The Big Bang Astrophysics Lesson 18

Learning Objectives To know:-  What is the big bang theory  What is the evidence supporting it including:-  Cosmological microwave background radiation.  The relative abundance of H and He.

Homework  Revise for the Exam! We have only one lesson on quasars left to cover after easter.

Expansion of the Universe The expansion of the universe implies that at the start of the expansion all galaxies, all radiation and matter must have been concentrated as a single point.  Hubble’s law implies that the universe is finite.  Cannot see light from galaxies further than 13 billion light years away: limit of observable universe.

The Big Bang

Olbers’ Paradox If the universe were infinite and contained stars spread more or less evenly throughout, then everywhere we look would intercept light coming from some star and so the sky would appear uniformly bright.  Speed of light is finite  Evidence that the universe must have had a beginning.

Olbers’ Paradox

The Big Bang If all the matter and radiation were concentrated at a single point about 13 billion years ago then an explosion must have occurred to start the expansion  this is known as the Big Bang. The Hot Big Bang Theory:- The universe started as an infinitely hot, infinitely dense point (a singularity) and has been expanding ever since.

Evidence for the Big Bang There are 3 strong pieces of evidence supporting the big bang theory:-  The expansion of the universe (cosmological redshift)  Hubble’s Law.  The Cosmic Microwave Background.  The relative abundance of H and He.

Cosmic Microwave Background Radiation The Hot Big Bang model predicts that a lot of e-m radiation was produced in the very early universe. This radiation is still observed today, redshifted to microwave wavelengths. Was observed by Penzias & Wilson in the 1960s.

Cosmic Microwave Background Radiation Can be thought of as the radiation released when the universe cooled sufficiently for matter and radiation to ‘decouple’, with the combination of protons and electrons to form neutral atoms. Matter and antimatter annihilating? Symmetry breaking.

Cosmic Background Explorer (COBE) COBE was sent into space in the 1980s to observe the CMB radiation. If found a perfect blackbody spectrum corresponding to a temperature of 2.73 K. The small fluctuations in temperature are thought to be the “seeds” of galaxies (WMAP).

Background Temperature Map with the map including the dipole and Galaxy on the top, Due to motion of Earth relative to distance matter in the universe. Signals ~1/1000 th of brightness of the sky.  the map with dipole removed.  the reduced map (dipole and galaxy subtracted)

The cosmic microwave background fluctuations are extremely faint, only one part in 100,000 compared to the 2.73 degree Kelvin average temperature of the radiation field.

Abundance of H and He

The ratio of hydrogen to helium in the universe is observed to be about 3:1 and spread evenly throughout the universe. This rather large abundance of helium can be explained by the big bang model. The early universe was hot enough at some point for hydrogen to fuse into helium. But then cooled too quickly for more heavy elements to form.

Summary of Evidence  Hubble’s law implies the universe started as an infinitely dense point.  The big bang theory correctly predicted the relative abundance of H and He in the universe.  The cosmic microwave background is though to be the background radiation from soon after the big bang.