Astronomers make a seemingly unbelievable claim, which is that the ordinary form of
matter that makes up you and me is just a small fraction of the energy budget of the
universe.
They claim that there is another form of matter, called dark matter, that is five times more
common than the familiar form.
So, what is the nature of dark matter?
And how will we find it?
That’s an interesting question and I think it’s worth taking some time to look into it.
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If dark matter is so much more prevalent than ordinary matter, surely it must be easy to
find, right?
I mean, after all, we can see stars and galaxies, so what’s the problem?
The problem is that dark matter is – well – dark.
It doesn’t emit or absorb light.
That makes it hard to detect by any manner other than its gravitational effect on the
familiar form of matter we can see.
And that’s why we believe dark matter exists.
Galaxies rotate faster than the observed, ordinary, matter can explain using the known
laws of physics.
Ditto with the motion of individual galaxies inside clusters of galaxies.
And the images of distant galaxies are distorted more than can be explained by the effects
of ordinary matter between the distant galaxies and our telescopes.
All of these are reasons why astronomers believe that dark matter is real.
But just what is dark matter?
It might surprise you that scientists just simply don’t know.
It could be a gas of unknown subatomic particles with a mass much lower than the mass of electrons,
or it could be made up of objects as heavy as stars.
If you want numbers, the possible range starts with a mass of about ten to the minus six
electron volts on the low end, or about ten to the minus seventy-one solar masses, up
to a mass of about a thousand solar masses on the high end.
That’s a ridiculously large range, with a difference from smallest to largest of about
ten to the seventy fifth power.
It wouldn’t be unreasonable for someone to hear that and conclude that scientists
don’t have a clue about the mass of individual dark matter particles.
That’s of course not the whole story.
After all, scientists have been looking for dark matter for many decades and some possibilities
have been ruled out.
For instance, astronomers have used telescopes to look for types of dark matter in the mass
range of asteroids, moons, planets, and stars.
While the dark matter can’t be seen – after all, it’s dark – if a dark matter star
passes in front of another star, you can detect its passage.
Using this technique, astronomers ruled out forms of dark matter in the ten to minus seven
times the mass of the sun to about thirty times the mass of the sun.
That’s about the mass of Mercury to thirty solar masses.
Give or take.
We can actually rule out all candidate forms of dark matter made from non-glowing versions
of ordinary matter, like gas, asteroids, planets, etc., simply because they wouldn’t be dark.
They’d heat up from the light of surrounding stars and emit infrared light.
So, we can rule out that form of dark matter.
Now for a very long time – most of my career, actually – scientists hypothesized another
form of dark matter, what are called WIMPS, short for Weakly Interacting Massive Particles.
WIMPs are thought to be subatomic particles, kind of like heavy and stable neutrons.
The hypothesized mass range was somewhere from slightly lighter than a neutron to maybe
ten thousand times heavier.
And scientists have looked for versions of this type of possible dark matter.
And, in spite of decades of looking, and huge improvements in detector sensitivity, researchers
have found absolutely nothing.
Now, that doesn’t mean that dark matter in this mass range is ruled out.
But it does mean that, if it exists, it interacts very weakly – maybe only via gravity.
And if dark matter interacts only via gravity, we may never see it.
That would be annoying.
However, we shouldn’t be negative yet.
Maybe dark matter interacts weakly with ordinary matter, but strongly enough that we can still
find it.
But that means looking in different places, using different techniques.
And that’s what we’re doing at Fermilab, in collaboration with a number of sister institutions.
Rather than the traditional searches for WIMPs, researchers here are trying different and
innovative approaches.
Let me tell you about two.
Sub-Electron-Noise Skipper-CCD Experimental Instrument, or SENSEI, uses ultralow-noise
silicon Charge Coupled Devices to look for dark matter particles with a mass as low as
the mass of the electron (that’s 500 thousand electron volts) if dark matter scatters off
electrons in the chips and as little as a single electron volt if the dark matter gets
absorbed by the electrons surrounding the silicon atoms.
And this isn’t just hypothetical.
These super quiet electronic chips are astoundingly powerful for finding low mass dark matter
that interacts with electrons.
A single SENSEI detector weighing just two grams outperforms a competing detector using
liquid Xenon with a mass of a ton.
And the SENSEI experiment will be upgraded to a mass of a hundred grams.
SENSEI is going to totally dominate searches for this form of possible low mass dark matter.
But, of course, dark matter might not have the properties that SENSEI is designed to
detect.
And, given that astronomical measurements have ruled out a lot of very heavy options
for dark matter, and the WIMP searches have ruled out a lot of possible heavier subatomic
particles, we need to explore more thoroughly the very low mass dark matter options.
In 1977, physicists Roberto Peccei and Helen Quinn were trying to solve a mystery involving
the strong nuclear force.
According to the theory governing the behavior of quarks and gluons, the strong force could
favor either matter or antimatter, but it seems to treat both equally.
Peccei and Quinn proposed a new energy field that forced the strong force to react equally
with matter and antimatter.
If that energy field is true, this means that a new particle exists – one that we’ve
never seen before.
This particle is called an axion.
Axions are very low mass, don’t emit or absorb light, and – in short – are a perfect
candidate for low mass dark matter.
Well, that is, if they exist.
So Fermilab scientists are participating in a collaboration called ADMX, short for Axion
Dark Matter Experiment.
This experiment uses a very strong magnet and a cavity that resonates at a certain microwave
frequency.
It’s a bit like when you blow across the top of a two-liter bottle, and it makes a
sound of a specific frequency.
Except, of course, it’s microwaves and not sound.
The experiment depends on a quirk of axion theory, which says that in the presence of
a strong magnetic field, an axion can convert into a microwave.
It’s a tiny effect, but the microwave cavity is surrounded by super powerful amplifiers
that make it possible to see the conversion of an axion to a microwave.
The ADMX experiment is designed to look for axions with a mass of about three millionths
of an electron volt.
This is at the bottom end of what scientists think is reasonable for dark matter.
So far, they have found nothing.
I think the real take away message is that scientists don’t have a preconceived notion
of what dark matter might be.
I mean- we know that there are a lot of astronomical observations that could be explained by dark
matter, so we keep looking for it.
But we’ve not found it, so we look in all possible places, under all metaphorical rocks.
The dark matter research community has looked for candidate particles from a millionth of
an electron volt, all the way up to much heavier than the Sun.
And we’ve found nothing so far.
That doesn’t necessarily mean that dark matter doesn’t exist.
It might just have properties that current and past experiments weren’t sensitive to.
And, of course, it might actually mean that dark matter isn’t real and that we need
another explanation for those astronomical observations.
But we need an explanation.
So I’m glad that my colleagues are looking so hard at possible answers.
I’m impressed that the SENSEI experiment, using a mere two grams of silicon, is more
sensitive to certain measurements than much larger detectors.
And I’m super glad that ADMX and similar experiments are looking for super light versions
of dark matter.
We wouldn’t want to get tunnel vision looking for the WIMPS that the theoretical community
favored for so long.
No matter the answer, the questions of why galaxies rotate so fast and why the image
of distant galaxies are so distorted are fascinating ones and we won’t understand the universe
until we’ve answered them.
Open questions are the most fascinating ones and I hope we’ll figure it out soon.
Asking hard questions is what scientists do.
I talked about dark matter in this video, but there are lots unanswered hard questions.
What’s your favorite?
Let me know in the comments and maybe I’ll pick one of the topics for a future video.
In the meantime, I hope you liked this video and you’ll subscribe to the channel and
share on social media.
I look forward to hearing your hard physics questions because they’re my favorite because,
well- you know- physics is everything.
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