Big Crunch

     The Big Crunch Theory does not differ from the Big Bang Theory, rather it relies on the Big Bang Theory.  It gives a prediction for the end and possible rebirth of the universe.  According the the article, "How the Big Crunch Theory Works," the universe is either open (expanding forever), flat (coasting from the energy of the big bang), or closed (expanding then coming back into itself).  The Big Crunch will happen if the universe is closed.  Once it reaches maximum expansion, it will start recoiling and collapse on itself, and the universe will end in hot, dense singularity.
     Gravity and density are the deciding factors as to whether the universe will expand or retract.  If the gravity within the universe ins strong enough, it could pull the universe to contract.  Also, if the density of the universe is greater than the critical limit, the universe will contract.  The reason scientists believe the universe is closed and the Big Crunch will happen is because dark matter, which cannot be seen, may be adding enough force to bring the universe back onto itself. However, dark energy may be speeding up the expansion.
     The problem with the Big Crunch Theory is that dark matter and dark energy cannot be seen and can only be estimated.

Ekpyrotic Theory

     The Ekpyrotic Theory gives the Big Bang Theory an alternate beginning.  According the the Ekpyrotic Theory, the universe arose from the collision of two three-dimensional worlds in a space with a fourth spatial dimension.  Instead of the universe beginning at infinite temperature and density, this theory suggests that the universe was cold and vacuous billions of years ago, and the collision made the universe hot and expanding. The theory does not cancel out the Big Bang Theory; it just offers a different beginning.  It fixes some problems with the Big Bang, specifically the magnetic monopoles. Monopoles are over produced in the Big Bang; with this theory, they are not produced at all.  The issue with this theory is that it is still in the works.  The article says, "Our proposal is based on unproven ideas in string theory and is brand new."  It still needs a lot of work to be proven and accepted.

Big Twang Theory

    The Big Twang Theory is a compromise between the Big Bang Theory and the String Theory.  It argues that the Big Bang did in fact form the universe and drove it into expansion, but it also argues that the universe expanded by vibration energies, a wave front of cosmic sound.  The "multi-dimensional harmonics account for the multivarient phenomena we perceive at all scales, e.g. galaxies, planets, atoms, and their interactions." In simple terms, the Big Twang Theory describes the formation of the universe as "not born of violence, but of music." The problem with this theory is that it is not well supported or credited by scientists.

Big Bang Theory

     The Big Bang Theory is a widely accepted theory that the universe began from an initial atom and has expanded over billions of years.  The Big Bang Theory discredited the Steady State Theory.  The Inflation Theory is a component of the Big Bang; it addresses the early moments of the universe following the Big Bang.  It suggests that the universe was created in an unstable energy state, so rapid expansion took place in the early stages of the earth's formation. Another element of the theory, CMB Radiation, is the remnant energy left over from the Big Bang.   There are some unaswered questions regarding the Big Bag Theory still.  The most frequent concerns are what existed before the Big Bang and what caused the Big Bang. There were some issues with Inflation Theory, such as homogeneity, flatness, and the overproduction of magnetic monopoles.  These issues have been potentially solved through various experiments. 

Steady State Theory

     The Steady State Theory was considered by George Gamow whose thoughts challenged those of the Big Bang Theory.  The back and forth triumphs and failures of both theories are demostrated in the article, "Big Bang or Steady State."  Gamow's work was followed up by Fred Hoyle, who fought to defend the Steady State Theory.  He claimed that the Big Bang theory, Steady State's rival, was a ridiculous concept.  The basic argument of the Steady State theory is that the universe was unchanging yet dynamic, like a smoothly flowing river.  The unchanging earth was dynamic because matter was continuously being created.  There were challenges that the Steady State Theory overcame and some for which the theory offered no answer.  The main obstacle that discredited the Steady State Theory was the idea of radiation.  Gamow had argued that the universe would be full of radiation and energy, but two scientists, Alpher and Herman, discovered that the temperature of the universe would now be about 5 degrees Kelvin.  Arno Penzias and Robert Wilson conducted  a similar experiment with microwave radiation which discredited the Steady State Theory, although it was already largely discredited.