Transcript Astrophysics

4. Whole new worlds
The Milky Way is our galaxy
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It is about 50 kpc in diameter
but only 1 kpc thick - with a bulge in the centre
We are about 8 kpc from the centre
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Henrietta Leavitt discovered that Cepheid variables had
a definite relationship between their period and
luminosity
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Hubble was able to use this to determine the
distance to galaxies
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His discovery revolutionised (that word again!) our
picture of the universe
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The universe was not static and unchanging – as
even Einstein had believed
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It was expanding!
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Hubble found a definite relationship between the distance
of galaxies and their redshift
This meant that the further away the galaxy, the greater
the rate at which it appeared to be moving away from us
v = Hod
Ho =
70 km/sec/Mpc
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Galaxies are not speeding away from us through
space, it is space which is expanding – carrying
the galaxies with it!
A 2D analogy of a 4D universe
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Astrophysics challenges many of our normal
assumptions – even the laws of physics
themselves
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This is a wonderful opportunity for students to
think about the assumptions they make all the
time
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could even have political consequences!
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For example, for the galaxies to move in the way they do
either Newton’s law (even when modified by Einstein) is
wrong or there is a lot of mass in the galaxies that we
can’t see – dark matter.
Is this dark matter?
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Where did galaxies come from? Why did they form?
Were Quasars present at the birth of a galaxy?
5. The expanding universe
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If the universe was expanding, what was it
expanding from? It seemed a very strange
idea to think it all came from nothing!
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Fred Hoyle came up with a brilliant solution:
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It had always been, it was infinite, matter was
continually being created to keep the density
constant
... at the rate of a few atoms per day per
Cathedral
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After all, how could it possibly have started
from nothing? - With a ‘big bang’?
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But how could we possibly tell the difference?
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Astrophysicists love hard questions!
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It must have been AWE FULL hot to start with!
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That heat radiation should still be bouncing
around the universe
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But would be MUCH colder by now.
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Sure enough, astronomers were aware of radio waves
coming from the sky
COBE and WMAP have mapped this
radiation. It agrees very precisely with the
predictions of the big bang model
WMAP has also given us a very
accurate value for the Hubble
constant and therefore the age of
the universe
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Age of universe – the time it has been expanding
at the observed rate – is equal to the reciprocal of
the Hubble constant (with a few adjustments)
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T = 1/Ho = 1/71 km/sec/Mpc = 13.7 billion yrs.
How will it end?
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Is it closed, open or flat?
The expansion seems to be accelerating!
So why and what is the
universe?
Who knows?
But it’s sure fun trying to find out!