# Big Rip implies new Big bang ?

1. Dec 22, 2013

### nicoo

Hi,

In the "Big Rip" scenario, the expansion of spacetime increases exponentially. As a result, it will torn apart galaxies, later the system solar, then the Earth itself, atoms....

If the expansion keeps on increasing, at some point would it be strong enough to separate virtual particles too ?
If so, would it look like a new Big Bang, where new particles are created from scratch ?

2. Dec 22, 2013

### friend

In which case how could we tell an original big bang from another big rip?

3. Dec 22, 2013

### phinds

Well, first, there is no proof that expansion will ever have any effect at the level galaxies, much less particles and even if it did, why would you think that would create a new big bang? We have no idea what the big bang was but your scenario does not sound like whatever it was (although, since we don't know, we can't absolutely rule it out)

4. Dec 23, 2013

### nicoo

I am not questionning whether the Big Rip scenario is right or wrong, in my question I suppose it is right. If the expansion rate grows exponentially, at some point it should be strong enough to separate virtual particles/anti-particlues. So suddently the Universe would be full of particles, just like if a new Big bang would have happened.

Good question, I am tempted to say
- classical Big Bang: volume of Universe is point-like with a Energy density almost infiny
- Big Rip --> Big Bang: volume of Universe is already infinite with a Energy density almost infiny
Yet, I have not idea if they would evolve the same and how to distinguish the 2...

5. Dec 23, 2013

### skydivephil

6. Dec 23, 2013

### nicoo

Thanks skydivephil, interesting article indeed.

I would be nice to have more info about how they make things to contact and if they have considered the expansion as a factor to possibly separate virtual particles..

7. Dec 23, 2013

### skydivephil

Well they claim its well motivated, you can read their original paper here:
http://arxiv.org/abs/astro-ph/0608138

Ive always wondered what would happen if the accelrated expansion of theu nvierse ever got the point where it was undergoing inflation, would we not see a new period of reheating and have something like the new big bang you mention?

8. Dec 23, 2013

### bapowell

The big rip scenario is distinct from ordinary inflation, in that the big rip is characterized by $\dot{\rho} > 0$ (whereas in ordinary inflation $\dot{\rho} \leq 0$.) Energy density that grows as the universe expands is called phantom energy, and it has been speculated that at the big rip all bound structures, including atoms, will be torn apart by the expansion.

"Virtual particles", if you choose to refer to them, are affected by ordinary inflation in much the same way as they would be during phantom-driven expansion leading up to the big rip. It is the effect of the inflating spacetime on ephemeral quantum fluctuations (virtual particles) that creates the large scale spatial perturbations that lead to structure formation. This process does not resemble a big bang event. Furthermore, at the future singularity of the big rip, the universe is essentially torn asunder -- it is a destructive rather than generative event.

9. Dec 23, 2013

### nicoo

I agree

Not sure to follow you here

If space enpansion rate is great enough to not let the virtual particles recombine and annihilate (because once created they are already too far from each other to recombine), then why would it not look like a creation of particles ? (this would happen because the singularity of the big rip)

10. Dec 23, 2013

### bapowell

I try to resist explanations in terms of virtual particles, because they are limited and their correspondence with reality is unclear. But, let's see how far we can get. During ordinary inflation, virtual particle pairs are separated by the exponential expansion of space, just as you mention. This is why de Sitter space has an associated temperature (the de Sitter temperature associated with the cosmological event horizon, analogous to the Hawking temperature of the black hole horizon). So during inflation, you have a de Sitter temperature associated with the vacuum on account of the expansion. But something else important is happening at the quantum level during inflation, and it is not captured by the virtual particle version of events. It involves the quantum fluctuations of the inflaton field itself: these fluctuations are amplified by the expansion to superhorizon scales where they manifest themselves as classical curvature perturbations. These perturbations form the seeds of structure formation after inflation ends. This is one of those cases where a naive interpretation in terms of virtual particles is unhelpful -- it's best to work on the level of the field fluctuation without adorning it with any particle interpretation. (In the case of the inflaton field, physically these fluctuations correspond to the uncertainty in the position of the field in field space -- not as actual quanta.)

So your question is essentially this: does the de Sitter temperature of an inflating spacetime resemble a big bang event? I'm saying no, since although particles are being created, they are being continuously diluted by the inflating background. The end of inflation -- reheating -- when the universe is populated with radiation (essentially from the latent heat of the inflationary phase transition), does resemble a big bang event and is taken by modern cosmologists to correspond to the effective "big bang" in our observable part of the universe.

I don't see how one reheats in a universe that's been torn apart in a big rip!

11. Dec 30, 2013

### timmdeeg

Just to understand you correctly, do these curvature perturbations correspond to the CMB anisotropies?

Another question, are there any theoretical ideas regarding fluctuations of the inflaton field on much larger scales, e.g. the size of the observable universe?

12. Dec 30, 2013

### phinds

Yes. As bapowel says, they are the seeds of the formation of large structures (galaxies, etc)

Very interesting question. I'll be interested to see what more knowledgeable folks, such as bapowell, have to say about that.

13. Dec 31, 2013

### bapowell

Yes. Inflaton fluctuations on the scale of the present-day universe were generated early on during inflation and were "stretched" by the expansion to super horizon scales. Only after inflation ended and non-accelerated expansion took over did the horizon grow to "catch up" to these fluctuations. The theory governing these fluctuations is the same as that governing those on all other scales; they only differ from fluctuations that are currently sub horizon in that they were generated earlier in the inflationary epoch. Fluctuations on the current horizon scale "left the horizon" around 60 e-folds before inflation ended; they contribute strongly to the quadrupole temperature anisotropy in the cmb.

14. Jan 1, 2014

### timmdeeg

Thank you. It helped to do a little search and according to that the separation due to the quadrupole anisotropy is about 90°. So, it seems that assuming perturbations on an even larger scale (if this makes sense at all) wouldn't be detectable.

I am astonished that while calculations based on the inflaton field yield precise predictions, it's nature and how it curves the space is in the dark. Hmm, in case the nature of this field itself is not somehow coded in the CMB, could it then be an arbitrary field?! It would be great, if you could comment on this.

15. Jan 1, 2014

### bapowell

Yes this is correct -- we cannot directly observe fluctuations that are outside the horizon but we expect them to be there.
Once we adopt a model of inflation: select a potential, fix the gravitational couplings, etc. we can understand not only the perturbations but also the evolution of the inflaton field itself. These dynamics include how the inflaton curves space and other aspects of its phenomenology. True -- we don't know what the inflaton is, but we do understand how scalar fields behave gravitationally according to GR.

Generally, yes, the properties of the inflaton are manifested in the perturbation spectra and these are encoded in the CMB.

16. Jan 2, 2014

### timmdeeg

However I guess, even the earliest possible quantum fluctuations of the inflaton field though causing energy density anisotropies much larger than the horizon, the cosmological principle wouldn't be violated. So, it seems, there is no causal connection between those quantum fluctuations and the energy density of the universe as a whole.

I would like to neglect the quantum fluctuations of the inflaton for the moment, because on scales large enough the anisotropies are vanishing.

Let's consider the scalar inflaton field at the moment just before it started to roll down the potential energy hill.
Do we have any reason to say that at this moment the whole universe must have had a definite energy density and thus a definite spatial curvature? I think so, but feel unsafe, because perhaps this era isn't governed by GR, but by QT. On the other hand at the Planck era (which I hopefully connect correctly with said moment, before inflation starts) the universe could even have been spatially infinte, obviously requiring a definite homogeneous energy density. Possibly the amount of which doesn't depend on the potential energy of the inflaton field but on its unknown nature. Would this reasoning be acceptable?

17. Jan 3, 2014

### bapowell

Yes, we generally suppose this to be true. The energy scale of inflation is fixed by the amplitude of a primordial gravitational wave signal (observed through CMB polarization). The absence of this signal at the sensitivity of the Planck satellite puts the energy scale of inflation below something like 10^15 GeV -- on or about the GUT scale, a couple of orders of magnitude below the Planck scale where we expect strong quantum gravitational phenomena to be operative. Therefore, GR is sufficient for understanding the gravitational degrees of freedom; QFT is still needed to treat to the matter fields which puts inflationary physics in the realm of semi-classical theories (classical gravity + quantum matter fields that don't back react on the geometry).

Given that the semi-classical theory is under control, we can understand homogeneous energy densities based on the underlying quantum field theory of the inflaton, and through Einstein's Equations, relate this density to a well-defined curvature.

18. Jan 3, 2014

### timmdeeg

Great, thanks for giving me new insights and for spending your time!