Is the universe expanding?
First, it’s probably a good idea to wrap your head around the idea that the universe is expanding and how this is even possible. We know that the Big Bank resulted in a (very rapid) expansion of a plasma of pure energy. Later, once it cooled, it started to coalesce and form basic atoms of matter, such as hydrogen. The key point is that even though matter as we know it (which comprises less than 5% of the universe) started to form, the universe did not stop expanding. And if the universe is expanding that means that everything in it is expanding – including matter (not just the ‘in-between stuff’ we refer to as dark matter and dark energy). Think of a balloon. Blow it up a little bit. Then write something on the balloon with a thick marker pen. Then blow it up some more. You will see the text (atoms) breaking up and expanding together with the the rest of the balloon (the universe). This raises questions on a sub-atomic level of how the four forces (gravity, electromagnetism, weak and strong nuclear forces) are able to respond to keep matter (as we observe it) intact. Or is it all just relative since we’re all expanding at the same rate (enter Mr Einstein!)?
How do we know the universe is expanding?
We now know from calculations such as the ‘Hubble constant’, that is based on the the observation of a cosmic microwave background (a faint glow emitted from the very early part of the universe after the Big Bang which we are able to see using the European Space Agency’s Planck telescope), that the universe is expanding. Mathematical calculations based on the observations of this period in the early life of the universe are used to predict what this constant (i.e. the speed of expansion) is today. This, combined with measuring distances between stars and other methods gives us a basis to substantiate the idea that the universe is expanding and at what rate.
The mystery of dark energy
That’s the easy part. The hard part is getting to grips with strange cosmic phenomena such as dark matter and dark energy. The latter seems to have the opposite effect of gravity so that rather than stars and planets following Newton’s Law of Gravitation (later expanded/redefined by Einstein) that holds that large objects (this theory doesn’t work as well on the sub-atomic level!) will attract each other based on their mass (the larger the mass the bigger the gravitational pull), dark energy has the effect of pulling them apart. Given the prevalence of dark energy in the universe it seems that this tug-of-war between gravity and dark energy (dark energy is winning) goes some way to explain why the universe is expanding.
Why this expansion is accelerating and what precisely dark energy’s role in this is remains unclear.