1. 4. Whole new worlds Continued from AstrophysicsDS#3
The Pin Wheel galaxy

2. The Milky Way is our galaxy
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
A 360 degree pan. We can’t see the centre because it is hidden by dust. Note that it is very ‘thin’.

3. Henrietta Leavitt discovered that Cepheid variables had a definite relationship between their period and luminosity
Cepheid’s are very bright and can be seen in other galaxies – hence their significance here. If we can find their period (easy) we can find their distance from luminosity (actual brightness) and apparent brightness.

4. Hubble was able to use this to determine the distance to galaxies
His discovery revolutionised (that word again!) our picture of the universe
The universe was not static and unchanging – as even Einstein had believed
It was expanding!
But how do we know? Next slides.

5. 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
Earth is travelling at 30 km/sec around the Sun, so this is not a huge speed relatively speaking.
A Mpc is Mega-parsec, a very big distance – distance to nearer galaxies.

6. A 2D analogy of a 4D universe
Galaxies are not speeding away from us through space, it is space which is expanding – carrying the galaxies with it!
Note the galaxies are not expanding, it is the space between them which is getting bigger.
The stars on the balloon are the same size, but the balloon is getting bigger.
A 2D analogy of a 4D universe

7. This could even have political consequences!
Astrophysics challenges many of our normal assumptions – even the laws of physics themselves
This is a wonderful opportunity for students to think about the assumptions they make all the time
This could even have political consequences!
Just imagine what would happen if we could somehow get politicians who actually could question the assumptions of their own party!

8. 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.
APOD : The total mass within giant galaxy cluster CL , about 4.5 billion light-years away, produces a cosmic gravitational lens -- bending light as predicted by Einstein's theory of gravity and forming detectable images of even more distant background galaxies. Of course, the total cluster mass is the sum of the galaxies themselves, seen as ordinary luminous matter, plus the cluster's invisible dark matter whose nature remains unknown. But by analyzing the distribution of luminous matter and the properties of the gravitational lensing due to total cluster mass, researchers have solved the problem of tracing the dark matter layout. Their resulting map shows the otherwise invisible dark matter in blue, and the positions of the cluster galaxies in yellow.
Is this dark matter?

9. Where did galaxies come from? Why did they form?
Were Quasars present at the birth of a galaxy?
This is a photo of an early galaxy being formed because it is something like 10 billion light years away – therefore 10 billion years old.

10. 5. The expanding universe
If the universe was expanding, what was it expanding from? It seemed a very strange idea to think it all came from nothing!
Fred Hoyle came up with a brilliant solution:
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
Don’t knock Fred’s theory! It was brilliant – and some astrophysicists are not convinced it is totally dead yet!

11. After all, how could it possibly have started from nothing
After all, how could it possibly have started from nothing? - With a ‘big bang’?
But how could we possibly tell the difference?
Astrophysicists love hard questions!

12. It must have been AWE FULL hot to start with!
That heat radiation should still be bouncing around the universe
But would be MUCH colder by now.
...just like expanding gases cool down. But more particularly, because the expansion of space means the wavelength of the radiation has expanded as well, which means cooler radiation - Wien’s law.

13. Sure enough, astronomers were aware of radio waves coming from the sky

15. COBE and WMAP have mapped this radiation
COBE and WMAP have mapped this radiation. It agrees very precisely with the predictions of the big bang model
This is the black-body radiation spectrum of a cold object.

16. WMAP has also given us a very accurate value for the Hubble constant and therefore the age of the universe
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)
T = 1/Ho = 1/71 km/sec/Mpc = 13.7 billion yrs.

17. How will it end? Is it closed, open or flat?
The expansion seems to be accelerating!
Dark energy appears to be accelerating the expansion.
The ‘critical’ universe will just keep expanding forever – any slower and it collapses again.

18. So why and what is the universe. Who knows
So why and what is the universe? Who knows? But it’s sure fun trying to find out!
© KB July 2004