What is the dark stuff in the universe? It's that which isn't giving off or even reflecting any light. This is important as light is the only information that we get from the distant universe; there's a lot of interesting stuff we just can't see. We don't know what dark energy actually is. In fact, "dark energy" is just a name that we have given it out of ignorance.
But first, let's look at light. How far away is a galaxy? We can't exactly just pop out into space with a measuring tape. Instead, we need a "standard candle": something that burns with the same brightness everywhere in the universe. Then, we can tell that brighter things are closer and dimmer ones are further away. Luckily, we have the type 1a supernova, which acts as our standard candle. This is actually the violent death of a white dwarf star.
The star accretes material from a companion star and once it has stolen enough mass, it destabilises the balance between gravity and thermo-nuclear reaction. The resultant runaway reaction leads to an explosion. Since they always explode in the same way, they always give off the same amount of light. This brightness can then be seen on Earth for around a month.
How fast are galaxies moving? The wavelength of the light coming from supernovae determines its colour. If it is moving towards us, the wavelength compresses and it appears blue. If it is moving away from us, it appears red. But stars can burn with all sorts of different colours so we need to know if it was burning red to begin with. So, we look at spectral lines, created when atoms absorb certain colours. Then we can see if those lines are shifted towards the red or blue end of the spectrum.
Einstein's General Theory of Relativity is our best current understanding of gravity. John Wheeler described it best when he explained, "spacetime tells matter how to move, matter tells spacetime how to curve." How the universe evolves depends on what it's made of. How far galaxies are away from us should be related to how far they are moving.
Quantum mechanics doesn't make any sense to us at our scale. For example, particles are never perfectly still – we can't know their location and their speed at the same time. Particles have a "quantum jiggle". Even in empty space, a particle could pop into existence and then disappear again. This means that even the vacuum has energy.
The universe cannot be made of ordinary matter as it would look different to what we actually observe. Around 30% looks like matter, but 70% looks like this vacuum energy. Of most of the matter, around 95% is actually dark matter; not standard model particles but it behaves like matter in terms of the evolution of the universe. As a brief aside, dark matter was first discovered by Vera Rubin in the sixties. As a woman, she was struggling to get onto any of the really interesting physics projects and so picked the most boring one she could find. She hoped this would mean she'd be able to work on it part time while she raised her family and no-one else would be interested so no-one would publish before her. That project led to the discovery of dark matter.
Quantum jitter energy is constant. Over the age of the universe both the energy from radiation and the energy from matter have receded. There used to be substantially more energy from both radiation and matter than from dark energy in the past. However, right now the levels of energy from dark energy and matter are about equal right now. This concerns physicists as, statistically speaking, we aren't supposed to live during special times. Also of concern is the fact that physicists' models predict that the constant level of dark energy should be much higher than it is. Yet, if it was at the level predicted by the models then galaxies would never have formed as the energy would have pushed apart all of the matter before it has the chance to coalesce.
We know that the expansion of the universe is speeding up faster than it should be and "dark energy" is the name we've given to whatever it is that's causing this phenomenon. However, we don't really know what dark energy is and none of our theories have a satisfactory explanation for it. The problems all come about when we try to marry quantum theory with gravitational theory. So, could it be that one of them is wrong?
How can we learn more about dark energy? One way is to do more tests on gravity. The way to do this is by looking at gravitational lensing; looking at how matter effects the light from distant galaxies. The other option is to modify our theories of gravity, but can we find something that works on a universal scale but also on Earth and within the solar system? This requires introducing new particles that effect the large scale but hide themselves at the small scale. Galileo taught us that things fall at the same rate. However, these new particles would mean that lighter things actually fall slightly faster. So, an atom would fall faster than both a hammer and a feather.
If all of that has got a bit much, Clare has an easy way to think about dark matter and dark energy. Imagine that the universe is a loaf of bread. Dark matter is the flour and dark energy is the yeast. The flour (dark matter) gives us the structure. The yeast (dark energy) lies in the spaces between the structures pushing them apart. While this is a fantastic analogy, it does leave the possibility that a future generation could find a book written by Clare and determine that Mary Berry is God.
SciBar returns to the Vat & Fiddle on Wednesday 28 June where David Cook will talk about The Research Behind Your Pint.
SciBar MeetUp website