COSMOLOGY
Why is space black?
The darkness of our night sky tells us more about the size, age and nature of the universe than we may expect. We are familiar with the twinkling of nearby stars in the night sky, but why does the night sky not glow with the colour and brightness of the average star? If the universe is infinite in size and age, then in every direction we look there should be a star or galaxy - well known to be powerful emitters of visible light. This puzzle is known as Olber’s paradox, after 19th-century German astronomer Heinrich Olbers, and has attracted many attempts to resolve it.
The first part in resolving this paradox is that the universe must have a finite age. This means only a finite number of stars can be observed from Earth in any direction – something which is true even if the universe itself has infinite size. The second part is from the expansion of the universe, an integral part of our understanding of cosmology, known as the Big Bang model. This expansion of space, together with a finite light travel time, causes the radiation emitted in this hot, dense beginning, as well as that from distant stars and galaxies, to be shifted to longer wavelengths than are visible to the naked eye. We observe this today as the cosmic microwave background - the faint glow of emission over the entire sky, observable only with microwave detectors such as the European Space Agency’s Planck mission. Ultimately, the night sky is dark to us due to the expanding, evolving and finite age of the universe.
SOLAR SYSTEM
Why don’t all planets have magnetic fields?
The magnetic field of a planet is generated by fluid motions in an electrically conducting region of its interior. Without such motions – or if they’re not vigorous enough – a planet will not have a magnetic field. For Earth, this region is the liquid outer core, which is dominantly composed of iron and nickel, and heat escaping the core powers turbulent convective flows. In a good electrical conductor, magnetic field lines tend to remain attached to material particles. These flows can twist and shear an existing magnetic field, and this is the process that can sustain it. Take away this stirring, and the field decays. For Earth this would take approximately 30,000 years. The other terrestrial planets also have metallic liquid cores, but only Mercury has a planetary magnetic field. This means that flows in the liquid cores of Mars and Venus are not vigorous enough at present, although we know that Mars had a field in the past from the magnetisation recorded in its crustal rocks. For the giant planets, the conducting region is metallic hydrogen for Jupiter and Saturn or ionised water and ammonia for Uranus and Neptune.
