Was our universe designed to be hospitable to life? In a recent Astrum video, we saw that
there is more dark energy in the universe than the energy from every single atom and particle of
ordinary matter combined. Yet, everything we know about particles, fields, and quantum mechanics
seems to suggest that this apparently large amount of dark energy could have been—and statistically
should have been—much, much larger. Decades before astronomers even had any observational evidence
for the existence of dark energy, physicists were shocked to see that the most successful theories
of the twentieth century predicted a level of dark energy so high that stars and galaxies could never
have come to exist… let alone life and humanity. So, how did our universe avoid this fate, and how
are we alive today to even ask these questions? I’m Alex McColgan, and you’re watching Astrum.
Join me today as we continue to learn about the effects of dark energy on the
cosmos and explore the maths that claims we really shouldn’t have existed at all.
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. By the same token, 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 [Show on screen: Acceleration ∝ -(ρ+3P)] 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 usually 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 as measured
by astronomers. Scientists believe its negative pressure has kept this density
roughly constant for billions of years. 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, beating out radiation. 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 for 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 10^45 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 10^45 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, fractions of a second after the universe began.
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 bring the answer for dark energy density back down towards 0. Perhaps
there is some massive negative potential energy source that evens 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, and 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 just such
a way that the universe was able to produce life. So… Was it a coincidence?
The answer to that question strays from what we know for sure into what we simply theorise,
however. It wanders into the realms of multiverse theory, and mankind’s quest to understand if we
are not here by chance, but by design. There’s a lot to cover - too much - to cram into the
end of this video, and so to do it justice I’ve split this one in two. I’ll cover the
rest when I have the time to do it full justice. In the meantime, what do you think? Our existence
so far appears to be excessively fortunate. How do you think it happened? Leave your answer in the
comments below, and see if other people’s ideas match or contrast with your own. It’s always
good to hear other viewpoints, particularly for a subject where there aren’t clear-cut answers.
You never know what you might find out. Our existence is surprising. Perhaps that
means the Universe has more surprises in store for us, if we can only find the answers.