Could the expansion of space tear us apart?

[Ghostcrown]

Is it possible that the forces behind the expansion of space could at some point overcome the fundamental forces holding our molecules together?

 

[mfb]

That idea is known as big rip. Possible, but it does not look likely.

 

[Chalnoth]

That idea is known as big rip. Possible, but it does not look likely.

I think that may be overstating the situation a bit.

The big rip scenario requires a violation of some of the energy conditions of General Relativity. I'm not sure it's possible to write a sensible theory of gravity if those energy conditions are violated.

So we can't say for sure that it's impossible, but it's probably impossible.

 

[PeterDonis]

I'm not sure it's possible to write a sensible theory of gravity if those energy conditions are violated.

Sure it is. Dark energy violates energy conditions, but we have perfectly sensible models in GR that include dark energy. Solutions that violate one or more of the energy conditions don't have certain "nice" properties, but they're still valid solutions.

 

[bapowell]

The negative kinetic energy of phantom fluids makes them unstable. It's all a matter of decay timescales: http://arxiv.org/abs/astro-ph/0301273

 

[Chalnoth]

Sure it is. Dark energy violates energy conditions, but we have perfectly sensible models in GR that include dark energy. Solutions that violate one or more of the energy conditions don't have certain "nice" properties, but they're still valid solutions.

Dark energy only violates the strong energy condition (a condition that is violated in other contexts as well), the "big rip" scenario violates them all. In particular, it violates the weak energy condition which requires that the observed mass density be non-negative for all observers. Negative mass can lead to all sorts of nasty consequences.

 

[PeterDonis]

Dark energy only violates the strong energy condition (a condition that is violated in other contexts as well), the "big rip" scenario violates them all.

Yes, agreed. Solutions that violate all of the energy conditions are most likely unphysical. But solutions that only violate some (like ordinary dark energy) can be physically reasonable. In post #3 you said "some", which was why I commented.

 

[Vighnesh Nagpal]

if my understanding is correct, the big rip hypothesis says that if the ratio between dark energy pressure and it's density is > -1 then the universe will eventually be pulled apart and that an instant before the end atoms will be destroyed

 

[Chalnoth]

if my understanding is correct, the big rip hypothesis says that if the ratio between dark energy pressure and it's density is > -1 then the universe will eventually be pulled apart and that an instant before the end atoms will be destroyed

Yes, essentially. In the model, the cosmological horizon is constantly decreasing in size. Eventually the horizon will get so small that it will be impossible for atoms and even atomic nuclei to hold together.

 

[marcus]

That idea is known as big rip. Possible, but it does not look likely.

Specifically, to judge for yourself how unlikely, look at the latest Planck results. The equation of state number w is measured to be WITHIN 5% of the value -1 which it has in the standard LambdaCDM model (cosmological curvature constant Lambda, no big rip)
The standard cosmic model has an INCREASING horizon distance that goes to an asymptotic value of 17.3 billion LY.
Not a DECREASING horizon distance that gets down to atom size. Huge difference.

Successive measurements keep nailing w down closer and closer to -1. So google [planck 2015 cosmological parameters] to get the report:

http://arxiv.org/abs/1502.01589
Planck 2015 results. XIII. Cosmological parameters
Planck Collaboration: ...
(Submitted on 5 Feb 2015...)
==abstract==
We present results based on full-mission Planck observations of temperature and polarization anisotropies of the CMB. These data are consistent with the six-parameter inflationary LCDM cosmology. ...
...
...The equation of state of dark energy is constrained to w = -1.006 +/- 0.045. Standard big bang nucleosynthesis predictions for the Planck LCDM cosmology are in excellent agreement with observations. We investigate annihilating dark matter and deviations from standard recombination, finding no evidence for new physics. The Planck results for base LCDM are in agreement with BAO data and with the JLA SNe sample. However the amplitude of the fluctuations is found to be higher than inferred from rich cluster counts and weak gravitational lensing. Apart from these tensions, the base LCDM cosmology provides an excellent description of the Planck CMB observations and many other astrophysical data sets.
==endquote==