Most of the Universe is Missing!

by Nicholas Mee on December 16, 2012

Neptune’s existence was originally deduced from its gravitational influence on the planet Uranus. Only later was it observed through a telescope.

It is a rather embarrassing fact that most of the universe seems to be missing!

There are two ways to determine the amount of material in the universe. One way is to measure its gravitational pull, the other way is to measure the amount of light that is being emitted by luminous objects. All objects have a gravitational attraction, but not all objects emit light, so we would expect the first measure to give an answer that is bigger than the second, and it does.

Dark Matter

Before these calculations were possible astronomers thought that most of the mass of the universe would be in the form of stars, which would emit light, so their expectation was that most of the material in the universe would be visible. It turns out however, that most of the material in the universe does not emit light. Astronomers call this stuff dark matter simply because we cannot see it.

Could There Be Some Mistake?

The Sombrero Galaxy
ESO (European Southern Observatory)

The first question is: could the measurements by wrong. Afterall, it can’t be that easy to measure the properties of galaxies that are millions of light years away. But astronomers have been working on this problem for many decades and the measurements are now quite accurate. For instance, the rate of rotation of a spiral galaxy that we view edge-on can be measured from the Doppler shift of the light from its two edges. The starlight from one edge will be red-shifted as the stars move away from us, while the starlight from the other edge will be blue-shifted as they move towards us. The rate of rotation can then be used to deduce the total gravitational mass of the galaxy. It is clear from these measurements that the rate of rotation of galaxies is so high that they would fly apart if they were solely composed of visible material. This implies that there must be additional material that is invisible to us. So there is no doubt that the universe contains a lot of material that we cannot see.

The Moon does not emit light, but it certainly has a gravitational attraction that raises the tides twice a day.

The Big Mystery

The big mystery is: What is it? And this is where everything becomes much more mirky, because all the obvious answers, such as dark gas clouds that have not yet formed stars or burnt out star remnants cannot be the answer. The Big Bang model of the early universe works very well, I am skipping over the evidence here, but there would be serious problems with it if the universe had formed with much more ordinary matter.

MACHOs and WIMPs

The possible sources of dark matter fall into two categories that are referred to as MaCHOs (Massive Compact Halo Objects) and WIMPs (Weakly Interacting Massive Particles). MaCHOs are the burnt out stars that we have just ruled out, so most physicists believe that the dark matter consists of vast quantities of WIMPs. A WIMP is a stable particle that only interacts very weakly with ordinary matter, which would explain why it forms a separate component of the matter in the universe and also why it has not already been discovered by particle accelerators or observed in cosmic rays.

Cosmic Relics

Most particles that are created in particle accelerators are unstable, which means that they rapidly transform into other particles. Eventually the only particles that are left are members of a small collection of stable particles. These include the protons and electrons from which atoms are formed, photons, which are particles of light and neutrinos. It is fortunate that these particles are stable because otherwise there would be no atoms and no light. Neutrinos were created in vast quantities in the early universe and are still being created by stars and supernovae. It was once thought that this might explain the origin of dark matter. However, we now know that the mass of a neutrino is too small to account for all the dark matter. Most physicists now believe that the dark matter is probably formed of another unknown type of stable particle that was produced in large quantities in the very early universe. Finding such a particle is one of the main targets of the research at the Large Hadron Collider.

Supersymmetry

Many physicists believe that the wimpy relic particle will turn out to be the lightest particle predicted by a theory known as ‘supersymmetry’. If they are correct then the LHC may soon make a discovery that is even more important than its discovery of the Higgs boson. I will be writing a brief article about supersymmetry in the near future.

More Information

I have recorded a video related to this article: Spotlight on Dark Matter on The Cosmic Mystery Tour YouTube Channel. Please don’t forget to subscribe to the YouTube channel.

 
For some sensational astronomical images, such as the one of the Sombrero Galaxy above, visit the website of the European Southern Observatory
http://www.eso.org/public/

The relic particle responsible for dark matter might be produced in the proton collisions at the LHC. But it is also being sought by dedicated experiments, such as this Dark Matter Detector in South Dakota:
http://www.ucl.ac.uk/news/news-articles/1211/121115-Dark-matter-detector-installed-and-submerged-in-South-Dakota

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