With dark matter,
dark energy, phantom matter and even a dark force, physics news can
sometimes sound like the voiceover for a superhero movie. So what’s
behind all the ominous-sounding jargon?
Over the past 40 years
astronomers have realised that everything we can see – all the stars,
planets and galaxies – make up less than 5% of the entire universe. What
is the rest? The short answer is, we have no idea.
What we do
know is there are two gaping holes in our understanding of our universe.
As a placeholder, physicists call them dark matter and dark energy.
In a nutshell, dark matter is the invisible stuff which we can only
detect from the way its immense gravity moves stars and galaxies.
Dark energy, on the other hand, is the mysterious something causing the universe to expand with ever increasing speed.
We don’t know if dark matter and dark energy are related – in fact they’re probably two completely different phenomena, both called “dark” just because we can’t see them.
Dark energy, on the other hand, is the mysterious something causing the universe to expand with ever increasing speed.
We don’t know if dark matter and dark energy are related – in fact they’re probably two completely different phenomena, both called “dark” just because we can’t see them.
Dark matter
How was it discovered?
Since the 1930s astronomers knew that the way galaxies spin did not
make sense. The stars at the edges of galaxies were moving much faster
than expected – so fast they should have been flung off the cosmic
merry-go-round and out into deep space.
But these strange motions
could be explained if there was a bunch of extra matter in and around
the galaxies – matter that we can’t see. It’s this “dark matter” that
holds galaxies together.
Since then, many other observations
beyond the scale of whirling galaxies, from the choreography of galaxy
clusters, to the collision of nebulae, all suggested the same thing.
Although some physicists have entertained other theories, such as
modifications to gravity, by now most are pretty sure dark matter
exists. It’s the only explanation that suits all the data.
What do we know?
We know dark matter doesn’t emit light (nor does it absorb or reflect
it), so it can’t be made of rogue planets or clouds of normal matter. We
know it’s “cold” (which in physics-speak means it moves slowly compared
with the speed of light). We know it has gravity. We also know it
doesn’t interact very strongly with anything, even itself – otherwise
the dark matter would collapse into flat structures such as galaxies,
rather than the spherical haloes we detect.
Oh, and it makes up about 27% of the universe
.
What could dark matter be?
.
What could dark matter be?
The bottom line is it is probably some new kind of particle (or a whole
family of particles) that we have never detected before. Dark matter
particles could be all around you, and floating through your body right
this second.
This means the answer to this grand cosmological
puzzle, affecting the universe on scales of mllions of light years,
could lie in the physics of tiny particles, much smaller than an atom.
Over the past 30 years physicists have sifted through dozens of
different dark matter candidates. The prime suspect at the moment is a
kind of particle called a weakly interacting massive particle, or WIMP.
This is a kind of heavy particles that feel only the weak force.
One of the goals of CERN’s Large Hadron Collider is to look for WIMPs
(the same way it found the Higgs Boson in 2013) – the elusive dark
matter particles might be created when protons are smashed together at
near the speed of light.
Can we detect dark matter?
Besides CERN, there are more than 30 experiments around the world devoted to finding dark matter.
Some of these are dedicated telescopes searching for the signature of
particles created when two particles of dark matter annihilate.
Others are giant vats of liquid xenon watching for a telltale flash when a dark matter particle nudges an atomic nucleus. None has yet made a convincing detection of a dark matter particle, although some of the experiments have ruled out various possibilities of what dark matter might be.
Others are giant vats of liquid xenon watching for a telltale flash when a dark matter particle nudges an atomic nucleus. None has yet made a convincing detection of a dark matter particle, although some of the experiments have ruled out various possibilities of what dark matter might be.
It remains a possibility that dark matter may never be
directly detectable – especially if it turns out to be a particle that
does not even feel the weak force.
The dark force and dark photons
Some physicists have proposed that dark matter particles can interact with one another via a new force of nature – called, yes, the dark force and transmitted by dark photons (aka dark radiation).
There may even be different kinds of dark matter, some of which feels the dark force, and some do not.
Some physicists have proposed that dark matter particles can interact with one another via a new force of nature – called, yes, the dark force and transmitted by dark photons (aka dark radiation).
There may even be different kinds of dark matter, some of which feels the dark force, and some do not.
Dark energy
How was dark energy discovered?
In the early 20th century, physicists including Albert Einstein
imagined the universe as static and unchanging. But in 1929 American
astronomer Edwin Hubble observed the motions of exploding stars and
discovered the universe was expanding. In fact the universe must have
had a beginning – a moment of creation called the big bang.
We
can imagine the big bang a bit like an explosion. But after that initial
burst, physicists thought the expansion should begin to slow down over
time, as gravity acted to pull everything back to a single point again.
The question was whether the universe would ever stop expanding and reverse direction, falling back into a “big crunch”.
Then, in 1998, things got a bit more complicated.
Using the same method as Edwin Hubble (and with the telescope named
after him) astronomers found that the expansion of the universe was not
slowing down, but instead was accelerating. Galaxies are flying away
from each other faster and faster each year.
It was a strange and
unexpected result. A bit like if you were driving on a flat highway,
took your foot off the accelerator – and then your car began to speed
up!
Yet the data were convincing. Physicists realised this
expansion must be driven by some sort of energy, and they called it
“dark energy”.
What we know
We know that dark energy
affects the universe as a whole. We know it acts a bit like a negative
gravity pushing galaxies away from one another.
We also know that
dark energy did not kick in until a few billion years ago. (For the
first half of its life, the expansion of the universe was slowing down
due to gravity pulling everything together.)
This makes
physicists think dark energy is somehow tied up with space itself. This
means its density in space is always the same, but as the universe
expands (that is as more space is created), the amount of dark energy
also increases.
This would explain why the amount of dark energy was insignificant when the universe was small.
What could it be?
The answer to the mystery
of dark energy might also lie in the minuscule quantum realm.
In quantum theory, “empty space” is not empty at all, but filled with a soup of particles continually popping into and out of existence. As weird as it sounds, physicists have actually measured the force created by these so-called “virtual particles” in the lab.
In quantum theory, “empty space” is not empty at all, but filled with a soup of particles continually popping into and out of existence. As weird as it sounds, physicists have actually measured the force created by these so-called “virtual particles” in the lab.
The problem
is, when physicists try to calculate how much energy these virtual
particles contribute to each cubic metre of empty space, they come out
with a number that’s a factor of 10120 too large when compared to the
density of dark energy (as measured from the accelerated expansion of
the universe). That's a 1 with 120 zeroes after it, a ludicrous answer
called “the worst theoretical prediction in the history of physics”.
Quintessence
Some physicists think dark energy could be akin to a fifth force of
nature, pervading all of space. They call it “quintessence”, after the
fifth element predicted by the Greek philosophers. As opposed to the
cosmological constant, the quintessence is imagined to change over time –
it was once attractive, but is now repulsive.
The big rip and phantom dark energy
In some theories, the quintessence can continue to grow stronger (in which case it’s called phantom dark energy).
This could destroy the universe.
If the expansion of the universe continues to accelerate, eventually
reach the speed of light – first galaxies and stars would be cut off
from one another, then eventually the space between the sun and the
Earth would expand faster than the speed of light and individual atoms
would be torn asunder as the space within them expanded at faster than
the speed of light. This is the big rip.
A new gravity?
Dark energy might not be a new force, it might just be a sign that, at
very large scales, gravity does not behave as Einstein’s theory of
general relatively describes.
The ΛCDM (lambda cold dark matter) model
This is the name for the astrophysicists’ current best picture of the way the cosmos is screwed together.
Λ (or lambda) stands for dark energy, while cold dark matter describes
the consensus that dark matter must be made up of some kind of slow
moving, previously unknown particle.
In this picture, dark matter
makes up 27% of the mass-energy of the universe, dark energy makes up
about 68%, and ordinary matter – that of the stars and galaxies and our
own flesh and blood – makes up less than 5%.
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