# I Realistic coasting cosmology from the Milne model

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1. Nov 8, 2016

### Moncy Vilavinal John

In the past few months, there appear some observational results supporting the linear coasting models for the universe.

http://www.nature.com/articles/srep35596

http://iopscience.iop.org/article/10.3847/0004-637X/828/1/35/meta

This brings the coasting models again to the centre-stage of cosmology.

(By the way, the paper by Kolb, E. W. "A coasting cosmology" Astrophysical Journal 344 (1989) is not the first eternally coasting model. Moreover, it contains only some exotic K-matter in the present universe, whereas in the early phases in this model there were only radiation and matter and the situation was not different from the erstwhile standard model'.)

The first eternally coasting model, with realistic matter/energy content is in

http://www.sciencedirect.com/science/article/pii/0370269396010738

http://iopscience.iop.org/article/10.1088/0264-9381/14/5/016/meta

http://journals.aps.org/prd/abstract/10.1103/PhysRevD.61.087304

They were studied in detail also in

http://journals.aps.org/prd/abstract/10.1103/PhysRevD.65.043506

http://iopscience.iop.org/article/10.1086/432111/meta

A recent paper discusses the above works, which are antecedents to the widely discussed
The Rh= ct universe and the A Concordant" Freely Coasting Cosmology"

https://arxiv.org/abs/1610.09885
Realistic coasting cosmology from the Milne model

2. Nov 9, 2016

3. Nov 9, 2016

### Moncy Vilavinal John

That's a wild overstatement.

I hope we agree that all statements have their own subjectiveness.
In my opinion, at present, the Lambda-CDM model is not the only actor on stage in cosmology.

4. Nov 9, 2016

### Bandersnatch

As far as I can see all the coasting models ignore CMB and nucleosynthesis data. As such, these are interesting in a purely abstract, academic sense, since they simply don't fit all available observations.

5. Nov 9, 2016

### Moncy Vilavinal John

In 1996, the main difficulty with the publication of this coasting model was that it does not fit the decelerating present universe of standard model'!

Sure, lot of details are to be worked out. But the global features like the absence of cosmological problems, such as the coincidence problem and even the recent `synchronicity problem' itself are to be considered.

6. Nov 9, 2016

### Chalnoth

In 1996, we had very little data for either the CMB or for the expansion history of our universe. Now we have both, and a coasting cosmology just does not fit very early-universe data.

You *might* be able to make a coasting cosmology work if it acts as a standard cold dark matter big bang model for the early universe, then transitions to a coasting model later on, but that's going to be an even more complicated model than the current LCDM model, because you'll have to add some new interaction that makes it so that the matter density ceases to impact the expansion after a certain time.

7. Nov 10, 2016

### Moncy Vilavinal John

I agree this argument has some substance. Now we have both data of present expansion history and the early-universe data. But the question is, do we have to compromise a model that fits (if it so happens) the present data of present expansion history, for the sake of very early-universe data, that requires a lot of extrapolations for its explanation? Or is it preferable in the opposite way?

Also please note that it is not yet proven that a coasting cosmology does not fit very early-universe data either.

8. Nov 10, 2016

### Chronos

Permit me a question. While mentioned in section 5 of your paper, https://arxiv.org/abs/1610.09885, it was unclear how "unnatural dimming' of SN 1 a at large reshifts" is resolved. This is a known issue in coasting cosmologies in that it implies a hyperbolic as opposed to flat spacetime. Since it is mentioned I assume it is also addressed. Perhaps I just missed it.

9. Nov 10, 2016

### Moncy Vilavinal John

The unnatural dimming of SN 1 a at large redshifts led to the discovery of accelerated expansion of the universe. This is just mentioned.

10. Nov 10, 2016

### Chalnoth

As somebody who has worked with CMB data for a number of years, yes, it is.

One way to see this is that the coasting cosmology is a universe where $\Omega_m = \Omega_\Lambda = 0$. Below is a plot of the constraints on $\Omega_m$, $\Omega_\Lambda$ from a few important data sets (attached, source).

The origin at the bottom left represents the coasting cosmology. Note that it's reasonably far from the supernova contour, but not ridiculously so. The BAO contour is an estimate of how much mass there is, and since in a coasting cosmology the amount of mass doesn't matter, it can be ignored for the purpose of this discussion.

The orange contour is the one I'm talking about: the CMB. The CMB contour doesn't come anywhere near the origin. I'd have to dig into the numbers to be sure, but it's easily dozens of standard deviations away. There just isn't any way to reconcile this.

11. Nov 10, 2016

### Moncy Vilavinal John

All coasting models are not with Ωm=ΩΛ=0. That is the entire point of the paper 1610.09885 discussed here .

12. Nov 10, 2016

### Chalnoth

In terms of their impact on the rate of expansion over time, that's precisely the definition of a coasting cosmology.

Sure, if you have some form of exotic dark energy which couples with matter in such a way to make it so that the pressure/energy density of our universe is always -1/3, then you could have a coasting cosmology without $\Omega_m=\Omega_\Lambda=0$, but in terms of the data based off of expansion rates over time, the results are still identical.

The fact that the CMB data depends on the matter density in other ways besides just the expansion history may make it a little less horrible for the model, but not by much. There's still a strong dependence of the CMB data on the expansion history alone, and I don't think there's any possibility of a model like this fitting that data.

13. Nov 10, 2016

### Moncy Vilavinal John

Sorry, no, the results are not identical. When we write the scale factor for a coasting model as a = mt, then the apparent magnitude-redshift (\mu -z) relation involves the parameter m. For details, please see the paper http://journals.aps.org/prd/abstract/10.1103/PhysRevD.65.043506