17th April 2011 Cosmic Inflation

Cosmic inflation is a hypothesis which explains

• why the universe appears to be flat
• why it is much bigger than is possible due to the restriction of nothing travelling faster than the speed of light
• why  the universe has the same properties throughout - temperature, density, average galaxy count.

Alan Guth came up with the idea

Imagine a Universe with different conditions everywhere. Some regions might be expanding, some might be contracting, and some might be stationary. Some might have positive curvature, some might have negative curvature, and some might be flat. Some might have a lot of matter, some might have little to none. Some could be very, very hot, and some could be practically at absolute zero. 

In other words, it doesn't matter what your initial conditions are to the Universe. What matters is that, at one point in space, in one of these regions, the right conditions to have inflation exist. Inflation takes this one region of space and expands it exponentially. [It Started with a Bang: The Greatest Story Ever Told -- 01 -- Before the Big Bang]

 We don't know what caused inflation to start and stop but there is a theory of a particle called the inflaton.
[The Greatest Story Ever Told -- 02 -- The Graceful Exit] We also don't know how it went from a sparcely populated universe with maybe a proton per galaxy to what it is today.

Cosmic Journeys: How Large is the Universe?

3rd April 2011 Nucleosynthesis in the Big Bang

Today, I've been digging a little deeper into nucleosynthesis, and looking at exactly how each element was thought to have formed in the Big Bang.  Using this information


 from Wikipedia's page on nucleosynthesis, I drew the following diagram.  Where a neutron or proton is produced, it is written by the side of the fusion lines.  The short star pattern of lines is gamma radiation. Where a proton or neutron is used in the reaction, it is joined by a line.

 

As to the question posed in the diagram, this post discusses it but doesn't seem to come to any conclusion.

2nd April 2011 Formation of Elements

Hydrogen, Helium, Lithium, Beryllium

The vast majority of hydrogen and helium are believed to have been formed in the Big Bang.

Protium, that is, ¹H is also formed  produced by spallation. Both helium-4 and protium  are formed by radioactive decay; protium by proton and neutron emission and helium as alpha particles. Helium is also formed in stars by the fusion of hydrogen.

Stars are clouds of hydrogen and helium which have collapsed under their own gravity.  At the core of the star, the atoms are under high temperature and pressure, and in these conditions can fuse together.

When our sun has run out, in 4-5 billion years, of hydrogen  to fuse into helium, it will collapse forming a white dwarf star, and then die.

The nuclei of these elements, along with some ⁷Li, and ⁷Be are believed to have been formed when the universe was between 100 and 300 seconds old. Because all this happened in a very short period of time, only the lightest of elements could form, that is, hydrogen, helium, lithium, beryllium and maybe boron.

The Elements up to & including Iron

The formation of elements other than isotopes of hydrogen and helium , and a small scattering of lithium, occurs in very massive stars.  When these stars run out of hydrogen at their core, they contract down which means their temperature increases and the helium can start to fuse together, gradually forming heavier and heavier atoms with successive collapses. This forms rings of different elements being compressed in to the core.

Image from Wikipaedia

The elements produced are those up to and including iron in the periodic table, that is, helium, lithium, beryllium, boron, carbon, nitrogen, oxygen, fluorine, neon, sodium, magnesium, aluminium, silicon, phosphorus, sulphur, chlorine, argon, potassium, calcium, tin, titanium, vanadium, chromium, manganese, iron.

Why does it stop at iron?

It's to do with something called binding energy.  This binding energy is calculated, using conservation of matter, from the mass deficit which is the amount of mass reduction when the atom forms compared with its component parts, namely (mass of individual protons and neutrons)- (mass of  atom).  The graph shows the average binding energy per nucleon (neutron or proton in the nucleus).

The Average Binding Energy per Nucleon

Because iron is at the peak of the graph, fusing iron atoms together doesn't release any additional energy, and furthermore it requires energy to occur. (Why can't two atoms of the left combine to jump past iron? Must be something like this: the gradients are such that (the binding energy per nucleon on left at x=a) > (binding energy per nucleon at x =2a) )

The Elements after Iron

Iron is the end of the process for energy producing fusion, since fusing iron atoms does not produce any energy for the star. Since the core is no longer producing outward thermal pressure, but gravity is still compressing things in, if the star has a mass of less than 5 times the mass of our sun, it collapses into a dense ball of neutrons.  This is called a neutron star and can be as little as 10km across. When the implosion is stopped by the neutrons, matter bounces off the iron core turning the implosion into an explosion and thus creating a type II supernova.  This explosion blasts the outer regions of the star into space.  It is thought that the heavier elements are formed in this explosion.

Note that if the mass of the core is greater than 5 times the mass of our sun, the neutrons can not stop the implosion and a black hole is formed.

Sources

1. Diamonds in the Sky - Universal Alchemy - a brief overview of the topic
2. Eureka: The Death of Stars - Balancing energy from fusion against gravity
3. Sxity Symbols: Element Creation - video explaning the formation of elements in the life of a star
4. Nuclear Binding Energy - explains the energy from fusion and why iron is significant
5. Supernovae - Types of supernovae, and what happens when they die
6. Type II supernova - the life of a star
7. Gravitational Energy
8. Nucleosynthesis 

1st April 2011 Big Bang Theory

 Yesterday's Starts with a Bang is about the Big Bang Theory. It ties together the stuff I've been reading about the alpha beta gamma paper, red shift and blue shift and some theories about the universe.  These are some possible explanations for the observed red shift.

• light gets tired, and simply loses energy over time,
• the Universe oscillates, contracting and expanding over time, and we are simply close to an expanding portion of it,
• physical constants, such as the speed of light, or the gravitational constant, have changed over time,
• the Universe grows steadily and evenly, and creates new matter as it expands, 
• the Universe is rotating rapidly, and that the galaxies that are moving away from us more quickly also have large -- unobservable -- translational motions
• the Universe was expanding, and that the expansion rate was faster in the past! As the Universe moves forwards in time, it cools, expands, and slows down.

The latter leads to the following prediction:

• In the beginning it was so hot the neutrons, protons and electrons couldn't stick together.
• It cooled some and the protons and neutrons got together to form deuterium, helium-3 and helium-4. 

When the cosmic microwave background was discovered, it could be explained as the left-over glow from the Big Bang, supporting this idea.