How Big is the Universe?

How Big is the Universe?


The Universe is a big, big place. Getting your head around mind-bendingly big and unimaginably small things is really hard, our brains just weren’t built to do it. To survive as a human, all evolution required of our ancestors was the ability to outrun and outwit predators long enough to survive and have offspring. Being able to mentally picture all the mammoths on the planet, wasn’t high on the list of essential skills.

Space….is big. Really big. You just won’t believe how vastly hugely mind-bogglingly big it is.

_____Douglas Adams, in The Hitchhiker’s Guide to the Galaxy

  At the moment we’re all struggling to comprehend something which appears totally the opposite, an unimaginably small virus that can’t be seen by the human eye, that occupies that strange border between non-life and life but that will nonetheless cause incomprehensible pain and suffering to many of us

Most of us can wrap our heads around the relative size of a thousand, a hundred thousand or a million, either up or down, but once we get past a few million it all gets harder to picture.

Its easier though to get a grip on just how huge or tiny something is if you just compare its size to the size of things, we are familiar with, like us.

On a scale of the smallest to the largest things we can see in the solar system, we sit pretty much dead in the middle, my artist friend, Alan Rossiter, at 1.60 metres is roughly 10 billion times bigger than an atom and one billion times smaller than the diameter of the Sun.

The average human cell is about 10-15 micrometres, which means we’re about 100,000 times bigger than our cells. If your cells were the size of a penny piece, you’d be two kilometres tall!

Bacteria are cells too, but only about 1/10th the size of our cells, and viruses are smaller again, they’re about 1/100th the size of our cells. So, we’re about 100,000 times bigger than our cells, a million times bigger than bacteria, and ten million times bigger than your average virus. If the virus was the size of a one penny piece, a bacterium would be the size of a large dinner plate, and you would be 200 kilometres tall!

Viruses may be tiny compared to all other living things, but they’re giants compared to atoms and molecules.

The one thing the micro world of the human body and the macro world of the universe have in common is they’re 99.9999% empty space!

So, when we think of the big things in the universe, we think of planets, stars and galaxies. Earth is big to us, but it is puny as far as the solar system is concerned, you could fit more than a million Earths inside the Sun.

Our galaxy, the Milky Way is a barred spiral over 100,000 light years across, containing 200 billion stars and within which our solar system revolves, about every 230 million years (1 Cosmic Year). The last time we were in this position in space, the Jurassic period was just beginning on Earth. The supercontinent Pangaea began splitting into Laurasia to the north and Gondwana to the south. Lush rain forests were created and on land, the dinosaurs came into their own, the first birds and the earliest mammals and lizards appeared.

The light from our second nearest star Proxima Centauri, takes approximately 4.2 years to reach us, therefore we can define it as roughly four light-years away. So, as you look at the star you are seeing it as it was 4 years ago.

We see all things in the universe as they were in the past, such as our nearest galactic neighbour, Andromeda (M31), 2.5 million light-years away, a very large galaxy that can be seen in the night sky with the naked eye. What you’re also seeing is a spiral galaxy of some 400 billion stars as it was 2.5 million years ago.

The whole universe is filled with galaxies like the Milky Way and Andromeda, and using our most powerful ground based and space telescopes, we can see light from galaxies that has taken more than 13 billion years to reach us!

Since a photon of light left one of these primeval galaxies, life sparked into existence and evolved. Dinosaurs ruled the Earth. Humans appeared, developed tools, art, science and technology, built the Hubble Space Telescope, put it into orbit and finally stopped that poor photon on its 13 billion year journey.

The universe is about 13.8 billion years old, so any light we see has to have been travelling for 13.8 billion years or less and this we call the “observable universe.”

However, the distance to the edge of the observable universe has a radius of 46 billion light-years because the universe is expanding all the time.

Imagine that a photon of light is emitted from a point on the edge of our observable universe.

While that photon has been travelling through space, the universe has expanded. We have moved away from the point where the light was emitted, and also it has moved away from us. Though the light might have only travelled for 13.8 billion years, the distance from us to the point it came from, is at present, 46 billion light years!

So how big is our universe? Well, we can say that from where we are, it traces a sphere roughly 93 billion light-years across, but we don’t actually know for sure.

It may be that it’s so big, that light hasn’t had time to cross it in nearly 14 billion years, and it’s still getting bigger all the time, so some of it, will be forever out of our reach.

Remember my fellow travellers, on spaceship Earth, the universe is big, very big, bigger than we could ever imagine, but we are part of it, we belong in it.

We will deal with this pesky virus lurking in the micro world and attacking us. However, unless we face up to our responsibilities to look after our planet and its resources and share our habitat with other planetary life-forms, we will sleepwalk into a tipping point, and an irreversible disaster.  Planet Earth will survive, we might not!

Scott Beadle FRAS

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