Surprisingly, only about one sixth of all the matter in the universe can be accounted for through observation. For example, when astronomers measure the hot gas clouds in the outer portion of galaxies they can calculate how much gravity that galaxy must have in order to keep the gas from escaping. Comparing that with how much matter is actually observed in the galaxy they come up short by a factor of five or six. This undetectable mass is called dark matter. Some of this dark matter is in the form of very dim dwarf stars, cold gas clouds, unlit planets or even black holes. These are called MACHOs (MAssive Compact Halo Objects). The halo refers to the outer part of a galaxy that extends beyond what is visible. Think of a large planet like Jupiter, for example. It can only be seen because it reflects the light of the Sun. If Jupiter was set adrift through interstellar space it would only be indirectly observable by its gravitational effect. When a MACHO passes in front of a background star, its gravity bends the starlight around it which causes the star to briefly shine much brighter. This focusing of the star’s light is called gravitational lensing. Astronomers have searched the skies for evidence of MACHOs by gravitational lensing, and it does not account sufficiently for the missing mass.
Another type of dark matter consists of theoretical particles which are yet to be detected. They are called WIMPs (Weakly Interacting Massive Particles), and astrophysicists are hoping to bridge the missing matter gap by finding evidence of their existence. Because WIMPs only interact through gravitational and weak forces, they are quite difficult to detect. Billions of them could pass through the Earth every second without leaving a trace. There are many experiments currently underway to detect WIMPs either directly or indirectly.
According to the Big Crunch theory, the universe
will end in an infinitely dense singularity.
There is much we don’t know about dark matter, yet the eventual fate of the universe depends on how much of it there is. We know that the universe has been expanding since the Big Bang, but will it continue to do so forever? That depends on its total mass which counteracts the expansion through gravity. With enough dark matter it will eventually collapse back in a Big Crunch. Otherwise, our universe will keep expanding forever and likely end in a Big Freeze. But don’t worry too much—either fate is at least 15 billion years away.
1) True or false: Most of the matter in the universe is dark matter.
2) MACHOs can be detected by their _________________.
a) reflection b) magnetic field c) spectral signature d) gravitational effects
3) True or false: MACHOs are theoretical particles.
4) WIMP stands for __________.
a) Weakly Involved Massive Planets b) Wandering Interactive Micro Particles c) Weakly Interacting Massive Particles d) Wandering Irradiated Micro Planets
5) Two eventual fates of the universe are ________________.