I was watching a video discussing possible ends of the Universe, namely the Big Freeze, the Big Crunch and the Big Rip.
For those that don’t know, the Big Freeze is when the last black holes have evaporated due to hawking radiation, all the hadrons have long since decayed and the energy distribution of the universe is uniform. The Big Crunch is where the expansion of the universe reverses and eventually converges to a singularity. The Big Rip is where the expansion of space is happening so quickly that it overcomes even strong force binding between particles who can no longer “communicate” with each other because the expansion is greater than c.
I was under the impression that the Big Crunch and Big Rip had been ruled out as possibilities. Is that not so?
Last I recall hearing about this subject was a program on Sol Perlmutter and his team, who had discovered that not only are all the stars and galaxies moving away from each other (Big Bang), but that the rate of movement is increasing. The likely result of this, according to Perlmutter, is that the universe will expand forever, gradually cooling, until only one or two lonely photons survive. When they go, game over.
There may have been more added to this theory lately, but I’m not aware of that. Sounds awful cold and lonely, but we won’t be here.
The evidence does show that we are not only expanding, but doing so at an increasing rate. As a result, the Big Rip theory is the best bet from what we know right now - a straightforward extrapolation of the observed evidence.
I don’t think we can rule anything out entirely, though. The rate of expansion is believed to have varied greatly in the past, both speeding up and slowing down at different times. To my knowledge, we have no explanation for what drives the change in expansion rates and so we can’t rule out further changes or even a reversal.
We are in the dark energy epoch, so the ultimate fate of the Universe will be decided by dark energy and in particular its pressure. Currently it has sufficiently large enough negative pressure to accelerate expansion. For the 3 possible fates you mention (though other possible fates are possible/have been theorized).:
For the big crunch to happen the pressure of dark energy must increase to a large enough positive value to reverse expansion.
For a big freeze, the pressure of dark energy must not increase or decrease too much.
For a big rip, the pressure of dark energy must decrease at a runaway rate so that it becomes arbitrarily close to negative infinity.
Not enough is known about dark energy to conclusively rule any of these fates out, but 1) is not thought very likely as it involves dark energy having a time dependence that is not due to expansion. 2) and 3) are possible with the pressure of dark energy only depending on the scale of the Universe (i.e. how much it has expanded). Simplicity and observational evidence favour 2), but it is far from settled.
None of them can be ruled out entirely, because trying to choose any of them depends on guessing what the dark energy will do in the future, and we’re worse than clueless about just what rules the dark energy obeys. The simplest assumption is that it’ll just continue to do what it’s doing now, which would mean either freeze or rip, depending on how you interpret the data (rip is actually a slightly better fit than freeze, but freeze is simpler). But hey, inflation ended, so maybe the current dark energy could end, too, in which case crunch might be back on the table.
BTW, I don’t understand why expansion only affects galaxies now, but will affect things like protons in 80 billion years or so (that’s the figure I heard).
Assuming dark energy operates the same on small scales as it does on large scales, then its effect is much, much smaller than local effects such as gravity, electromagnetism on a proton. In the big rip scenario however, the energy density and negative pressure of dark energy run away in a finite time and so overwhelm any local effects. If dark energy is in the form of a cosmological constant (which is one of the forms of dark energy that lead to the big freeze), then the energy density negative pressure remain constant and so never overwhelm local gravity, etc, however effects such as evaporation will disperse things such as galaxies, stars, black holes after an insanely long amount of time.
There’s a hypothesis that protons are not stable and decay with a half-life of 10^38 years or something absurdly huge like that. I’m not sure whether that has ever been verified, but it always shows up in discussions of the end of the universe for some reason.
It’s never been verified, but proton decay shows up in some form or another in all models to date which unify the Weak and Strong forces. There’s considerable circumstantial evidence that the Weak and Strong forces should unify, and nobody has any inkling of how that might happen without allowing for proton decay, so most physicists still consider proton decay to be fairly likely.
If the rate of expansion continues increasing along the observed path, then over infinite time, the rate will reach infinity.
The expansion is a certain rate multiplied by distance, so two galaxies are moving away from each other pretty quickly - enough that gravity is too weak to keep them bound together. That’s the state today (though some galaxies are still gravitationally bound to each other in clusters). But if the rate of expansion does grow towards infinity, then eventually no force will be strong enough to bind anything together and you end up in a Big Rip.
It depends on how much (negative) pressure vs energy density dark energy has.
If w = (pressure)÷(energy density) of dark energy and w<-1 then the time left is approximately
11.5 billion *|1+w|[sup]-1[/sup] years.
Observational evidence gives -1.25 < w <0.95, which gives a minimum time for the big rip as 46 billion years. However that doesn’t constrain the maximum time to the big rip (or indeed if w ≥ -1 then the big rip is avoided completely, which many suspect is the case). A value of w = -1.1 gives the times to the big rip as 115 billion years and if w is only very slightly less than -1 then the big rip could occur in the very, very, very far future.
The time when it occurs is important to how it plays out. If the big rip occurs in a matter of billion years there will still be stars left close to the end (before they are ripped apart). If it occurs in the very far future then the Universe in late times might be a near featureless void , much like in the big freeze scenario.
Last I heard (though there’s doubtless better data now), a Big Rip could plausibly come as soon as a few million (with an M) years from now. The first signs would come from distant supernovae, like we use to study the dark energy now, but as time went on, we’d see evidence in nearer and nearer objects, including our own Galaxy, solar system, and planet. The last stages would progress really quickly, though, so we wouldn’t have much time to ponder the Milky Way being shredded before we, too, were shredded.
EDIT: Yup, there is indeed better data now. Listen to Asymptotically Fat, not to me.
If the Big Rip happens in 46 billion years, when, I believe, there are still stars around and we can observe other galaxies, what would an observer see assuming she was immune to the effects?