The current scientific story of the origin of the universe begins with an explosion which made space
itself expand. About 15 billion years ago, all the matter and energy that, today, make up the
observable universe were concentrated into a space smaller than the head of a pin. The cosmos blew
apart in one inconceivably colossal explosion: the big bang. And the stuff of the universe,
together with the fabric of space itself, began expanding in all directions as they do today.
But there is a force at work in the universe that casts a singular shadow over this
explanation. Rather than travelling in a uniform manner, that expansion is behaving strangely.
Something unusual is happening at the edges, causing matter to travel away from us faster
and faster, the further away from us it is. Most explosions don’t speed up the longer they go on.
Scientists believe that dark energy is at play.
But what is dark energy? Why are so many scientists sure that it’s there? And what
does this mean for our understanding of the universe as we know it?
I’m Alex McColgan, and you’re watching Astrum. Join with me today as we uncover what we know of
dark energy, and explore some of the ways scientists are trying to understand it.
Our universe is much more mysterious than it appears at first glance.
Understanding the structure of this universe is something we only really began to do in
the last 100 years or so. We had plenty of understanding of stars before that point,
but it was yet to be discovered how far away some of those stars actually were. Scientists
before then largely believed that the universe was just a single galaxy, filled with stars.
Although some astronomers began to spot little fuzzy patches (actually galaxies) in the sky and
postulated them to be “island universes” external to our own, nothing was proven until the 1920’s,
when American Astronomer Edwin Hubble began studying one such object for himself.
While studying what we now call the Andromeda galaxy in 1923, Hubble realised that one of the
stars he was looking at was a Cepheid variable star – a type of star that blinks brighter and
dimmer over the course of weeks. Importantly, it had been proven that these stars’ periods were
linked to their luminosity, so by measuring how regularly the star brightened and dimmed,
Hubble could calculate how bright the star should have been. By comparing that to how
bright it actually was, he could mathematically calculate how far away it was. Sort of like
knowing that a candle that’s further away from you is dimmer than one that’s close by.
Suddenly, he had a ruler with which he could measure the universe.
And what he found surprised him. The star he was examining was so far away,
it had to exist outside of our galaxy.
This opened up our understanding, but it wasn’t long before another surprise arrived. Hubble
noticed another strange fact. He was able to see an expanding Universe. It felt like an affront;
a Universe almost like a mobile, dynamic, organic, and expanding entity. It was an observation so
disturbing that even Einstein refused to accept it. The German theoretical physicist had just
shaken the structures of the scientific foundation by proposing an unimaginable transformation to our
old notions about space and time. And even then, he could not accept that the entire Universe was
something dynamic and changeable. In fact, he was so convinced that our Universe was
a static entity, Einstein set out to modify his original equations by adding an extra element to
stabilize the mathematics of the Universe. This addition was called the Cosmological Constant.
For Einstein, the idea that the universe was expanding was so
distasteful that he initially couldn’t bring himself to accept it. And yet,
time and Edwin Hubble proved his static model wrong, making a cosmological constant unnecessary.
There is an effect known as the Doppler effect, that says that the wavelength of radiation
stretches if its origin is moving away from you, and compresses if it is moving towards you.
Because Hubble knew what frequencies the light from these galaxies ought to be arriving at,
he could calculate their direction of travel using this doppler effect.
And he discovered that all the galaxies in the Universe were moving away from us.
And it wasn’t long before scientists realised that if they were moving away from us now,
they must have once been a whole lot closer. The theory of the Big Bang was born.
But another strange fact troubled scientists, one that the Big Bang did not quite account for.
It wasn’t just that galaxies were moving away from us – they were accelerating
away from us. The further away from us they were, the faster they were moving.
And acceleration requires energy. Where was this energy coming from?
Nobody seemed to know. But it seemed to permeate everywhere.
To fill this gap in their knowledge, scientists came up with the idea of Dark Energy.
Applying general relativity to this expanding model universe, and comparing it to the latest
observational data, scientists realised that the particles we could see – known as baryonic
matter – made up only 5% of the universe. Somewhere out there is a shortfall of 95%.
It was the only thing that made sense. Matter was congregating strangely,
as if something invisible with gravity was pulling it closer together. Scientists called
this substance dark matter, and attributed to it 27% of that mysterious 95% shortfall.
But matter was also being pushed apart. The remaining 68% of the universe had to be the energy
that was responsible for that. Undetectable, invisible, but everywhere. The maths proved it.
There are a few different ideas about the nature of this energy. For starters,
Einstein’s cosmological constant raised its head again, this time with a new application.
Einstein originally came up with the idea of such a constant as a way of perfectly counteracting
gravity, a push to balance its pull, to ensure the universe neither expanded and contracted.
But if you start with a bigger cosmological constant, you’d get a universe that tipped
into constantly accelerating away from itself – just like we see in real life.
Einstein hypothesised that empty space might not truly be nothing, but that it held energy,
and it could and did create more of itself. This creation of space would create more energy,
increasing the pushing power as time went on. It’s unclear why space would have this property,
though. It was more a recognition that it seemed to, than an explanation.
To fill the gap, quantum mechanics stepped up to the plate. Scientists of this school hypothesised
that perhaps virtual particles were to blame for this extra energy. Virtual particles are
pairs of atoms that simply pop into existence as a quirk of the quantum wave function that exists
everywhere. They don’t stick around for long, as they meet and cancel each other out again, but in
the moment before their annihilation they could conceivably push against things, exerting force.
This theory had some experimental evidence to back it up – there is some suggestion that
virtual particles do in fact exist, at least in some form – but when scientists tried to work
out how much energy such particles would create if this was happening in the vacuum of space,
their answer proved to be too large – And not just by a little; by about 120 zeros.
When your answer is too big by 120 zeros, there’s likely something wrong with your maths. Indeed,
this answer got the dubious accolade as the “worst theoretical prediction in the history of physics.”
The third main idea is that of a varying cosmological constant – so,
not really a constant at all – but rather an adjusting field like an electrical field or a
gravitational field that accounted for this repelling force throughout space.
This force was called “Quintessence”, and it might be the 5th fundamental force of existence,
if it really does exist. The problem with this idea, however, is the same
as the issue with the cosmological constant – there is no explanation as to why it exists.
In short, this is still a very poorly understood field of study. More
information would be needed before accurate conclusions can be drawn.
Spotting something invisible is naturally hard, but studying dark energy through its impact
on the universe around it is already underway. Different observatories are attempting this in
different ways. BINGO – the Baryon acoustic oscillations from Integrated Neutral Gas
Observations telescope in São Paulo, Brazil – is a land-based radio telescope that intends to monitor
the radiation given off by hydrogen atoms in the dark patches of space between galaxies, to see
if there are any unusual distributions of matter that might point to the influence of dark energy.
A curious fact about the universe is that matter clusters in waves;
it is statistically more likely that there is a distance of around 500 million light years
between any two galaxies than any other number. It’s thought that this is to do
with ripples in the early universe right after the big bang that have coalesced into a more
solid form of matter as the universe cooled, locking those ripples into place. Seeing the
gaps between such ripples – even in the darkest corners of space – could be very instructive.
However, if you really want to spot dark energy’s hand over time and space,
you need to go big. And it’s for that purpose that ESA has just launched the Euclid space telescope.
Euclid is a wide-angled telescope with a 600-megapixel camera, designed to capture
visible light and infrared radiation from over a 3rd of the sky. Euclid’s mission is to survey this
massive patch of sky with a level of detail and sharpness that’s 4 times greater than previously
achieved by ground-based telescopes. It wants to map everything¸ going back 10 billion years.
The hope is that in so doing, it’ll be possible to create a 3D image of the influence of dark energy,
with the capacity to see how dark energy’s influence has changed or remained constant over
all of that time. Euclid will look at individual stars too, but its main objective is sheer volume.
This way, the pattern of dark energy will hopefully emerge.
Euclid launched on the 1st July 2023, and has since arrived at its designated L2 Lagrange point.
It has already started taking its first images, although these photos of star-scapes and galaxies
are only tests, and do not represent the levels of fidelity Euclid will be able to manage once
it has finished its tuning. Once the calibration process is complete, however, it will begin its
6 year mission to map out our stars, and hopefully answer important questions like what is the nature
of dark energy, and whether our understanding of the laws of gravity is really complete.
So, fingers crossed that this mission can shed some light on the mystery that is dark energy.
For now, though, this enigmatic force remains a mystery. It is only because of the universe’s
expansion that we are aware of it at all. It is otherwise undetectable. But as it makes up over
60% of all that exists in the universe, it must be all around us right now. It pushes on us,
just as it pushes on every other galaxy. And slowly but surely, it forces all things apart.
Perhaps one day, thanks to dark energy, the only galaxy we will be able to see is our own.
All others will have been pushed out past the limits of detectable light. And then,
there truly will be only a single galaxy in the universe… at least, as far as we can see.
One day, we may wonder where all the other galaxies have gone. Perhaps we
will question if they ever existed at all.
Whether the universe eternally expands, or whether at some point dark energy will run out,
we don’t know. At that point, there would be nothing but gravity, leading to everything
compressing back in with a Big Crunch. What is dark energy? For now, only time,
and possibly a space telescope called Euclid, may tell.