Expansion of the Universe
The expansion of the Universe is not constant. Recent measurements have shown that the rate of the Universe's expansion is increasing.
This is currently explained by An unidentified form of matter that accounts for galaxies rotating faster than their visible mass should cause. and An unknown form of energy, put forward as a solution to the problem of why the expansion of the Universe is accelerating..
The evidence for this area of physics comes from observations of the rotation of galaxies and planets.
Our Milky Way is a spiral galaxy rotating around a central point. Our Sun is on one of the spiral arms as shown in the diagram:
The Sun's orbital speed is determined almost entirely by the gravitational pull of matter inside its orbit. The expected orbital speed should decrease with orbital radius but observations indicate the orbital speed is constant at most radii.
The orbital speed of the Sun and other stars gives a way of determining the The amount of matter an object contains. Mass is measured in kilograms (kg) or grams (g). of our galaxy.
Physicists noticed that the effects of gravitation suggested a greater mass that the mass calculated from 鈥渓uminous matter鈥 that could be seen 鈥 stars, gas and dust. They suggested that this extra mass was made up of 鈥榙ark matter鈥 - a form of matter with a strong 'negative pressure'
Observations of the rotation of galaxies indicated that most of the mass of the universe was originally dark matter.
There is much conjecture and theorising about dark energy and dark matter. For example, some research indicates the proportion of dark energy in the universe is increasing allowing increased expansion:
Unlike gravity where the A push or a pull. The unit of force is the newton (N). of attraction between masses would slow the expansion down, dark energy works to speed the expansion up.