There’s a question in theoretical physics
that you may never have considered, but unlike many other hot science questions, this is
one that hardly any physicist will answer ‘no’ to. The question? ‘Is there more to the universe than what
we’re able to see?’ The observable universe is a sphere (we think),
with planet Earth at its centre – it encompasses the limits of what we are able to observe
with current technology – that’s around 46.5 billion light-years in all directions. The idea that our universe stops there, that
if we were to hop into SS Superluminal and travel 46.5 billion light-years in one direction
all we’d find is an impassable loading screen, is, well… it goes far beyond egocentricity
and into outright lunacy - with a little dash of geocentricity thrown in for good measure,
of course. So, if you were to ask any theoretical physicist
or astronomer ‘is the observable universe all there is?’ Not one would answer ‘yes’, and if they
did, there’s a good chance you’ve taken a wrong turn and are no longer in the University,
but have gotten lost in the local Aldi, down the cheese aisle. No, the cardigan-fueled arguments only arise
when you ask professors a slightly different question - not, ‘is there more,’ but ‘how
much more is there?’. If the answer you receive is ‘well we’ve
got another ten packs of double Gloucester in the storeroom’, then seriously, you need
to leave the guy at the deli counter alone. He doesn’t know shit about the universe. ‘How much more is there?’ is currently
one of the most hotly debated questions on the science scene, right after ‘how can
this triple-stack sandwich be so delicious when it doesn’t obey Occam's razor?’. MIT theoretical physicist Alan Guth is one
of the most influential minds in the field - the universe, that is. Not sandwiches. He estimates the size of the unobservable
universe, all that is beyond our field of view, to be 3 × 1023 times the radius of
the observable universe. Now, Alan Guth is, it’s fair to say, a rather
smart cookie. He doesn’t just enjoy spending his time
making our brains hurt by peppering them with large numbers, he wants to liquify them entirely. You see, Guth is one of the leading proponents
of the idea that our universe may be one of many within a larger multiverse. But Guth was far from the first to play around
with this idea. The Ancient Greeks no less, postulated in
the 5th century BC that all matter is composed of unseen particles called atoms, from the
Greek ‘atomos’ meaning ‘indivisible’. Over the following centuries philosophers
such as Chrysippus would go on to suggest that when atoms collide they create an infinite
number of multiple universes that eventually expire and regenerate in an eternal cycle. Deep stuff. But there wasn’t a great lot those peplos-wearing
polymaths could do with this mind-bending information. They couldn’t whip out their Olympus electron
microscopes – not that Olympus – and zoom in to the atomic level to disprove their hypothesis. Only now, more than two millennia on, can
we measure all the weird invisible bits of our world. Things like cosmic background radiation and
gravity. And what have we discovered? We inhabit an expanding universe that, like
an unplanned child, was banged into existence some 13.8 billion years ago. But to take this disturbing analogy a little
further - who, or more likely what, was our parent? The most obvious way to answer this question
is to ask what happened during, and even just before the big bang. But it turns out attempting to figure out
what existed before anything existed, when that thing is your entire existence, is well,
about as easy as it sounds – the last time we tried we ended up with God, which answered
absolutely nothing. But that didn’t stop Alan Guth from having
a stab at it, along with his colleagues Andrei Linde, Paul Steinhardt, and Andreas Albrecht. In the eighties they came up with Inflation
Theory, which combines ideas from quantum physics and particle physics to posit that
just prior to the big bang there was a period of faster-than-the-speed-of-light inflation
- funnily enough the same thing happened in Germany after the first world war. During these initial moments – less than
a millisecond after the big bang – the universe went from the size of a billionth of a proton,
to roughly the size of an orange. After which, traditional big bang expansion
took over, in a period sometimes referred to as the ‘hot big bang’. Inflation theory neatly answers the consistency
conundrum. You see, our universe is remarkably uniform. Up close it looks like a morning after an
interstellar barn dance, on acid with a galaxy here, a black hole minding its own business
over there, and a nebula pissing about somewhere in the background. But if you were to zoom out - and I mean really
zoom out - like Hubble Ultra Deep Field and then some zoomed out, you would begin to notice
a pattern – or more accurately, a lack of one. Space basically all looks the same, with the
googleplex of assorted items chilling out in our universe being very, very evenly distributed
- like grains of sand on a perfectly flat beach. In fact, if you measure the intensity of the
background radiation in any two randomly sampled patches of the universe, they will come out
pretty much exactly the same, with a maximum variation of 0.001%. If the big bang just happened, with no pre-expansion
period, this homogeneity wouldn’t be possible. It’s difficult to know exactly what a non-inflation
universe would look like, but according to some models we would see a super-concentrated
mass of galaxies surrounded by vast areas of nothing interspersed with the odd bit of
matter. Inflation allowed the universe to achieve
uniformity before it ‘banged’, which wasn’t a bang per se, but a slightly slower expansion
of space over billions of years. Think of it like thoroughly mixing your batter
before baking a cake instead of just tossing a load of flour and eggs directly in the oven
and praying they turn themselves into something that’s not total shit. Because Inflation Theory neatly answers so
many ‘well that’s not quite right’ dilemmas that exist in astrophysics, it’s since been
incorporated into the Big Bang Theory proper.When you plug inflation into a chalkboard covered
in the kind of pretty gibberish sometimes known as a scientific model, to say the output
is ‘interesting’ would be the biggest understatement in the history of mankind. Because almost all inflationary models of
the universe result in the existence of a multiverse. Take the Eternal Inflation model, for example,
which suggests that inflation is, you guessed it, eternal. According to the Eternal Inflation model,
as space inflates, increasing in size at an exponential rate, it creates many big bangs,
each of which results in a brand-new universe, just like our own. Our universe, therefore, is one of many so-called
‘pocket’ or ‘bubble’ universes within the eternal inflation of the parent universe,
whatever the bloody hell that is.. It’s a very reasonable inference that if
inflation was the precursor to our own universe, then it could be to countless others too. And if inflation really is eternal there must
be an infinite number of pocket universes, too - one of which is ours. But this creates more questions than it answers,
such as: If there are infinite universes, how come
we find ourselves in this one? Do all the other universes contain life, or
only some? Do the other universes obey the same fundamental
laws as ours? And, which one is Star Trek set in? Those are some big questions, so how about
we scale things down a bit by substituting the word ‘universe’ with ‘planet’
in each of them. Luckily, science has been pondering the planetary
varieties of these posers ever since ancient astronomers first realised a few of those
pinpricks of light in the night sky weren’t stars at all, but nearby planets. And now here we are, in 2020, with 4,354 exoplanets
- that is, planets outside our own solar system - identified in our big book of things we
found lying around space. If our home is not as unique as once thought,
why do we live on this planet and not any one of an estimated 50 sextillion in the observable
universe? Well, scientists do have a fairly robust answer
to that - the goldilocks zone, also known as the habitable zone, which is the range
of orbits in which a planet is the perfect distance from its host star for surface temperatures
to support liquid water and, therefore, life. But a planet worthy of biological stirrings
needs other attributes besides just God-damn-impossibly-lucky-positioning. Such as a magnetic field to protect organisms
from deadly ionising cosmic radiation and a healthy dose of nitrogen in the atmosphere. The planet’s mass must fall within a very
specific range, too, so cell division is not impeded by its gravitational pull, or allowed
to just float around bumping into shit. Put simply, unless a planet is dealt pocket
aces, every hand, it can’t support life. Luckily for us, it just so happens our own
lovely planet had aces for days. If that all sounds kind of random, that’s
because it is. We only exist on this planet, in this quiet
corner of space, because we can exist here, so we do. We weren’t designed for our environment,
our environment designed us. We didn’t get lucky, we’re just a byproduct
of our planet arbitrarily ending up just where it needed to be in order to support life. Turns out, it’s kind of the same deal when
it comes to multiple universes. It’s likely that each universe would have
its own laws of physics, and could therefore potentially look and behave nothing like our
own. Other universes could have more, or fewer,
dimensions than the three we observe. If there are indeed an infinite number of
universes, we may just happen to live in one of the infinitesimally small percentile of
contenders that can host life – a goldilocks universe, if you will – with just the right
laws of physics, the ideal temperature and concentration of background radiation, and
the perfect size and shape to allow for life to germinate. We don’t know what that percentage is, yet,
but for every universe like ours, there could be a billion others that are dead - just empty
voids, perhaps with a few rocks and the occasional Cthulhu-esque interdimensional being floating
around, looking in vain for someone to hang out with. It’s actually the near-perfect conditions
and physical laws of our own universe which in many ways give credence to and a need for
the multiverse theory. If this was the one and only universe, it
would be awfully darn convenient that it just so happened to pop into existence 13.8 billion
years ago with the ideal conditions to support planets and lifeforms, on its very first attempt. If the universal strength of gravity varied
by just 1%, life as we know it could not exist in our universe. The same is true for any number of other parameters,
from the cosmological constant to the efficiency of fusion from hydrogen to helium and even
the number of dimensions, all of which had to be just so for us humans to exist. If, on the other hand, our universe is just
one of many, each with varying conditions, it’s rather less remarkable that we bagged
ourselves a goldilocks. It stands to reason that if we were to exist
at all we would do so in one of the potentially infinite universes with conditions perfect
for our kind of life, since we couldn’t have existed in any of the others. So it seems ‘multiverse’ is a neat way
for theoretical physicists to answer their unanswerables. Why is our universe like this? Multiverse. Why does life exist? Multiverse. Why does Donald Trump exist? Multiverse. But could we ever empirically prove our universe
is just one of many? Does this Truman Show have an outer wall,
perhaps even an exit? Or are we an eternally-lonely snowman trapped
in a snow globe – suspicious of another existence, but never quite sure? Well, like the snowman, we may only be able
to confirm other planes of existence if our own is destroyed. Or, at the very least, if it receives a good
wallop. It’s very possible that only if our bubble
universe collides with another will we, post-impact, be able to measure the effects of the collison
and definitively prove we have other universe pals out there. As you can imagine an entire fucking universe
smashing into another would be sort of a big deal. Depending on the velocity of the collision
– or the strength of other external forces that don’t even exist in our universe – it
could spell the end of both universes or merely leave a flesh wound. It really would be the cosmic equivalent of
a car crash, either a galactic bumper scrape or a multi-dimensional write-off. If it’s the latter, and we’re still here
afterwards wondering why the stars look a bit wobbly, we could measure the inevitable
results in the form of heat, radiation and other tell-tale signs. But it just so happens, we’ve already stumbled
across something a bit odd whilst poking around our cosmic backyard. In early 2000 the recently-launched Wilkinson
Microwave Anisotropy Probe, was – well – probing, when its data revealed something odd. A huge region of space within the constellation
Eridanus that’s unusually cold, a full 0.00007 Kelvin colder than the average temperature
of the cosmic microwave background radiation – which is 2.7 Kelvin. OK, that doesn’t sound all that much. If my eyes were 0.00007 millimetres further
apart than the average, I wouldn’t be branded a mutant and publicly flogged. But remember, earlier I said our universe
is remarkably homogeneous. Compared to pretty much all the other nooks
and crannies we’ve probed... in space - get your mind out of the gutter! - – this particular
region is abnormally cold. On closer inspection, we found out why. Matter emits heat, and there’s 20% less
matter in this particular galactic neighbourhood than pretty much everywhere else. And this isn’t some pokey, out of the way
corner that can be explained away through some other anomaly. This thing is vast. At 1.8 billion light-years across, the aptly
named ‘Cold Spot’ is a relative cosmic desert. As yet, we have absolutely no idea what’s
actually there, but whatever it is, the cold spot is the largest contiguous structure we’ve
ever found in the observable universe. There are other cold regions of space - well
OK, all regions are space of cold, but you know what I mean - ‘colder’ regions of
space - but according to all popular scientific models of the universe, the existence of one
as large as this should be near impossible. Yet it very much does exist, raising the obvious
question, why? Well, a commonly espoused hypothesis is that,
at some time or other, another bubble universe collided with our own. When this happened matter was pushed away
from the impact site, leaving an imprint on our universe – a ‘space bruise’ if you
will – the cold spot. Colder spot. Despite eternal inflation, the cold spot,
and a few other clues, the existence of the multiverse remains no more than an idea tossed
around university chalkboards by a few cult-like believers. It’s viewed with scepticism by the scientific
masses and it’s not even a standalone hypothesis - the multiverse is the inevitable outcome
of more complete ideas, such as eternal inflation and string theory. We still have no way of testing models that
directly predict a multiverse, and until we do it will be relegated to a kind of grand
thought experiment and byproduct of sturdier models. Even the word itself is an oxymoron, when
you think about it. ‘Universe’ means ‘everything’, so
by implication ‘multiverse’ means ‘multiple everythings’ – I’ll let you think about
that for a moment… Of course, this is no more than a linguistic
trap, and not quite grounds to call up Mr. Guth and tell him he’s a silly little muffin
who doesn’t know what he’s talking about. And if it does turn out to be true the implications
of a multiverse are equally incredible and terrifying. Infinite universes would mean there is a universe
identical to our own, with another you, and that there’s a universe where squirrels
are our despotic overlords and force us to farm acorns twenty hours a day. And, believe it or not, there’s even a universe
where the Star Wars reboots weren’t total shite. Thanks for watching.
