If all of the normal (non-dark) matter in the Universe was turned into humans, and they were equally spread throughout the Universe, the gap between them would be about 1.7 million miles.
However, if all of the dark matter was turned into “dark humans”, and they were also equally spread throughout the Universe, the gap between the “dark humans” would be only 1 million miles.
The Universe has expanded by 0.47% in the 65 million years since the (non-bird) dinosaurs became extinct.
No, I’m not talking about a decline in your neighbourhood. Or something unmentionable that happens when the lights go out.
I’m talking about the evolution of the Universe, and how it depends on the dark sector.
The gravitational effects of the dark sector that contribute to the evolution of Universe, which is how we “observe” the existence of the dark sector, depend only on the total dark sector.
If there are different theories for the dark sector that have the same total properties, then they are identical as far as the evolution of the Universe is concerned. Even if one of the theories has the dark sector made up of 12 different things, and another of the theories has a dark sector made of a single thing. This is what is known in physics as a degeneracy.
A degeneracy occurs when there are two things that are not exactly the same in all ways, but are the same in some specific ways that are important for physics. If a particular system only cares about the ways in which the things are similar, then the system will behave the same way no matter which is present. Moreover, in this case our observations of the system will be unable to tell which of the two things is actually there.
The dark degeneracy refers to the fact that the split we have made between dark energy and dark matter is sort of arbitrary.
In other words, we explain the Universe by having the dark sector composed of two parts (dark matter and dark energy), but the Universe only really cares about the total combination of the two of them. So we could instead write down a single (more complicated) “dark thing”, that does the job of both dark matter and dark energy. Or maybe there are actually 5 or 6 “dark things” that all behave in different ways, but the combined effect of these 5 or 6 is the same as having dark matter plus dark energy.
Consider going to pay for something in a shop; for arguments sake, let’s say the item costs £5. The shopkeeper doesn’t care what coins you pay with, as long as they are worth £5. You could pay with 5 £1 coins, or 2 £2 coins and a £1 coin, or 25 20p coins… there are loads of possibilities. Now imagine you told your friend that you had bought something for £5; your friend would have no idea what coins you had paid with.
This is the situation we are in with the Universe. The Universe is telling us the total effect of the dark sector (i.e. what amount has been spent), but it is very difficult for us to actually work out the breakdown of that total effect (i.e. what coins someone paid with).
In a recent work (HERE), the authors show how two different types of dark sector model can have the same total properties, and therefore give rise to the same Universe.
Specifically, they show how a model where dark matter and dark energy talk to each other (as discussed HERE), can give exactly the same Universe as a different model, where the properties of the dark energy change with time.
If you remember the split of the Universe into the “average” Universe and “different from average” parts (discussed HERE), this new paper shows that these two theories can give the same “average” Universe and “perturbed” Universe. The authors also work out what might cause these two theories to be the same at the background level, but not at the perturbative level, and thus show that it might be possible to determine between these two different ideas.
The only real way to fix or “break” the dark degeneracy is to get non-gravitational evidence for at least part of the dark sector, i.e. to actually see it directly. For example, if we found a dark matter particle on the Earth and measured its properties, then the degeneracy would be gone.
This would be like being told that 3 £1 coins were used in our earlier example: it hugely reduces the number of possibilities. If we assume that there are only two types of coins, it almost completely breaks the degeneracy. Therefore we could be much more certain of the contents of the Universe.
Until we actually see dark matter or dark energy though, the possibility remains that there is maybe just a single “dark thing”… or maybe even 100 “dark things”…
Figure credits
(1) Pie chart from the Planck satellite HERE
(2) Hand holding money, stock photo, HERE
Imagine that we lived in our usual 4 dimensions (3 space and 1 time), but gravity (and nothing else) could also see an extra space dimension. This would make gravity seem weaker over large distances, as gravitational force would “leak” into the dimension we can’t see. This idea (called DGP-Dvali Gabadadze Porrati gravity, after its creators) was proposed in 2000, and could possibly replace the required dark energy in the Universe. However, later theoretical and observational results mean that this is now a disfavoured explanation.
On 14 September 2015, humanity received an entirely new signal from the Universe.
Until then, the only information we had received from far away objects was light, i.e. electromagnetic radiation. Whether radio waves, visible light, infra-red, gamma rays… these signals are all just light waves with a different frequency.
Then, for the first time, we detected a different kind of signal: a gravitational wave. This is literally a ripple in space itself (or, if you read THIS blog post, a ripple in the trampoline fabric). Detecting these waves gives us an entirely new way of looking at the Universe.
These waves are one of the predictions of Einstein’s theory of gravity, General Relativity. This replaced replaced the absolute space and time of Newton’s “clockwork” Universe with the notion of space-time: the very fabric of space and time themselves can be bent and twisted. This allows waves to travel through the fabric of space itself, in a way that just couldn’t happen in Newton’s theory of gravity, no matter how many apples fall on one’s head…
The key source of the gravitational waves that have been detected so far, are so-called “binary systems”, i.e. systems containing two very heavy objects. Typically these are neutron stars or black holes. These objects are so massive that, as they spiral in towards each other (eventually merging), they release vast amounts of energy in the form of strong ripples in space-time (see figure 1).
One of the things that we are always trying to measure better is the expansion rate of the Universe, and how it has changed over time. This is because the contents of the Universe (i.e. how much of it is normal matter, dark matter and dark energy) decides how the expansion rate of the Universe changes with time (as discussed in THIS blog post).
As a result, if we can measure how fast the Universe is expanding at different points in time, then we can say something about how much dark matter and dark energy there is, and even what the properties of these things are.
In practice, what this means is that we are trying to detect signals from which we can work out (1) how long they have been travelling towards us (i.e. at what point in the history of the Universe the signal was emitted), and (2) the distance that the signal has travelled (i.e. the amount of space it has travelled through, and therefore how much space has expanded).
The gravitational wave signals give us one half of this puzzle. The waves that we detect have a particular shape, which tells us how big the wave was at the point it was created (see figure 2). Gravitational waves have the property that the size of the wave decreases in proportion to the square of the distance that it travels, so a wave that travels 2 metres is 2×2=4 times weaker than a wave that has travelled only 1 metre. Since we know how big it is when we detect it, and we know how big it was when it started, we can work out how far it must have travelled.
The other half of this puzzle comes from using “normal” telescopes. When the binary systems merge, they also emit lots of light. This light can be measured and used to compute the “redshift” of the galaxy hosting the binary merger (see THIS post for more about redshift). This redshift tells us how long ago in the Universe’s history the light (and also gravitational waves) were emitted. So now we have the two parts.
Astronomers have long used light signals for the first part of this puzzle as well, using so called “standard candles”, which are light sources whose original brightness can be worked out from the signal we receive, and therefore the distance to them can be worked out in the same way as with the gravitational waves. In analogy with this, these gravitational wave signals are called “standard sirens”. The advantage of the standard sirens is that they aren’t affected by a lot of the complications and assumptions that go into standard candles, so they give us an independent check of some of our most important measurements.
There are lots of ideas for what the dark energy could be if it turns out not be Einstein’s famous cosmological constant. One of the options is that there could be an extra constituent in the Universe, and theorists like a particular type of these called scalar fields. These are fields that exist everywhere in space, providing an extra source of energy in the Universe that affects the speed at which the Universe expands, thus allowing them to explain the observed dark energy phenomena.
A recent paper (see HERE) points out an interesting possibility: some of these fields will affect light and gravitational waves differently. This means that, if we use both the “standard candles” (emission of light) and “standard sirens” (emission of gravitational waves) to measure the expansion of the Universe, we will get two different answers! Conversely, if we make these two measurements and they agree, then we will have disproved some possible explanations of dark energy.
This is a really interesting possibility that we wouldn’t be able to investigate if we hadn’t managed to detect gravitational waves. I doubt that this idea could even have been imagined when Einstein’s gravity was first written down…
Figure credits:
(1) Binary merger (HERE) from Alta online (HERE).
(2) Figure of the waveform (HERE) from Institute of Mathematics and its Applications website (HERE).
Dark Matters 