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PORTSIDE  April 2012, Week 5

PORTSIDE April 2012, Week 5

Subject:

Why Should There Be Dark Matter?

From:

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Date:

Mon, 30 Apr 2012 00:17:36 -0400

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Why Should There Be Dark Matter?
by Ethan Siegel
Starts With A Bang
April 27, 2012
http://scienceblogs.com/startswithabang/2012/04/why_should_there_be_dark_matte.php

[moderator: to view the rich graphics which accompany
this article please use the link above]

    "And what I wanted to do was, I wanted to explore
    problems and areas where we didn't have answers. In
    fact, where we didn't even know the right questions
    to ask." -Donald Johanson

You can learn an awful lot about the Universe by asking
it different questions than you asked about it
previously. If all you ever used were your own senses,
there would be an awful lot to learn, but you would be
severely limited.

Even from the highest mountaintops, for example, you'd
never be able to distinguish whether the Earth was round
like a sphere or flat as a pancake, if all you used were
your eyes. But by looking at the Earth on a larger scale
than you could achieve otherwise, its roundness becomes
both apparent and indisputable. 

The same thing applies to the Universe, both on large
scales and small. If you want to know what the overall
structure is of the Universe, you have to look at it on
the largest scales. Looking at individual galaxies or
even large clusters of galaxies won't get you there at
all; if you want to know what your Universe looks like,
you need to look at it on the largest and grandest of
all scales, spanning billions of light years in all
directions. 

In the above image, still showing just a fraction of the
Universe scanned and measured by the Sloan Digital Sky
Survey, each pixel represents an entire galaxy. By
measuring how galaxies cluster and clump together -- how
they are distributed throughout the Universe -- we can
determine what it takes to create a Universe that looks
like ours. 

What we learn, as you can go through in detail, is that
the structure of the Universe requires that there be a
type of matter in it that does not collide with either
normal matter or with photons, that outnumbers our
(normal) matter by a factor of five- or six-to-one, that
don't respond to either electric or magnetic fields, and
that... frustratingly, can not be any of the known
particles in the Universe! 

This would be a very, very big problem under one
condition:

If the known particles and laws of physics explained all
of the observed phenomena in the Universe.

In other words, if there's no new physics out there
(beyond the standard model), then there's no need for
any new particles out there, and so, why would there be
any dark matter? There simply wouldn't be a strong
motivation, not from an elementary physics standpoint.

And yet the opposite of that is also true: if there is
physics out there that isn't explained by the standard
model, then there must be new types of particles out
there! And if there are new particles out there, there
are good candidates for this dark matter. You've
probably heard of some of the speculations that abound:

For example, if there's a symmetry of nature known as
Supersymmetry (or SUSY, for short), then there ought to
be twice as many fundamental particles as the ones we
currently know about. Moreover, the lightest one is a
perfect candidate for dark matter! Until we know what
this particle's properties are, however, we don't know
exactly what predictions to make as far as particle-
particle interactions go.

While dark matter may or may not be supersymmetric in
nature (many argue that SUSY may not even exist), this
last part -- that until we know what dark matter's
particle properties are, we don't know what predictions
to make for dark matter's interactions -- is generally
true. But there are plenty of other ideas. Two more
speculative ones, first, and then two definitive ones.

The electromagnetic, weak, and strong nuclear forces
could all unify at some high energy, in what's called a
Grand Unified Theory, or GUT. One of the universal
consequences of GUTs is that they all predict that
protons will decay, and so that's one of the things we
look for. In many variants of GUTs, there are candidates
for dark matter that emerge naturally. 

Same case for extra dimensions; they may or may not
exist, but if they do, then there are plenty of new
particles and interactions that certainly exist, and one
(or some) of them may make excellent dark matter
candidates.

But those -- supersymmetry, grand unification, and extra
dimensions -- are speculative ideas, and may not
describe our Universe. But there are two observations
that we have already made in the Standard Model that
already cannot be explained by the particles and
interactions we know today. This means there are new
particles out there, yet undiscovered, that could easily
solve the dark matter problem. 

For one, neutrinos have mass! According to the Standard
Model, there should only be one type of neutrino -- a
left-handed one -- and they should have zero mass. But
this is not the case!

They are observed to have non-zero mass. In fact, all
three types of neutrinos have non-zero mass, meaning
there is new physics and there are new particles out
there! Right-handed (or sterile) neutrinos could very
easily make up the dark matter; we are searching for
them as you read this! But perhaps the new physics that
explains neutrinos isn't also what explains dark matter.
There's another problem. 

There are a couple of fundamental symmetries of nature
that, at least in everyday life, seem pretty obvious.
One is that the laws of physics in a mirror -- where
left and right are reversed -- are the same as our
normal laws of physics. (We call that Parity, or P-
symmetry.) Another is that matter and anti-matter obey
the same laws of physics. (We call that Charge
Conjugation, or C-symmetry.) Most laws of physics that
you know, like gravity and electromagnetism, always obey
these symmetries.

According to the standard model, they have to; it's
coded into the physics. But these symmetries don't exist
for the nuclear (weak and strong) forces in the standard
model. If I took something like a muon, reflected it in
the mirror (applying P-symmetry), and replaced that
image with an anti-muon (applying C-symmetry), I'd be
testing whether the combination of CP-symmetry was a
good one or not. 

If it were a good symmetry, then if all the muons
decayed with one orientation, all the anti-muons would
decay with that specific, mirrored orientation. But they
don't, and so that CP-symmetry is violated. This is good
for the Universe, because CP-violation is one of the
necessary things to make more matter than anti-matter.
But if it happens for an interaction like this -- the
Weak nuclear interaction -- then it stands to reason
that it should also happen for the strong nuclear force.

But it doesn't! Why wouldn't it? 

The same reason this unicycle toy doesn't tip over:
there must be some sort of extra, hidden weight that
provides extra balance, or in the particle's case,
crushes the amount of strong CP violation.
Theoretically, the standard model allows you to violate
both C and P together here, but we've looked, and to
something like one part in a billion, we don't see any.
So something -- and this means there's new physics --
has got to be forbidding it!

This outstanding problem, known as the Strong CP
problem, is the second hint of new physics that must go
beyond the standard model. And at least one class of
solutions to it produces an outstanding dark matter
candidate, known as the axion. 

There's definitely physics in this world that's beyond
the standard model, there's definitely more to neutrinos
than we know, and there's definitely something stopping
CP violation from occurring in the strong interactions.
There may also be extra dimensions, grand unification,
supersymmetry, or something even more exotic or
surprising. But all of these possibilities require new
particles, many of which make good dark matter
candidates, and all of which have unknown particle
parameters.

When you combine this information with our astrophysical
knowledge of dark matter, you can see why I prefer the
approach of using the astrophysics to try and
reconstruct/determine some of the particle properties of
dark matter, and try to guide us as to what we should
look for. (No, really, I sometimes research that!)

We've got lots of options and lots of searches going,
but there's so much we don't know about it at this
point! Cross-sections, masses, reaction rates,
lifetimes, etc., they're all mysteries at this point. We
may not know what dark matter is, exactly, but we've got
lots of strong possibilities for what it could be, and
some hints that simply can't be ignored. We're
desperately trying to be able to detect it directly, and
solve this mystery once and for all. Welcome to the
cutting edge!

___________________________________________

Portside aims to provide material of interest to people
on the left that will help them to interpret the world
and to change it.

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