Look up at the night sky and it is easy to think of the universe as simply there.

Stars. Galaxies. Planets. Space.

We grow up surrounded by a universe that already exists, so its existence can begin to feel completely ordinary.

But it isn’t.

Before we even get to the extraordinary conditions that allow a planet such as Earth to exist — and before we ask the still greater question of how life emerged — there is a much bigger story.

The Universe Is About 13.8 Billion Years Old

The universe is estimated to be approximately 13.8 billion years old.

More precisely, this refers to the age of the universe in its present expanding phase, measured backwards towards the extraordinarily hot and dense early state described by the Big Bang model.

That distinction becomes important.

When people hear “the Big Bang”, it is very easy to picture an enormous explosion occurring somewhere in a gigantic black emptiness.

That isn’t what modern cosmology describes.

And the further backwards we try to go, the stranger the story becomes.

1. There Is a Limit to How Far Back We Can See With Light

The oldest light we can directly observe comes from approximately 380,000 years after the Big Bang.

This is the Cosmic Microwave Background, usually abbreviated to the CMB.

Before this period, the universe was so hot that matter existed largely as an ionised plasma. Photons continually interacted with charged particles, making the universe effectively opaque to electromagnetic radiation.

Then the universe cooled sufficiently for electrons and atomic nuclei to combine into neutral atoms.

Light could finally travel much more freely.

That ancient radiation has been travelling through the universe ever since and is still detectable today.

It therefore creates something rather like a wall of light.

Using ordinary electromagnetic astronomy, we cannot simply build a more powerful telescope and look through it.

There may eventually be other ways of obtaining information from earlier periods — including primordial gravitational waves or neutrinos — but detecting such signals is extraordinarily difficult.

2. The Man Who Named the Big Bang Didn’t Support It

The expression “Big Bang” was coined in 1949 by British astronomer Fred Hoyle.

Interestingly, Hoyle didn’t support the theory.

He favoured a competing cosmological model known as the Steady State theory, in which the universe had no beginning and maintained broadly similar large-scale properties as it expanded through the continuous creation of matter.

It is frequently claimed that Hoyle used the phrase “Big Bang” mockingly.

The historical reality is less certain, and historians have questioned whether he genuinely intended it as ridicule.

Either way, the name stuck.

And it may also have contributed to one of the biggest misconceptions about what actually happened.

3. The Big Bang Wasn’t Really a Bang

The Big Bang wasn’t an explosion in the ordinary sense.

There wasn’t a bomb-like object that suddenly exploded and threw matter outwards into empty space.

Instead:

space itself expanded.

As the universe expanded, distances between sufficiently separated regions increased.

This distinction is fundamental to understanding modern cosmology.

An explosion occurs within space.

The expansion of the universe is an expansion of space.

That is a very different idea — and one that becomes increasingly difficult to visualise using our everyday experience.

4. There Wasn’t a Giant Cloud Floating in Empty Space

Another tempting picture is of all the matter in the universe compressed into an unimaginably dense ball or cloud, surrounded by an infinite amount of empty space.

Again, that isn’t what the standard Big Bang model describes.

The early universe was extraordinarily hot and dense, but that state existed throughout space as described by the model.

There wasn’t an empty region surrounding it waiting for the universe to expand into.

The universe wasn’t an object sitting somewhere else.

Space was already part of the universe.

This leads to another strange consequence.

5. There Is No Known Centre of the Universe’s Expansion

If the Big Bang were an ordinary explosion, we would expect there to be a location where the explosion occurred.

But there is no known centre of cosmic expansion.

From our position, distant galaxies generally appear to be moving away from us.

But that does not mean Earth is at the centre.

An observer in another sufficiently distant galaxy would also see distant galaxies generally receding.

The expansion is occurring throughout space.

So asking: “Where did the Big Bang happen?”

has an unusual answer.

In an important cosmological sense: everywhere.

Every region of today’s observable universe traces its history back towards that earlier hot, dense state.

6. There Is No Known “Outside” of the Universe

This produces another question that sounds completely reasonable:

What is the universe expanding into?

The answer may simply be:

nothing.

Not “nothing” as in an enormous empty black room.

Rather, the universe doesn’t necessarily require an external container at all.

A useful way of expressing this is:

There is no container.

Space itself is part of the universe.

The universe therefore doesn’t have to be an object expanding through some greater surrounding space.

This is why asking “Where is the universe?” becomes a surprisingly difficult question.

In one sense, it is rather like asking where the surface of Earth is while standing on it.

You’re already there.

There may be speculative theories involving additional dimensions, multiverses or larger structures, but none of those are required simply for the universe to expand.

7. As the Universe Expanded, It Cooled

The early universe was extraordinarily hot.

Expansion changed that.

As space expanded, radiation was stretched towards longer wavelengths and the universe progressively cooled.

The ancient radiation represented by the Cosmic Microwave Background now has a temperature of only about:

2.725 kelvin

— approximately 2.7 degrees above absolute zero.

The universe we inhabit today is therefore dramatically different from its earliest observable state.

Cooling was also essential to what happened next.

Without sufficient cooling, stable atoms could not have formed.

Without atoms, there could be no conventional chemistry.

Without chemistry, there could be no planets like Earth.

And without chemistry, there could certainly be no life as we understand it.

8. The Universe Contains an Almost Unimaginable Number of Stars

Once matter began gathering under gravity, stars and eventually galaxies could form.

Today the observable universe contains an extraordinary number of stars.

The precise number cannot be known, but estimates commonly place it somewhere around:

10²² to 10²⁴ stars.

That means tens of sextillions of stars, potentially extending towards approximately a septillion.

The numbers rapidly lose intuitive meaning.

Even if someone could count one star every second, continuously, without ever sleeping, counting 10²² stars would take vastly longer than the present age of the universe.

And many of those stars have — or have had — planets.

9. The Distances Between Stars and Galaxies Are Enormous

The universe is not packed uniformly with stars.

Stars within galaxies are separated by enormous distances, while the distances between separate galaxies are greater still.

Our nearest neighbouring star system, Alpha Centauri, is more than four light-years away.

Even light — travelling at approximately 300,000 kilometres per second — takes years to cross the relatively small distances between neighbouring stars.

Cosmic expansion becomes particularly significant on still larger scales between gravitationally unbound groups and clusters of galaxies.

This is an important distinction.

The universe is expanding.

But everything in the universe isn’t expanding with it.

Earth isn’t expanding.

The Solar System isn’t expanding.

The Milky Way isn’t simply being stretched apart by cosmic expansion.

Local gravitational and other binding forces dominate on these scales.

10. The Expansion of the Universe Appears to Be Accelerating

For much of the twentieth century, an enormous question remained:

Would gravity eventually slow the expansion sufficiently for it to stop?

Might the universe eventually begin collapsing back in on itself?

Observations beginning in the late twentieth century produced a surprising result.

The expansion appears to be accelerating.

The phenomenon responsible is represented in our current cosmological models by what we call dark energy.

That name should not be mistaken for a complete explanation.

We can observe its apparent effects on cosmic expansion, but the underlying physical nature of dark energy remains one of the major unresolved problems in modern physics.

Based on current observations, however, the universe appears much more likely to continue expanding than eventually collapse.

11. Eventually, the Stars Will Go Out

Stars cannot shine forever.

They survive because nuclear fusion converts lighter elements into heavier ones while releasing enormous quantities of energy.

Eventually their usable fuel runs out.

Different stars meet different endings depending largely upon their mass.

Some become white dwarfs.

Others explode as supernovae and leave neutron stars or black holes behind.

New stars continue to form today, but the raw material available for star formation will eventually become increasingly depleted or inaccessible.

On unimaginably long timescales, the universe will enter an era in which conventional star formation effectively ceases.

The age of starlight will end.

Even black holes are not necessarily eternal.

According to theoretical physics, they should eventually lose energy through Hawking radiation and evaporate — although for large black holes this would take timescales so immense that ordinary human descriptions of time become almost meaningless.

12. The Most Likely Ending Is the Big Freeze

If our current understanding remains broadly correct, the universe’s most likely long-term future is generally described as the Big Freeze or heat death.

The universe continues expanding.

Matter becomes increasingly dispersed.

Usable differences in energy become progressively harder to exploit.

Stars disappear.

Temperatures trend towards extraordinarily low values.

Eventually the universe approaches a state in which there is insufficient accessible free energy to perform useful thermodynamic work.

This doesn’t happen merely millions, billions or even trillions of years from now.

The final stages occur across timescales so enormous that numbers such as trillions of years barely begin the story.

The universe would not necessarily cease to exist.

It would become increasingly cold, dark, dilute and inactive.

13. Could the Universe Eventually Begin Again?

There are other possibilities.

One of the most fascinating families of ideas involves a universe that does not simply expand forever.

In Big Crunch, Big Bounce or other cyclic cosmological models, cosmic expansion might eventually reverse or transition into another state.

The universe could become increasingly compressed, hot and dense before undergoing some form of new expansion.

These are genuine areas of theoretical cosmology.

They should not, however, be presented as equally supported descriptions of our universe’s future.

Current observational evidence favours continued expansion, rather than an eventual gravitational recollapse.

But cosmology remains unfinished science.

We do not yet possess a complete theory describing every stage or explaining every fundamental feature of the universe.

And that brings us to perhaps the biggest question of all.

Did the Universe Actually Have a Beginning?

People often hear:

“The universe is 13.8 billion years old.”

and understandably interpret this as:

“13.8 billion years ago there was absolutely nothing — and then the universe suddenly appeared.”

That is not something the Big Bang model itself establishes.

The approximately 13.8-billion-year figure describes the history of our observable expanding universe backwards towards an extraordinarily hot and dense early state.

It does not demonstrate that there was once literally nothing and then suddenly something.

There are serious cosmological proposals in which reality extends beyond what we ordinarily call the Big Bang.

These include Big Bounce models, cyclic cosmologies, emergent-universe models and other proposals involving a past-eternal underlying reality.

Other cosmological models do involve some form of beginning.

At present, we simply do not know which description is ultimately correct.

This leaves open a fascinating possibility.

Perhaps existence never began.

One philosophically reasonable interpretation is that the universe — or some deeper underlying reality from which our observable universe emerged — has always existed in some form.

Under that interpretation, asking: “What happened before the beginning?” contains a false assumption.

There wasn’t necessarily a beginning.

There was therefore never a moment when absolute nothingness existed and something subsequently appeared from it.

That does not mean we know that the universe is eternal.

We don’t.It means that modern cosmology has not established that existence emerged from absolute nothingness 13.8 billion years ago.

The ultimate origin — if there even was an origin — remains unknown.

But This Creates an Even Bigger Question

So far we have only described what happened.

The universe expanded.

It cooled.

Atoms formed.

Gravity gathered matter.

Stars ignited.

Galaxies developed.

Stars manufactured heavier elements.

Those elements were scattered through space.

Later generations of stars and planetary systems emerged.

Eventually, approximately 4.6 billion years ago, one relatively ordinary star formed within the Milky Way.

We call it the Sun.

Around it formed a collection of planets.

One of them was Earth.

But there is another layer to this story.

Because none of this was inevitable merely because a universe existed.

For stars to exist, gravity had to behave appropriately.

For atoms to exist, fundamental particles had to interact in particular ways.

For chemistry to exist, stable atomic structures had to be possible.

For carbon to exist, stars needed to manufacture it.

For planets to form, matter needed to organise into long-lived systems.

And for anything remotely resembling life to emerge, all of these conditions had to be compatible with one another.

This leads into one of the deepest questions in science and philosophy:

Why is the universe capable of producing complexity at all?

The fundamental constants and laws of physics fall within ranges that permit stable matter, stars, chemistry and complex structures.

Change some of those properties sufficiently and the universe could look radically different.

Perhaps no long-lived stars.

Perhaps no stable atoms.

Perhaps no complex chemistry.

Perhaps no planets.

Perhaps no life.

This is where discussions about the fine-tuning of the universe begin.

Fine-tuning does not, by itself, tell us why the universe has these properties.

Different explanations have been proposed — necessity, chance, deeper physical laws, selection effects, multiverse hypotheses and philosophical or theological interpretations.

Science has not settled that question.

What we can say is something simpler and extraordinary: we inhabit a universe whose properties permit complexity.

And eventually, that universe produced a planet on which complexity reached an entirely different level.

From the Universe to Earth

The universe supplied the stage.

Stars supplied many of the elements.

Gravity assembled solar systems.

Chemistry supplied the possibilities.

But those ingredients alone were not enough.

A planet still needed conditions in which complex chemistry could persist for enormous periods of time.

Liquid water.

A suitable atmosphere.

Appropriate temperatures.

Long-term climate stability.

A relatively stable star.

The right planetary mass.

Geological activity.

Protection from some forms of radiation.

A Solar System whose architecture allowed a habitable planet to survive for billions of years.

And approximately 4.54 billion years ago, one such planet formed.

Earth.

It isn’t a perfect planet.

It can be violent, unstable and extraordinarily hostile.

Yet among all the worlds we currently know, it possesses an astonishing combination of conditions.

Conditions that allowed something even more extraordinary to happen.

Chemistry became life.

Continue the Journey

Next: The Perfect Conditions of Planet Earth →

How Earth’s position, Sun, Moon, water, atmosphere, magnetic field, geology and Solar System combined to create a world capable of supporting life.

Then: The Extraordinary Existence of Life →

How non-living chemistry somehow crossed the threshold into biology — and ultimately produced organisms capable of contemplating the universe from which they came.

A Note on What We Know — and What We Don’t

Cosmology is one of the most extraordinary achievements of modern science, but it is not a completed subject.

We have powerful evidence for cosmic expansion, the Cosmic Microwave Background, the evolution of stars and galaxies and the hot early universe.

But major questions remain unresolved.

We don’t yet know the fundamental nature of dark matter.

We don’t know what dark energy ultimately is.

We don’t possess a complete theory of quantum gravity.

We don’t know whether our observable universe represents all of physical reality.

And we don’t know whether existence itself had an absolute beginning.

That uncertainty isn’t a weakness.

It’s part of what makes the universe so interesting.

The more we discover, the more remarkable the questions become.