In case you’re right up to date on your cosmology mysteries, I’m going to give you
the quick answer right now: filaments of oxygen gas at 1 million degrees Celsius in intergalactic
space.
Boom.
You’re welcome.
Answer starts at zero zero colon zero five.
For everyone else who likes context with their groundbreaking scientific discoveries, let’s
get into it.
Astronomers have finally solved one of the outstanding mysteries in cosmology.
Forget about all the dark matter and dark energy, where’s all the missing regular
mass in the Universe?
This has been called the “missing baryon problem”.
They’ve been chipping away at the problem, finding missing matter in the vast gulfs between
galaxies, and in the last few months, a team of astronomers have finally found the outstanding
missing matter.
What we don’t know about the Universe could fill a universe.
For example, if you take all the matter, energy, everything and add it up to 100%, more than
half, 68.3% is dark energy; a mysterious force that’s accelerating the expansion of the
Universe.
Of the remaining portion, about 26.8% of that is dark matter.
Astronomers think it’s some kind of particle that doesn’t emit radiation of any kind,
and doesn’t interact with regular matter in any way.
If you’re doing the math, that leaves you with 4.9% of regular mass.
You know, stars, planets, gas, dust, cats, smartphones, bread, protons, neutrons.
Stuff, things, what scientists called baryonic matter.
But here’s the thing, until the last few years, astronomers had no idea what this baryonic
matter was.
When you add up the mass of all those galaxies and their stars and cats, you only end up
with 10% of the mass that’s out there.
Before we get into the discovery, I’d like to talk about how astronomers knew this mass
was missing in the first place.
See, I warned you, this is going to take a while.
The answer, of course, comes from the Cosmic Microwave Background Radiation.
This is the diffuse background radiation that we can see in all directions.
It was a time, about 380,000 years after the Big Bang when the entire Universe had cooled
off to the point that light was finally able to escape and travel through space.
What started out as dull red visible light got stretched out, or red-shifted, by the
expansion of the Universe so that we see this radiation in the microwave spectrum now.
Although this background is about the same temperature in all directions, there are tiny
variations - fractions of a degree difference.
NASA’s Wilkinson Microwave Anisotropy Probe, and later the European Space Agency’s Planck
Mission have mapped the entire sky with incredible precision, and by doing so have provided astronomers
with a way to calculate many of the Universe’s properties.
And one of these is the total number of protons and neutrons that should be present in the
Observable Universe.
If there were more or less, then you would see different temperature variations in the
background temperature.
The shape, size and movements of galaxies would be different.
You would see different amounts of hydrogen and helium compared to lithium, beryllium
and other elements.
That number, by the way is 1 x 10^80.
That’s a one followed by 80 zeros.
On average, across the entire Universe, there’s 1 proton for every 4 cubic meters of space.
Just for comparison, there are 5 x 10^21 atoms in a drop of water.
So, just in case you weren’t aware, we live in a dense part of the Universe.
Once astronomers were able to calculate this number, they could then compare the amount
of mass in the Universe they could see, with the amount that should be out there.
The most obvious mass we can see are the vast galaxies and galaxy clusters strewn across
the Observable Universe.
How do you measure the mass of a galaxy, you might ask?
Astronomers have several techniques which they can use to check and double check.
They measure the speed that the galaxies are turning, and use that to calculate the mass
of the galaxy.
Imagine the Sun quadrupled in mass, the Earth would need to double its orbital velocity
to stay in orbit.
The Sun is orbiting the Milky Way at a speed of 225 km/s (which, by the way, is 29 times
faster than the International Space Station is hurtling around the Earth).
If the Milky Way were more massive, the Sun’s orbital velocity would need to speed up to
stay in orbit.
Another technique that astronomers use is gravitational lensing.
This is where the gravity of a galaxy cluster can distort the light from more distant galaxy
clusters.
By measuring the amount the light is distorted, they can calculate the amount of mass in the
cluster.
The point is, astronomers had done the math and had determined the following amounts of
matter in the Universe.
Remember, these are fractions of the roughly 5% regular stuff, not that 95% dark matter/dark
energy.
7% is stars in galaxies, 1.8 % cold gas in galaxies, 5% is hot gas in galaxies, 4% is
hot gas in galaxy clusters.
That still left about 83% of baryonic matter as mystery mass.
Astronomers have chipped away at those remaining amounts, and in the last few years have been
able to account for another 28% as cool intergalactic gas.
The theory is that the remaining half of missing baryonic matter had to be some kind of heated
gas, hiding in the vast voids between galaxies.
The technical term was WHIM, or warm-hot intergalactic medium.
In 2017, a team of astronomers announced that they’d found some of this WHIM.
They overlaid the Cosmic Microwave Background Radiation with a 3-dimensional map of the
Universe created using the Sloan Digital Sky Survey.
Within this, they found the telltale signature of scattered radiation that had passed through
the warm-hot intergalactic medium.
This accounted for some, but not all of the missing matter.
And now, finally, it’s time to talk about this brand new discovery, the one that fills
in the final missing piece.
And I’ll get to that in a second, but first I’d like to thank:
Victor Georgiev Shane Quinlan
Abram Lea Aireyean Smith
And the rest of our 838 patrons for their generous support.
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This month, a team of astronomers from the INAF Rome Astronomical Observatory and the
Harvard-Smithsonian Center for Astrophysics published a paper in the journal Nature called
Observations of the missing baryons in the warm-hot intergalactic medium.
It was the announcement that they had finally turned up the last missing piece of regular
matter in the Universe.
They used the European Space Agency’s XMM-Newton space observatory to observe a distant quasar
in X-ray wavelengths.
Quasars, of course, are supermassive black holes at the hearts of galaxies which are
actively feeding on material.
The material piles up, and emits enormous amounts of radiation which shines brightly
in X-rays.
They watched the quasar for a total of 18 days, the longest X-ray observation ever made
of a quasar.
And when they tallied up all the photons they’d collected, they found the telltale signature
of oxygen atoms at two different locations along the line of sight.
What that means is that there were two separate clouds of oxygen, heated to more than a million
degrees Celsius that got in the way of their view to the quasar.
From this, the astronomers were able to calculate that these huge clouds of hot gas in intergalactic
space more or less account for the final amount of missing mass in the Universe.
Case closed, matter found.
We now know what 5% of the Universe is.
Phew.
Time to figure out what dark matter and dark energy are.
Of course, you might be wondering how atoms of hydrogen, heated to more than a million
degrees Celsius were found out in the vast mostly empty gulfs between galaxies, and that’s
a really good question.
In fact, the researchers are planning to try and figure that out next.
I guess the story isn’t over yet.
What do you think?
Let me know your thoughts in the comments.
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