Just How Big is the Universe

By G. O. Okeng'o

Ever since the origins of mankind the questions as to “How big?” and “How far?” things are in the universe have always attracted much attention and spun curiosity down the spines of the human race. On a beautiful clear night, one is likely to see a myriad of stars that seem innocently close and so reachable that he/she may feel the temptation of reaching out to them or even touching them! This then begs answers to the questions: how far are those stars? What lies beyond/behind those stars? What is their composition and what mechanism powers them? How large is the universe as a whole? What is the universe composed of and how do we humans fit in into the whole picture of the cosmos? Well, if you find these and many related questions interesting then you are not alone! You are only doing what the greek philosophers did several years ago that gave birth to the well-known `scientific process or method', which was later to be extended by great scientists amongst them; Nicolaus Copernicus (the father of the model that put the sun at the center of our solar system), Tycho Brahe (the first observational astronomer to obtain the most accurate data of all time), Johannes Kepler (a student of Tycho Brahe who used his data to propose how planets move), Galileo Galileo (the first man point a telescope-a light gathering and focusing machine- to the sky), Isaac Newton (the genius who formulated laws of gravitation and invented calculus a branch of mathematics that describes motion), Albert Einstein (the famous clerk-cum-scientist who invented the the most well tested theory of physics to date- general relativity) and Stephen Hawking (the greatest theoretical physicist of our time after Newton), just to name a few! It was on the shoulders of these and many other giants that the laws of science sprung, and with them followed the technology that we all enjoy today. But how do astronomers do their thing?

Sir Arthur Eddington, a famous british astrophysicist who lived between 1882-1944 (may God rest his soul!), once used an interesting analogy. He imagined of a large ship sailing across the ocean carrying sacks full of potatoes and a potato bug inside one of the potatoes trying to understand the nature of the ocean in which the ship was moving. He then likened the activities of the potato bug to scientists who study the universe (astronomers and cosmologists). Whereas he might have been reasonably spot on in terms of sizes, he was ultimately wrong in the spirit of his comparison because as we know today, the so-called 'potato bugs', have gathered lots of information about the universe and our knowledge about the workings and functioning of the universe continues to grow by the day, thanks to better technology and cutting-edge study techniques!. Astronomers therefore, have a simple mission; to understand the physical laws that govern the universe and their main tool is a combination of physics, chemistry, computer science and mathematics.

The question of size and location of things in the universe can be well illustrated by classifying cosmic structures in terms of hierarchy; from the smallest scales (our solar system), to the very largest scales (superclusters of galaxies) and then assembling a form of cosmic distance ladder. The universe is a big, big place and as a word of caution, in order to be a good student of the universe you will need to get accustomed to a new system of units that can get really large and sometimes mind-boggling compared to what you're used to! This is the origin of the term 'astronomical' that you may have probably heard some politicians and ordinary folks use.

Let us begin our feel for size with the planet Earth our `sweet home' which is about 6,400 km in size. A small jetliner would cover this distance in 40 hours. The Earth is the third planet from the Sun after Mercury and Venus in our solar system. It is orbited by the moon, the closest cosmic object to the Earth at a distance of about 400,000 kilometers (km). It took Apollo 11, the first spacecraft to land humans on the moon, (although some critics dispute this!) approximately 4 days to cover this distance. The Earth moves around the Sun at a distance of about 150 million km. Apollo 11 could have taken 5 years to travel this distance to the Sun. The farthest object in our solar system is the dwarf planet Pluto which sits at a distance of about 6,000 million km, approximately 60 times the distance between the Earth and the Sun.

At this point, the km becomes a small unit to measure distances and astronomers graduate to a larger unit of distance called the light year, defined as the distance that light travels in one year at it's known average speed of about 300,000 km per second, and it's equal to about 10 million million kilometers; the number 10 followed by 12 'zeros' or simply written as 10¹³ km in scientific form. However, the Sun is just one star in about 100,000 million stars that light up our galaxy, the Milky Way. All the stars observed in the night sky belong to the Milky Way galaxy and studies by astronomers indicate that they show properties similar to our Sun. The nearest star to us after the Sun is called Proxima Centauri and is at a distance of about 4 light years. It would have taken Apollo 11 about 1 million years to fly to Proxima Centauri! (would this have been possible?). Examples of other stars visible on the sky are; the brightest star in the night sky called Sirius at a distance of about 8 light years and the Pole star located at the North Pole and whose distance is about 700 light years. This would translate to about 2 million and 18 million respectively, were the Apollo mission to visit the two stars. The Milky Way galaxy is only but a medium-sized spiral galaxy about 45,000 light years across. At this point, the light year also becomes a small unit to measure distances on scales of galaxies. An even larger unit called the kiloparsec (kpc) equivalent to about 3,000 light years is introduced. This is the standard unit for measuring galactic distances which puts the size of our galaxy at about 15 kpc.

But the Milky Way galaxy belongs to a group of about 30 galaxies, some of which are considerably small in size called 'dwarf galaxies' and contain more than one million stars. This cluster of galaxies is called the 'Local Group' and the closest neighbour to our galaxy to is the 'Andromeda galaxy' in the constellation of Andromeda, at a distance of about 700 kpc. Andromeda has a size similar to the Milky Way and also contains about 100,000 million (10¹¹) stars. The Local Group cluster has a size of about 1000 kpc, called a Megaparsec (Mpc).

Observations using powerful telescopes show that galaxies are very social 'beings', preferring to assemble in groups rather than existing as isolated systems and that there are over 100 million galaxies similar to our galaxy in the universe. Apart from the Local Group, another cluster known as the 'Coma cluster' with a membership of about 1000 galaxies has also been discovered among others. However, further observations also point to clusters of galaxies existing in groups to form even larger clusters called 'Superclusters' and our cluster, the Local Group is thought to belong to the 'Virgo Supercluster' which is about 30-60 Mpc in size. This then leads to our initial question; how big is the universe? Assuming that the size of our universe has a scale similar to that of superclusters, we can put the lower limit of the 'observable' universe to be about 6,000 Mpc or 6,000,000 kpc or 18,000,000,000 light years or approximately 18,000,000,000,000,000,000,000 kilometers! And you can clearly see that the universe is indeed very BIG!

 References

1. Roger A. Freedman and William J. Kaufmann, "Universe", W. H. Freeman, 8th Edition.
2. www.wikipedia.org
3. T. Padmanabhan, "After the first three minutes: The story of our universe" Cambdidge University Press (1998)