OPTION E - ASTROPHYSICS E6 Galaxies and the expanding universe Galactic motion

Galaxies – not assessed A galaxy is a collection of a very large number of stars mutually attracting each other through the gravitational force and staying together. The number of stars varies between a few million and hundreds of billions. There approximately 100 billion galaxies in the observable universe. There are three types of galaxies: - Spiral (Milky Way) - Elliptical (M49) - Irregular (Magellanic Clouds) Spirals / barred spiralsEllipticalIrregular Shape Flattened disc, central bulge from which spiral arms start. In barred galaxies, arms start from the ends of the bar. Halo. Spherical or ellipsoidal in shape with stars fairly uniformly distributed in the galaxy. No obvious structure. Star content The disc contains both young and old stars. Halo has mainly old stars. Contain mainly old stars. Contain both young and old stars. Gas and dust The disc contains significant amounts of both. The halo does not. Contain little or no gas and dust. Contain a lot of gas and dust. Star formation Takes place in spiral arms. No significant new star formation in the last 10 billion years. Very significant star formation.

E.6.1 Describe the distribution of galaxies in the universe. Our galaxy, the Milky way, is part of a group of galaxies or cluster called the Local Group, made of about 20 galaxies. The nearest galaxy to us is the Large Magellanic Cloud at a distance of 2x10 6 ly away. The Local group extends over a distance of 1x10 7 ly (10 million ly). Clusters are in turn are grouped into super-clusters – collection of clusters of galaxies. Our Local group belonst to a supercluster of about 15x10 6 pc across.

E.6.1 Describe the distribution of galaxies in the universe.

E.6.2 Explain the red-shift of light from distant galaxies. Due to the expansion of the Universe, the light received from galaxies is red-shifted. The shift in a spectral line from a galaxy emission spectrum is given by: The speed of the galaxy is given by: Where: λ’ is the wavelength measured on Earth λ is the wavelength emitted by the galaxy c is the speed of light in vacuum v is the recession speed of the galaxy E.6.3 Solve problems involving red-shift and the recession speed of galaxies.  see AHL worksheet Q2, 8 and 16.

OPTION E - ASTROPHYSICS E6 Galaxies and the expanding universe Hubble’s Law

Hubble’s Law states that distant galaxies are moving away from the Earth with a speed that is proportional to their distance. Where: d is the distance to the galaxy (in Mpc) v is its recessional speed (in km s -1 ). H is known as Hubble’s constant and its average value is 72 km s -1 Mpc -1 This means that for every megaparsec to a galaxy, the galaxy's speed away from us will increase by 70 kilometers/second. E.6.4 State Hubble’s law.

E.6.5 Discuss the limitations of Hubble’s law. Hubble’s Law has a few limitations: It’s constant in space but varies with time. This means that the rate of expansion of the Universe was not the same throughout its expansion. There are uncertainties in the distances measured precisely because it is quite difficult to measure distances to remote galaxies accurately.

E.6.6 Explain how the Hubble constant may be determined. Hubble’s Law is determined by: Measuring the distance to distant galaxies Measuring their recessional speed using Doppler effect Plotting a grapg of v against distance. Hubble’s constant is equal to the slop of the graph

E.6.7 Explain how the Hubble constant may be used to estimate the age of the universe.

E.6.8 Solve problems involving Hubble’s law.

E.6.9 Explain how the expansion of the universe made possible the formation of light nuclei and atoms. We can “work backwards” and imagine the process that took place soon after the Big Bang. Very soon after the Big Bang, the Universe must have been very hot. As the universe expanded it cooled. It had to cool to a certain temperature before atoms and molecules could be formed. The Universe underwent a short period of huge expansion that would have taken palce from about s after the Big Bang to s. At the very high temperatures of the early universe, only elementary (fundamental) particles could exist and expansion gave rise to cooling to temperatures at which light nuclei could be stable.

In the first s after the Big Bang the four fundamental interactions (gravity, weak force, electromagnetic force and strong force) were unified. At s (T=10 32 K) gravity appeared a separated force. At s (T=10 27 K) strong nuclear interaction separated from weak and electromagnetic interaction. Between s and s the Universe underwent a rapid expansion increasing its size by a factor of (Inflationary Epoch). Matter outnumbers anti-matter. At s (T=10 12 K) the electromagnetic interaction separated from the weak interaction. E.6.9 Explain how the expansion of the universe made possible the formation of light nuclei and atoms.

At s temperature has dropped enough for individual protons and neutrons to exist. At about 2 s (T=10 10 K) neutrinos ceased to interact with protons and neutrons. By 3 minutes after the Big Bang all the primordial He had been produced. After some years the Universe temperature had cooled enough for H and He atoms to exist. High energy photons no longer interact with atoms. The Universe became transparent to photons and it is these photons which now give rise to the 3K background radiation. 1 billion years after the Big Bang, some matter can be brought together by gravitational interactions. If this matter is dense and hot enough, then nuclear reactions can take place and stars are formed. E.6.9 Explain how the expansion of the universe made possible the formation of light nuclei and atoms.

1. The beginning of the universe.. the big bang The History of the Universe in 10 Minutes The First Second After The Big Bang - Universe Documentary