The Big Bang The universe appears to be expanding – cosmic background radiation and red shift.

Olbers’ Paradox 1826 The universe is infinite and therefore contains an infinite number of stars. Whatever direction we look in, we should see a star in our line of sight. So the universe should look uniformly bright in all directions.

Heinrich Olbers 1758-1840 His theory contradicted Newton’s idea of an infinite, static universe. What a legend!

So the universe cannot be infinite. This was ‘solved’ by the author Edgar Allan Poe in 1848. He argued that the universe was so massive that the light from the most distant stars had to travel rather a long way and hasn’t reached us yet.

Poe’s argument - verbatim “Were the succession of stars endless, then the background of the sky would present us a uniform luminosity, like that displayed by the Galaxy –since there could be absolutely no point, in all that background, at which would not exist a star. The only mode, therefore, in which, under such a state of affairs, we could comprehend the voids which our telescopes find in innumerable directions, would be by supposing the distance of the invisible background so immense that no ray from it has yet been able to reach us at all.”

Edgar Allan Poe 1809-1849 A legend of literature. Not of cosmology.

The cosmological principle The universe is: Homogenous -that matter is distributed evenly on a large scale. Isotropic - uniform in all directions on a large scale. Universal – the laws of physics are the same at all points.

Homogenous vs isotropic

Homo vs iso Homogenous – of uniform density Isotopic – of uniform structure

On left iso on right homo

Homogenous Homogeneous (usually pronounced homo-GEE-nee-us) literally means "to be the same throughout," no matter where you are in the universe. If you look at the universe from Earth or from a galaxy a million light-years away, it will look the same.

Isotropic Isotropic (pronounced eye-so-TRO-pic) means to appear the same in every direction or viewing angle. This approximation breaks down when viewing the night sky from Earth since our planet is located inside of the Milky Way, but if you were able to stand at any point outside of a galaxy the universe would look the same in all directions.

Cosmological principle What this means is that we are not in a special place in the universe. We are in an insignificant part of an insignificant galaxy. We are not at the centre of the universe as the medieval church believed.

Luckily what we observe about the universe seems to fit the cosmological principle We have no reason to suspect that the laws of physics are different in different galaxies. Light from distant galaxies appears as we’d expect. On a large scale (Hubble deep field) the universe appears fairly uniform.

The Doppler effect Often experienced when a distant siren is approaching. The pitch of the siren: Is higher during approach. Normal at the point of passing. Is lower during the recession.

Doppler effect as applied to light The more distant galaxies appear to be redder than we would expect. They must be moving away from us. The further away they are, the faster they are receding.

A few galaxies appear blue. For example, the andromeda galaxy. Blue shifted and moving towards us.

Red Shift Distant galaxies appear to be redder than expected. The light is shifted towards the red end of the spectrum 

Doppler effect maths. When v<<c Where f’ is the observed frequency f is the emitted frequency c is the speed of light v is the relative speed (when v is positive, the galaxy is moving further away.)

Doppler effect maths Also we can use: Where Δλ is the change in wavelength. For these equations, if v is positive, the object is moving away – receding. This formula is on the formula sheet.

Example A galaxy shows a redshift from 500 nm to 515 nm. What is the redshift? What is the speed at which this galaxy is moving?

Moving away, red, moving towards blue.

Redshift Spectra from The Sun a distant galaxy

Edwin Hubble 1889-1953 Proved that the universe was expanding in 1929. Measured the distances and the redshift of distant galaxies. Had he not died in ’53 he would probably been awarded the Nobel prize that year. It is fitting that the Hubble telescope is named after him. What a legend!

Example Hubble observed a galaxy 0.50 Mpc away and estimated its recessional velocity to be 290 km/s from its redshift. Use this data to calculate the change in frequency and wavelength for 500 nm green light.

Hubble’s Law The speed of recession is proportional to the distance away. v = H0d Current value for H0 is 74.2 +/- 3.6 kms-1Mpc-1

Age of the Universe Using this most precise value of the Hubble constant we get an age of the universe to be 13.8 billion years. The value is likely to be modified in future as more powerful telescopes look further back in space and time.

Cosmic Microwave Background Radiation If we look at the spaces between stars with a telescope in the optical band we see nothing but darkness. If we use a telescope tuned into the radio/microwave bands we get a very faint, near uniform ‘glow’ – the cosmic background.

Cosmic Background

The cosmic background was discovered in 1964 by Arno Penzias and Robert Wilson What legends 

Cosmic Background Is at a measured temperature of about 2.7 K and emits as a black body at 1.9 mm wavelength. This is in the microwave region.

How does Cosmic Background help explain the big bang? If the universe was initially small, hot and dense then as it expanded it would have cooled. Intense radiation left over from the big bang has increased in wavelength and become the cosmic background that we see now.

The Big Bang Redshift  the universe is expanding. Cosmic background radiation  can only be explained by an initially hot and dense universe, expanding to what we see now. The big bang!

The Big Bang Is not a ‘bang’ in the conventional sense but a rapid expansion powered by the internal energy of the big bang. Everything in our universe once existed as a singularity. Why this is will probably never be unravelled and certainly not in our lifetimes.

The fate of the universe. We are sure that the universe is expanding  redshift. We are fairly confident that it started as a singularity  cosmic background radiation. What will happen to it in the future?

The universe’s fate depends on its mass. Its internal energy drives it outward. Gravitation attempts to haul the universe back to its starting point. Whether gravity succeeds depends upon the gravitational force and so the mass.

Fate of the universe. The mass of the universe is not at all well known. There seems to be dark matter – detected by gravitation but not emitting radiation. Knowing the mass of the universe is vital to understanding its fate.

Critical density of the universe –open universe model If the overall density of the universe is less than the critical density, the universe will expand forever, not being pulled back in by gravity. Eventually it will cool down to 0K and suffer thermodynamic heat death  no work can be done since there is no source of energy. 

Density of the universe equals the critical density – flat universe The rate of expansion will slow and approach a maximum size. The speed of the galaxies tends towards zero. It is in stasis.

Density of the universe is greater than the critical density – closed universe The gravitational force pulls the universe back to its starting point. The universe, currently expanding will slow and the collapse back on itself in a big crunch. Maybe the whole process starts over again?

3 possibilities for the fate of the universe.

Critical density of the universe The density of the universe is currently believed to be close to the critical value of the universe meaning it may asymptotically approach a maximum size. The universe may well be flat. From about 7.5 billion years ago the rate of expansion seems to have increased  dark matter?