, we still wonder and debate to determine our assumptions on what the Universe is, and how massive it might be. Till the present day, we are still assembling fresh evidence suggesting that the Universe is much larger than we have till now conceived. 


In this blog, we will address the question of how big the universe is and the debates and theories surrounding it. 



The Great Debate on the Milky Way


It all began in 1920 when while addressing the Washington DC audience, the American astronomer Harlow Shapley indulged in the legendary “Great Debate” with his fellow scientist, Heber Curtis regarding the scale of the Universe. 


Shapley suggested that the Milky Way galaxy was 300,000 light years across. Although these calculations are actually 3 times off in terms of the latest reasoning yet they seemed quite feasible in those times. He actually measured broadly accurate proportional distances within the Milky Way, such as the position of the Sun in relation to the centre of the galaxy. 


Yet in the initial 20th century, 300,000 light years seemed a pretty bizarre and humongous figure to Sharpley’s associates as did the assumption that spiral galaxies like the Milky Way can be observed through telescopes. Shapley stuck to the assumption that the Milky Way galaxy was unprecedented in comparison to other galaxies. 


Curtis, on the contrary, disagreed with this presumption. He believed, accurately, that there were also other galaxies as colossal and massive as the Milky Way, across the universe. Curtis stuck to the belief that the Milky Way was actually quite smaller than what Shapley had measured. As per his calculations, the Milky Way was merely 30,000 light years in diameter, according to modern measurements.  


Considering that these astronomers debated on this issue almost a century back it is natural for them to be a little off with their figures. In the present day, we have been led to believe that the Milky Way is around 100,000 light years across. Let’s take a look at how big the Universe is in the present scenario and how it has come to be measured. 


How Big is the Universe?

We’ve often wondered how far away the moon and the sun could be from the earth. In the third century B.C Aristarchus of Samos inquired about the distance of the moon from the earth, measuring the distance by observing the Earth’s shadow cast on the moon, amidst a lunar eclipse. 


Around 300 years ago, Edmund Halley, well known for predicting the return of Helly’s Comet, figured out a way of measuring the distance to the Sun and to the planet of Venus. He was aware that Venus only once in every 121 years passed directly between the Sun and the Earth.


Its position, corresponding with the disk of the Sun following it, shifts in accordance with where we are on Earth. The varying shift is dependent on the distance of the Earth from the Sun and from Venus. In recent days, this phenomenon of the shift of Venus took place on June 8, 2004.


We are all well aware of the fact that the Milky Way galaxy has much more to it beyond the stars and planets that we can observe amidst the solar system.


The galaxy is actually booming with stars, so massive that even if one travels at light speed, they will take around 100,000 years to get across it. The stars we glimpse in the night sky, including the Sun, are just a handful of the inhabitants existing in this galaxy, alongside millions of other stars that are too distant or dim to be glimpsed. 


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The further a star is located, the more dimmer it appears. This can help astronomers in figuring out the distance to stars located far away. The dilemma of a dim star being dim because of its far location or owing to it not being that bright in itself was solved by Henrietta Leavitt in 1908 who figured out a way of disclosing the “wattage” of specific stars which transformed their pulse rate depending on their wattage. This has enabled their distance to be calculated throughout the Milky Way. 


A single light year equals 9 trillion kilometers or 6 trillion miles, and our Universe is around 93 billion light-years in diameter. By Universe, we mean the observable portion of the Universe, the portion that we can presently see. There is a possibility that the entire Universe is far larger than what we have observed and are aware of. 


By making use of the Bayesian model, scientists have estimated that the whole Universe is at least 250 times bigger than the observable Universe, or at least around 7 trillion light-years in diameter.


The Bayesian model is fixated on ascertaining the chances of a model being precise, in accordance with the data, instead of inquiring how well the model itself is fitting the data. While not exactly being the most accurate approach of measuring the universe, it does help to conclude that the actual universe is much larger than what we observe.



The observable universe


A team of astronomers using National Science Foundation’s Atacama Cosmology Telescope (ACT) recently came to the conclusion that the universe is 13.77 billion years old, give or take 40 million years. This number was reached while measuring the cosmic background radiation, the oldest light of the universe. 


This result greatly complements the results put forward by the European Space Agency’s Planck satellite, which had calculated the remains of the Big Bang from the period between 2009 - 2013. 


Owing to the correlation between the light speed and distance, this implies that scientists can observe till a space region lying 13.77 light-years beyond. This further implies that Earth's astronomers can adjust their telescopes to survey 13.77 billion light years in any direction, putting Earth within an observable realm having a 13.77 billion light-years radius. Let us keep in mind the word “observable” here which emphasises that this realm restricts what the scientists can observe and yet not what is actually there. 


On a logarithmic scale, we can illustrate the entire Universe, going all the way back to the Big... [+] Bang. Although we cannot observe farther than this cosmic horizon which is presently a distance of 46.1 billion light-years away, there will be more Universe to reveal itself to us in the future. The observable Universe contains 2 trillion galaxies today, but as time goes on, more Universe will become observable to us.

The observable universe Illustration Credit: Wikipedia, Pablo Carlos Budassi

The above image shows a glimpse of the universe, falling all the way back to the Big Bang. Scientists acknowledge the constant expansion of the universe. Though we can’t observe beyond the cosmic horizon that presently stands at a distance of around 46.1 billion light-years away,  which is a diameter of 540 sextillion (or 54 followed by 22 zeros) miles, the Universe has a great degree of scope to expand in the future. 


The observable Universe holds around 2 trillion galaxies in the present day and there is a high probability of additional portions of the universe becoming visible with the passing time.



The shape of the universe


The universe's size relies quite a lot on its shape. We've often heard about scientists proposing how the universe can be closed like a sphere, flat and infinite or infinite and negatively curved.


 As per NASA, scientists believe that the universe is flat, having barely a 0.4 percent error margin (till 2013).  This could considerably shape our assumptions regarding how big the universe actually is. 


"This suggests that the universe is infinite in extent; however, since the universe has a finite age, we can only observe a finite volume of the universe. All we can truly conclude is that the universe is much larger than the volume we can directly observe."

- NASA says on their website


What is the farthest galaxy in the universe?


Have you ever wondered about the furthest object we can see in the Universe and how far it could be? The farthest galaxy that we know of is GN-z11. The galaxy is around 31.96 billion light years away from Earth. 


While we may wonder how it is possible for us to view an object 31.96 billion light years away, with the universe itself being 13.8 billion years old yet the thing is we aren’t really viewing the object from that far. 


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The light of GN-z11 has come to us from 13.4 billion years ago, back when the Universe was 407 million years old. The distance from GN-z11 to the Earth, which is only probable when the expanding Universe is taken into account, is a massive 32.1 billion light years, and is only probable owing to a serendipitous lack of light-blocking dust alongside the line-of-sight to this galaxy.

The image shows GN-z11, taken by the Hubble Telescope

GN-z11 taken by the Hubble Telescope - NASA

So how big is the universe actually? We don’t really know if the universe is infinite in size or even if this is the universe that exists in the galaxy. But hopefully this blog has managed to shed light on what we know about the theories surrounding the size of the Universe. 

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