In science, we like to do our best to prove hypotheses.
But just because it’s not proven yet, doesn’t mean it’s not real.
Take black holes. Karl Schwarzschild saw the possibility for black holes
within the framework of General Relativity back in 1916.
John Michell – using Newtonian physics – hypothesised their existence even earlier,
all the way back in 1783.
But that means that for 55 years using relativity, or 188 years using Newtonian physics,
we’d known about the theoretical possibility of black holes, but never actually saw any.
The first black hole – Cygnus X-1 – was only spotted for certain in 1971.
If it happened once, it’s no stretch to believe it could be happening again.
In the vast sea scientific possibilities and outlandish ideas,
exotic stars and strange forms of matter perhaps exist
that are predicted, prolific… but are currently unperceived.
What are these objects?
…Who knows? That’s just it, they’re unproven.
But I have a few ideas.
I’m Alex McColgan, and you’re watching Astrum.
And in today’s supercut, we’ll be looking at some of the strangest objects
or states of the universe that could be real, if only we could find one to know for sure.
Be sure to check back in after the next 188 years, to see how many did turn out to exist.
This video may not age well. Unsurprisingly for astronomers,
one of the first things we’ll turn our gaze towards are stars.
Stars are some of the most beautiful and awe-inspiring objects in the universe.
Through violent thermonuclear reactions, they emit enormous amounts of light,
illuminating the night sky and heating up any nearby planets in the process.
Through mergers and powerful explosions, stars are also responsible for producing
the heavy elements that make up much of the matter we see around us.
Astronomers and astrophysicists have come a long way toward understanding how stars operate,
to the point where we can even predict how long
it will be before our Sun runs out of fuel and transforms into a Red Giant.
But today, we’re not just going to talk about the types of stars we’re familiar with.
Instead, we’re going to contemplate stars we’ve never seen before.
Ones that astrophysicists say could hypothetically exist—
including ones that are quite literally made of vacuum.
First, we need to lay some groundwork and understand how stars work in general—
starting with the most important star of all: the Sun.
Since the Sun is made of gas or plasma and not solid matter,
you couldn’t stand on its surface like you would on the Earth. The Sun’s gravity would
pull you down toward the centre. So, why doesn’t the gas at the surface get pulled inwards too?
It turns out that there is a pressure pushing outward from the Sun’s core that counterbalances
this pull of gravity. The main source of this pressure is the radiation created by fusion
reactions in the core—the same radiation responsible for carrying light and heat
from the Sun to the Earth. Whether you think of radiation as individual photons or waves in
the electromagnetic field, its path from the core toward the outer layers of the Sun will
cause an outward push in all directions. This radiation pressure wouldn’t be enough to stop
you from falling into the Sun, but it’s just enough to stop the Sun’s own gas (which is
relatively light) from collapsing in on itself. This balancing act between inward-pulling gravity
and outward-pushing pressure is a feature of pretty much all known types of stars. What
differentiates different types of stars is where that pressure comes from. For our Sun,
the pressure comes from fusing hydrogen into helium. When the Sun runs out of hydrogen in
about 5 billion years, it will become a red giant star, and begin to generate its pressure by fusing
helium into even heavier nuclei like carbon and oxygen. Eventually, the Sun will also run out of
helium and any other light elements that could be used for fuel for nuclear fusion. At that point,
it will collapse into a small star known as a white dwarf. Since no more fusion is occurring
anymore, white dwarfs are sometimes called stellar remnants instead of stars. But if
there’s no more fusion, where does the pressure to support these stellar remnants come from?
It turns out that gravity pulls the gas of a white dwarf inward until it’s so dense
that a new type of pressure emerges, known as electron degeneracy pressure. This is a purely
quantum-mechanical effect, and it’s the same effect that’s responsible for you not being
able to pass your hand through solid objects: electrons simply cannot get too close to each
other without an extraordinary amount of energy. But some stellar remnants are so massive that even
electron degeneracy pressure isn’t strong enough to counteract the pull of gravity. This is called
the Chandrasekhar Limit: it’s absolutely impossible to have a stable white dwarf any
heavier than 1.4 times the mass of the Sun. If a star has burned through all its nuclear fuel and
its core is still heavier than the Chandrasekhar Limit, that core will collapse in on itself until
it becomes so dense that an even stronger type of quantum pressure emerges, known as neutron
degeneracy pressure, which - as you may have guessed - prevents neutrons from getting too
close to each other. A star that reaches this state is called a neutron star, the smallest
and densest known class of stellar objects. So now you might be wondering—is there a maximum
mass that a neutron star can have while still being supported by neutron degeneracy pressure?
And there is! This time, it’s about 2.2 times the mass of the Sun. Any heavier than that,
and the neutron star would keep collapsing under the influence of its own gravity, until… well,
actually, not until anything. As far as we know, neutron degeneracy pressure is the strongest
possible pressure that ordinary matter can exert, so there’s nothing left to prevent a sufficiently
heavy stellar remnant from collapsing in on itself forever. This is the fate that might await some of
the most massive stars we’ve observed in the universe. From the perspective of an
outside observer, these stars would get so dense that all we’d be able to see is a black hole.
That’s a lot of pressure.
Now, you might feel like you resonate with that,
but if so, perhaps it’s time to start finding yourself a new career.
Something a little more laid back and fun.
You may want to try software developing.
And yes, training for that can be surprisingly fun,
thanks to the sponsor of today’s video, Boot.dev.
Boot.dev is no ordinary coding course.
Their website feels more like you’re playing a fantasy adventure,
with quests and gamified elements that made each interactive task feel whimsical and entertaining.
But by the end of it you’ll have coded entire projects,
making it a great way to gain real-life experience and skills.
If you get stuck, there’s a great community who can help,
or you can talk to the AI bear (yes, there is one of those,
and he is surprisingly good at code).
The first few chapters with all the interactive activities are free to trial.
Even after the free demo, everything on their website is completely free to read,
so go check it out for yourself by scanning my QR code
or clicking this link in the description below.
If you use my code ASTRUM, you can get 25% off your first payment for boot.dev.
But then scientists started to wonder—
what would happen to a star that wasn’t made of ordinary matter?
If exotic forms of matter could hypothetically produce even
larger pressures than neutron degeneracy pressure,
then what new kinds of stars could exist? You might think that we could just continue
the pattern that we were starting to see earlier: stars could get even smaller and
even denser than neutron stars, as long as they were supported by some even greater pressure.
You might think that the only limit on how small these stars could get would be the point
of no return where they would turn into black holes. That point of no return is when a star
becomes smaller than its own Schwarzschild Radius. This is almost true, but not quite.
Instead, Physicist Hans Adolf Buchdahl showed in 1959 that a star made of any “realistic” form of
matter, with positive energy density and pressure, can’t be smaller than 9/8 times its Schwarzschild
Radius. It might not sound like a big difference, but that factor of 9/8 means that any “realistic”
star has to be noticeably larger than a black hole, making it much easier to tell them apart.
That’s when physicists began asking about stars that Buchdahl would call…
less realistic. For example, what if a star was made of matter that allows for pressure
to be negative in some regions and positive in others? Then Buchdahl’s limit wouldn’t apply,
and that star could be less than 9/8 times its Schwarzschild Radius—in fact,
it could be arbitrarily close to the size of a black hole with the same mass! As a side note,
theoretical stars like these that could mimic the appearance of black holes came to be quite
a nuisance for astrophysicists, since now every time they discovered something they
thought was a black hole, they couldn’t just say they discovered a black hole; they had
to qualify that it’s either a black hole or an extremely compact object mimicking a black hole,
even though in their hearts most of them are quite convinced that it’s a black hole. But,
back to the main point—what would these exotic stars with negative pressures even look like?
The answer turns out to be even weirder than you might have guessed. Tolman, Oppenheimer,
and Volkov derived an equation in the 1930’s that describes how the pressure of a stable
star changes as you go outward from its centre. These days, it’s just called the TOV equation for
short. If the centre of a star has a significantly negative pressure, which would exert some sort
of attractive pull instead of a push, the TOV equation says that this star can only be stable
and in balance with its own gravity, if its pressure does something really wonky. First,
the pressure has to blow up to negative infinity somewhere in the star. Then, it has to flip sign
to positive infinity and decrease outwards from there until it reaches zero pressure,
marking the star’s outer surface. The exact location of the infinite pressure depends on
what the pressure is at the centre, so if you tune that central pressure just right, you can
keep the pressure finite right up until the edge of the star. That happens when the pressure at
the centre is exactly equal and opposite to the star’s energy density there. In this special case,
the TOV equations say that the pressure and energy density remain constant, equal,
and opposite throughout the entire volume of the star, right up until its surface.
If you’ve seen some of our other episodes, this might ring a bell or two. A constant
positive energy density, paired with a constant but negative pressure—that doesn’t sound like any
star we’ve seen before, but it’s starting to sound more like a mysterious component of our universe:
Dark Energy. And since the pressure and energy density in a gravastar are equal in magnitude,
it’s not just any kind of dark energy: those are the exact properties of vacuum
energy. That’s why this special exotic star came to be known as a “gravitational
vacuum star,” or gravastar for short. Inside of a gravastar, there is precisely
zero gravitational force, because the positive energy and negative pressure respectively exert
a gravitational pull and push that cancel each other out. And since the pressure in gravastars is
constant, there is no pressure gradient to exert any sort of push in one direction or another,
either. This means that if you found yourself inside of a gravastar, you would neither be pulled
in nor pushed out: you could float freely, just as though you were in a real vacuum.
Another curious feature of stars with negative pressures is that the closer you get to the
special limit of a gravastar, which if you recall is when the infinite-pressure surface
gets pushed out to the edge of the star, the more their size approaches that of a
black hole with the same mass. This raises two key questions that have recently gotten a lot
of attention from physicists: First, how can we tell whether the extremely compact objects
astronomers have observed in merger events and at the centre of large galaxies are black holes
or just exotic stars with nearly the same size ? And second, if gravastars are real and scattered
throughout the entire universe, could they be the dark energy that’s been making the universe
accelerate for the past five billion years? To answer the first question, physicists rely
on the main distinction between black holes and other compact objects: the existence of an event
horizon. You see, the defining feature of a black hole is that nothing that falls inside
or even touches the surface of a black hole can ever come back out. On the other hand,
if an exotic star is even slightly larger than its Schwarzschild Radius, it would not have an
event horizon, and it would potentially be able to emit information outwards in the form of light.
Not only that, but objects without event horizons could have different signatures
in their gravitational wave emissions than those of black holes during a merger. This would allow
us to tell them apart by measuring and analyzing those gravitational waves (using observatories
like LIGO). But even with this battery of tests, it turns out that sufficiently small and dense
stars can still be difficult to distinguish from black holes. So far, the best we can do is to say
that the radius of the black hole candidates we see in the universe is at least approximately
equal to their Schwarzschild Radius—in some cases, that’s just a rough estimate,
but in others, we’ve calculated that radius to within 40 decimal places.
As for the second question about dark energy, that’s where things get really heated.
In early 2023, an analysis led by physicists at the University of Hawaii found that astronomical
observations of massive black holes showed them to be growing in mass over time, at exactly the same
rate that dark energy would be expected to grow over time due to the expansion of the universe.
They came to the inevitable conclusion that black holes must be made of dark energy,
and that they are really more like gravastars than ordinary black holes with infinite-density
singularities at their centres. This conclusion was fiercely contested! Astronomers came out
arguing that the analysis of the observational data wasn’t quite right, and theoretical
physicists came out arguing that the expansion of the universe should have nothing to do with
the growth of black holes in any case. So while the idea that dark energy could exist in the form
of gravastars sounds exciting, it looks like the scientific community isn’t going to be convinced
unless it’s simultaneously supported by better data and more compelling theoretical models.
So, while gravastars may exist, we’re far from proving them – and they might look too much like
black holes to be able to easily spot in the first place. But the next exotic object I’ll
explore today should be as recognisably distinct from a black hole as it’s possible to get. They
are black holes’ opposite – the push to a black hole’s pull. They are white holes – and there’s
no reason in physics why they couldn’t exist. I want to start off with a simple analogy. If
I ask you to imagine a cone, you’ll probably think of either something that points up, like
a traffic cone, or something that points down, like an ice cream cone. These are what we think
of as cones in the real world. But if you write down the most general mathematical equation for
a cone and try graphing it, you’ll get something that looks like two cones glued together. You see,
a mathematical cone has two halves, one that points up and one that points down,
connected at their tips—and what we think of as cones in the real world are actually just
chopped-up pieces of the full mathematical cone. Black holes are the exact same way. Let me
explain. Many of the black holes we see out in the universe are the remnants of massive stars,
whose extreme gravity caused them to collapse in on themselves. Once a black hole is created,
it becomes permanently separated from the world around it by a spherical barrier known as the
event horizon: anything can fall into the black hole, but nothing—not even light—can come out.
This structure of spacetime can be neatly summarized in what physicists call a Penrose
Diagram, which is a way of representing an infinite spacetime in a finite drawing.
The key feature of a Penrose Diagram is that light travels upwards at 45º angles,
making it easy to distinguish regions where light can or cannot enter.
In this diagram, Region 1 is the regular universe, containing the original star (before it turned
into a black hole) and everything outside of it, from the infinite past, to the infinite
future. Region 2 is the black hole, and it is separated from the rest of the universe by its
event horizon. You can see how it’s easy for light to enter the black hole (region II), but it’s
impossible for light to exit—it will inevitably hit the singularity instead. The same holds true
for ordinary matter, which moves slower than light and so in this diagram can only travel upwards at
angles steeper than 45º. What’s so special about this Penrose diagram is that it accounts for the
formation of the black hole at some specific point in time, when the star collapsed in on itself.
In the far past, there was no event horizon or black hole to speak of, and light could freely
travel across all of space. But while this diagram represents the kind of black hole we’re used to
seeing in the real world, it’s only one piece of the full mathematical description of black
holes in General Relativity. In other words, it is the lone ice cream cone of black hole diagrams.
So what happens if we forget the real world and imagine an ideal black hole with an infinite past,
leaving behind the baggage of collapsing stars and everything else in the physical universe? What is
the black-hole analogue of the full mathematical cone? It might look like a simple extension of
our original Penrose diagram, but this diagram comes with physics that’s a little… upside down.
Region 1 is still our regular universe, and Region 2 is still a black hole with an inevitable
singularity in its future. But the maths also describes something that looks like the inverse of
a black hole in region 3. Instead of a singularity in its future, region 3 has a singularity in its
past. And if you draw the motion of light rays at 45º angles, you can see that it’s straightforward
for light to leave region 3, but it can never enter it! This type of region is exactly what
we would call a white hole—it’s the spacetime analogue of a traffic cone. But that’s not all!
If you look beyond the white hole, you’ll see there is a patch of spacetime labelled region
4 with the same connections to the black and white holes in the middle as the regular universe. This
patch is a parallel universe, and it’s entirely disconnected from the universe in region 1,
since it’s impossible for even light to travel from one region to the other.
The upshot of all this is that the same mathematical equation describing the regular
universe and the black hole also describes the parallel universe and the white hole. This doesn’t
mean that every black hole comes attached to a white hole, though. Just like with cones, the real
universe can contain chopped up pieces of the full mathematical solution. But this does mean that
even if white holes are less common than their black hole counterparts—or even if there aren’t
any white holes connected to our universe at all—it doesn’t make them any less scientific, in
the sense that they’re equally consistent with the laws of physics. In our analogy, if you imagine
that ice cream cones remain extremely popular, while traffic cones become difficult to produce
and are phased out of existence, that doesn’t make the idea of a traffic cone any less real. This is
more or less the status of white holes today: the notion of a white hole makes perfect sense, but
it’s possible that any white holes that existed in the past were unstable and were similarly phased
out of existence. If you’re wondering whether there could be some more stable white holes
that have just escaped our sight, you’re not the only one—and you should stick around for when we
talk about the tiniest possible white holes in just a few minutes. But first, buckle up,
because the Penrose diagram can describe more than just white holes. We’re about to go to a
whole new level of warped spacetime – into the realm of wormholes and parallel universes.
The No Hair Theorem says that a black hole can have up to three intrinsic properties: mass,
charge, and spin. But the Penrose diagram I showed you only described the simplest kind
of black hole, which was electrically neutral and completely stationary.
The diagrams for charged or spinning black holes are much more complicated, but they’re also much
more exciting, because they completely alter our understanding of singularities, and they give rise
to new and fascinating regions of spacetime. Let’s have a look at the Penrose diagram for a
spinning black hole as an example. Instead of four distinct regions of spacetime,
there are now 8 types of regions that repeat themselves in an infinite pattern of universes.
Regions 1 through 4 are the same ones that we saw before: these are the regular universe, the black
hole, the white hole, and the parallel universe. But now that the black hole is spinning, its
geometry no longer has an unavoidable singularity in its future—and the white hole similarly loses
the singularity that existed in its past. So, what actually happens after you cross
the event horizon of a spinning black hole? At first, you’ll be drawn toward the centre
of the black hole, just like you’d expect. But eventually, you’ll cross a threshold known as
the inner horizon, beyond which the geometry of spacetime un-warps and lets you stop yourself from
falling even deeper. In these innermost regions, marked 5 and 7, you can float freely toward or
away from the centre of the black hole. You could even touch the singularity if you wanted to—though
I wouldn’t recommend it. Here in these innermost regions of a curved, rotating spacetime, you are
officially in a wormhole, and if it weren’t for the fact that you are permanently separated from
everyone who stayed in Region 1, this would have been the perfect time for you to brag about your
wild spacetime adventures. At this point, you’re also faced with a bit of a fork in the road.
One option is to keep moving inward and go around the singularity, which takes the shape of a ring,
instead of a point, for spinning black holes. This path would take you to region 6 or 8 in the
diagram. But it’s quite possible that the existence of this path is actually a flaw
in the predictions of General Relativity, rather than a true description of reality,
because going even in a single loop around the singularity can lead to causal paradoxes where
you visit your own past. A safer bet would be to turn around and move outwards, crossing back into
the inner horizon that you just came out of. This would take you into region 3—a white hole—which
would then carry you all the way out to a brand new universe, similar to the structure to the one
you started out in originally, but sadly without your friends or family waiting for you. In fact,
by the time you get here, your old universe will have already experienced an infinite
amount of time. You would be living in a whole new definition of the future.
Okay, let’s step back into reality. As fun as that journey through the wormhole was in theory,
you probably couldn’t survive it in practice—and honestly, there’s a good chance that the wormhole
itself would collapse once you disturbed its spacetime. So, how much of this mathematical
structure can we expect to see in the real world? Spinning black holes with event horizons appear to
be everywhere in the universe—but what goes on inside of them is still largely unknown. There
are two main reasons for why the physics inside real black holes might be different from what the
Penrose diagram would lead you to believe. First, when black holes form from the turbulent collapse
of massive stars, some assumptions that went into making the Penrose diagram are violated—like that
the black hole existed forever, or that the universe around it is perfectly symmetric and
doesn’t contain random infalling humans. And second, the theory of General Relativity used
to construct the Penrose diagram might itself only be approximately correct, breaking down
near the singularity where the curvature gets most extreme. So there’s a good chance that
some predictions of the diagram—especially the most problematic ones involving singularities
and time travel paradoxes—aren’t real features of a highly curved spacetime. But the existence
of white holes is more of an open possibility, leaving plenty of room for speculation about how
they might come to be and whether they might leave any observational footprints for us to discover.
One hypothesis that’s recently gained some traction is that a white hole is born whenever
a black hole dies. And no, this isn’t some magical story of reincarnation or voodoo—it’s a genuine
attempt to understand the full life cycle of black holes, with at least some level of mathematical
backing. You see, even an isolated black hole in an otherwise empty space will continuously emit
particles known as Hawking radiation, causing it to gradually shrink and lose mass over time.
But once the black hole reaches the super tiny Planck mass—just about 20 micrograms—we have no
way of predicting what future awaits it; at least, not without a full theory of quantum gravity. This
is where Loop Quantum Gravity comes in—a proposed link between gravity and quantum mechanics,
in which space itself is made of discrete loops. In short, the equations of Loop Quantum
Gravity predict that instead of continuing to shrink even smaller than the Planck mass,
such a tiny black hole is instead more likely to quantum tunnel into a white hole, spewing out its
contents back into the universe over an extended period of time. Incredibly, even though this white
hole will be similarly small and light, it can contain an enormous amount of entropy, because
its interior geometry will have been stretched into a thin tube with an extremely large volume.
This store of entropy could potentially open a pathway for any information that falls into a
black hole to be recovered in the distant future, solving a decades-long problem in theoretical
physics known as the information paradox. As the white hole releases this information,
it begins to slowly fade out of existence; the spacetime around it loses its curvature,
and its life cycle comes to a close. In General Relativity, our whole universe
is predicted to have had a singularity in its past, namely 14 billion years ago, at the moment
of the Big Bang. In this way, the universe is like an uncharged, stationary white hole. But
if Loop Quantum Gravity says that white holes can be born when black holes collapse, could the same
be true about the creation of our universe? Could it be that our expanding universe was born from
the ashes of an older, contracting spacetime? As crazy as this seems, Loop Quantum Gravity
says that may well be the case. So, perhaps white holes as a concept are not so farfetched. Maybe
it’s time to start believing in white holes… because in a way, maybe we were in one.
So, white holes certainly could exist in the real world, even if we haven’t definitively
nailed one down yet via observation. But I’ve glossed over another part of the penrose diagram
that might have made you raise an eyebrow. The penrose diagram I used had no issue referencing
parallel universes – but that’s a big claim. Could entire universes – inaccessible to ours
yet running alongside us, just out of reach – actually exist?
There’s some logical reasons to believe so. But before we get to that, we need to
turn our gaze to a baffling cosmic mystery, and the worst prediction in physics, ever.
It starts with a deceptively simple question: Why is our universe expanding?
One of the first things you might learn in a physics class is that gravity is an attractive
force between two objects, whose strength is proportional to each of their masses. Gravity
keeps planets locked in orbit around the Sun, it keeps stars grouped together in the Milky Way,
and it even attracts galaxies together to form galaxy clusters and superclusters. But if we
zoom out further, this gravitational attraction begins to act in reverse, causing galaxy clusters
to accelerate away from each other rather than drawing each other in. So, what’s going on here?
Einstein’s theory of General Relativity gives us a lens through which we can understand this
strange behavior. In General Relativity, space isn’t just some background in which other things
can move around; instead, space itself can stretch and warp and evolve over time.
Einstein showed that in this framework, gravity is no longer a force, but rather a distortion of an
object’s inertial path in a dynamical spacetime. For example, when a planet orbits around the Sun,
in Einstein’s description, it’s merely following a straight-line path in a curved space. This
means what we observe as a repulsive force between galaxy clusters should really be thought of as an
accelerated stretching of the space between them. But what could possibly cause the expansion of
space to accelerate like this on the largest scales of the universe? To begin to answer this
question, we need to know what causes spacetime to stretch and warp to begin with. In Newton’s
theory of gravity, all objects with mass exerted a gravitational pull, and in General Relativity,
all objects with mass curve the spacetime around them. But Einstein showed that in
addition to mass, any form of energy or pressure will also influence the dynamics of spacetime.
Energy density is how much energy is found within a given volume - how much pop, heat,
zap and motion exists within a given space - and in our universe has always been found to
be positive. Pressure is how much force over an area the contents of that space sends pushing
into the rest of the universe around it – think interstellar dust pushing on each other whenever
they bump into each other in the void. As you can imagine for objects so far apart, this pressure
value hovers a little over 0 on a cosmic scale. When these values are cumulatively positive,
as in the following formula, their influence on the space around them causes the space around
them to contract in – essentially, they create gravity by decelerating the stretching of space.
However, conceivably, if you were somehow to set the values of this equation so that
pressure was negative, and greater than the positive energy density, that minus sign would
cause the whole thing to flip, and you would accelerate the stretching of space. In effect,
you would end up with a volume of space filled with a kind of anti-gravity. A Dark Energy.
It's a little nebulous, as it’s tricky to visualise anything with a truly negative
pressure – how can you have less than 0 atoms in a patch of space, after all? But atoms are not
the only things that exerts pressure. After all, pressure is simply a force applied across an area.
Fields can also apply forces - think a magnetic field, dragging a piece of iron in towards a bar
magnet, or two magnets pushing each other away. If dark energy were some kind of field that pushed
away not just other magnets, but everything, then this would match what we see the universe doing;
vacuum itself having dark energy, and enough of it to slowly, very gently push the universe apart.
Scientists have thought a lot about dark energy over the years, and have even figured out what
its combined energy density and pressure needed to be, to create the rate of spatial expansion
that we witness. It’s around 10-9 in SI units, which is a very small number – which is why we
don’t notice it here on Earth, and only spot it on the grand cosmological scale of the universe.
This is actually good news, because it turns out that if the number was much
higher or lower than this, things would get very bad for our chance of existing.
Let me show you what I mean, with a quick sketch of the history of the universe. Here is the full
history of the universe, from the moment just after the Big Bang to the present.
This is the dark energy density measured by astronomers. Scientists believe its
negative pressure has kept this density roughly constant for billions of years,
and although there is new evidence coming to light that shows dark energy may be weakening over time,
for our purposes we can just keep the value consistent – it doesn’t change the
point we’re making, as you’ll see in a moment. This is the density of ordinary matter in the
universe, which has little to no pressure. The density decreases over time because the
expansion of space dilutes the matter within it. Finally, this is the density of radiation in the
universe, including photons and other extremely light particles. The positive
pressure of radiation makes it dilute away even more quickly than ordinary matter.
Towards the beginning of time, for a very short period, there were no atoms. Very quickly though,
protons and neutrons fused together to create the first atomic nuclei in the very early universe.
Shortly after matter took over as the dominant form of energy, the universe cooled down enough
for nuclei to attract and hold onto electrons, forming the first atoms of hydrogen and helium.
These atoms were the building blocks of the very first stars, and these stars were pulled together
by gravity to form the very first galaxies and clusters. Only later did dark energy take over,
triggering the accelerated expansion of space and preventing the formation of larger structures.
But what would have happened if the energy density were larger? Consider what would have happened if
the pushing force of dark energy were stronger. It could have been enough to halt the formation
of those early galaxies – pushing apart stars more powerfully than their own gravity could pull them
together. A bit larger than that, and it would have been much more difficult for any stars to
form, either. And if the dark energy density were large enough, we would hardly have had any atoms
or even nuclei produced in the universe. So, that’s what it could have been. What
should it have been? Using some clever maths and the principles of quantum field theory,
scientists attempted to predict how much the energy density of dark energy ought to have
been. Their result was… larger than the result we see in nature. How much larger? Their value came
in at an astounding 1045 Joules/m3. Where does the predicted energy density of 1045 Joules per cubic
meter fall on this graph? It’s (quite literally) off the charts. If that prediction were correct,
the universe today would have no structure, no features, no life—it would be a giant void
filled with nothing but dark energy. And if the dark energy density were instead a negative 1045
Joules per cubic meter, the fate of the universe would be no more promising,
as it would be forced to rapidly collapse in on itself in what’s known as a Big Crunch.
So here lies the mystery. Why is dark energy density so much lower than we predict; at the very
level it needed to be at for life? Scientists have attempted to account for their number being so far
off by hypothesising that there exist particles with positive energy and negative pressure (as yet
undiscovered) that even out the maths and brings the answer for dark energy density back down
towards 0. Perhaps there is some massive negative potential energy source that evens all the maths
out, a cosmic stretched spring that somehow reigns in all that rampant energy. Either that,
or quantum field theory is fundamentally wrong. But as there’s actually quite a lot of evidence
supporting quantum field theory, it seems imprudent to completely throw the idea out.
Instead, we are left contemplating the marvellous nature of this cosmic coincidence. What were
the odds that the energy density of dark energy would be so low, when it was predicted to be so
much higher? That this great universal balancing act occurred, in such a way that we weren’t torn
apart or crushed into a singularity? Without it being so low in magnitude, we wouldn’t exist – our
very atoms would never have come together, torn apart by surging, expanding space. According
to our predictions, we have got very lucky. This coincidence happened in such a way that
the universe was able to produce life. So… Was it a coincidence?
Of course, there are different explanations for this dark energy density problem,
often known as the cosmological constant problem. Those of a religious mindset would point to the
idea of intelligent design - that this wasn’t so much luck, as intent. A god, or extremely advanced
being, has set the value of dark energy density to be just right for life to come into being for
the purpose of creating us. Of course, this idea doesn’t sit well with everyone. The existence of a
God-like being by nature is difficult to prove or disprove, and taking such matters on faith is not
really what science is about. For many scientists, a different sort of answer was needed.
And that answer came in the form of multiverses. Let’s go back to 1987, when the precise value
of the dark energy density in our universe had not yet been measured, but it was clear that
it was extremely small or zero, since, well, we existed, and so did everything around us.
That year, physicist Steven Weinberg published a paper connecting the universe’s fortuitously
low dark energy density with something known as the anthropic principle. Essentially, the only
universes that we can evaluate the probabilities of are the universes that met the right conditions
to produce us. If the universe hadn’t rolled such an unlikely roll of the dice, there would be no
“us” to say whether these things were more or less probable. If it takes a massively unlikely roll
of the dice to produce humans, then a universe where that roll of the dice actually happened
is the only universe we could possibly see. Weinberg could be more specific than that.
He was confident that the threshold of dark energy density for forming life would be no
lower than the threshold for forming galaxies. In other words, by the time galaxies have formed,
you’ve probably crossed the threshold to where life can form too. After all,
how else was life supposed to arise if there were no stars and galaxies to provide complex building
blocks and energy? At the time of his publication, observations of an extremely old and distant
quasar had indicated that galaxies had already begun to form just one billion years after the Big
Bang. Weinberg used this information to calculate that the formation of these ancient galaxies
required the dark energy density to be smaller than around 10-7 J/m3. That meant that, assuming
that galaxies were indeed necessary for life to arise, the threshold dark energy density for life
to exist was less than or equal to this value (while not being so low as to be negative).
Weinberg initially reasoned that if the dark energy density was significantly smaller than
this value, something else other than anthropic reasoning must explain the dark energy density’s
smallness. However, as we now know, the real dark energy density is closer to 10-9
J/m3, which isn’t too far off from the threshold of 10-7 calculated by Weinberg.
In short, life and dark energy density correlate surprisingly well.
Of course, this ignores one vital point. Just because we can see universes where
we exist doesn’t mean it is guaranteed that we should exist. We could have just… not existed
at all. After all, the odds of the cosmological constant being at its recorded level is incredibly
unlikely. Could the various contributions to the dark energy density have cancelled out to
a value compatible with life just by chance? …Probably not. Here’s what would have needed
to have happened: just imagine generating 3 random real numbers between -1 and 1,
adding them up, and hoping that the magnitude of the sum is at most
0.00000000000000000000000000000000000000000000000000000001
(with 55 zeros). It sounds just about impossible. But what if we had more than one go at it—what
if we could generate those numbers over and over and over again until we got
lucky? It might take us something like 1056 tries, but given enough time, the unlikely
cancellation of three randomly generated numbers will turn into an inevitability.
We can use the same logic to potentially explain why the dark energy density is so bizarrely small.
Instead of just one universe, imagine there exists a vast multiverse, with each universe
having a different, random value for the dark energy density. Only a tiny fraction
of them would have a dark energy density small enough to be compatible with life,
but that’s exactly where we humans would expect to find ourselves. This explanation requires no
coincidences, no fine-tuning, no intelligent design—just a lot of different universes.
On its own, the idea of a multiverse would still be too vague to be good
scientific theory. For starters, what kind of multiverse are we talking about? How do
they exist in the first place? Fortunately, there are actually a few different concepts
of multiverse that physicists can describe. Firstly, we have a multiverse separated by
time. If you are familiar with the idea of the Big Bang and the Big Crunch, you will know that it’s
often believed that this creation and destruction of the universe is an eternally repeating process,
with new universes arising each time. If the underlying numbers of each universe are a bit
different with each iteration - for instance if dark energy density can change from universe to
universe - then with enough Big Bangs and Big Crunches, you could inevitably end up
with a universe with a dark energy density exactly like our own. There are a few assumptions at play
here, of course, as we don’t know for sure that the universe does contract into a Big Crunch yet.
But if it does and if the rules of the universe are a bit different with each new iteration,
then this is a viable model for a multiverse. A second multiverse described by physicists
(although one I find personally a little bit more iffy) is a multiverse separated by space.
While the rules of physics seem to be consistent across all of space that we can see, there are
realms of space beyond our sight – sections of the universe that are accelerating away
from us so quickly, we never will see light from them. It’s conceivably possible that the rules
of these parts of the universe could differ from our own. And if this occurs, then we are
essentially in another sort of multiverse. I find this model a little less satisfying,
though: both because it feels strange to call a further away part of our universe a different
universe just because we can’t see it, and also because there is no evidence that the laws of
physics differ across space. This seems to me to be the least likely sort of multiverse.
But finally, modern physics describes a third type of multiverse, one that is
disconnected from our spacetime entirely. In the Many-Worlds interpretation of quantum mechanics,
every macroscopic interaction or measurement of a quantum state causes the wavefunction of the
universe to branch into multiple non-interacting parts. If you’ve heard of Schrodinger’s Cat,
you can imagine the universe splitting into one universe where the cat is alive
and one universe where the cat is dead. With every such interaction, the total number of
universes grows exponentially, and the quantum multiverse becomes ever richer and more diverse
in its features. To be clear, we still don’t know whether this branching process is the correct
physical description of quantum mechanics, but it is arguably the simplest, and it is perfectly
consistent with all experimental data – or rather, we have no proof that it doesn’t exist that way.
If it does turn out to be correct, then it would add a whole new layer of complexity
to our understanding of the multiverse. Multiverses are useful in that they remove
the need for massive coincidences or intelligent design as a way of explaining our presence in
the universe. However, are they actually real? Unfortunately, the answer is an unsatisfying,
“we don’t know.” While differing multiverses do help solve the cosmological constant problem,
we actually have no evidence that they exist beyond the offering of an explanation for that
coincidence. We have no proof that the laws of physics change across space or time or quantum
universes - if other universes exist, they might have exactly the same laws of physics and values
for dark energy density as our own. Which would put us right back to the same problem we started
with - how are we here, when the odds of the galaxy itself existing is 1056 to one? Or worse;
the value for dark energy density is 1056 to 10120 times smaller than what our theory predicts.
And getting that proof will be difficult. We will need to either survive through a Big Crunch,
or pass into a part of the universe that’s so far away that not even light can travel quickly
enough to get back from it, or enter a different quantum universe somehow. All seem impossible,
or at least very difficult, and potentially the kind of trip you can’t return from. This might be
just as unprovable as the existence of God. So, in a way, you are free to believe
whichever you find most comforting. Or perhaps, continue to wait until a better
way of answering the question can be found. Ultimately, whether it’s parallel universes,
wormholes, white holes or gravastars, the theoretical landscape of science is brimming
with possibilities. And remember, there is plenty of sound precedent for hypotheticals becoming
realities. Black holes, the planet Neptune, the existence of antimatter or the cosmic background
radiation itself – all were found in theories before being spotted in practice. We are in an
age where our mind’s eye frequently is outpacing our physical vision, where maths and equations are
leaving any telescope or detection device in the dust. To me, it’s marvellous that we live
in a universe founded on logical principles that allow such deductions to be possible,
where we can Sherlock Holmes reality. The rules are so consistent, that once we understand them
fundamentally, the rest of the universe just… naturally arises. I love the order of it, and the
wonders that become possible because of it. So, could these things be real?
Yes – if we have understood the theory correctly. And if we haven’t? Then I can’t wait to see what
strange and jaw-dropping phenomena science will next predict as our understanding improves…
And of course, the day when what we imagine is no longer just imaginary. When we eventually
spot those real-life counterparts, waiting for us in among the stars.