Key Areas covered Evidence for the expanding Universe We can estimate the mass of a galaxy by the orbital speed of stars within it Evidence for dark matter from observations of the mass of galaxies Evidence for dark energy from the accelerating rate of expansion of the universe

What we will do today Discuss the evidence that shows the universe is expanding at an accelerated rate. Describe how we can work out the mass of a galaxy. Discuss the evidence for ‘dark matter’ Describe how ‘dark energy’ affects the acceleration of the expansion of the universe

The expansion of the universe

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What did the universe do at the start? The universe began with the big bang. All matter started the size of a grain of sand and then rapidly expanded. How would this expansion behave now? Everything in the universe that has mass has a gravitational pull associated with it, the bigger the mass, the greater the pull. As the universe is so full of mass, scientists thought that all the mass and subsequent gravity would start to slow the expansion of the universe down.

So the universe is expanding, is it speeding up or slowing down? Scientists believed that exploding stars where key to finding out about the expansion of the universe and so decided to look for exploding stars (know as type 1a supernovae) Just like car headlamps, you can tell how far away a star is by observing its brightness Exploding stars always give off approximately the same brightness. The process of the explosion takes a few weeks so they can be observed over this time period.

What happened when we found exploding stars? After finding some exploding stars scientists discovered that the exploding stars where actually dimmer than they should have been – they had to be moving away In fact, they were so far away that the only explanation would be that they were accelerating away and not slowing down.

Was there any other evidence? Using calculations, scientists calculated the mass of the universe to be a negative value – this was impossible. They then realised that their calculations were based on the expansion of the universe slowing down. Re-doing the calculations, assuming the expansion to be speeding up, showed that the universe must be expanding

How can we know what the mass of the universe is? The mass of objects in space can be estimated using their orbital speed. The masses measured seem to be bigger than the mass that can be accounted for by the number of stars present in our galaxy. Therefore it is theorised that there is something else making up this mass – something we can’t see called ‘Dark Matter’

Did everyone agree with this? Doubters suggested that the stars weren’t moving away and just appeared to be farther due to space dust The Hubble telescope, positioned out of the Earth’s atmosphere, showed this was not the case.

So what exactly happened? Scientists now think this: When the universe was born in the Big Bang, it was expanding extremely fast. It was also much smaller and thus denser than it is today. Because the universe’s mass was so close together, gravity began slowing the universe down. But in the background the entire time was this other, mysterious, repulsive force – the force scientists would come to attribute to “dark energy.”

So what did ‘dark energy’ do? As the universe continued to expand, though slowly, the galaxies containing the bulk of the universe’s mass became farther removed from one another. As the distance between mass became greater, the gravitational pull faded. As gravity weakened, the repulsive force took over and began forcing the universe to speed up.

So what happens in the future? There are two possible outcomes: 1.Closed universe: the universe will slow its expansion and eventually begin to contract. 2.Open universe: the universe will continue to expand forever.

Einstein’s “greatest blunder” The increased expansion appears to verify Einstein’s inclusion of the cosmological constant in his Special Theory of Relativity. He described this as his ‘greatest blunder’ – he only added it because he couldn’t agree that the universe would eventually collapse (a popular theory at that time). Unfortunately he died before the evidence to support his theory was uncovered.

The Expanding Universe Experiment The Universe - The Expanding Universe What you need: a balloon a marker pen a ruler a stopclock

The Expanding Universe Experiment Galaxyd1 - distance from Milky way (cm) d2 – distance from Milky way (cm) Time taken to travel d1 – d2 (s) Ave. velocity (cm/s)

Conclusion The further away the galaxy, the _______ the average velocity. This shows that the universe is accelerating.

Is the universe expanding? The Universe has been expanding since the Big Bang. The objects within the Universe (ie galaxies) are not expanding – rather the space between them is. The expansion of the Universe is actually accelerating. Most galaxies are moving away from each other

What evidence is there to support this? We can estimate how far away something is by its brightness – exploding stars in our galaxies are moving further away at an accelerated rate. Furthermore, light from some distance galaxies was observed to display a redshift, showing they were moving away at speed.

What is the mass of our galaxy – The Milky Way? One method that astronomers have used to calculate the mass of the Milky Way galaxy is to use Kepler's 3rd law that relates orbit radius and period with central mass. We figure out the radius of our orbit relative to the centre of the galaxy and the period (the time it takes to complete one complete orbit). Plugging those numbers into Kepler's 3rd law we can estimate the mass inside our orbit. Doing the maths we come up with a number that is just shy of 100 billion solar masses, with one solar mass being equal to the mass of our Sun. Overall mass estimates of the Milky Way are on the order of 750 billion to 1 trillion solar masses. NB: We do not need to calculate the masses of galaxies, just how it is done.

How can you estimate the mass of a galaxy? Measurements of the velocities of galaxies and their distance from us lead to the theory of the expanding Universe. Gravity is the force which slows down the expansion. The eventual fate of the Universe depends on its mass. The orbital speed of the Sun and other stars gives a way of determining the mass of our galaxy.

How is the mass of the universe related to its expansion? It was thought that the gravitational pull of all the mass in the universe would slow the expansion. However as the universe expands, the force of gravity is decreasing. Additionally there must be another invisible force pulling the universe – this is know as ‘Dark Energy’.

What is Dark Energy? Measurements of the mass of our galaxy and others lead to the conclusion that there is significant mass which cannot be detected — dark matter. Measurements of the expansion rate of the universe lead to the conclusion that it is increasing, suggesting that there is something that overcomes the force of gravity — dark energy

Open-ended question Specimen paper

Possible answer Yes, the universe is getting even bigger. Astronomers believe that the universe is expanding - that all points in the universe are getting farther apart all the time just like a balloon blowing up. By drawing galaxies on the balloon, you can observe that it is not the galaxies themselves that are expanding (as you blow the balloon up over time) but the space between them, just like in the universe (over time). It can also be used to work out speed of each galaxy by recording the distances between galaxies and the time taken to increase these distances when blown. The more you blow up the balloon – the faster the speed ie acceleration. However, this model shows how the expansion works but it is only a 2D model whereas the universe is 3D.