The concept of accelerating expansion does get you wondering just how much it can accelerate. Theorists think there still might be a chance of a big crunch, a steady-as-she-goes expansion or a big rip. Or maybe just a little rip?
The concept of accelerating expansion does get you wondering just how much it can accelerate. Theorists think there still might be a chance of a big crunch, a steady-as-she-goes expansion or a big rip. Or maybe just a little rip?

Cosmology, Dark Energy

Astronomy Without A Telescope – Big Rips And Little Rips

2 Jul , 2011 by

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One of a number of seemingly implausible features of dark energy is that its density is assumed to be constant over time. So, even though the universe expands over time, dark energy does not become diluted, unlike the rest of the contents of the universe.

As the universe expands, it seems that more dark energy appears out of nowhere to sustain the constant dark energy density of the universe. So, as times goes by, dark energy will become an increasingly dominant proportion of the observable universe – remembering that it is already estimated as being 73% of it.

An easy solution to this is to say that dark energy is a feature inherent in the fabric of space-time, so that as the universe expands and the expanse of space-time increases, so dark energy increases and its density remains constant. And this is fine, as long as we then acknowledge that it isn’t really energy – since our otherwise highly reliable three laws of thermodynamics don’t obviously permit energy to behave in such ways.

An easy solution to explain the uniform acceleration of the universe’s expansion is to propose that dark energy has the feature of negative pressure – where negative pressure is a feature inherent in expansion.

Applying this arcane logic to observation, the observed apparent flatness of the universe’s geometry suggests that the ratio of dark energy pressure to dark energy density is approximately 1, or more correctly -1, since we are dealing with a negative pressure. This relationship is known as the equation of state for dark energy.

In speculating about what might happen in the universe’s future, an easy solution is to assume that dark energy is just whatever it is – and that this ratio of pressure to density will be sustained at -1 indefinitely, whatever the heck that means.

But cosmologists are rarely happy to just leave things there and have speculated on what might happen if the equation of state does not stay at -1.

Three scenarios for a future driven by dark energy - its density declines over time, it stays the same or its density increases, tearing the contents of the universe to bits. If you are of the view that dark energy is just a mathematical artifact that grows as the expanse of space-time increases - then the cosmological constant option is for you.

If dark energy density decreased over time, the acceleration rate of universal expansion would decline and potentially cease if the pressure/density ratio reached -1/3. On the other hand, if dark energy density increased and the pressure/density ratio dropped below -1 (that is, towards -2, or -3 etc), then you get phantom energy scenarios. Phantom energy is a dark energy which has its density increasing over time. And let’s pause here to remember that the Phantom (ghost who walks) is a fictional character.

Anyhow, as the universe expands and we allow phantom energy density to increase, it potentially approaches infinite within a finite period of time, causing a Big Rip, as the universe becomes infinite in scale and all bound structures, all the way down to subatomic particles, are torn apart. At a pressure/density ratio of just -1.5, this scenario could unfold over a mere 22 billion years.

Frampton et al propose an alternative Little Rip scenario, where the pressure/density ratio is variable over time so that bound structures are still torn apart but the universe does not become infinite in scale.

This might support a cyclic universe model – since it gets you around problems with entropy. A hypothetical Big Bang – Big Crunch cyclic universe has an entropy problem since free energy is lost as everything becomes gravitationally bound – so that you just end up with one huge black hole at the end of the Crunch.

A Little Rip potentially gives you an entropy reboot, since everything is split apart and so can progress from scratch through the long process of being gravitationally bound all over again – generating new stars and galaxies in the process.

Anyhow, Sunday morning – time for a Big Brunch.

Further reading: Frampton et al. The Little Rip.

By  -      
Steve Nerlich is a very amateur Australian astronomer, publisher of the Cheap Astronomy website and the weekly Cheap Astronomy Podcasts and one of the team of volunteer explainers at Canberra Deep Space Communications Complex - part of NASA's Deep Space Network.



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Lawrence B. Crowell
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Lawrence B. Crowell
July 3, 2011 3:26 AM
The FLRW equation for the scale parameter a = a(t) is (a’/a)^2 = 8?G?/3 – k/a^2 where a’/a = H, the Hubble parameter, and flatness means k = 0, spherical geometry is k = 1 and hyperbolic geometry is k = -1. There is an equation of state for the material in the spacetime d(?a^3)/dt + pda^3/dt = 0 which is most relevant for the case of radiation and matter dominated universe with p = ?/2 and ? a^3 = constant for radiation. The general equation of state is p = w?. Since we are interested primarily in the cosmological event horizon I will consider the de Sitter spacetime, which has ? = constant. The FLRW equation for… Read more »
Lawrence B. Crowell
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Lawrence B. Crowell
July 3, 2011 12:06 PM
That is basically how it works. I wrote some years ago papers which in this context had a sum(all) = 0, so the net mass-energy content of the universe is zero. This was something I realized after the discovery of the accelerated expansion of the universe. Energy conservation requires there to be some time translation invariance principle. In the case of general relativity this means there must be some isometry condition for a vector projected on any geodesic or path in spacetime. This vector is called a Killing vector. These vectors are eigenvectors of the Weyl curvature, and for a spacetime one can compute them. As a rule of thumb, if the metric terms contain some dependency on… Read more »
Anonymous
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Anonymous
July 4, 2011 9:30 PM

“the net mass-energy content of the universe is zero” – ?!?

Do you mean the M-E content remains constant or do you really mean total M-E is zero?
Right after the big bang dark energy was ~zero and there was a lot of ‘normal’ energy/mass. Does what you say imply that after the big bang there was a kind of “potential dark energy” just as large as (but opposite or negative in value) the amount of ‘normal’ energy/mass, and that potential dark energy is gradually turning into ‘active’ dark energy?

Lawrence B. Crowell
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Lawrence B. Crowell
July 5, 2011 3:02 AM

Dark energy was 10^{100} times what it is today during the inflationary period. The accelerated expansion of the universe was such that any region with a radius R increased in radius by e^{63}R or 2.3×10^{27}R, or so called 63 efolds. The dark energy “crashed” in the reheating period which generated the matter-energy we observe, and a residual dark energy remains in the cosmological constant. This reheating period is similar to a thermodynamic phase transition with the release of latenet heat of fusion. That latent heat is the thermal form of the big bang.

The total energy of the universe is constant, which is basically set to zero.

LC

Anonymous
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Anonymous
July 5, 2011 9:57 AM

Wow Mr Crowell. I wonder how many people know this. I didn’t.
Is there consensus among physicists that this is how it went, and that if you add up everything in the universe the sum is zero?
If there is, then this is such a fundamental insight, it should be basic education for everyone.
Thank you, and Mr Nerlich, very much for teaching us.

Lawrence B. Crowell
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Lawrence B. Crowell
July 5, 2011 12:22 PM
I should say that the mass-energy associated with the de Sitter vacuum is a new zero. Other mass-energy is a perturbation on the de Sitter spacetime, and the conservation of this can’t be established by general relativity. However, it may be that this is a “frozen” quantum fluctuation on the de Sitter spacetime, which will attenuate away over a long time period. This gets into deeper issues, for the de Sitter vacuum or spacetime is actually a boundary for an anti-de Sitter space of one dimension higher, and this gets into string & M-theory of Dp-branes. The observation that the universe is a net zero was first mentioned by E. P. Tyron back in 1973, who argued that… Read more »
Torbjorn Larsson OM
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Torbjorn Larsson OM
July 5, 2011 6:15 PM
Hold your galloping universe! =D I don’t think there is a generally agreed on test for this as of yet. I come from this with another perspective than lcrowell, because the inner workings of general relativity is unfamiliar to me. However, a few years after standard cosmology was more or less accepted (with the WMAP 2004 results, I would say), I heard about this idea in a cosmological setting. The reference is work by Faraoni et al on the energy of FRW universes as dynamical systems. It turns out Minkowski and inflationary de Sitter spaces are zero energy. So our universe is too. Note that this is a general result and on the resulting total energy. Now this… Read more »
Anonymous
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Anonymous
July 5, 2011 7:57 PM

So my galloping universe probably has zero energy.
It does seem to be real and it is galloping like mad. I don’t know how to hold it… Perhaps it would annihilate when it would stop – the only way to maintain the sum at zero

Anonymous
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Anonymous
July 4, 2011 9:30 PM

“the net mass-energy content of the universe is zero” – ?!?

Do you mean the M-E content remains constant or do you really mean total M-E is zero?
Right after the big bang dark energy was ~zero and there was a lot of ‘normal’ energy/mass. Does what you say imply that after the big bang there was a kind of “potential dark energy” just as large as (but opposite or negative in value) the amount of ‘normal’ energy/mass, and that potential dark energy is gradually turning into ‘active’ dark energy?

Anonymous
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Anonymous
July 3, 2011 4:03 AM

Consider that “dark energy” may have passed over a (next) dimensional horizon just beyond the distant red shifted galaxies. This curvature could conceal further massive galactic materials (we call “dark energy”) that interact gravitationally with our current visible universe. Imagine the curvature of the Earth stepped up one more dimension. This horizon would exist at the periphery, in all directions, of our currently perceived four dimensional universe.

Multiple dimensions have been foretasted. This simply involves further dimensionality and gravitation. Do the thought experiment: Imagine.

Olaf
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Olaf
July 3, 2011 10:13 AM

If you are assuming that there is a massive hidden sphere of massive amounts of mass just beyond the visible universe then you have to realize that this sphere will collapse on itself and any gravitational effects inside the sphere gets cancelled out by the opposite side jut like a charge inside a metal sphere is zero.

peter fraser
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peter fraser
July 3, 2011 5:32 PM

I expect that there will be an eventual gravitational implosion to singularity followed by a reflective big bang, followed by a gravitational implosion, followed by…

Lawrence B. Crowell
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Lawrence B. Crowell
July 3, 2011 11:18 PM

The de Sitter spacetime will exponentially expand into a vacuum state, which will obtain in about 10^{100} years. There will still be a horizon at r = sqrt{3/?}, which will over a huge period of time quantum mechanically decay and the universe will end up as an empty flat spacetime as time — > ?. The universe does not appear to be such that it will recollapse.
LC

peter fraser
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peter fraser
July 4, 2011 12:31 AM

Is the reach of gravity infinite in time and space? Lets you and I wait this situation out and see what happens and pick up this conversation in a trillion years. We should have some clarity of pattern by then! Thanks for contributing to the thought process.

Lawrence B. Crowell
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Lawrence B. Crowell
July 4, 2011 3:10 AM

It is similar to escape velocity. For the Earth with mass M a projectile moving out at v > sqrt{2GM/r} will escape Earth. The expansion of the universe is similar, and further with the de Sitter metric it keeps accelerating.

LC

Jamie Kitchen
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Jamie Kitchen
July 4, 2011 2:47 PM

Hello Icrowell. I am trying to follow your mathematics and it has been a long time since college for me. One way out question I have is that is there any way that the Dark energy that is being created (And cancelled out by negative work) could potentially turn into the equivalent mass? I remember that energy and mass are equivalent from a universal content perspective and can be converted under the correct conditions.

Lawrence B. Crowell
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Lawrence B. Crowell
July 4, 2011 3:40 PM

The dark energy of the universe does bleed away. It quantum tunnels into the formation of nascent cosmologies. Eventually the de Sitter vacuum will decay into a Minkowski spacetime. Attempting to mine dark energy will prove to be tough. In a cubic meter of space there is about one proton equivalent of dark energy. This is about 10^{-11}joules, which would not be enough light the smallest spark. Trying to “grab” that energy would be tough, for it would require some “unobtainnium” field which couples to it strongly.

LC

Torbjorn Larsson OM
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Torbjorn Larsson OM
July 5, 2011 7:17 PM
Is the reach of gravity infinite in time and space? Classically long-range forces like EM and gravity has to be exactly infinite (r^-2 dependence). That is one of the few things that can be “proved” by math – if the theory is tested as valid already. =D Of course there is no guarantee that this is a fact of an actual universe, since there are constraints such as cosmological scales. As I mentioned in another comment here, FLRW universes are zero energy objects so guaranteed existence for infinite time, and standard cosmology which belongs to that class looks to specify that explicitly. It also looks like the natural state of inflationary universes are infinite volume, and again our… Read more »
peter fraser
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peter fraser
July 7, 2011 12:40 AM

Thank you for your response.

Raimonds Cirulis
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July 3, 2011 11:30 PM

pls dont forget that gravity/time/space is strongly inerconnected. What we call time, within that time it flows differently,we dont have zero point to measure the real time , and , within zero point, the time even exist in categories we are unable to name it,and non-exist, thanks to the same time/gravity/space interconnections.So the change within universe dont change the universe, we simply are watching a kind of inner changes, it does not mean that universe expands, as is absolute and ended, all what we see is projections

Torbjorn Larsson OM
Member
Torbjorn Larsson OM
July 5, 2011 7:02 PM

What we call time, within that time it flows differently,

Evidently time doesn’t flow, if you take its derivative it is constant: dt/dt = 1.

It is precisely that which general relativity (GR) explains, by taking the idea of local clocks seriously. Time proceeds uniformly (by GR), space is flat (by GR applied as standard cosmology), so instead spacetime has to curve to explain observation (of different reference frames or clocks).

Raimonds Cirulis
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July 3, 2011 11:30 PM

pls dont forget that gravity/time/space is strongly inerconnected. What we call time, within that time it flows differently,we dont have zero point to measure the real time , and , within zero point, the time even exist in categories we are unable to name it,and non-exist, thanks to the same time/gravity/space interconnections.So the change within universe dont change the universe, we simply are watching a kind of inner changes, it does not mean that universe expands, as is absolute and ended, all what we see is projections

bugzzz
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bugzzz
July 3, 2011 6:04 PM
As a curious observer with no science credentials I find all of this pretty interesting and at the moment there is a lot of room for imaginative speculation. My main issue is trying to “resolve” the many “supernatural” experiences I have had. I have observed and experienced inexplicable physical phenomena, even in the presence of others, that lead me to wonder whether there is really some form of consciousness holding the universe together. I don’t expect most scientists would like this explanation but even quantum weirdness implies the universe is quirky. And one can never seem to solve the chicken/egg problem of what came first. So to me the universe and all life is based on a contradiction… Read more »
peter fraser
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peter fraser
July 3, 2011 6:46 PM

This reality is big, complicated and shiny; even tending toward blinding. Personally, I would just assert that there is conscientious in this universe and this conscientious is this universe: Full stop.

bugzzz
Member
bugzzz
July 4, 2011 2:21 AM
I don’t believe I have expressed any incredulity or suggested anyone lacks imagination just b/c they haven’t had a “supernatural” experience. But the things I’ve seen came around periods of great relaxation and clarity in my mind so I’ve admitted that i don’t know if they could be scientifically verified under controlled conditions. But if I found someone open to exploring some form of testing with an open mind I’d probably try it. By the way, I have no interest in organized religion and was agnostic before weird things started happening. I couldn’t give you an exact answer on what I think is “out there” other than to say I believe there’s more than we can see and… Read more »
bugzzz
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bugzzz
July 4, 2011 1:01 PM
No problem. No offense taken. Thanks for hearing me out. I do believe this experience, or similar, is available to anyone. If this counts as an ‘experiment’ try posing a question to the universe sometime. Sounds ridiculous I know. (But not really since existence is already pretty crazy.) If you are able to do this under quiet circumstances as a kind of meditation then it would be similar to my ‘test conditions.’ I can’t promise any result since I don’t know how this works. But I think openness to new and far out ideas and a basic humility in the face of creation may be factors. Taking a scientific approach is perfectly reasonable. I’m intrigued by the possible… Read more »
Torbjorn Larsson OM
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Torbjorn Larsson OM
July 5, 2011 6:56 PM

Unfortunately subjective experience doesn’t count as an experiment in that sense.

If you go by way of biology, it is evident that the brain is a chemical machine which logically can have no “ghost in the machine” soul operating the mind (no substrate; infinite regress) and experimentally is shut down by such things as sleep or narcosis. The question have been posed many times and amply answered; every time you go to sleep for example.

Anonymous
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Anonymous
July 6, 2011 4:15 AM

Actually, I’ve heard of this tickling experience before. I was told it is a very rare and unusual form of restless leg syndrome. From what I’ve heard it’s harmless, but in it’s most extreme form can take the form of a tickling/burning sensation with a duration of minutes. It’s a temporary malfunction of the nervous system.

There are a couple other explanations I’ve heard for this but they are a bit more exotic and less reliable.

Richard Kirk
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Richard Kirk
July 3, 2011 11:25 AM
You can get the feel that we are narrowing in on what holds the universe together. However, I recently came across a reminder of how close people thought they were to a solution over a hundred years ago. The book was W.W.Rouse Ball’s “Mathematical Recreations”, reprinted in 1905, and now on Project Gutenberg. The last chapter is called “Matter and Ether theories”. This has lovely descriptions of how the ether may be a membrane in 4-space, with fundamental particles as perpetual vortex loops. The idiom feels surprisingly modern, bearing in mind we haven’t yet got to relativity or quantum theory. Here’s a quote (page 329)… “It is alleged that the theory accounts for the known phenomenons of gravity,… Read more »
Lawrence B. Crowell
Member
Lawrence B. Crowell
July 3, 2011 12:16 PM

Raleigh, Reynolds and others worked on a proposal with loops and knots in aether, and tried to show that different knot topologies corresponded to different particles or atoms. Interestingly there have been some developments analogous to this in recent times, which were worked by Witten. The Yang-Baxter equations and Jones polynomial for knots has been employed to characterize certain amplitudes.

The dark energy is “dark” because we do not know how to characterize the quantum vacuum for it. We know that dark energy, or at least the phenomenology it induces, exists. However, a complete description of the eigenstates does not exist as yet.

LC

Torbjorn Larsson OM
Member
Torbjorn Larsson OM
July 5, 2011 6:50 PM
I am not particularly worried that any kind of “rip” will be the result, though it can’t be excluded as of yet. One reason is because when you relax the requirement that w = -1 it naturally comes down on the shy side. (Though when all observations are included, WMAP 7 year sets it plunk on -1 and with symmetrical uncertainties.) Another reason is because rips injects all sorts of problems such as problems with universal energy (can it then be defined), problems with singularities (the mentioned circumventing of cyclical cosmologies natural problem), problems with the nature of dark energy (if not vacuum energy, what then), problems with vacuum energy (if not dark energy, why is it then… Read more »
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