how big is the universe

As transient lifeforms, we are normally not fully equipped to apprehend the unimaginable stretches of space around us. This is largely due to the limits of perception imposed by our own brains. What our senses define and register is nothing but a mere figment of the entire vastness of the universe around us. As time went on however, our sense-extending capabilities of scientific instruments kept growing outwards and, all the more, knocked man’s pretentious self-hood off its pedestal.

We can fully appreciate the vastness of space with the relative comparison of scale and with the perspectivity of time. Our local cosmic backyard, the solar system, is located around an insignificant star among a hundred billion others in the corner of an insignificant galaxy that is among a hundred billion others. But, if we could just scale our entire solar system to a point that our minds could conceivably handle such that the sun is a reference point to establish the scale, we would be amazed by the sheer expanse of our humble solar system alone.

The Solar System in perspective. This scale represents relative distances in astronomical units (AU). One astronomical unit is the distance of the Earth from the Sun, or about 150 million kilometres. Credit: NASA

One such scale model does, in fact, exist at the National Mall in Washington DC, extending along a 2,000-foot path at a scale of 1:10,000,000,000! At this scale, the Sun would be the size of a grapefruit (around 15 cm across) and the Earth would be the size of a pinhead 15 metres away and orbiting at that radius. The moon would be a mere dot 0.03 cm across and orbiting the Earth at a radius of around 4 cm. In our scale, 4 cm would be the farthest distance that a human being has ever travelled! Mars would be 7 more metres to go. And, Neptune the furthest member of the solar system would be some 435 metres away.

But, how far would our closest star system, Alpha Centauri, be positioned on this scale? Quite amazingly, one would have to walk a distance akin to 4000 kilometres (the entire distance across the US) in order to reach it. The actual trip, however, with current spacecraft technology would take an astonishing 100,000 years!

Even light itself, the high priest of speed, takes years to cross these vast distances between the stars. When we say that a particular object is some quantity of light years away, we mean to say that light takes this amount of years to just reach us. One light year is equivalent to a distance of about 10 trillion kilometers. If our closest star system, Alpha Centauri is located at 4.4 light years away, that means we can only see it as it was 4.4 years ago because the light would have taken this amount of time to reach us.

Our own Milky Way galaxy, although an average-sized galaxy, stretches 100,000 light-years across. It belongs to a cluster of thirty other galaxies, called the Local Group which also includes the Andromeda galaxy (M31). At a distance of 25,540,000,000,000,000,000 kilometres away, it is the closest galaxy to us and we are seeing it as it was 2.5 million years ago. Hundreds of billions of other galaxies populate the universe and stretch out to our cosmic horizon. This horizon is defined by 13.7 billion years, when the Big Bang occurred. We cannot see beyond it, even with the most powerful telescopes, because light that is older than 13.7 billion years hasn’t had enough time to reach us yet. Light that is just 13.7 billion years ago is light that just formed after the Big Bang before any galaxies existed and light from 13.2 billion years ago is light that is just reaching us from the earliest galaxies that have ever existed.

The Hubble Space Telescope has allowed us to see 13.2 billion years into the past. Credit: ESA/Hubble

Are there any galaxies beyond our cosmic horizon? It is plausible in fact that most galaxies are well beyond our own cosmic horizon. That is because in the first billionth of a trillionth of a trillionth of a second after the Big Bang, the universe started expanding faster than the speed of light and continued so as far to this day (Note that in expansion, matter is not moving but the the space between the galaxies is moving faster than the speed of light which it can and which does not violate Einstein’s laws). This suggests that distant galaxies beyond our own horizon are so far away that light from them has not even reached us yet and that in fact, they are still continuing to recede further away from us faster than the speed of light. Thus, our own observable universe is just an inconceivably tiny fraction of an infinite universe.

As immensely vast the notion of hundreds of billions of galaxies containing hundreds of billions of stars is, all of that and all ordinary matter that we can observe through our telescopes constitutes just 4 % of the current universe. Studies suggest that most mass in the universe is unseen and made up of some mysterious dark matter. Even though it is unseen, we can detect its effects on gravity through gravitational lensing techniques. This huge amount of unseen mass is thought to hold galactic clusters together and is thought to constitute 22 percent of all the matter-energy content of the universe. The remaining 74 percent of the universe is thought to be made up of equally mysterious dark energy  that counteracts the gravitational pull of dark matter, causing the accelerated expansion of cosmic bodies.

Humans have walked the Earth for the tiniest fractions of the universe’s briefest of fleeting instances. Indeed, if we were to scale the entire 14 billion year history of the universe into 1 year, humans would have just evolved in the last hour of New Year’s Eve and all of recorded history would have happened in the last few seconds. But, in those last few seconds, we have, nonetheless, made remarkable progress! As insignificant as contemplating those vast stretches of space makes me feel, the mere fact that we, as humans, can contemplate the vastness of space alone instantly drives my feelings the opposite way. We are indeed remarkably, fortunate, that we live at such moments of time, albeit incredibly short-lived, where cosmic messengers from the opposite side of the universe can reach us and recount to us the entire story of the universe. 150 billion years from now, all the light from all the other galaxies will have faded beyond our cosmic horizon and all traces of these cosmic messengers would have long been gone. Any future civilizations would not know of this past hidden world. So, we are, indeed, truly privileged to be alive at this instant of time to even consider this.

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