Go outside on a clear night and the universe does not look particularly crowded.
A few thousand stars are potentially visible to the naked eye from Earth under good conditions. Between them lies what appears to be mostly darkness.
But this is profoundly misleading.
Our Sun is only one star among hundreds of billions in the Milky Way. The Milky Way is only one galaxy among billions in the observable universe.
NASA estimates that the observable universe contains at least 100 billion galaxies. If galaxies contained roughly similar numbers of stars to our own, that would suggest something like 10 sextillion stars — a 1 followed by 22 zeros.
And the observable universe may be only a small part of the whole universe.
The numbers become so enormous that they almost stop meaning anything.
Why does reality need to be so extraordinarily big?
Could a much smaller universe have contained stars, planets, life and eventually beings capable of looking back at the universe and wondering why they exist?
Trying to Imagine the Numbers
Consider one million.
If you counted one number every second without stopping, reaching one million would take about 11½ days.
One billion seconds is about 32 years.
One trillion seconds is more than 31,000 years.
Now consider a sextillion:
1,000,000,000,000,000,000,000
And estimates of the number of stars in the observable universe can reach ten times that.
Human intuition simply did not evolve to comprehend quantities like these.
We can write the number.
We can calculate with it.
But actually imagining it is something else entirely.
More Stars Than Grains of Sand?
A famous comparison says that there are more stars in the observable universe than grains of sand on all the beaches of Earth.
The exact comparison depends upon assumptions: how many galaxies and stars there are, what counts as a beach, how deep the sand is and the average size of a grain.
So it shouldn’t be treated as a precise astronomical measurement.
But the comparison is entirely plausible.
And photographs from the Hubble Space Telescope provide an extraordinary way of appreciating why.
The Hubble Ultra Deep Field was produced by pointing Hubble towards what appeared to be an almost empty patch of sky.
The patch was tiny — roughly the apparent area of an eye of a needle held at arm’s length.
Hubble stared deeply into it.
What appeared almost empty contained approximately 10,000 galaxies.
Not stars.
Galaxies.
Each is potentially home to millions or billions of stars.
And that tiny patch represents only a minute fraction of the entire sky.
The darkness above us is not really empty.
We simply cannot see most of what is there with our eyes.
From the Centre of Everything to One Planet Among Billions
For much of human history it was natural to place ourselves near the centre of the cosmos.
The Earth appeared stationary.
The Sun, Moon, planets and stars appeared to move across the sky around us.
Ancient cosmological systems therefore commonly placed Earth near the centre.
The development of the heliocentric model associated particularly with Nicolaus Copernicus changed that picture.
Earth became a planet orbiting the Sun.
Then the Sun itself lost its privileged position.
It is one ordinary star among the enormous population of the Milky Way.
Then the Milky Way lost its apparent uniqueness.
It is one galaxy among billions.
At each stage, humanity seemed to move farther from the centre of the picture.
This history is sometimes presented as a story of our increasing insignificance.
But that conclusion does not necessarily follow.
Location is not the same thing as importance.
Are We at the Centre of the Universe?
There is something strange about looking deeply into space.
Whichever direction we look, we can see enormous distances into the universe.
And surrounding us in every direction is the cosmic microwave background — the oldest light we can directly observe.
It is tempting to conclude:
Surely that means we are at the centre.
But it does not.
Imagine dots drawn on the surface of an expanding balloon.
As the balloon expands, every dot sees the other dots moving farther away.
There is no special dot at the centre of the surface.
The centre of the balloon exists in the three-dimensional space surrounding it, but it is not a location on the two-dimensional surface itself.
The analogy is imperfect, but it helps illustrate an important cosmological principle.
From any sufficiently typical location in an expanding, broadly homogeneous universe, an observer would see distant galaxies receding and would find themselves apparently surrounded by their own observable universe.
There is no evidence that Earth occupies a privileged central position.
Every Observer Has Their Own Observable Universe
This gives us a useful distinction between the universe and the observable universe.
The observable universe is the region from which light or other signals have had enough time to reach us since the early universe.
An observer in a galaxy billions of light-years away would have a different observable universe centred upon them.
Their cosmic horizon would overlap ours, but it would not be identical.
So in one limited sense, you really are at the centre of the observable universe.
But so is every other observer.
That is not because everyone occupies the same place.
It is because an observable universe is defined from the position of the observer.
Looking Across Space Means Looking Back in Time
There is another extraordinary consequence of the universe’s size.
We never see distant objects as they are now.
Light travels at a finite speed: approximately 300,000 kilometres per second.
The Moon is therefore seen as it was about 1.3 seconds ago.
The Sun appears as it was about eight minutes ago.
The nearest stars appear years in the past.
The Andromeda Galaxy appears roughly 2.5 million years in the past.
When we observe galaxies billions of light-years away, we see them billions of years ago.
A telescope is therefore also a kind of time machine.
The deeper we look into space, the further we look into the past.
The Sky Is a Historical Record
This produces one of the most beautiful consequences of modern astronomy.
The night sky does not show us one moment in the history of the universe.
It contains many different times simultaneously.
A nearby star might show us the universe a few years ago.
A distant galaxy shows us the universe millions or billions of years ago.
The deepest astronomical observations reveal galaxies when they were young and still developing.
The Hubble Extreme Deep Field, for example, contains thousands of galaxies in a tiny region of sky, including galaxies seen as they existed when the universe was only a small fraction of its present age.
The farther away we look, the younger the universe becomes.
Eventually we reach a remarkable boundary.
The Oldest Light We Can See
We cannot use ordinary light to see all the way back to the Big Bang itself.
For roughly the first 380,000 years, the universe was so hot and dense that light could not travel freely through it.
Eventually the universe cooled enough for electrons and atomic nuclei to combine into atoms.
Light could then travel much more freely.
That ancient radiation has been travelling through the expanding universe ever since.
Today we detect it as the cosmic microwave background, or CMB.
It surrounds us in every direction.
The CMB is therefore sometimes described as a kind of baby picture of the universe.
But it is important to understand what we are seeing.
The cosmic microwave background is not a physical shell surrounding Earth.
We are looking backwards in time towards an era when the universe became transparent.
An observer in a distant galaxy would also see a cosmic microwave background surrounding them.
They too would appear to occupy its centre.
How Can the Observable Universe Be Bigger Than 13.8 Billion Light-Years?
This creates another wonderfully counter-intuitive question.
The universe is about 13.8 billion years old.
Surely, then, the most distant things we could see should be at most 13.8 billion light-years away.
But the observable universe has a present-day radius of roughly 46 billion light-years.
How can light have travelled farther than the age of the universe seems to allow?
Because space itself has expanded while the light has been travelling.
Imagine beginning a long walk while the road beneath you is simultaneously being stretched.
The distance between your starting point and destination can become much greater than the distance you personally walk.
Similarly, ancient light has been travelling towards us while the universe has expanded.
The objects or regions that emitted some of the most ancient light are therefore now much farther away than 13.8 billion light-years.
Our observable universe is consequently about 93 billion light-years across today.
And that still may not be the entire universe.
How Big Is the Whole Universe?
We do not know.
The observable universe has a horizon because the universe has a finite age and light travels at a finite speed.
Beyond that horizon there may be vastly more universe.
Possibly infinitely more.
If you stood on a ship in the middle of an ocean surrounded by fog, you might be able to see only a few kilometres.
The boundary of your vision would not be the edge of the ocean.
Our cosmic horizon may be something similar.
We see only as far as information has had time to reach us.
The universe beyond may continue enormously farther.
Does the Universe Have an Edge?
Probably not in the ordinary sense of a wall or boundary.
A universe can be finite without having an edge.
The surface of Earth provides a simple lower-dimensional analogy.
Earth’s surface has a finite area.
But if you walk around it, you never reach an edge and fall off.
You eventually return to where you started.
Three-dimensional space could, in principle, possess analogous global properties that are difficult for us to visualise.
One possibility investigated by cosmologists is a topology known as a 3-torus.
The name can be confusing.
It does not necessarily mean that the universe is literally shaped like an ordinary doughnut sitting in some larger space.
Rather, it describes a possible way in which spatial directions could connect back upon themselves.
Travel far enough in one direction and, in principle, you might eventually return from the opposite direction.
It is a fascinating possibility.
But observations have not established that the universe has a 3-torus topology.
What Shape Is the Universe?
Cosmologists distinguish between the geometry of the universe and its topology.
Measurements of the cosmic microwave background indicate that, on the largest observable scales, space is extremely close to geometrically flat.
That doesn’t necessarily tell us whether the whole universe is finite or infinite.
A flat sheet can extend forever.
But space could also possess a more complicated topology while appearing locally flat.
Imagine the screen of an old video game where leaving the right side causes you to reappear on the left.
Locally everything looks ordinary.
Globally the space connects with itself.
Whether our universe possesses anything comparable remains unknown.
The important point is that asking “What shape is the universe?” is much harder than asking whether it resembles a sphere or a doughnut.
Dante’s Universe
Long before modern cosmology, writers and philosophers struggled to imagine realities that exceeded ordinary three-dimensional intuition.
Dante’s Divine Comedy is particularly fascinating in this respect.
In Paradiso, Dante constructs a cosmos in which the arrangement of the heavenly spheres and the divine realm becomes difficult to represent using ordinary spatial intuition.
Modern writers have occasionally noted intriguing mathematical similarities between Dante’s imaginative cosmology and higher-dimensional geometries.
We should not turn Dante into a medieval cosmologist who somehow anticipated modern relativity.
He did not.
But the comparison is interesting for another reason.
Both medieval theology and modern cosmology can push human imagination beyond the kinds of spaces our brains evolved to navigate.
Sometimes reality — or our attempts to describe it — becomes impossible to picture using ordinary geometry.
Why Does the Universe Need So Much Empty Space?
Even galaxies themselves are mostly empty space.
Stars are separated by enormous distances.
Galaxies are separated by even greater ones.
Why all the room?
Part of the answer comes from how structures form.
Gravity gradually drew matter together from small variations in the early universe.
Gas formed stars.
Stars collected into galaxies.
Galaxies formed groups and clusters.
On enormous scales, matter forms something resembling a cosmic web, with galaxies and galaxy clusters concentrated along filaments surrounding vast cosmic voids.
A universe capable of developing complex structures requires time and space for these processes to occur.
And complex chemistry requires stars.
We Are Made From an Old Universe
The early universe did not contain planets, oceans or people.
It consisted primarily of simple particles and eventually mostly hydrogen and helium.
Many of the heavier chemical elements necessary for rocky planets and life were produced later inside stars or during violent stellar events.
The carbon in living organisms, the oxygen we breathe and many of the elements making up the Earth have a cosmic history.
Stars had to form.
Some had to evolve and die.
Their material had to be dispersed.
New stars and planets had to form from enriched material.
Life on Earth appeared only after billions of years of cosmic evolution.
So a universe containing observers like us could not simply begin yesterday.
It needed a history.
And that history naturally produced an enormous cosmos.
Does a Large Universe Make Life More Likely?
Now we reach a fascinating possibility.
Suppose the development of life requires an unusual combination of circumstances.
A suitable star.
Long-term stability.
The right chemical elements.
A planet with appropriate conditions.
Complex chemistry.
The emergence of self-replicating life.
Billions of years of evolution.
Eventually perhaps intelligence and consciousness.
We do not yet know how improbable each of these steps is.
We currently have only one confirmed example of life: Earth.
That makes estimating the probability extraordinarily difficult.
But imagine, purely as an illustration, that the probability of conscious life developing around any particular star were extremely small.
In a universe containing an enormous number of stars, rare events receive an enormous number of opportunities to occur.
A lottery with fantastically poor odds becomes a different proposition if there are enough tickets.
This does not demonstrate that the universe is large in order for consciousness to exist.
But it does raise a legitimate question:
Could the enormous scale of the universe be part of the reason observers like us can exist at all?
The Anthropic Principle
This leads to an idea known as the anthropic principle.
At its simplest, it makes a surprisingly modest observation.
We can only find ourselves in a universe compatible with beings capable of finding themselves in it.
If the laws or conditions of the universe made observers impossible, nobody would be here wondering about them.
Imagine millions of planets, almost all completely unsuitable for life.
A conscious observer should not be surprised to discover themselves on one of the rare planets where conscious observers can survive.
They could not have found themselves on the others.
The same reasoning can be extended to properties of the universe.
We observe a universe capable of producing complexity, chemistry, stars, planets and eventually ourselves because only such a universe could contain observers making the observation.
But this does not explain why such a universe exists in the first place.
The Fine-Tuning Problem
Modern physics has discovered that the universe depends upon fundamental constants and physical laws.
If some properties of nature were substantially different, the universe might develop very differently.
Perhaps stable atoms would not exist.
Perhaps stars would never form.
Perhaps matter would behave in ways incompatible with complex chemistry.
This has produced the fine-tuning problem.
Why does our universe have conditions compatible with complexity and life?
Several broad possibilities have been proposed.
Perhaps the fundamental constants could not actually have been otherwise.
Perhaps we are extraordinarily fortunate.
Perhaps an enormous or multiple-universe reality provides many different opportunities and we inevitably observe ourselves in a life-compatible region.
Perhaps some deeper physical theory will eventually explain the apparent fine-tuning.
And some philosophers and religious thinkers regard fine-tuning as evidence suggestive of purpose or design.
At present there is no universally accepted answer.
Does the Universe Exist for Consciousness?
We can now ask a more radical question.
Perhaps consciousness is not merely an accidental late arrival in the universe.
Perhaps it is somehow fundamental to reality.
There are philosophical traditions that place mind or consciousness at the foundation of existence rather than treating it solely as something produced by matter.
Modern discussions of consciousness have revived some versions of these ideas.
Quantum mechanics has sometimes been brought into this discussion because measurement plays an unusual role in the theory.
The famous double-slit experiment shows that quantum systems can display wave-like interference until interactions or measurements reveal which-path information.
This is sometimes described as though human consciousness itself causes the wavefunction to collapse.
That is not an established conclusion of quantum physics.
Modern quantum theory does not require a conscious human observer simply to make physical events occur, and different interpretations of quantum mechanics understand measurement in different ways.
Nevertheless, the relationship between observer, measurement and reality has encouraged genuine philosophical questions.
What is an observer?
What is a measurement?
Why does a universe governed by physical laws eventually produce beings capable of discovering those laws?
Those questions remain fascinating without claiming that consciousness literally creates the physical universe.
The Universe Becoming Aware of Itself
There is another way of thinking about consciousness that requires no assumption that minds caused the universe to exist.
For billions of years after the Big Bang, as far as we currently know, the universe contained matter and energy but no human beings.
Stars formed.
Galaxies collided.
Planets developed.
Then, at least on one small planet, chemistry became biology.
Biology evolved nervous systems.
Some nervous systems eventually became capable of constructing internal models of the world around them.
And eventually a creature looked towards the stars and asked:
What is all this?
Something extraordinary had happened.
Matter organised itself into a form capable of studying matter.
Atoms forged inside ancient stars became brains capable of discovering how stars work.
The universe produced, within itself, observers capable of observing the universe.
In that poetic but meaningful sense, conscious beings allow the universe to become aware of itself.
Whether this is merely an extraordinary consequence of evolution or reflects something deeper about the nature of reality is a philosophical question.
Science does not currently tell us that consciousness was inevitable.
But neither do we yet know how common consciousness is.
We have only begun looking.
Does Our Smallness Make Us Insignificant?
The scale of the universe can produce an uncomfortable reaction.
Earth is tiny.
The Sun is one star among an unimaginable number.
The Milky Way is one galaxy among billions.
A human life occupies an almost absurdly small amount of cosmic space and time.
It is tempting to conclude:
We are insignificant.
But size and significance are completely different concepts.
A human brain is smaller than a mountain.
That does not make a mountain more conscious.
A book is smaller than a planet.
That does not make the information in the book less meaningful.
A child is smaller than a star.
That tells us nothing about the value of the child’s experiences.
Physical scale measures size.
It does not measure importance.
A Universe Seen From Billions of Perspectives
If conscious life exists elsewhere, something even more remarkable follows.
Every conscious creature would experience the universe from its own location.
Different skies.
Different stars.
Different senses.
Perhaps completely different ways of experiencing reality.
There would be no single privileged perspective.
The universe could, in effect, be observing itself from countless locations.
Even here on Earth, every conscious life provides a perspective that has never existed before in precisely the same form.
That does not establish that the universe needs observers.
But it gives us a different way to think about our place within it.
We do not need to be geographically central to be interesting.
Perhaps There Is No Centre
The history of astronomy repeatedly displaced humanity from imagined centres.
Earth was not the centre of the Solar System.
The Sun was not the centre of the universe.
The Milky Way was not the only galaxy.
The observable universe may itself be only part of something much larger.
Perhaps the lesson is not that humanity has become less important.
Perhaps it is that the universe may not require a privileged centre at all.
Every location provides another perspective.
Every observer sees their own cosmic horizon.
And every conscious being begins from the same peculiar position:
here, now, looking outwards.
So Why Is the Universe So Big?
We do not know that the universe has a particular size for a reason.
Its enormous scale emerges from its history, expansion, physical laws and the way cosmic structures developed.
The observable universe is vast because the cosmos has been expanding for billions of years.
The entire universe may be vastly larger still — perhaps even infinite.
But its size also creates extraordinary possibilities.
An enormous universe contains enormous numbers of stars.
Those stars create chemical elements.
Some develop planets.
Given enough time and opportunities, at least one of those planets produced life.
And at least one form of that life became conscious enough to wonder why the universe was so large.
Perhaps consciousness is a rare accident in an unimaginably large cosmos.
Perhaps it is common.
Perhaps it is an almost inevitable consequence of sufficiently complex matter.
Perhaps it reveals something deeper about reality that we do not yet understand.
Those possibilities should not be confused with one another, and science has not yet given us the answer.
But the size of the universe need not make us feel insignificant.
Quite the opposite.
Consider what is happening at this moment.
A tiny collection of atoms on a small planet orbiting one ordinary star is contemplating a universe tens of billions of light-years across.
Those atoms were themselves produced by the history they are trying to understand.
The universe is unimaginably larger than us.
Yet somehow we are capable of imagining it.
And perhaps that is one of the most remarkable things about the universe of all.