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You, the computer you’re sitting at, the air you breathe, even the distant stars are all made up of protons electrons and neutrons. For a long time this ordinary matter, or what physicists like to call baryonic matter, was thought to be the main constitute of the universe. However, in the past twenty years evidence has been accumulating to the contrary, that in fact the universe is much stranger than ever thought of before and is almost entirely made up of something that we can’t see.
For a long time astronomers weren’t concerned about the mass of objects that they couldn’t see. For example the earth is too small and dim to see from any great distance and all the planets in out solar system make up less than one percent of the total mass of the sun. However it soon became a concern when astronomers began to measure the mass of galactic clusters and it became apparent that there was a significant amount of matter unaccounted for.
In the thirties, astronomers named Zwicky and Smith both examined closely two relatively nearby clusters, the Coma cluster and the Virgo cluster. They looked at the individual galaxies making up the clusters individually, and the velocities of the clusters. What they found was that the velocities of the galaxies were about a factor of ten to one hundred larger than they expected.
In a cluster the main force is the gravitational pull of the galaxies on one another which gives rise to their velocities. By knowing the velocities of the galaxies the total mass of the cluster can be determined.
If your web browser is Java-aware -- e.g., Netscape 2.0b or higher, try this experiment. It allows you to vary the mass inside a galaxy cluster, and watch the individual galaxies.
( Courtesy of John's Homepage http://www.astro.queensu.ca/~dursi/dm-tutorial/dm1.html)
Now like all observations there is a certain amount of error involved. In this case, watching the galaxies in a cluster takes years of observation and the velocities are hard to determine due to the expanse of the cluster. It’s not like the experiment were the dots are whizzing around. Also some of the galaxies measured may not be in the cluster but are just in the line of site of the telescope.
So even as strong as this evidence seemed to appear it was mostly ignored due to the errors of the observation.
Forty years later people like, Rubin, Freeman, Peebles, started to measure the rotation curves of galaxies to determine their mass. Masses such as galaxies spin around their center and follow Kepler’s Laws for orbits about a center axis. These laws state that the rotational velocity around the center depends only on the distance to the center, and the total mass that is contained within the orbit.
Here is another experiment. This one allows you to vary the mass inside an orbit, which changes the rotation velocities.
( Again courtesy of John's Homepage http://www.astro.queensu.ca/~dursi/dm-tutorial/dm2.html)
The mass discrepancy found through observing the velocities and rotations of galaxies has been confirmed through gravitational lensing, the bending of light predicted by Einstein's theory of general relativity.
By measuring how the background galaxies are distorted by the foreground cluster, astronomers can measure the mass in the cluster. The mass in the cluster is more than five times larger than the inferred mass in visible stars, gas and dust.
Non-Baryonic dark matter comes in two type; hot dark matter and cold dark matter. Hot dark matter (HDM) is thought to be made up of particles traveling near the speed of light and possessing little or no mass and have zero charge. One likely candidate is the neutrino. A neutrino, until recently, was thought to be a massless particle, but experiments in Japan and elsewhere have determined that some types of neutrinos may actually have between one million and one thousandth the mass of an electron. These tiny particles are theoretically zipping around everywhere in the universe and could account for a large part of the missing dark matter but unfortunately they very rarely interact with baryonic matter and have never actually been detected.
Cold dark matter (CDM) on the other hand is made up of WIMPs (Weakly Interacting Massive Particles). Scientists have postulated the existence of photinos--partners of photons--with an expected mass 10 to 100 times that of a proton; axions, carriers of force that have mass, or even quarknuggets, odd non-baryonic aggregates of quarks. None of these particles have been detected, either in space or in particle accelerators.
Baryonic dark matter is any ordinary matter, matter that is made of protons, electrons and neutrons, and doesn’t emit any radiation that is currently detectable on earth. One source of such matter is the primordial helium and hydrogen that permeates the universe. It is believed that this primordial matter equals or exceeds all other measured baryonic matter.
MACHOs (Massive Compact Halo Objects) are another source baryonic matter that does not emit light and could exist in huge numbers in the halos surrounding galaxies. These can be brown dwarfs, masses of hydrogen that is held together by gravity but it not massive enough to start a nuclear reaction and therefore doesn’t emit radiation. Others can be planets that are not quiet large enough to start a nuclear reaction, or even black holes. So far not enough of these objects have been found to prove or disprove this theory.
Why is knowing how much dark matter is out there important? Well the shape of the universe is dependent on the amount of matter present or what is called the Critical Density of the Universe, denoted by Omega. If Omega is less than one then the universe is thought to be an “open universe” and has negative curvature like a torus. In this scenario the universe will continue to expand forever because there isn’t enough mass present to slow down the outward expansion caused by the Big Bang.
If on the other hand Omega happens to be larger than one, then the universe is said to be a “closed universe” and has positive curvature like that of a sphere. In this scenario the universe will expand to a point then the gravitational pull of the universe will cause it to collapse in on its self into what is called the Big Crunch. But for the special case that the Omega equals one, the universe has no curvature and is said to be flat.
Recent observations from probes such as WMAP, has determined that the universe is flat from which it follows that the mean energy density in the universe is equal to the critical density (within a 2% margin of error). This is equivalent to a mass density of 9.9 x 10 -30 g/cm 3, which is equivalent to only 5.9 protons per cubic meter.
Of this total density, we now know the breakdown to be:
> 4% Atoms, 23% Cold Dark Matter, 73% Dark Energy. Thus 96% of the energy density in the universe is in a form that has never been directly detected in the laboratory. The actual density of atoms is equivalent to roughly 1 proton per 4 cubic meters.
> Fast moving neutrinos do not play any major role in the evolution of structure in the universe. They would have prevented the early clumping of gas in the universe, delaying the emergence of the first stars, in conflict with the new WMAP data.
> The data places new constraints on the Dark Energy. It seems more like a "cosmological constant" than a negative-pressure energy field called "quintessence". But quintessence is not ruled out.
So most of our universe is of a material we know little about, have never seen or experimented with in a laboratory. The universe is a much stranger place than we have ever imagined. Science has barley scratched the surface of existence and the deeper we explore the more we realize that we have no clue.