OPTION E - ASTROPHYSICS

E6 Galaxies and the expanding universe

Galactic motion

OPTION E - ASTROPHYSICS

E6 Galaxies and the expanding universe

Galactic motion

E.6.1 Describe the distribution of galaxies in the universe.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 2x106 ly away.

• The Local group extends over a distance of 1x107 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 15x106 pc across.

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

E.6.2 Explain the red-shift of light from distant galaxies.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

'

c

v

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

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.

Hdv 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.6.4 State Hubble’s law.

E.6.5 Discuss the limitations of 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.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.6.7 Explain how the Hubble constant may be used to estimate the age of the universe. • If we assume that the expansion of the Universe has been

constant up to now, then gives an upper bound to the age of the Universe.

• This is only an upper band as the Universe expanded faster at the beginning (this would imply a younger Universe).

• The time , known as Hubble time, is about 14 billion years. • The Universe cannot be older than this.

and

E.6.7 Explain how the Hubble constant may be used to estimate the age of the universe. E.6.7 Explain how the Hubble constant may be used to estimate the age of the universe. • The value of Hubble’s time with H = 70x103 m s-1 Mpc-1 is

E.6.8 Solve problems involving Hubble’s law.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 10-35s after the Big Bang to 10-32s.

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 10-43 s after the Big Bang the four fundamental interactions (gravity, weak force, electromagnetic force and strong force) were unified.

• At 10-43 s (T=1032K) gravity appeared a separated force.

• At 10-35 s (T=1027K) strong nuclear interaction separated from weak and electromagnetic interaction.

• Between 10-35 s and 10-32 s the Universe underwent a rapid expansion increasing its size by a factor of 1050 (Inflationary Epoch). Matter outnumbers anti-matter.

• At 10-12 s (T=1012K) 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 10-6 s temperature has dropped enough for individual protons and neutrons to exist.

• At about 2 s (T=1010K) neutrinos ceased to interact with protons and neutrons.

• By 3 minutes after the Big Bang all the primordial He had been produced.

• After some 300 000 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 banghttps://www.youtube.com/watch?v=VOz4PkdY7aA <iframe width="560" height="315" src="https://www.youtube.com/embed/VOz4PkdY7aA" frameborder="0" allowfullscreen></iframe>

• The History of the Universe in 10 Minuteshttps://www.youtube.com/watch?v=-Ip5BAEfZuA<iframe width="560" height="315" src="https://www.youtube.com/embed/-Ip5BAEfZuA" frameborder="0" allowfullscreen></iframe>

• The First Second After The Big Bang - Universe Documentary 2014https://www.youtube.com/watch?v=MfpH3Zox6m4<iframe width="560" height="315" src="https://www.youtube.com/embed/MfpH3Zox6m4" frameborder="0" allowfullscreen></iframe>