The Universe-7.
In 1915, one month after Einstein published the theory of General Relativity,
the physicist Karl Schwarzschild found a solution to Einstein’s equations,
which is now known as the Schwarzschild metric.
The Schwarzschild metric describes the structure of spacetime around a perfectly spherical object.
There are two interesting features of Schwarzschild’s spacetime.
One occurs at a particular distance from the object, known as the Schwarzschild radius.
For distances less than the Schwarzschild radius, space and time are distorted in such a way,
that the entire future of anything that falls in will point inwards.
This sounds weird, but remember that space and time are mixed up in Einstein’s theory.
In technical language, we say that the future light cones inside the Schwarzschild radius,
all point to the centre.
This means that, as inexorably as we here on Earth march into the future,
if we were to cross the light defined by the Schwarzschild’s radius,
you would inexorably march inwards towards the object, that is bending spacetime.
There would be no escape, not even for light itself,
in the same way that you cannot escape your future.
This surface, defined by the Schwarzschild radius, surrounding the object,
is known as the event horizon.
But what happened to the object itself?
This is the second interesting feature of the Schwarzschild metric.
Let’s first think about the Sun.
If you asked what the Schwarzschild radius for the star with the mass of the Sun is,
it would be 3 kilometres.
This is inside the Sun!
So there is no problem here, because you can’t get that close to the Sun,
without actually being inside it, at which point all the mass outside you doesn’t count anymore.
But what about an object like a collapsing neutron star, getting smaller and smaller,
and denser and denser?
What if you could have an object that was dense enough to have the mass of the Sun,
and yet be physically smaller than the Schwarzschild radius.
It seems there are such objects in the Universe.
In these stars, even neutron degeneracy pressure will not suffice to resist the force of gravity.
These objects are called black holes.
At the very centre of the black hole, at r=0, the Schwarzschild metric has another surprise in store.
The spacetime curvature becomes infinite.
This is known as a singularity.
In physical theories, the existence of singularities signals the end of the applicability of the theory.
In simple language, there must be more to it!
This has led many physicists to search for a new theory of gravity.
Quantum theories of gravity, such as string theory,
may be able to avoid the appearance of singularities,
by effectively setting a minimum distance scale,
below which spacetime does not behave in the manner described by Einstein’s equations.
As yet, we do not know whether any of these current theories are correct,
or even if they are on the road to being correct.
But what we do know is that black holes exist.
At the centre of our galaxy and possibly every galaxy in the Universe,
there is believed to be a super massive black hole.
Astronomers believe this because of precise measurements of a star known as S2.
The star orbits around the intense source of radio waves known as Sagittarius A star,
that sits at the galactic centre.
S2’s orbital period is just over 15 years.
This makes it the fastest known orbiting object, reaching speeds upto 2% of the speed of light.
If the precise orbital path of an object is known, the mass of the thing it is orbiting,
can be calculated.
The mass of Sagittarius A Star is enormous - 4.1 million times the mass of our Sun.
Since the star S2 has a closest approach to the object of only 17 light hours,
it is known that Sagittarius A Star must be smaller than this,
otherwise S2 would literally bump into it.
The only way of cramming 4.1 million times the mass of a Sun,
into a space less than 17 light years across, is as a black hole.
This is why astronomers are confident that a giant black hole sits at the centre of the Milky Way.
These observations have been confirmed and refined by studying a further 27 stars,
known as the S-Stars, all of which with orbits taking them close to Sagittarius A Star.
Black holes are fascinating objects.
We don’t understand them, and yet we know they exist.
They are of immense importance, because despite the fact that we will never encounter one directly,
the physics that rise inside the event horizon is undoubtedly fundamental.
These are objects that will require a new theory of gravity, indeed a new theory of space and time,
to describe.
One of the holy grails of observational astronomy is to find a pulsar orbiting around a black hole.
Such a system surely exists somewhere.
To be able to observe the behaviour of one of these massive cosmic clocks,
in the intensely curved spacetime,
close to a black hole would surely test Einstein’s theory of General Relativity to its limit.
It may even, if we are lucky, reveal flaws that point us to a new theory.
The passage of time is the story of something so fundamental,
that is impossible to imagine a Universe without it.
Yet it is a property of the Universe that modern science struggles to explain.
Time is something that feels very human.
It regulates our days and its relentless and unavoidable passing, drives our lives forward.
It is why each one of us has a beginning and an end.
But time isn’t a human creation.
We evolve with its passing, but so does the rest of the Universe.
Time is woven into the very fabric of the Cosmos.
Even with our incomplete understanding,
our exploration of time has allowed us to do something remarkable.
Just by investigating the nature of time and the natural world as we find it here on Earth,
we been able to not only glimpse the beginning of the Universe, but to imagine how it might end.
Everyday we wake up to the rhythm of our planet as it spins at over 1500 kilometres an hour,
relentlessly rolling us in and out of the Sun’s glare.
But a day is the 24 hours it takes Earth to rotate once on its axis.
It is the 86,400 seconds it takes for a person standing on the Equator,
to be whipped around the 40,074 kilometres circumference of Earth.
This rhythm of the Earth, which comes about because of the spin rate of our rocky,
iron-core ball that was laid down in Earth’s formation in it’s 4.5 billion year history.
Travelling at 108,000 kilometres an hour, we move through space in orbit around our star.
Racing around the Sun at an average distance of 150,000 kilometres,
we complete our 970 million kilometre journey, in 365 days, 5 hours, 48 minutes and 46 seconds.
We return regularly to an arbitrarily defined starting point.
This marks the beginning and end of what we call as a year.
Everywhere we look in space we see celestial clocks marking the passage of time in rhythms.
Our moon rotates around Earth every 27 days, 7 hours and 43 minutes.
Because it is tidally locked to Earth it takes almost the same amount of time,
to rotate on its own axis.
Further out in the solar system, a Martian day lasts one Earth day and 37 minutes.
But because Mars is further from the Sun, a Martian year is 687 Earth days.
Further away from the Sun, the length of the year gets progressively greater.
Neptune takes around 165 Earth years to make its way around the Sun.
In 2011, Neptune completed its first full orbit of the Sun, since it was discovered in 1846.
Just as the Earth and other planets mark out the passing of the years, as they orbit the Sun,
so our entire solar system traces out its own vast orbit.
We are just one star amongst at least 200 billion in our galaxy.
All these stars are making their own individual journey’s around the galactic centre.
We are all in orbit around the super-massive black hole that lies at the heart of the Milky Way.
It is estimated that it takes us about 225 million years,
travelling at 792,000 kilometres per hour to complete one circuit.
This is known as the galactic year.
Since Earth was formed 4.5 billion years ago, it has made 20 trips around the galaxy.
So Earth is 20 galactic years old.
The entire history of human species, of 200,000 years, is a blink of an galactic eye.
Humans are yet to complete a galactic day.
Interestingly here on Earth there are creatures that have existed for lengths of time,
that span the grandest of rhythms.
The Ostional wildlife refuge on the Pacific coast of Costa Rica,
is home to one of nature’s most spectacular sights.
It is one of the few beaches in the world were large numbers of sea turtles make their nests.
Here the turtles haul themselves from the ocean, as they have done year after year,
for over 120 million years, or half a galactic year.
The ancestors of the turtles, waiting for the right movement to crawl into the land,
did so, when the continent were very different.
The continents on Earth were slowly on the move.
North America was close to Europe.
South America was connected to Africa.
Australia was joined with the Antarctic.
Collectively they have witnessed the reshaping of our planet, and the heavens above.
The pattern of stars must have looked very different from the other side of the galaxy.
The first attempts in chronometry, began 30,000 years ago.
Stone Age humans used the Lunar cycle to mark time.
By following it through its phases, they were able to create the first calendars.
They were able to create the structure of the year beyond the day night cycle.
This is how classification and division of the cycles of the Cosmos began.
The first clocks were simple pieces of technology.
Using nothing more than a stick, ancient civilisations were able to use sundials,
to track the passing of time during the day.
But sundials did not work during the night.
Ancient Egypt was the first civilisation that took measuring time beyond the sundial.
The technique of using the flow of water to measure time, dates back to 6000 BC.
The oldest physical evidence of a water clock can be found,
in the reign of Pharaoh Amenhotep III in 1400 BC.
These elegant devices allowed water to escape at a near constant rate from a hole in the base.
Inside the clock were 12 markings which could be measured as the water level dropped.
These are permanent clocks that gave accurate measurements during both day and night,
so that priests could perform their rituals at the appointed hour.
Water clocks continued to be refined and used for many centuries,
Hourglasses using the flow of sand was also used extensively.
The Portuguese explorer Magellan used 18 hourglasses as a navigation tool on his ship,
when he circumnavigated the globe in 1522.
Time keeping was elevated to completely new level of accuracy,
with the invention of the pendulum clock.
Galileo was the first scientist to investigate the physics of a swinging pendulum.
The key property of the pendulum, which makes it a useful time keeping device,
is that the period of the swing, depends only on the length of the pendulum,
and Earth’s gravitational pull.
This means that all you need to make the clock tic accurately,
is to get the length of the pendulum right.
Most grandfather clocks have a pendulum with a swing of 2 seconds, and a length of about 2 meters.
The dutch astronomer Christiaan Huygens invented the first pendulum clock in 1656.
It remained the most accurate way of telling the time until the 1930’s.
Today we rely on atomic clocks to measure time with extraordinary accuracy.
Atomic clocks used the frequency of light emitted when electrons jump around in atoms like caesium.
This is highly accurate because the structure of atoms is unchanging.
Therefore the light emitted from them always has the same frequency.
This light can be used to keep an oscillator ticking at a precise rate.
This allows atomic clocks to tell the time with an accuracy,
of one thousand-millionth of a second per day.
The second itself has been defined since 1967 using the theory behind atomic clocks.
One second is defined as the duration of 9,192,631,770 periods of the radiation,
corresponding to the transition between the two hyperfine levels,
of the ground state of the caesium 133 atom.
This means that the second is the time it takes for 9,192,631,770 peaks of light emitted,
in an atom of caesium, to fly past you.
Atomic clocks allows us to measure incredibly small periods of time.
The shortest period we are able to measure is 12 attoseconds, or 12 quadrillionths of a second.
This is how long it takes for light to travel past 36 hydrogen atoms.
For all the accuracy and precision we have achieved in keeping time,
we have never managed to do anything more than observe it.
From the earliest solar calendars, to the electrons jumping around in caesium atoms,
one thing about the nature of time is clear.
We can measure its passing, but we cannot control it.
It moves inexorably forward.
It cannot be stopped.
This tells us something profound about our Universe.
A glacier operates in a highly ordered sequence.
As time passes, snow falls, ice falls, the glacier gradually inches down the valley,
and when the ice meets the water, pieces break off and fall into the lake creating waves.
In many ways the ordering of events into a sequence is the simplest way to think about time.
The fact that sequences of events always happen in a particular order,
is a fundamental part of our experience of the world.
If things happen out of order, we know that there is something wrong.
Yet there is a legitimate question about what we mean by events happening ‘in order’.
The sequence of events in the glacier cannot happen in reverse order.
We however have a scientific explanation for why such a dramatic reversal never happens.
We call it ‘the arrow of time’.
The phrase was first used by the physicist Arthur Eddington in the early 20th century,
to describe this deceptively simple and yet profound quality of our Universe.
It always seems to run in a particular direction.
Eddington was instrumental in bringing Einstein’s theory of relativity,
to the English speaking world, during the first World War.
He was also one of the first scientists to directly confirm the findings of relativity,
when he led an expedition to observe the total solar eclipse in May 1919.
In 1928, he published ’The nature of the physical world’, in which he introduced two great ideas,
that have endured in popular scientific culture to this day.
The first was the image of the infinite monkey theorem.
This states that given an infinite amount of time,
anything consistent with the laws of physics will happen.
If an army of monkeys were strumming on typewriters,
they might write all the books in the British Museum.
This is related in a deep way to the arrow of time, which Eddington described as follows:
Let us draw an arrow arbitrarily.
If as we follow the arrow we find more and more of the random element in the state of the world,
then the arrow is pointing towards the future.
If the random element decreases, the arrow points towards the past.
This is the only distinction known to physics.
This follows at once if our fundamental contention is admitted,
that the introduction of randomness is the only thing that cannot be undone.
We can use the phrase ’times’ arrow’ to express the one way property of times,
which has no analog in space.
Eddington’s arrow vividly and economically expresses a key property of time.
It only goes in one direction.
But what does he mean by randomness?
It seems obvious that the Universe is constantly evolving, but what drives this evolution?
How should we quantify how random something is?
Why is the past different from the future?
Why is there an arrow of time?
Time is something that we all understand, and yet a plausible scientific reason,
as to why time marches inexorably forward wasn’t offered till the late 19th century,
coming about as the solution to a practical problem on Earth.
In 1712, the english inventor Thomas Newcomen created the first commercial steam engine,
paving the way for the industrial revolution.
This accolade is more usually awarded to the Scottish inventor James Watt.
In 1763, Watt was asked to repair a Newcomen engine by the University of Glasgow.
In doing so, he developed a new steam engine which transformed the landscape of modern life.
Watt’s steam engine was more efficient and more flexible then it predecessor.
It used far less coal than the Newcomen for a given power output,
and was therefore much cheaper to run.
More importantly still, Watt’s engine could do more than pump water out of wet mines.
It could also generate the rotary motion that was needed to power the machines on the factory floor.
No longer did a factory have to be situated by a river to run its equipment.
With the help of Watt’s engine a factory could be situated anywhere,
catalysing the emergence of the modern industrial landscape.
Steam powered machines changed the course of history.
Yet engineers who followed Watt struggled to improve them.
There seem to be fundamental principles that restricted their efficiency.
With profit margins to maximise, even a small increase in there effectiveness,
would be highly valuable.
So understanding how hot the fire should be, or what substance should be boiled in the engine,
were problems that were not only interesting from a scientific perspective,
but were also critical for businesses.
It was out of these questions of engineering design, that the science of thermodynamics arose,
and with it the concepts of heat, temperature and energy entered the scientific vocabulary,
in a precise way for the first time.
One of the scientists working on these problems was Rudolf Clausius, who was interested in heat.
Till the first half of the nineteenth century, it was thought to be a fluid,
that flowed from hot things to cold things.
Clausius and others realised that this description was not able to explain the cycle of a steam engine.
The foundation for Clausius’s theoretical advances was laid by James Joule.
Joule was working to improve the efficiency of the steam engines in his brewery.
The quest for cheaper beer motivated him to investigate the relationship,
between the work his steam engines could do, and heat.
In doing so he managed to reduce the costs of beer production,
and lay one of the corner stones of the science of thermodynamics.
Using a series of beautifully simple experiments,
Joule was able to demonstrate that mechanical work could be converted into heat.
From his experiments he found that it took the same amount of work,
to raise the temperature of a fixed amount of water, by 1 degree Fahrenheit.
Inscribed on his tombstone is the number 772.55.
This was his measurement of the amount of work done in foot-pounds force,
that is required to raise the temperature of 1 pound of water, by 1 degree Fahrenheit.
The reason that Joule’s work was important is that it demonstrated,
that heat is not a thing that can be created or destroyed.
It doesn’t literally flow between things or move around.
It is in fact a measure of something else.
Even today, this is perhaps not obvious because we still speak of the flow of heat,
from hot to cold things.
Heat, we now understand, is simply a form of energy.
Just as a ball resting on a table, has gravitational potential energy,
so a hot thing has energy that can be released, at least in part, by putting it next to a cold thing.
To heat something up, you simply have to transfer energy to it, by doing work on it.
It doesn’t matter how the work is done.
It can be a falling weight, a shinning light or an electric current,
but as long as you do the same amount of work, the temperature increase will still be the same.
This was all quantified, as a result of Joule’s work, into the first law of thermodynamics.
This is a statement of the fact that energy cannot be created or destroyed.
It can be changed from one form to another.
Clausius made the first explicit statement of the law,
and laid down the foundation of the science of thermodynamics, in his landmark 1850 publication.
The first law of thermodynamics can be written down mathematically as
Delta U = Q minus W.
This says that the increase in the internal energy of something, Delta U,
is equal to the heat flow into it, Q minus the work performed by it, W.
15 years after writing down the first law of thermodynamics,
and far more importantly for our understanding of the arrow of time,
Clausius introduced a new concept known as entropy,
which lies at the heart of the second law of thermodynamics.
Clausius’s statement of the second law,
does not at first sight sound as if it has profound implications for the future of our Universe.
He simply stated that ‘no process is possible whose sole result is the transfer of heat,
from a body of lower temperature, to a body of higher temperature’.
This simple proposition occupies a profound position in modern science.
Arthur Eddington said of the second law :
’If some one points out to you that your pet theory of the Universe,
is in disagreement with Maxwell’s equations, then so much the worse for Maxwell’s equations.
If it is found to be contradicted by observations, well,
these experimentalists do bungle things sometime.
But if your theory is found to be against the second law of thermodynamics, I can give you no hope:
’There is nothing for it but to collapse in deepest humiliation.
The concept of entropy enters when the second law is written down in quantitive form.
The change in entropy of a system, such as a tank of water,
is simply the amount of heat added to it at a fixed temperature.
In symbols Delta S = Delta Q divided by T.
Delta S is the change in entropy as a result of adding a small amount of heat, Delta Q,
at a fixed temperature T.
It may still be unclear what this has to do with the Universe,
but here is the profound point discovered by Clausius.
In any physical process at all, you find that entropy either stays the same or increases.
It never decreases.
Here is the thermodynamics of the arrow of time.
Clausius had discovered a physical quantity that can be measured and quantified,
which only ever increases in practice, and never decreases in theory,
no matter how cleverly you design your experiments, or piece of machinery.
This is extremely useful information, if you are designing a steam engine,
because it puts a fundamental limit on the efficiency.
It also prevents the construction of the so called ‘perpetual motion machines’.
You could say that the second law tells you that you cannot get something for nothing.
The second law is more profound than this,
because it introduces a difference between the past and the future.
In the future, entropy will be higher than it is in the present, because it always increases.
In the past entropy was lower than it is now, because it always increases.
Clausius introduced the concept of entropy because he found it useful.
What exactly is entropy, and what is the deep reason, that it always increases?
What was the meaning of Eddington’s cryptic quote, about the randomness and the arrow of time?
He seem to be equating entropy with the amount of randomness, in the world, and indeed he was.
Understanding this will make it clear why the second law of thermodynamics,
mandates that our entire Universe must, one day, die.
To understand entropy we need a more intuitive definition of entropy, than given by Clausius,
known as the statistical definition of entropy.
It was developed by Ludwig Boltzmann in the 1870’s.
A sand castle is made of lots of little grains of sand, arranged into a distinctive shape -
a castle.
Let us build a sand castle, in a windy desert, with say, a million grains of sand.
We could take those million grains and, instead of carefully ordering them into a castle,
we could just drop them on to the ground.
They would then form a pile of sand.
We would be surprised, if we dropped our sand grains on to the ground,
and they assemble themselves into a castle.
Why does this not happen?
What is the difference between a pile of sand and a sand castle?
They both have the same number of sand grains.
Both shapes are obviously possible alignments of the grains.
Boltzmann’s definition of entropy is essentially a mathematical description of the difference between, a sand castle and a sand pile.
It says that the entropy of something,
is the number of ways you can rearrange its constituent parts, and not notice you have done so.
For a sand castle, the number of ways you can arrange grains,
and still keep the highly ordered shape of the castle is quite low, so therefore has low entropy.
For a sand pile, on the other hand, pretty much anything you do to it,
will still result in there being a pile of sand in the desert,
indistinguishable from any other pile of sand.
The sand pile therefore has an higher entropy than the sand castle,
simply because there are many more ways of arranging the grains of sand,
such that they form a pile of sand, then arranging them into a castle.
Boltzmann wrote this down in a simple equation, which is written on his gravestone.
S equal to k suffix B ln W.
S is the entropy.
ln is the natural logarithm.
W is the number of ways you can arrange the component bits of something,
such that it is not changed.
k suffix B is a number known as Boltzmann’s constant.
The equation simply relates to the number of ways you can arrange things, to the entropy.
This may seem a bit complicated, and not entirely illuminating yet, but here is the key point:
As long as each particular arrangement of the sand grains is equally likely,
then if you start moving sand grains around at random,
they are overwhelmingly likely to form a shapeless pile of sand, then a sand castle.
This is because most of the arrangements you create at random looked like a formless pile,
and very few look like a sand castle.
This is common sense, of course, but now think about what this looks like at a microscopic level-
the level of individual sand grains.
There is nothing at all in the laws of nature to stop the wind blowing a grain of sand,
of one of the turrets of our castle, and then picking up another grain from the desert,
and blowing it back on to the turret again, leaving our castle perfectly unchanged.
Nothing at all, that is, other than pure chance.
It is much more likely that the grains of sand blown off the castle,
are not replaced with others from the desert.
So our castle slowly disintegrates in a windy desert.
This is to say it gradually changes into a formless sand pile.
In Boltzmann’s language, this is simply the statement that the entropy of the castle,
will increase overtime.
The castle will become more like a sand pile.
Why?
Because there are more ways of arranging the grains into a sand pile, then there are into a castle.
So if you randomly blow sand grains around, they will tend to form piles more often then castles.
Here is the deep reason that entropy always increases:
it is simply more likely that it will!
Notice there is nothing in the laws of nature, that prevented it from decreasing.
It is possible that the wind will build a sand castle,
but the chances are akin to tossing a coin billions of times, and each one coming up heads.
It simply not going to happen.
Boltzmann’s statistic definition of entropy is the key to understanding Eddington’s arrow of time.
This is such a key concept, with such profound consequences, that it is worth repeating it once more,
in a slightly different way.
If there are a million different ways of arranging a handful of sand grains,
with 999, 999 of the ways producing disordered sand piles,
but only one producing a beautifully ordered castle,
then it you keep throwing the sand grains up in the air,
they will usually land in the form of a disordered pile.
So, overtime , if there is a force like the wind that acts to rearrange the things,
things will get more messy or disordered,
simply because there are more ways to be disordered than ordered.
This means there is a difference between the past and the future.
The past was more order , and the future will be less ordered,
because this is the most likely way for things to play out.
This is what Eddington meant by his statement that the future is more random then the past.
His description of the arrow of time,
is as the thing that points in the direction of increasing randomness
This is why entropy always increases.
There is still a great deal of debate and research surrounding entropy.
It is centred on something we have dodged slightly.
We have only spoken about the entropy differences.
The past had a lower entropy than the future.
Ordered things become more disordered as time ticks by.
But one might legitimately ask where all the order in the Universe came from, in the first place.
In the case of our sand castle it is obvious.
We made it.
But how did we get here?
We are very ordered.
How did Earth get here?
It is very ordered too.
How did the Milky way appear if it is composed of billions of ordered worlds,
orbiting around billions of ordered stars?
There must be some reason why the Universe began in such a highly ordered state,
such that it can gradually fall to bits.
The answer is we don’t know.
Why the Universe began with sufficient order in the bank,
to allow planets, stars, and galaxies to appear.
We understand how gravity can create local order in the form of solar system and stars,
but this must be at the expense of creating more disorder somewhere else.
So there must have been a lot of order to began with.
In other words, the Universe was born in a highly ordered state, and there should be a reason for it.
Its unlikely to have been chance, because by definition a highly ordered state,
is less likely to pop into existence then a less ordered one.
A sand castle is less likely to be formed by the desert winds, then a pile of sand.
Since the Universe is far less ordered today, then it was 13.75 billion years ago,
this means it is far more likely that our Universe popped into existence a billionth of a second ago, fully formed with planets, stars, galaxies and people, than it is at the Universe popped into existence, at the Big Bang, in a highly ordered state.
There is clearly something in the early Universe that we have yet to understand.
Our Universe follows the law of any living thing.
It develops in stages from birth through life and ultimately to death.
We understand the early stages of its life,
because observations by scientists have provided valuable information,
as to how the Universe was created, and also fill in the crucial facts of the Universe thus far.
We are living in the early phase of our Universe, the Stelliferous Era,
with many more stages of life and change still yet to come.
Yet we can confidently make predictions about our Universe’s future.
By observing the life cycles of the stars above us,
we can map out the remaining years of our Universe’s life.
1: Primordial Era.
0 to 10 power 5 years.
The Big Bang, inflation, and Nueclosynthesis takes place.
Towards end of this era the Universe become transparent for the first time.
2. Stelliferous Era.
10 to the power of 6 to 10 to the power of 14 years.
Our current Era.
Matter is arranged in stars, galaxies, and galaxy clusters.
3. Degenerate Era.
10 to the power of 15 to 10 to the power of 39 years.
Galaxies no longer exist.
This is the Era of brown dwarfs, white dwarfs, and black holes.
The Sun becomes a black dwarf.
White dwarfs will assimilate dark matter and continue with minimal energy output.
4. Blackhole Era.
10 to the power of 40 to 10 to the power of 100.
Organised matter will remain only in the form of black holes.
Black holes slowly evaporate away the matter contained in them.
By the end of this Era only extremely low energy photons, electrons,
positrons, and neutrinos will remain.
5. Dark Era.
10 to the power of 101 to infinity.
The Universe will be nearly empty.
Photons, neutrinos, electrons, and positrons will fly from place to place.
Electrons and positrons will occasionally form positronium atoms.
These structures are unstable, and their constituent element eventually annihilate each other.
6. Heat death.
Considered to be the most likely fate of the Universe.
It will occur if the Universe continues expanding as it has been.
The continued expansion will result in a Universe that approaches absolute zero temperature.
6. Big bounce.
This is a cyclical repetition interpretation of the Big Bang ,
where by the first cosmological event was the result of the collapse of a previous Universe.
6. Big crunch.
The expansion of space will reverse and the Universe will re-collapse,
ultimately ending as a black hole singularity.
6. Multiverse.
Our Universe is merely one Big Bang,
among the infinite number of simultaneously expanding Big Bangs,
that are spread out over endless distances.