Asked by: Lewis Rylands, Liverpool
By making observations of fluctuations in the cosmic microwave background (the ‘leftover’ radiation from the Big Bang), astronomers have shown that the Universe is ‘flat’. This means that its density appears to be close to the critical density, which is the density needed for gravity to just halt its expansion after an infinite time.
The expansion rate we see today indicates that the critical density of the Universe is about 9×10-27 kg m-3. This density, however, is the total density of both matter and energy. Observations have shown that ordinary matter accounts for 4.9 per cent of this density, while 26.8 per cent of it is due to dark matter, a form of matter not directly visible.
The remaining 68.3 per cent is due to dark energy, a mysterious energy field causing the Universe’s expansion to accelerate. So, ordinary matter has a density corresponding to about one proton for every four cubic metres of volume. The nature of dark matter and dark energy are not fully understood.