The universe is big…really, really big. You have no idea just how gargantuan it actually is. The universe is everything in existence, so it is thought. Unless there are others, and ours is just one among trillions, but that’s for another article all together. Ok, so you know we live in a galaxy called the Milky Way containing a few hundred billion stars of which our Sun is just one. If the Milky Way was reduced to a disk measuring 10 metres in diameter, then our entire solar system would be no more than 0.1 of a millimetre across. If it’s nice and clear tonight and you look up into the sky, you’ll only ever see 0.000003% of all the stars that are in our Galaxy. So let’s take one of those stars, the nearest one to our Sun in fact. This is a red dwarf star called Proxima Centauri, at 4.24 light years away. It’s too small to see with the naked eye but its light takes 4.24 years to cross space and get to us. So if you wanted to travel to Proxima Centauri with the rocket technology we have now, it would take you between 70,000 and 80, 000 years to get there. If we actually managed to accomplish light speed travel (186,000 miles per second) it would still take you four and a quarter years to get there, that’s nine and a half years for the round trip. Now this is just the star next door, our nearest stellar neighbour.We’re not really travelling across the universe here.
If you wanted to cross your very own home spiral, the Milky Way from one side to the other, it would take an astonishing 100,000 years. That’s roughly 1,250 human lifetimes. No this is not with our present fastest rocket engines, this is at blisteringly fast light speed. Travelling this fast, you could go seven times around the Earth in a second. What if you wanted to visit our nearest major galaxy at this speed? That’ll be the Andromeda Galaxy, and it would take you every one of 2.5 million years to get out of the Milky Way and arrive at Andromeda. Kind of puts things into perspective, and the universe is a whole, whole lot bigger than that…this is still just small potatoes!
This is where the numbers start to get slightly terrifying. There are billions, upon billions, upon billions of other galaxies out there just like the Milky Way and Andromeda, each with up to hundreds of billions of star systems, and their trillions of planets.
If you pull out further and look at the even bigger picture, then you’ll see that galaxies seem to clump together, these are the galaxy groups and larger galaxy clusters. They are collections of somewhere between 50 and a 1,000 galaxies. The next scale up from the galaxy clusters are even more gigantic conglomerations…galactic superclusters. These are the largest things known to humankind, made up of galaxy groups and clusters into huge cosmic super structures spanning hundreds of millions of light years, even up to a billion light years across. They’re made up of colossal walls, sheets, and filaments of galaxies. They are objects of unimaginable scale, that they take up pretty sizable chunks of the observable universe. Pull out even further and these clusters and superclusters form around large voids in the universe, resembling a structure like a sponge or a loaf of bread. The Galaxy clusters are the bread, and the voids are the air bubbles.
So how big is the whole universe? Well nobody really knows the answer to that. We can only see so far in all directions, but it’s here where we hit upon a “brick wall”. Distance is not the issue when we’re looking to the edge of the observable universe, it’s actually time. The cosmos is known to be 13.7 billion years old and the thinking is that anything past 13.7 billion light years, then the light hasn’t had enough time to cross space and reach us since the Big Bang. So that makes a sphere of visible universe around the Earth of about 13 billion light years in radius. At the time of writing 13 billion light years is roughly the distance of the furthest galaxy yet seen.
But what about the rapid expansion of the universe, and that ever accelerating expansion due to mysterious dark energy? Well this expansion has stretched space during this time, and makes for an actual observable universe of a whopping 78 billion light years, a visible sphere 156 billion light years across. Just say a star at say 13.7 billion light years away emitted a photon (although this is impossible as this is at the beginning of everything, but just for this example). Then by the time that photon gets to Earth, then the starting point of that photon is no longer at 13.7 billion light years away, but at around 78 billion light years away. The light hasn’t actually travelled across 78 billion light years, but the space between us and its beginning point has been stretched. So the photons of light we see from very distant objects are at much further points away than they were when the light was emitted.
There is a point where it was impossible for light or any kind of radiation to travel, this point is earlier than 380,000 years after the Big Bang. Astronomers would never see any kind of light from this time, not visible light, not gamma-rays, not radio, or ultraviolet, nothing on the electromagnetic spectrum. You might have seen that strange looking green and blue map of blobs(below), this is the Cosmic Microwave Background Radiation. This is the afterglow of the Big Bang itself, and the Cosmic Microwave Background Radiation marks the point whereby the universe became transparent to radiation. It was opaque before this time, opaque like the centre of a star is opaque.
The varying colours on the CMBR show tiny temperature variations as the infant universe cooled and became transparent. These small temperature variations imply varying density, and this pattern matches the places where the giant galaxy clusters and superclusters are situated today.
So astronomers look out to the edge of our 78 billion light year radius observable sphere, but even this is probably just a miniscule part of what may actually exist. Nobody knows for sure but scientists estimate it may account for just one ten-thousanth of what’s actually out there. What is the universe like beyond? Is it the same everywhere or do the physics vary? What worlds and other civilisations lie out there forever out of our reach? Does it go on for infinity, or does it have an edge? It seems that most of the universe is beyond our telescopes, our comprehension, and maybe even our imagination.
By John Brady
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