5.2b The Universe

The 'universe' contains everything! The observable universe is believed to consist of billions of galaxies.

This famous picture below (figure 1) was taken by the Hubble Space Telescope. It shows a very small patch of the night sky magnified to an enormous degree. Nearly every object in the image is an entire galaxy! It gives you some idea of how many galaxies there must be.

Figure 1: The famous 'deep field' image from the Hubble Space Telescope

There are billions of stars in most galaxies. Our Sun is in a huge galaxy called the Milky Way. It has about 100 billion stars in it, and is about 100,000 light-years across. The galaxy nearest to us is called the Andromeda galaxy. Both the Milky Way and Andromeda are spiral galaxies, with the outer stars orbiting around a very heavy central mass of stars.

By examining distant galaxies and radiation, astronomers have gathered evidence to tackle a significant question: Did the universe have a beginning, or has it existed forever? Recent findings have convinced scientists that the universe indeed had a distinct origin, known as the Big Bang:

The Big Bang Theory

This theory states that the universe started in a very small region in an extremely hot dense state. In an enormous release of energy (the Big Bang) all matter was created and moved outward from this point. Eventually the matter formed dust clouds, stars and the galaxies we see today. However, these galaxies are still moving outwards - the universe is still expanding. The current age of the universe is estimated to be close to 13.8 billion years.

Initially, this theory was one of several that was put forward to explain the way the universe looks today. However, key pieces of evidence led to this being the main theory that is supported today.

Here is a great summary video about the Big Bang Theory, from National Geographic:

Youtube video: National Geographic - origins of the universe 101

Evidence for the Big Bang Theory: Redshift

Have a look at this video showing the most distant galaxy discovered (as of 2018), called GN-z11:

Youtube video: GN-z11 - the most distant galaxy yet discovered

Did you notice how red the galaxy looks?

When very distant galaxies were first observed, astrophysicists noticed that certain tell-tale frequencies produced by the hydrogen in stars had been shifted to longer wavelengths. In fact, all of the mixture of colours that normally appear white have been shifted to the longer-wavelength red end of the spectrum. This effect is called redshift.

A source of waves moving away from us produces a shift in the wavelength towards longer wavelengths. That is what is being observed here - distant galaxies are moving away from us at very high speeds. In fact, the further the distance to the galaxy, the faster the recession velocity (recession means 'moving away'). This higher velocity leads to a higher red shift. As galaxy GN-z11 is so far away, it is receding at an enormous speed.

This red shift of galaxies is clear evidence for the Big Bang Theory: If all of the galaxies are moving away from each other, then space is expanding. Originally, it must have been much more compact and dense, and we can even use the speeds of galaxies to work out roughly when they were all together in one place. This gives us an approximate age of the universe to be about 13.8 billion years since the Big Bang.

For the double course, you will not be expected to explain 'redshift' as evidence for the Big Bang Theory. It is included here as additional information.

END OF TEXTBOOK!

 

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