BIG BANG SUMMARY NOTES

Transcription

1 BIG BANG SUMMARY NOTES

2 BIG BANG THEORY Studies of red-shifts of distant galaxies show that the universe is expanding. This and other observations has led to the Big Bang Theory The Big Bang Theory claims that the universe has expanded from a very dense, very hot state that existed at some time in the past.

3 Big Bang Model Running the expansion backward allows us to calculate the temperature and density of the universe during its earliest moments. The known laws of physics can be used to determine the behavior of matter and energy at these temperatures and densities. The model is then used to make predictions that can be compared to observations.

4 Where did matter in the universe come from? E = mc 2 Mass is just condensed energy Energy mass Albert Einstein A particle antiparticle pair can be created if the available energy equals the mass of both particles times the speed of light squared A very BIG number!

5 BIG BANG TIME LINE

6 BIG BANG MODEL We will begin our discussion at about one millionth of second after the universe began its expansion. It is at this time that the universe had cooled enough for protons and neutrons, the building blocks of matter, to exist as individual particles.

7 ABOUT A MILLIONTH OF A SECOND Temperature is about K (ten trillion Kelvin) a lot of energy Protons, anti-protons, neutrons and anti-neutrons begin to form As a proton or neutron collides with its anti-particle they annihilate and are converted to energy in the form of photons neutron proton anti-proton anti-neutron

8 ABOUT A MILLIONTH OF A SECOND Because of the large amount of energy available, as fast as these particles annihilate, new protons, anti-protons, neutrons and anti-neutrons form A billion and one protons and neutrons form for every billion anti-protons and anti-neutrons protons anti-protons + 1 billion 1 billion

9 About one ten-thousandth of a second... Temperature has fallen to about K (one trillion Kelvin) It is no longer hot enough to produce protons and anti-protons (or neutrons and anti-neutrons) spontaneously from pure energy to replace those that annihilate each other. Almost all particles and anti-particles annihilate and produce gamma ray photons. anti-proton proton

10 ONE TEN-THOUSANDTH OF A SECOND... CONTINUED Annihilation results in a billion photons for every proton or neutron Photons are constantly scattered by free particles with an electric charge like electrons or protons These photons increase in wavelength as the universe expands and will eventually become the majority of photons that make up the cosmic background radiation Immediately after annihilation there are equal numbers of protons and neutrons

11 ONE TEN-THOUSANDTH OF A SECOND... CONTINUED High energy collisions between protons, neutrons and other particles like electrons can transform one particle into another. These constantly occurring reactions that transform protons and neutrons into each other initially maintain equal numbers of protons and neutrons... p + + e n + e n + e + p + + e

12 ... however, the mass of a proton is slightly less than the mass of a neutron, so... About a tenth of a second... As the temperature (and available energy) drops, transformation to protons is favored over neutrons About one second Transformation reactions can no longer occur. Neutrons begin to decay into protons n p + + e + e

13 ABOUT 100 SECONDS Temperature is about 10 9 K. Neutron decay results in a 1:7 abundance of neutrons to protons at this point. Universe is now cool enough for protons and neutrons to bind together. This is called fusion. proton deuterium neutron tritium helium This process creates new, heavier atomic nuclei and is called nucleosynthesis.

14 AT THE BEGINNING OF NUCLEOSYNTHESIS protons 2 neutrons At the end of nucleosynthesis hydrogen nuclei 1 helium nucleus Atomic mass = 12 Atomic mass = 4 Mass ratio 75% 25%

15 About 10 minutes... the end of big bang nucleosynthesis After the temperature drops below about 10 9 K (one billion Kelvin), very little happened in nucleosynthesis for a long time as temperature and density are too low for fusion. It required star formation for the production of heavier elements.

16 ABOUT 380,000 YEARS Temperature drops to 3000 K Universe is cool enough for electrons to bind with nuclei and form stable atoms H He With most electrons now bound in atoms, photons can travel large distances without being scattered by free electrons. Photons now travel in all directions, resulting in what is called the cosmic background radiation.

17 NOW With continued expansion, temperature drops to about 3 K (Three degrees above absolute zero) Photons that make up the cosmic background radiation are now microwaves most of these photons were produced by the particle antiparticle annihilation at about one tenthousandth of a second

18 Big Bang Model Predictions The only elements in the early universe were hydrogen and helium (and a tiny amount of lithium). The hydrogen-helium mass ratio was about 75-25%. Microwaves with an energy corresponding to a temperature of about 3 K will be found everywhere in space. From Earth they will be seen across the entire sky.