# Why is the ratio dark matter / normal matter bigger today than in the past? Is it the dark matter that increased or the normal matter that decreased?

These two pie charts clearly show that the ratio dark matter / normal matter is bigger today than it was in the past.

But why? How is it possible?

And does it mean that the amount of dark matter increased or that the amount of normal matter decreased?

The only explanation that I could come up with is that there are extra dimensions and dark matter contrary to normal matter can freely move in these extra dimensions. So dark matter outside our 3D universe came inside our universe, which made the amount of dark matter in our universe increase.

• The two ratios are approximately the same. They're not exactly the same because the left plot is based on incomplete data. In fact, the source for that image explicitly states "Warning! This graph will continue to change slightly as better and better data is collected" – lemon May 14 '15 at 17:07
• The ratio 63/12 is bigger than 23/4.6 ? – Rob Jeffries May 14 '15 at 17:45
• These graphs actually clearly show that the dark matter/normal matter ratio is smaller today than in the past (not significantly so). I don't know where you got larger today from. – Jim May 14 '15 at 17:58

The ratio of dark to baryonic matter is 5.25 in the first diagram and 5 in the second diagram, but I don't think the difference is significant. We don't know the densities with absolute certainty, especially near the Big Bang, and the small difference between the ratios is probably just down to the uncertainties in the densities.

We would expect the ratio to stay fixed because the densities both scale as $a^{-3}$, where $a$ is the scale factor. For comparison the density of relativistic matter (photons and neutrinos) scales as $a^{-4}$ and density of dark energy stays constant i.e. it doesn't depend on $a$ at all. That's why the photons and neutrinos have disappeared in the second diagram while a large chunk of dark energy has appeared.

As far as I know there is no mechanism for dark and baryonic matter to interconvert, or for either to disappear.

• New experiments showed neutrino oscillations and therefore the neutrino has a non-zero mass (also confirmed by new results of the Planck mission). In my opinion, it will change the picture dramatically: neutrinos are not relativistic matter anymore. But then, the ratio increases from 2.8 to 5.4. Is anything wrong in the argumentation? – Torsten Asselmeyer-Maluga Sep 29 '17 at 18:08

In the FLRW model which is used today normal (baryonic) matter and dark matter together add up to matter. It dilutes with the growing radius to the third power (because volume is proportional to radius³). So the ratio matter : dark matter is and was always the same, at least in the plot you showed and the model that was used there.

Edit: Maybe the 5.25 to 5 change John Rennie mentioned comes from matter which has been transformed into radiation with time (the sun for example radiates away 4e26 Watt, which means it converts a little more than 4e9 kg of baryonic matter to photons every second. Dark Matter on the other hand can or at least does not transform into radiation energy like normal matter does inside stars).

• If this were the explanation, then there would be an extra sliver of photons in the second pie chart. In fact, despite the question, the ratio of DM/BM has decreased between the two charts, so making BM disappear doesn't help. – Rob Jeffries May 14 '15 at 17:48
• Right, then it would have to be the other way round. In this case forget part 2 of my answear. – Yukterez May 15 '15 at 1:14

The following graph indicates the amount of radiation, matter (which almost is entirely made of dark matter), and dark energy vs time. We are at a point in time where dark energy has just started to dominate. For instance, if you go back to the first few years, radiation was the most abundant source of energy.

I the very early universe there was very little atoms. Atoms are mainly made from exploding stars. I the first pie chart there are less atoms and more photons. This is because that was the initial state and the universe. It was highly fence with photons and neutrinos going in all directions. They will be more of all materials in the second pie chart. For matter and universe has increased in volume with starts ending and expelling larger atoms. The Higgs field continues to create matter. Its just the ratios that have changed. Its not worth put the photons and neutrinos on the second pie chart. Although there quantity has dramatically increased the ratios now are so much lower that the atom ratio. More humility learning needs to come before your question about dark matter ratios can be answered.

• The Higgs field does not create matter. – pfnuesel Nov 5 '16 at 11:01

## protected by Qmechanic♦Feb 19 '17 at 19:15

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