# The Big Bang Model - Chapter 12

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• The Big Bang Model of the Universe
• Hubble's Ideas
• Recessional Velocity is Proportional to Distance
• The spectra from galaxies all show redshift.
• The amount of redshift gives the recessional velocity.
• HUBBLES LAW: v=H0d where v = recessional velocity,      H0 = Hubble's Constant,      d = distance
• UNITS:          v = km/s        d = Mpc      H0 = (km/s) / (Mpc)
• Mpc is a megaparsec. 1Mpc = 3.09E+22m
• SI Unit for H0 is 1/s therefore...       v = m/s       d=m
• When Hubble plotted the recessional velocity of galaxies against their distance from Earth (Cepheid Variables) he found that they were proportional.
• The Universe is Expanding
• By showing that objects in the Universe are moving away from each other, Hubble's work is strong evidence that the Universe is expanding.
• The rate of expansion depends on the value of H0.
• The way cosmologists tend to look at this, the galaxies aren't actually moving through space away from us.
• Instead, space itself is expanding and light waves are being stretched along with it.
• This is called cosmological redshift to distinguish it from redshift produced by sources that are moving through space.
• The Universe is expanding uniformly away from us, creating the illusion that we are at the centre of the universe.
• The Age and Observable Size of the Universe depend on H0.
• If the Universe has been expanding at the same rate for its whole life, the age of the Universe is                    t = 1/H0
• This is only an estimate as it is unlikely that the universe has been expanding at the same rate.
• Since no on know the exact value of H0, we can only guess the universe's age.
• Approx. 14 billion years old.
• The absolute size of the Universe is unknown. There is a limit on the observable size of the Universe.
• This is a sphere, where radius = maximum distance that light can travel during its age (radius approx. 14 billion light years).
• Evidence: The Red Shift  of Galaxies
• If you assume that the Universe has always been expanding, if you trace back far enough it can be implied that the universe must once have been contained in a single point.
• Evidence: Cosmic Microwave Background Radiation
• Properties of the Cosmic Microwave Background Radiation
• Cosmic Background Explorer set up to look at spectrum.
• Continuous spectrum corresponding to a temperature of 2.73k
• Radiation is ISOTROPIC AND HOMOGENOUS - same intensity whichever direction you look at it
• Very tiny fluctuations in temperature, at the limit of COBE detection.
• Due to tiny energy density variations in the early universe and are needed for the initial seeding of galaxy formation.
• Also shows a Doppler Shift
• Milky Way is rushing towards an unknown mass (GREAT ATTRACTOR) at over a million miles per hour.
• Penzias and Wilson, 1960's
• HBBM predicts that EM radiation was produced in the very early Universe. This radiation should still be observed today.
• Because the Universe has expanded,, the wavelengths have been stretched and are in the microwave region.
• The HBB (HOT BIG BANG MODEL)
• The Universe started off very hot and very dense (perhaps as an infinitely hot, infinitely dense singularity)and has been expanded.
• Widely accepted
• Since Hubble's findings were published,the redshifts of other objects have been measured and fit with his predictions.
• Evidence: the amount of Helium in the Universe
• The HBB Model also explains the large amount of helium in the Universe.
• The early Universe had to have been very hot and at some point it must have been hot enough for hydrogen fusion to occur.
• This means that, together with the theory of the synthesis of heavier elements in stars, the relative abundances of all of the elements can be accounted for.