Dark Matter and Dark Energy
The
New York Times has an article on dark matter and dark
energy that I think might be slightly overhyping the degree to which these ideas
cause a problem for the foundations of physics.
The author suggests that these will require a fundamental overthrow of
how physics is typically thought of, which, while possible (after all,
relativity explained a very small perturbation in Mercury’s orbit), is likely
not to be as dramatic as the author suggests.
I thought I’d write a response, explaining some of the various hypothesis to explain these things, and how dramatic of a
shift they would require in our conceptual foundation of physics.
Dark matter-
While trying to explain the stability of galaxies, astronomers have concluded
that there is more mass in the universe than can be explained by the stuff we
see at night. So, they introduced “dark
matter” as something to make up the substantial difference between the mass we
measure and the mass we observe.
Initially, dark matter was thought to be burnt out stars that no longer
give off light (dead black dwarfs, for example), except then people realized
that, according to our calculations, the rate stars form and die and their
density, etc, there wasn't quite enough stars for that to explain all the mass
we observe. However, people aren't absolutely certain what percentage of
dark matter is comprised of these things, because we just don't have all the
initial conditions known that well. (These are called MACHOs-
massive compact halo object- they're made out of baryons.) However, as
said, most people feel like there is not enough of
these to account for all the observed mass we see. Hence, people have
started asking about whether there's other stuff out there that's not baryonic.
While leptons tend to have relatively small mass (electrons, taus, muons, than their neutrinos), the most uncreative
proposal is that there is, for each family, a type of massive neutrino, and
these provide the missing mass. Ignoring
this, we think we have a good idea of the simplest combination of things in the
standard model- and complex combinations of quarks and antiquarks decay
quickly, so it's UNLIKELY that there's some extremely stable complex configuration
of quarks that was formed in such a HUGE quantity compared the lower energy,
stabler configurations we can make. Hence, most people would introduce a
new elementary particle,which
wouldn't participate in the electromagnetic interaction, but would in
gravitational. A lot of people have used non-standard model particles
that are theorized as solutions to particle physics issues (for example, the axion is supposed to solve the strong-CP problem in quantum
chromodynamics). WIMPs- weakly interacting
massive particles (named before the MACHOs, in case
you were wondering) are predicted by supersymmetry, which is an extension to
the standard model (which predicts lots of particles that haven't been observed
yet). So, while dark matter, other than MACHOs,
is still unknown, it's also not really as "overthrow physics" as the
author makes it, because the standard model has a lot of room to accommodate new
particles. In fact, there are a lot of particles that we don't see much
(if at all) on earth...hmmm...reminds me of helium once upon a time.
Dark energy-
This is the more exciting part. So we think that the universe’s expansion
is increasing...so what's pushing it? That's the question we don't have real
answer for. Dark energy- energy does work, and pushing planets and stars
and dark matter requires doing work- has been proposed as a shorthand for what
it is. However, we have at least one candidate- Einstein's infamous cosmological
constant, which he introduced to make the size of the universe constant- to
stop it from expanding. In the end, he decided it was the worst mistake
of his life, but if we switch the sign, it describes an increasing acceleration
for the universe. However, this would be a constant energy that homogenously
fills space, which we're not sure that dark energy is. The model with this
constant in is the Lambda-CDM model. In this case, it's LIKE (oversimplification
coming) if we found out that
changing slowly, or from one place to another. This is taken really
seriously by the physicists, so I suspect there's probably a really strong
mathematical formulation of it somewhere, because it certainly doesn't have much
of a concrete interpretation. However, other things can predict similar
phenomena. While supersymmetry demands that the cosmological constant be
zero, other ideas have come from string theory, brane
theory (the universe is a brane vibrating in a bigger
brane, and those bigger branes
attract stuff in our universe, hence the outward expansion), and something
called a holographic principles from quantum gravity. Also, the more
conservative physicists think that this shows that some of our assumptions that
things are negligible fail, but about the only of these hypotheses still standing
is that "this has something to do with the electroweak symmetry breaking
that happened early in the universe". This relates to the Higgs mechanism
and Higgs boson, which they are looking for at LHC. (The Higgs boson is
also what gives things mass.)
The interesting thing is that this
COULD require a change in our idea of gravitation- such as the cosmological
constant, which, since gravity is physics favorite subject since