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Structure of the Universe

Olbers' Paradox

Why is the sky dark at night? This question was credited to Olbers, but was first suggested by Kepler. If the Universe is infinite, then whichever way we look into the night sky, we should see a star. It is like standing in the middle of a large forest. The tree trunks seem to go on forever as they can be seen in every direction. Your line of sight will always end on a tree trunk. If the Universe were infinite then your line of sight would always end on the surface of a star and the whole sky would look as bright as the surface of a star. Clearly this is not the case as the night sky is dark.

Several explanations for this were put forward, including dust between us and the distant stars. However, if the dust were blocking the light, it must be absorbing energy, which would eventually heat up the dust and make it glow - the sky would still look bright.

In order to solve this problem, we have to understand that the universe has a finite age, and so the visible Universe has a finite size. This doesn’t mean that it has an edge, it simply means that the Universe is not old enough for the light from distant stars to have reached us yet. Although light moves very quickly, it still takes a long time to reach us from distant stars. The farthest that can be seen is the distance light has travelled in the age of the Universe, i.e. 15 billion years. Light from more distant objects hasn’t had time to reach us yet. This boundary of visibility continues to increase as new light reaches us, and new galaxies come into view.

If stars lived forever, this would mean that the sky would get brighter and brighter with time, until the sky was indeed bright at night. However, stars only live for about 10,000,000,000 years, so by the time the light from the distant stars has reached us, the stars closest to us have reached the end of their lifetime and cooled to the point where they can no longer be seen.

In order to make the sky bright, we would have to be able to look out to a distance of about 1023 parsecs, which means that the Universe would have to be about 1023 years old. Neither the stars, galaxies nor the Universe itself is that old.

The sky will therefore still be dark at night.

The Cosmological Principle

The Cosmological Principle states that the Universe is “homogeneous” and “isotropic” on large scales. This is simply a scientist’s way of saying that it is the same from wherever you look at it and in whichever direction you look. This is important because it means that we have no special place in the Universe.

Hubble's Law

In 1929 Edwin Hubble discovered that all of the galaxies that are not in our local group are moving away from us, and that those furthest away from us are moving the fastest. Hubble’s Law states that:

V=H0 R

Where V is the velocity of the galaxies, R is the distance to the galaxies and H0 is Hubble’s constant, which represents the current rate of expansion of the Universe.

In order to make sure that the Cosmological Principle is not broken, the same expansion in all directions must be seen from every point in the Universe. In fact the galaxies are not actually moving away from us through space, instead it is space itself that is expanding. This is quite hard to picture, but imagine a balloon with lots of dots drawn on it. When the balloon is flat, all of the dots are fairly close to each other, but when it is blown up, the dots get further apart. They are not moving across the surface of the balloon because they are fixed wherever they were drawn, but they move apart because the space on which they are drawn is expanding. The same thing is happening to the galaxies in the Universe. They move through space fairly slowly, but they can be moving away from us very quickly because the space in which they all lie is expanding.
Galaxies move relatively slowly through space - instead space itself expands
This expansion can be seen by looking at the light from distant galaxies. The light is “red shifted”, meaning that the wavelength of the light seems to be longer than it would be in a laboratory. The light is shifted towards the red end of the spectrum. This is all due to the expansion of the Universe, which causes the light from distant galaxies to be stretched as it travels towards the observer. The amount by which the light is stretched depends on the amount the Universe has expanded since the light was given out. The light takes longer to travel to us from more distant galaxies, so the Universe has had time to expand further. This means that the shift in wavelength of the light is also greater. The light from nearby galaxies hasn’t been travelling for as long, so the Universe hasn’t expanded so much since it was given out, and the light is therefore red shifted by less.

Red-shifted spectrum, showing that the source is moving away from the Earth Red-shifted spectrum
Spectrum as it would appear in the laboratory, in other words, if the source was not moving relative to the observer The spectrum as it would look in a laboratory
Blue-shifted spectrum, showing that the source is moving towards the Earth Blue-shifted spectrum

The Cosmic Microwave Background (CMB)

The Cosmic Microwave background is a faint glow of microwaves coming from all directions in the sky. It has an almost constant temperature of about –270C, and it is one of the strongest pieces of evidence in support of the Big Bang theory.

When light has a lot of energy, it is able to interact with atoms, knocking electrons out of the outer shell to form ions. In the very early Universe, the light was so energetic that these reactions were happening all the time, which meant that light could not travel very far. When the Universe was 300 000 years old, the energy of the light dropped enough to stop these reactions, which meant that the light could now travel over long distances. The CMB is a snapshot of the Universe as the last interactions took place.

Very small variations in the temperature of the CMB were detected in 1992 by the Cosmic Background Explorer satellite (COBE). The picture at the top of the page shows a temperature map of the microwave sky obtained by COBE. The different colours show very slightly different temperatures. These temperature variations are due to slight density variations in the early Universe, with the hot areas showing regions of higher density. It is these regions that eventually collapsed to create the galaxies we see in the Universe today.

Two recent balloon missions called BOOMERanG and MAXIMA studied the CMB in more detail, and the new missions MAP and Planck Surveyor will map the entire sky to find much smaller variations than COBE.

Dark Matter

How much mass does the Universe contain?

There are two different ways of measuring this. The first is to look at the amount of light coming from a galaxy. This tells us roughly how many stars are in that galaxy and, because we know the mass of an average star, gives us the amount of mass in that galaxy. By looking at a large number of galaxies, we can measure the amount of visible matter in the Universe.

The other way is to look at how galaxies behave in clusters. Clusters are groups of galaxies that are quite close to each other in space. The way that the galaxies move within the cluster depends on the gravitational force between them. This force depends on the mass of the galaxies, so by looking at their movements we can find the mass of the cluster and therefore the mass of the Universe.

If we compare the masses found using the two methods, we get an interesting result. The mass of the Universe that we can see (i.e. the amount giving out light) is much, much smaller than the amount we know is there due to the measurements of clusters. This means that there is probably a lot of “dark matter” in the Universe that we cannot see. We now think that at least 90% of the mass in the Universe is in the form of dark matter.

Scientists think that there are two types of dark matter – hot and cold. Hot dark matter is made up of particles created in the Big Bang that have very low masses and move very fast (at almost the speed of light). These particles are called “neutrinos”. They are thought to have a tiny mass, which may contribute to the missing mass in the Universe.

Cold dark matter was also created in the Big Bang, but is moving much more slowly. The particles that make up the cold dark matter probably have a higher mass than those in hot dark matter. There are many possible types, including particles called axions and photinos, and low mass black holes. So far no-one has been able to detect any of these, but research is still going on in these areas.

One other source of unseen matter is “brown dwarf” stars which are too small to start nuclear burning and therefore don’t shine. They make up a small amount of the missing mass that we are unable to see, but are not really dark matter.

Click on the links below to find out more about the Universe.

Introduction to the Universe

Early Models of the Universe

Evolution of the Universe

Gamma Ray Bursts

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Authors: Carolyn Brinkworth and Claire Thomas

Last updated: July 2001