# The dark cosmic coincidence problem

If the Big Bang theory and dark stuff theory is correct then what are the odds that humans are alive on planet Earth and in this universe when you have amazing cosmic coincidences involving the expansion/acceleration of the universe and dark matter/energy?

Welcome to the dark cosmic coincidence problem.

Invisible dark matter and dark energy make up around 95 per cent of the universe between them. What’s more, their densities are roughly equal – a state of affairs so unlikely that cosmologists have taken to calling it the cosmic coincidence problem.

Is this a genuine conundrum? At first blush, it seems contrived. Dark matter, which gravitates like normal matter, accounts for about 27 per cent of the universe. Meanwhile some 68 per cent is dark energy, the stuff that is causing the expansion of the cosmos to speed up. Not quite so equal after all, then.

But the values are still close enough to be perplexing – and according to our standard cosmological model, the similarity is relatively new.

Cosmic dark matter and energy balance – for now. Coincidence? | New Scientist

### Fine tuning a dark problem

While the dark energy helps us explain a great many things, it also resurrects an old problem once thought buried—the idea that our universe is the product of a highly unlikely cosmic coincidence. During the decades following common acceptance of the Big Bang model, physicists and astronomers tried very hard to measure the composition of the universe. According to theory, the average density of the universe would determine its ultimate fate. A universe with too little matter would expand forever, and its average density would eventually drop to zero. A universe with too much matter, on the other hand, would one day collapse under its own gravity (the ‘Big Crunch’). Only one special value, the critical density, could prevent both a Big Crunch and the unchecked expansion of the universe ...

The Big Bang model, however, still had a big problem: our low-density universe could only arise from a highly unlikely coincidence of initial conditions. An expanding universe is fine in principle, but it mustn’t expand too quickly! For galaxies, stars, and planets to form, the average density of matter has to stay relatively high for at least a few billion years. To satisfy even this one vague constraint, it turns out that the initial density of the universe would have had to be very close to the critical value1.

How close? The answer is a bit hard to swallow even to a disinterested physicist! A difference of one part in a million billion (1015) would allow galaxies to form before the expansion of the universe pulls everything too far apart for new structures to form. This is known as a fine-tuning problem: to explain the observed properties of the universe under the Big Bang model, physicists had to assume a very specific value for its initial density.

A Cosmic Coincidence Resurrects the Cyclical Universe | Phys Org

## cosmic coincidence problems and solutions

We suggest a paradigm that might allow for a non-anthropic solution to the cosmic coincidence problem of why the density of vacuum energy and matter are nearly equal today. The fact that the half life of Uranium 238 is near to the age of the solar system is not considered a coincidence since there are many nuclides with a wide range of half lives implying that there is likely to be some nuclide with a half life near to any given time scale.

Toward a Possible Solution to the Cosmic Coincidence Problem (PDF)

A coincidence of the constants of the Friedmann cosmology is recognized and identified as an epoch-independent symmetry relation between vacuum and matter. A possible physical explanation of the relation is discussed on the basis of the freeze-out process at the electroweak temperatures in the early Universe. The symmetry relation is used to suggest a simple solution to the ‘cosmic coincidence problem’.

Physical vacuum and cosmic coincidence problem

Among the suggested solutions to the cosmological constant problem, we Ønd the idea of a dynamic vacuum, with an energy density decaying with the universe expansion. We investigate the possibility of a variation in the gravitational constant as well, induced, at the cosmological scale, by the vacuum decay.

The cosmic coincidence in Brans-Dicke cosmologies (PDF)

We show that holographic dark energy could explain why the current dark energy density is so small, if there was an inflation with a sufficient expansion in the early universe. It is also suggested that an inflation with the number of e-folds N≃65N≃65 may solve the cosmic coincidence problem in this context. Assuming the inflation and the power-law acceleration phase today we obtain approximate formulas for the event horizon size of the universe and dark energy density as functions of time. A simple numerical study exploiting the formula well reproduces the observed evolution of dark energy. This nontrivial match between the theory and the observational data supports both inflation and holographic dark energy models.

Dark energy, inflation and the cosmic coincidence problem