More often than not, new physics is uncovered when someone notices a discrepancy through
observation.
An example of this is in regards to the physics of Isaac Newton.
For many years, his physics held up as, in so far as anyone knew, a perfectly accurate
description of the universe that could be used to predict the behavior of objects in
motion.
But as time went on, discrepancies were noticed and as they mounted new theories by way of
relativity and quantum theory came in, though not to really replace Newtonian mechanics,
it’s still well used, but to expand upon it to give a clearer picture of how the universe
works.
Another example is Albert Einstein and the expansion of the universe.
He absolutely hated the idea, as did most scientists at the time, that the universe
wasn’t static and eternal, but had some sort of beginning.
But as evidence, both theoretical and observational, mounted, it became clear that we live in an
expanding universe kicked off by the big bang and the scientific consensus revised itself.
The result was a better description of the universe, with fewer discrepancies, that is
widely accepted today.
Yet, even with these new theories, there are still discrepancies popping up, such as situations
like the subatomic world where relativity breaks down and quantum theory takes over,
or the conditions inside a black hole where the laws of physics as we know them stop predicting.
One such discrepancy that has yet to be fully rectified is known as the missing baryon problem.
Baryonic matter is what we see and are made of, the term covering protons and neutrons
and everything made of them.
Here we have a discrepancy between the amount of baryonic matter that can be seen shortly
after the big bang, versus what’s seen in more recent periods.
There are limits to the amount of baryonic matter that resulted from the big bang however,
known through studies of the cosmic microwave background radiation and studies of big bang
nucleosynthesis.
Big Bang nucleosynthesis is where in the very early universe, basically the first few seconds
to almost a half an hour after the big bang, nuclei formed other than hydrogen.
This is where the universe got its original helium, but also small amounts of deuterium
but also a small amount of lithium.
It’s also thought that the radioactive tritium and beryllium 7 were formed, but then subsequently
decayed into isotopes of helium and lithium.
All other elements we see in the universe were subsequently created in the nuclear fires
of stars.
Given this state of affairs, big bang nucleosynthesis should have produced predictable ratios of
helium and lithium versus hydrogen.
This ultimately means that about 4-5 percent of the universe’s energy content should
be baryonic matter.
This agrees with evidence from the CMB, where baryonic matter at the beginning of the universe
has left an imprint.
The numbers here agree, coming in at around 5%.
So there are two pieces of solid evidence establishing how much baryonic matter was
present in the early universe.
But that’s not what we see, at least apparently, today.
Somehow, in the intervening time, a bunch of baryonic matter has just disappeared.
This should not be.
Known as the missing baryon problem, and it should not be confused with dark matter, which
is non-baryonic in nature so far as we know, more on that in a bit.
This is approaching the point that half of the matter the universe started with is simply
missing today.
Determining the density of baryonic matter in the universe today is tricky.
Remember, it’s all normal matter, so that can range from highly visible things like
stars and galaxies, to hard to see things like rocky planets and the dust and gas clouds
of the universe, to rogue brown dwarfs and objects plying the intergalactic medium and
so on, to, of course, black holes of which estimating how many of those are out there
is a subject of much debate, especially if primordial black holes formed during the early
universe exist or do not.
But a decent estimate can still be derived from what we know.
Several useful phenomena here are things like gravitational microlensing, where you can
look at how much of that phenomenon you see overall, and clouds of dust and gas can be
measured in several ways, such as if they are backlit, or emit radio as hydrogen clouds
do along with other techniques enough to notice a large discrepancy, especially when looking
at relatively recent galaxy surveys.
After all of this is accounted for, the universe is still missing on the order of 39% of its
original baryons.
So where did they go?
There have been some proposed solutions.
One was put forth in 2017 where very difficult to detect cool matter exists in intergalactic
space between galaxies that might account for it, or at least most of it.
Further evidence from 2021 supports this, but it’s not quite to the point of being
case closed and needs more confirmation.
This would mean that in addition to most of the matter of the universe being composed
of invisible non-interactive dark matter, a further huge portion of normal matter is
nearly invisible due to circumstance, forming trailing tendrils through the depths of space
between the galaxies.
We live in a dark universe indeed.
Only time will tell if this answers the missing baryon question, but herein lies another mystery,
another discrepancy, about matter in the universe.
Where is all the anti-matter?
This mystery is known as the baryon asymmetry problem, baryonic matter is normal matter,
but anti-baryonic matter is anti-matter.
By all rights, the big bang should have produced equal amounts of both, and in principle all
matter might have annihilated if it did.
But that’s not what we see, and both the standard model of particle physics and general
relativity offer no insights on why the universe appears to only have produced normal matter,
at least in large quantities during the big bang.
There are many theories as to just why this is.
The first is that somehow the laws of physics acted differently back then, or that some
unknown factor played in, creating the asymmetry that we just don’t fully understand yet.
But there are other options.
One idea is that the universe was only slightly off and that enormous amounts of matter and
anti-matter were indeed produced in the big bang, but slightly more baryonic matter was
produced.
As a result, there would have been a huge annihilation event that left only a residue
of normal matter left behind, which makes up the baryonic matter we see in the universe.
But there is another option, a rather spooky one.
Perhaps the universe is made up of both matter and anti-matter, and the two are just widely
separated concentrated in different areas of the universe.
The strange thing here is that you could look at an anti-matter galaxy from afar that has
no normal matter within it, but it would be identical to a normal matter galaxy and look
exactly the same.
So long as the matter and anti-matter components of the universe were kept apart, and never
came into contact, they could independently exist, and probably do so forever as the universe
expands.
But if two galaxies of different flavors were to come into contact somehow, the result would
be the wholesale annihilation of both galaxies, leaving only a giant release of energy in
a perfect reaction and a residue of whatever form of matter happened to be from the more
massive galaxy.
But if this were the case, there should be some interaction in the intergalactic borderline
between the two areas.
Call it the matter zone and the anti-matter zone.
The borderland between should result in gamma radiation production from annihilating material
deep in intergalactic space.
No such radiation has ever been detected, and the general consensus these days is that
anti-matter galaxies probably do not exist.
Another possibility is that conditions could exist that allow for matter and anti-matter
to decay at different rates, allowing for the asymmetry.
But perhaps the most interesting idea regarding this is the idea of a mirror anti-universe.
There are several takes on the concept of a two-sided universe, one with a normal universe,
and the other stretching backwards from the big bang as an anti-universe.
One common theme is that the anti-universe would have reversed time, it grows in size
the further back in time it goes as a mirror to how the normal universe works.
This concept has an interesting effect, in that it would actually explain the minute
temperature fluctuations in the cosmic microwave background, implying that the universe and
the anti-universe may not be true mirror images of each other.
Moreover it might explain dark matter, in that such a paired universe would produce
heavy neutrinos that could fit the bill, and also provide a source for very high energy
cosmic rays.
And, it could also provide a home, so to speak, for the predicted anti-matter product of the
big bang.
The anti-matter is simply in the anti-universe, going backwards in time.
This dovetails with some suggestions in the past that in a way anti-matter behaves like
normal matter going backwards in time, though that’s somewhat of an over-simplification.
It’s an interesting thought exercise to imagine such a dualistic anti-matter anti-universe,
where it may mirror our own universe so closely that it might be possible for anti-humans
on the other side to create small amounts of normal matter in their labs and marvel
at how it annihilates on contact with anti-matter.
And of course, the greatest mystery of all facing astrophysics is dark matter itself,
where you see the gravitational effects of an enormous amount of matter in the universe,
but not the matter itself that’s responsible for the effect.
So far, an explanation or detection of dark matter has not been forthcoming.
So this provides essentially three potential forms of matter that should be here, or once
was here but may no longer be, that we don’t see, which makes up the overwhelming vast
majority of matter in the universe, both normal and otherwise.
Or, if you will, most of the universe is effectively missing, and it’s conceivable we may never
know what exactly, or where exactly, everything went.
Thanks for listening!
I am futurist and science fiction author John Michael Godier currently adding in more missing
matter from the universe.
The obvious one here are socks, missing socks at this point may make up a percentage of
dark matter, but there are also such things as missing cookies and donuts for which there
is no explanation as to where they went, or at least that’s what I tell ANNA during
the health food audits over at Event Horizon.
And, when in doubt, blame the possum and be sure to check out my books at your favorite
online book retailer and subscribe to my channels for regular, in-depth explorations into the
interesting, weird and unknown aspects of this amazing universe in which we live.