Physics
Cambridge IGCSE
Cambridge IGCSE
TOPIC 6: SPACE PHYSICS
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 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:
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.
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. The main piece of evidence you should understand is to do with the redshift of distant galaxies.
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 14 billion years since the Big Bang.
To understand how the redshift of light occurs, we need to understand what is known as the Doppler effect:
A small dropper is used to drip water into a pond. The waves move outwards, making circles in the pond as shown in figure 1 in the left-hand diagram. However, what would happen if we slowly move the dropper left to right as we continue to drip water into the pond?
As you can see from the diagram on the right, we still produce circular patterns of waves, but the circles bunch together on the right of the diagram. As they reach a detector labelled 'Y', the waves hit the detector more frequently than at detector X. They are also closer together on the right-hand side.
Figure 2. The Doppler effect in water.
This means that frequency of waves detected at Y has increased, and the wavelength has decreased. The speed of the waves through the water remains the same. This all happens because the source of the waves is moving towards the detector.
What do you think would happen if the source moves away from the detector? In this case the opposite happens: The velocity still remains the same, but the wavelength increases as the frequency decreases.
This effect is called the Doppler effect, and is most often noticed when listening to moving vehicles, like police cars or ambulances. As the vehicle moves towards us, the frequency is higher (higher pitch of sound), and as it moves away the frequency decreases and the pitch is lower.
Have a listen to this YouTube clip to see and hear the Doppler effect in action, using a car horn:
YouTube clip showing the Doppler effect
(hherhold 2007)
Figure 3: Animation of the Doppler effect
(Charly Whisky - 2007 - CC BY-SA 3.0)Link
The Doppler effect occurs with all waves, including light waves. If the source is moving away from us, the observed wavelength of light will be longer than the original wavelength. In section 3.3 you will have learnt that red light has the longest wavelength for visible light, so we say light has been 'redshifted' if the wavelength increases. The faster the source is moving away, the greater the change in wavelength, and this allows us to calculate the speed of 'recession' (moving away).
This is the reason that distant galaxies look red, as the source of light is moving away from us. Astronomers believe that space itself is expanding, moving galaxies apart.
How can astronomers find the distance to some of the very remote galaxies seen in figure 1? They are so far away and we only have telescopes to give us data on these objects. The answer to this came from a special type of supernova (called a type 1a supernova). When these stars explode, they give out the same amount of energy each time. By studying the brightness of these supernovas in distant galaxies, we can work out how far away the galaxy is. The dimmer the supernova, the higher the distance to the galaxy.
In 1929, Edwin Hubble collected data on these distant galaxies, and showed a simple connection: The further the galaxy is, the larger the redshift and hence the larger the recession velocity. He plotted this on a graph, shown here in figure 4:
Figure 4. Hubble's graph showing a galaxy's recession velocity against distance
Brews ohare (adapted) CC BY-SA 3.0
The straight line shown in the graph indicates that the recession velocity of a galaxy is approximately proportional to the distance to that galaxy. The original data had some anomalies (e.g. galaxies in the Virgo Cluster), but more recent data has confirmed the straight line fit. Hubble added a constant to make a formula now called Hubble's law, and this constant is now known as the Hubble Constant H0:
| Hubble Constant = | recession velocity |
| distance |
| H0 = | v |
| D |
where H0 = 2.2 x10-18 per second
This formula allows us to calculate the distances to other galaxies as shown in the example below:
Example:
A galaxy redshift is measured, and found to indicate a recession velocity of 3000 km /s.
Answer:
a) The formula is:
| H0= | v |
| D |
| D = | v | = | 3000 (km/s) |
| H0 | 2.2 x 10-18 (per sec) |
which gives D = 1.36 x 1021 km to 3 sig figs.
Note that we did not need to change units as seconds were used for the speed as well as for the Hubble constant. The answer is in km as the speed is given in km/s.
Hubble's law gives evidence that the universe is expanding. The galaxies are moving apart as space expands, and the further away a galaxy is, the faster it is moving away. Imagine watching this on a video. What if we could play the video backwards? We would see the galaxies moving closer together. A galaxy a distance D away would move towards us at a velocity v. It is simple to then work out the time taken before all galaxies were together at one point - the Big Bang!
Using speed = distance /time, then the time taken for any particular galaxy to have reached the point it currently occupies is T= v/D. This value of T is an approximate age of the universe. Notice that the age of the Universe T is the inverse of the Hubble constant H0!
| age of Universe, T = | v | = | 1 |
| D | H0 |
Using our value of H0 = 2.2 x 10-18 (per sec) gives:
T = 4.5 x 1017 seconds
or T = 14 billion years.
Note: This is an approximate value as it assumes all of the galaxies have been moving at a constant rate. The current age of the universe is estimated to be close to 13.7 billion years.
Questions:
1. Light from distant galaxies appear to be redshifted.
a) redshift occurs when (white) light is emitted from a galaxy moving away from us (at high speed). The wavelength of light increases if the source of light is moving away from us (the Doppler effect).
b) If all observable distant galaxies are moving away, the universe must be expanding. This means in the past it was much smaller. It therefore started in a very small region of space in an event called the Big Bang.
2. Describe the relationship between the distance to a galaxy and:
The further a galaxy is away:
a) the higher the speed it is moving away.
b) the more the light is redshifted. (To a longer wavelength).
3. A distant galaxy is found to be 9.0 x 1020 km away.
a) By measuring the brightness of (one type of) supernova using telescopes, the distance to the galaxy can be measured. The dimmer the supernova, the further away the galaxy is.
b) Using the Hubble formula and rearranging gives:
v= H0D
so v =
H0 = 2.2 x 10-18 (per sec) x 9.0 x 1020 (km)
v = 1980 km/s
In 1964, two scientists (Penzias and Wilson) were taking readings using a sensitive microwave detector, as shown in figure 5:
Figure 5: Microwave detector used by Penzias and Wilson in 1964
By Fabioj CC-BY-SA-3.0 from Wikimedia Commons
When the two scientists were using the detector to try to take radio wave readings, they noticed a background noise of microwaves coming from all directions, day and night. However, this microwave background had already been predicted by other scientists as a 'left-over' from the big bang:
If the big bang happened, releasing a vast quantity of matter and radiation, then the radiation remaining should now be enormously red-shifted into the microwave region of the EM spectrum. (You can think of this as the universe no longer being white hot or red hot, but having cooled over time until only microwave radiation is present). Penzias and Wilson had discovered what we now call the cosmic microwave background radiation (CMBR), exactly as predicted by the Big Bang theory. We now understand that the CMBR was produced shortly
after the Universe was formed and that this
radiation has been expanded into the microwave
region of the electromagnetic spectrum as the
Universe expanded.
The graph below (figure 6) is quite famous but takes a little time to understand: The line shows the predicted intensity of radiation at different frequencies based on current understanding of the big bang model. The 'x' points are the CMBR data points from a recent satellite (COBE). The match is nearly perfect, and looks like the line was drawn after the data points were obtained.
Figure 6: COBE satellite CMBR data compared to predicted radiation from the Big Bang
The CMBR, along with the red shift of distant galaxies has provided astronomers with enough evidence for them to be reasonably certain that the Big Bang theory is correct. The universe did have a starting point!
Questions:
4. In 1964, CMBR was discovered by astronomers.
a) CMBR is cosmic (from space) microwave background (all around us) radiation.
b) This radiation is predicted from the big bang theory: Any radiation produced in the big bang should now be red-shifted to microwave radiation, but should be observable in all directions.
Now test your understanding using these quick, 10 minute questions on the Universe:
Here's an end of unit quiz on Space Physics:
