Currently, Dark energy (68.3%) and Dark matter (26.8%) together constitute about 95.1% total matter-energy content of the universe while only 4.9% is ordinary baryonic matter. Was this always the case? Are these ratios constant since the big bang or did they change over time?


Short answer: The ratios have changed over time... drastically. This is a consequence of the expansion of our universe.

Initially (and by that I mean after the conjectured inflationary epoch, which I will not consider here), radiation dominated all other forms of energy by far. However, as the universe expands---as measured by the increase of the ''scale factor'' $a(t)$, the radiation energy density scales as $\rho_\text{rad}\propto a^{-4}$ while non-relativistic matter has an energy density that scales as $\rho_\text{mat}\propto a^{-3}$. Note that this includes dark matter, which is currently widely thought to be composed of matter that does not interact electromagnetically but is otherwise similar to ''normal matter''. Furthermore, dark energy corresponds to a constant energy density per unit volume, i.e. it does not depend on the scale factor at all: $\rho_\Lambda\propto a^0$.

To understand the scaling law for matter, simply think about particles in an expanding box of volume $V(t)\propto a^3(t)$ where $a(t)$ is a characteristic ''side length''. The particle density, which is proportional to the energy density for non-relativistic particles, scales as $V^{-1}\propto a^{-3}$: The energy density decreases purely due to dilution. For radiation there is not only dilution, but also the fact that the energy of e.g. a photon (or another relativistic particle) is proportional to the wavelength, which also depends on $a$ as $\lambda\propto a^{-1}$, i.e. $\rho_\text{rad}\propto a^{-3}a^{-1}=a^{-4}$ due to both redshifting and dilution.

Considering these facts, it is clear that although radiation initially dominated, (non-relativistic) matter and dark energy become more and more important in time (as $a(t)$ keeps on growing). Hence, at some point matter started dominating (initially $\rho_\text{mat}$ was significantly higher than $\rho_\Lambda$) but eventually most of the energy density of the universe is concentrated in dark energy. This is the epoch we are currently entering, since $\rho_\Lambda$ is now more than 50% of the total energy density, but not yet orders of magnitude higher than all other forms of energy.

As an aside, it is interesting to note that there could potentially have been a fourth energy component, due to spatial curvature, which would scale as $a^{-2}$. However, it is an experimental fact that the spatial curvature of the universe is either exactly zero or negligibly small.

  • $\begingroup$ When you're talking about "a" universe instead of "the" universe (or "the multiverse", to use the phrase popular since the theory of inflation appeared in the 1980's), do you think that the converse would necessarily have occurred in a "different part of space", or in a "different epoch of time", or might not necessarily have occurred at any physical time or in any physical space? (Sorry to be asking for a reconstruction of ideas 3 yrs. old, but the view of spacetime as less than fundamental, or as mass-dependent, is hard to wrap the mind around.) Is any bias toward symmetry valid? $\endgroup$
    – Edouard
    Aug 26 '19 at 10:50
  • 1
    $\begingroup$ For all we know, our universe is the only one, and there is no evidence that there are other universes of any kind, in which other laws of physics hold, or other conditions are present. Nevertheless, one can theoretically imagine things like a multiverse. $\endgroup$
    – Danu
    Aug 26 '19 at 16:58

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