Weighing the Entire Universe: Dark Matter and Dark Energy
Counting all the electrons and all the protons in the Universe
sounds like a daunting task. MAP will be able to get a pretty good estimate
and, in addition, determine the ratio of "ordinary" matter to dark matter
and a mysterious force that scientists call dark energy. There's more to
the Universe than meets the eye. In fact, some scientists think that all
we can see is really only 3% of the entire Universe. Just what exactly is
Ordinary matter is the stuff that we are made of, protons, neutrons, and electrons. Stars, planets, trees, animals... everything we can see or detect with telescopes is ordinary matter, which scientists call baryonic matter.
Dark matter is sometimes called exotic matter. We cannot see dark
matter, but we know it is present in the halos of galaxies and in vast
galaxy clusters because of its gravitational effect on ordinary,
baryonic matter. All particles of matter are attracted to each other
through the force of gravity. Dark matter seems to be the "glue"
holding galaxies together with all their ordinary matter, for the mass
of ordinary matter alone is not enough. Galaxies would fly apart if not for the extra, dark mass.
Some dark matter may be baryonic but simply too dim to see with telescopes.
Dark matter may also be exotic, composed of something other than electrons
and protons. While MAP may constrain some of the properties of the dark matter,
its main contribution will likely be accurately measuring the mass density of ordinary matter.
Dark energy refers to the force that is accelerating the expansion of the
Universe. This is an "anti-gravity" force, sometimes referred to as quintessence
or a cosmological constant.
Gravity pulls things together, and this familiar force should act to slow
down the expansion of the Universe. Yet the Universe continues to fly wide
open faster and faster. No one knows why.
So, what will MAP do? MAP will determine the number of electrons and
protons relative to the number of photons, in the cosmic microwave
background. Remember that denser regions in the primordial fog of the early
Universe attracted more matter. Protons rolled in, and photons (radiation
pressure) pushed them back out. This ebb and flow created the vibrations,
or sound waves, that rippled through the fog. The density or thickness of
that fog, as you might imagine, determines how easily sound waves travel.
Now, remember those "bumps" in the cosmic microwave background
? These are peaks in temperature differences from region to region. Just
as the position of those bumps determine the shape of the Universe, the size
and distance of the bumps relative to each other tell the relative concentrations
of electrons, protons and photons. (The number of electrons and protons
are more or less equal, which is why the Universe doesn't have an electrical
Bumps in the cosmic microwave background also speak of dark energy. If
the Universe is flat and composed entirely of matter, then photons will behave
in a specific way. Photons, like matter, are attracted by gravity. Einstein
described gravity as a well or dent in the fabric of spacetime. Imagine
a photon rolling into a well. It gains energy going in and loses energy
rolling out. In a flat universe with only matter contributing to the force
of gravity, the net energy difference is zero. Roll in, gain; roll out,
The path of light being bent by the
gravitational field of a massive object.
If there is dark energy acting in an opposite way to gravity, the
depth of those gravitational wells become less deep over time. Matter forms
the gravitational well, but anti-gravity forces slowly smooth out the well
by pulling matter out. Now, when the photon rolls into the well, it has
a shorter climb out. The photon picks up energy going in and loses just
a little energy coming out. The net energy difference is positive.
The denser regions in that early fog (seen today as slightly warmer regions
in the microwave background) had gravitational wells. The low-density regions
had "hills" as opposed to wells. Photons roll up the hill and roll down
a slope that is less steep because of the action of dark energy. These photons
lose energy overall.
Now, back to the bumps in the cosmic microwave background. MAP will be
able to trace photon energy loss and energy gain from region to region --
hot spot to hot spot or cold spot to cold spot. If there is dark energy,
MAP would see its effect on the energy of microwave photons at various temperatures
and angular scales. That is, MAP may detect properties in the microwave
light that suggest the influence of dark energy. The greater the influence,
the higher the contribution of dark energy to the total mass-energy content
of the Universe.
Earlier experiments have suggested that the Universe is about 5% ordinary
matter, 30% dark matter, and 65% dark energy. If the stuff we can see is
only 1 to 5% of what's out there, the Universe may be even more fantastic
than we ever imagined.