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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 radiation, 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 pc, an age of about 1023 years. Neither the stars, galaxies or the Universe itself is that old.
The sky will therefore still be dark at night.
The Cosmological Principle states that the Universe is “homogeneous” and “isotropic” on large scales. This simply means 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.
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:
| Red-shifted spectrum, showing that the source is moving away from the Earth |  |
| Spectrum as it would appear in the laboratory, in other words, if the source was not moving relative to the observer |  |
| Blue-shifted spectrum, showing that the source is moving towards the Earth |  |
Hubble’s constant links the distance of galaxies to the rate of expansion of the Universe. Recent measurements give it a value of about 70 km/s/Mpc. The inverse of the Hubble constant (1/H02) gives us an age for the Universe, and therefore the time the light from distant galaxies has had to travel towards us. This tells us the size of the visible Universe.
The Cosmic Microwave background is a faint glow seen in the microwave part of the spectrum, coming from all directions in the sky. It has an almost uniform apparent temperature of 2.725 K, and its existence can only be explained by a hot, dense and uniform early Universe. It is therefore one of the strongest pieces of evidence in support of the Big Bang theory.
The photons that make up the CMB last interacted with matter when the Universe was 300 000 years old, showing us what the Universe was like at that time. Prior to this the photons were energetic enough to ionise the atoms, which meant that they didn’t travel very far before interacting with another atom. As the Universe cooled, the photons became less energetic, and when the Universe had dropped to a temperature of 3000K, the electrons were captured by the remaining ions and the photons scattered for the last time. The Universe became transparent and light could travel freely. This is the moment seen as the CMB.
The CMB was imaged by the Cosmic Background Explorer satellite (COBE), and shows temperature variations of approximately 30 microKelvin. 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 correspond to slight density variations in the early Universe, with the hot areas representing regions of higher density. It is these regions that eventually condensed to create the galaxies we see in the Universe today.
The recent balloon missions BOOMERanG and MAXIMAstudied the CMB in more detail, and the new missions MAP and Planck Surveyor will map the entire sky to a much higher resolution than COBE.
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 therefore the amount of mass. By looking at a large number of galaxies and their separations, a value for the density (mass/volume) of visible matter in the Universe can be calculated.
The way to detect both visible and dark mass is through its gravity, as all mass gravitates. This is done by measuring the motion of galaxies in clusters. The galaxies move within the cluster depending on the gravitational force between them. This force depends on the mass of the two galaxies, so by comparing their separations and velocities we can calculate the mass of the cluster and therefore the density of the Universe as a whole.
Comparing the masses calculated using the two methods gives us an interesting result. We find that the mass of the Universe that we can see (i.e. the amount giving out light) is much smaller than the amount we know is there due to gravitational measurements. This suggests that there is a lot of “dark matter” present in the Universe that we cannot see. It is believed that at least 90% of the mass in the Universe is in the form of dark matter.
There are thought to be two types of dark matter – hot and cold. Hot dark matter is made up of particles created in the Big Bang that have low masses and move very fast (at almost the speed of light). These particles are believed to be “neutrinos” and until recently were thought to have no mass. Recent experiments have shown that they may have a tiny mass after all, which means that they could 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 are likely to have a higher mass than those in hot dark matter. There are many possible types, including axions, photinos and low mass black holes. As yet no-one has been able to detect any of these, but this is an ongoing area of research.
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 true dark matter.
Click on the links below to find out more about the Universe.
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Authors: Carolyn Brinkworth and Claire Thomas
Last updated: July 2001