Scaling Cosmologies: Solving the Coincidence Problem

[wolram]

http://arxiv.org/abs/astro-ph/0605488

Challenges for scaling cosmologies
Authors: Luca Amendola, Miguel Quartin, Shinji Tsujikawa, Ioav Waga
Comments: 14 pages, 3 figures

A cosmological model that aims at solving the coincidence problem should show that dark energy and dark matter follow the same scaling solution from some time onward. At the same time, the model should contain a sufficiently long matter-dominated epoch that takes place before acceleration in order to guarantee a decelerated epoch and structure formation. So a successful cosmological model requires the occurrence of a sequence of epochs, namely a radiation era, a matter-dominated era and a final accelerated scaling attractor with $\Omega_{\phi} \simeq 0.7$. In this paper we derive the generic form of a scalar-field Lagrangian that possesses scaling solutions in the case where the coupling $Q$ between dark energy and dark matter is a free function of the field $\phi$. We then show, rather surprisingly, that the aforementioned sequence of epochs cannot occur for a vast class of generalized coupled scalar field Lagrangians that includes, to our knowledge, all scaling models in the current literature.

 

[Garth]

An interesting paper about one outstanding problem with the mainstream $\Lambda$CDM model.

Questions arise because of the so-called coincidence problem: why two components that are completely unrelated and scale with time in a different way, namely dark energy and matter, appear to have roughly the same energy density just now and only now.

Either the densities of dark energy and matter are linked through all time (the scaling solution) or there has been a coincidence in the initial conditions of the system.

So, in the 1970's the resolution of one problem of GR, the suite of the horizon/smoothness/flatness problems, required the invocation of Inflation , but even after much searching the Higgs boson/inflaton has never been discovered.

Then the problem of hidden gravitating mass required the invocation of non-baryonic DM, but the DM particle has never been discovered.

Then the problem of making up the density component of a flat universe and apparent cosmic acceleration required DE even though no one is sure what DE is. Now that solution itself has a problem - the problem of coincidence.

It is rather remarkable that the sequence of two scaling regimes cannot be realized for such a vast class of scalar-field Lagrangians (although, to be fair, we did not investigate thoroughly the consequences of Eq. (24) having infinite terms). This underlines how difficult it is to solve the problem of coincidence: although cosmological scaling solutions have been studied for over a decade now, no successful case has been identified and this paper shows that even a large generalization of the models does not help.

This problem seems rather difficult to resolve.

The search for a good scaling cosmology is not over yet, though.

So how much more untested and undiscovered physics will this standard model require I ask?

Garth

 

[wolram]

An interesting paper about one outstanding problem with the mainstream $\Lambda$CDM model.Either the densities of dark energy and matter are linked through all time (the scaling solution) or there has been a coincidence in the initial conditions of the system.

So, in the 1970's the resolution of one problem of GR, the suite of the horizon/smoothness/flatness problems required the invocation of Inflation , but even after much searching the Higgs boson/inflaton has never been discovered.

Then the problem of hidden gravitating mass required the invocation of non-baryonic DM, but the DM particle has never been discovered.

Then the problem of making up the density component of a flat universe and apparent cosmic acceleration required DE even though no one is sure what DE is. Now that solution itself has a problem - the problem of coincidence.

And so this problem seems rather difficult to resolve. So how much more untested and undiscovered physics will this standard model require I ask?

Garth

Wait for Gpb, garth, i will be suprised if frame draging is confirmed.

 

[Garth]

Wait for Gpb, garth, i will be suprised if frame draging is confirmed.

I will be surprised if it is not confirmed, after all the factor
(1 + $\gamma$)G has been tested in other experiments and confirmed to within 1%.

The interesting measurement will be the geodetic N-S precession in which the corresponding factor is (1 + 2$\gamma$)G, which has never been tested before.
(Edit: i.e. this will also test whether the G that enters into the description of the metric is actually the Newtonian constant - this may not be the case in a non-metric theory)

Garth

 

[blackcat007]

well i don't understand onething that is what exactly do you mean by scaling? take a look at this http://arxiv.org/abs/0904.0877 although other things are fine.. but the last two critical points one of them is termed as scaler-fluid scaling solution. what exactly do you mean by scaling?