In 1929 Edwin Hubble discovered that the galaxies are all moving away from each other. The more distant they are, the greater their speed of recession (v). This is known as Hubble’s Law:
v=Hor, where r is the distance.
The Hubble Constant, Ho, was found using measurements of nearby galaxies. The formula then allowed the distance of far-away galaxies to be calculated. This enabled astronomers to map the universe. It has a foam-like structure with galaxies arranged in sheets and clusters. Each structure is about 300 million light years across.
On even larger scales, the universe is uniform. However, at a distance of about 8 billion light years, Hubble’s law underestimates galaxy distances. It seems that the universe is now expanding faster than it was in the past. It is probably accelerating, driven by a mysterious form of energy, dubbed dark energy.
Observations show the universe to be 73% dark energy and 4% matter like ourselves. The other 23% is dark matter that is very different from the atoms around us. It cannot be seen but can be detected by the effect of its gravity.
When the universe was young, the much higher density of matter allowed gravity to slow the rate of expansion. The above-average density of matter inside galaxies and even in clusters of galaxies halted the expansion of space, but elsewhere gravity has lost control and the universe may well expand for ever.
We believe the universe came into being 13.7 billion years ago in a very-high-energy event, known as the Big Bang, from which it has expanded and cooled. It became transparent to photons when it was 380 000 years old. These energetic photons cooled to give us the view we see today as the cosmic microwave background. Its average temperature is only 2.735 degrees above absolute zero. The slight fluctuations in this temperature have their origins in quantum fluctuations in the early universe. These fluctuations have become the large-scale structures we now see in the universe.
Back to Gravity Homepage