Posted on 08/30/2006 1:01:48 AM PDT by snarks_when_bored
An event like the Big Bang is about as likely as billions of coin tosses all coming up heads. Explaining why that is might take us from empty space to other universes--and through the mirror of time.
by Sean Carroll • Posted August 28, 2006 11:53 AM
From the SEPTEMBER issue of Seed:
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The nature of time is such that the influence of the very beginning of the universe stretches all the way into your kitchen—you can make an omelet out of an egg, but you can't make an egg out of an omelet. Time, unlike space, has an obvious directionality—the view in a mirror makes sense in a way that a movie in reverse never would.
The arrow of time in our universe is puzzling because the fundamental laws of physics themselves are symmetric and don't seem to discriminate between the past and future. Unlike an egg breaking on the side of a frying pan, the journey of the planets around the sun would look basically the same if we filmed them and ran the movie backwards. Rather, it must be due to the initial conditions of the universe—a fact that makes the nature of time a question for cosmology. Remarkably, the answers we're beginning to discover are telling us there may be other universes out there in which the arrow of time actually points in reverse.
For some reason, our early universe was an orderly place; as physicists like to say, it had low entropy. Entropy measures the number of ways that you can rearrange the components of a system such that the overall state wouldn't change considerably. A set of neatly racked billiard balls has a low entropy, since moving one of the balls to another location on the table would change the configuration significantly. Randomly scattered balls are high entropy; we could move a ball or two and nobody would really notice.
Low-entropy configurations naturally evolve into high-entropy ones—as any billiards-break shows—for the simple reason that there are more ways to be high entropy than low entropy. The very beginning of time found our universe in an extremely unnatural and highly organized low-entropy state. It is the process by which it is inevitably relaxing into a more naturally disordered and messy configuration that imprints the unmistakable difference between past and future that we perceive.
Naturally, this leads one to wonder why the Big Bang began in such an unusual state. Attempts to answer this question are wrapped up with the question of time and have led me and my colleague Jennifer Chen to imagine another era before the Big Bang, in which the extremely far past looks essentially the same as the extremely far future. The distinction between past and future doesn't matter on the scale of the entire cosmos, it's just a feature we observe locally.
If time is to be symmetric—if the direction of its flow is not to matter throughout the universe—conditions at early times should be similar to those at late times. This idea has previously inspired cosmologists like Thomas Gold to suggest that the universe will someday recollapse and that the arrow of time would reverse. However, we now know that the universe is actually accelerating and seems unlikely to ever recollapse. Even if it did, there is no reason to think that entropy will spontaneously begin to decrease and re-rack the billiard balls. Stephen Hawking once suggested that it would—and he later called that the biggest blunder of his scientific career.
If we don't want the laws of physics to distinguish arbitrarily between past and future, we can imagine that the universe is really high-entropy in both the far past and the far future. How can a high-entropy past be reconciled with what we know about our observable universe—that it began with unnaturally low entropy? Only by imagining that there is an ultra-large-scale universe beyond our reach, where entropy can always be increasing without limit, and that if we went far enough back into the past, time would actually be running backwards.
Such a scenario isn't as crazy as it sounds. Our universe is expanding and becoming increasingly dilute, and the high-entropy future will be one in which space is essentially empty. But quantum mechanics assures us that empty space is not a quiet, boring place; it's alive and bubbling with quantum fluctuations—ephemeral, virtual particles flitting in and out of existence. According to a theory known as the "inflationary universe scenario," all we need is for a tiny patch of space to be filled with a very high density of dark energy—energy that is inherent in the fabric of space itself. That dark energy will fuel a spontaneous, super-accelerated expansion, stretching the infinitesimal patch to universal proportions.
Empty space, in which omnipresent quantum fields are jiggling back and forth, is a natural, high-entropy state for the universe. Eventually (and we're talking about a really, really big eventually) the fluctuations will conspire in just the right way to fill a tiny patch of space with dark energy, setting off the ultra-fast expansion. To any forms of life arising afterward, such as us, the inflation would look like a giant explosion from which the universe originated, and the quiescent background—the other universes—would be completely unobservable. Such an occurrence would look exactly like the Big Bang and the universe we experience.
The most appealing aspect of this idea, Chen and I have argued, is that over the vast scale of the entire universe, time is actually symmetric and the laws truly don't care about which direction it is moving. In our patch of the cosmos, time just so happens to be moving forward because of its initial low entropy, but there are others where this is not the case. The far past and the far future are filled with these other baby universes, and they would each think that the other had its arrow of time backwards. Time's arrow isn't a basic aspect of the universe as a whole, just a hallmark of the little bit we see. Over a long enough period of time, a baby universe such as ours would have been birthed into existence naturally. Our observable universe and its hundred billion galaxies is just one of those things that happens every once in a while, and its arrow of time is just a quirk of chance due to its beginnings amid a sea of universes.
Such a scenario is obviously speculative, but it fits in well with modern ideas of a multiverse with different regions of possibly distinct physical conditions. Admittedly, it would be hard to gather experimental evidence for or against this idea. But science doesn't only need evidence, it also needs to make sense, to tell a consistent story. We can't turn eggs into omelets, even though the laws of physics seem to be perfectly reversible, and this brute fact demands an explanation. It's intriguing to imagine that the search for an answer would lead us to the literal ends of the universe.
—Sean Carroll is a cosmologist at the University of Chicago and the author of a popular textbook on general relativity. He is also a regular contributor to the physics blog Cosmic Variance.
The low-entropy beginning of our cosmos does appear to be very highly improbable...Considering that it's a veritable law of the universe that entropy increases over time, it appears to me that a low-entropy beginning of the cosmos is not only not improbable, but is in fact inescapable.
But Carroll's point is that the physical state of the far-distant past is likely to have been similar to what the physical state of the far-distant future is likely to be: that is, a state of high entropy (that is, very disordered)! The quantum vacuum is just such a state.
The problem he's addressing is this: how do we get a low-entropy beginning for a cosmos? His tentative answer: as a negative pressure fluctuation of the high-entropy quantum vacuum. (He's not the only person to have suggested this, of course.)
Taking the analogy of time as a dimension like spatiality, it is the distance between two points. In thermo, the probability of a state compared to the present state is equal whether measured forward or back. That is, noting the present entropy and that entropy tends to increase, it tends to increase in either direction. That is, the universe could be coming into order just now as well as going into disorder from now on. It is not the physics that gives us the impression of the arrow of time, but our intuitions.
I don't know if it is a front. It might just be that the air mass is settling in and cooling in situ. The aurora was excellent last night, a sign that winter is coming since it is now dark enough for a couple hours at night to see the aurora. There was a bright swirling curtain with red on the bottom edge. Not bad.
couple of key phrases in the article...
"imagine a"
"science not only needs facts, but needs to tell a
consistent story"
Since when in western science, is the imaginings of a theorist
considered true? (unless proven empirically...at a later "time")
Who decides what the "consistent story" is, if the
new facts discovered may overturn the old "consistent story"
Are the new facts considered "not pertinent"?
The whole rambling literary exercise just demonstrates how little
we can prove about our universes origin, history, and
final direction.
Science is Latin for Epistemology. What do you know and how do you know you know it?
but aren't our "intuitions" caused by the physical
processes going on in our brains? (not branes).
The two articles referenced in Post #6 contain more details.
Intuition has a definite meaning as the sense we make of our perceptions. Sense perceptions fall into the category of aesthetics, or literally feelings. Imagination is what intuition gives us directly; fancy is images we make within ourselves beyond or in spite of intuition. Truth is entirely within, the relation between our intuition and imagination. As to what causes intuitions: it is sensory input. The brain seems to be in the neighborhood of these phenomena but the relation is not established.
THAT... is fantastic. Creative genius at work.
I always thought time was more a measure of location than anything else.
Example: You're in NYC and make a phone call to Denver and you tell the person on the other end of the phone to give a count down and then say "bang". 3......2......1....."Bang", you both say it at the same "time"....yet when you both look at you watches, you are seperated by a few hours.
A good cook/chef would never start a recipe knowing only 10 percent of the recipe.. you could end up with "GooP"... O.K.... O.K... a conservative chef not a liberal one.. granted..
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6 Discussion
In this paper we have addressed the question of why the entropy of our observable universe appears to have been extremely small in the past. It has long been suspected that inflation might have something to do with the answer, although the fantastically low entropy of the proto-inflationary universe was a significant obstacle to constructing a convincing picture. The answer to this conundrum must lie in the process by which inflation begins; we have proposed a scenario in which this happens naturally from the evolution of some arbitrarily-
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chosen state. It is interesting to note that that the recently discovered cosmological constant plays a crucial role in our scenario for generating the arrow of time.
The basic ingredients of our picture are as follows. We consider an arbitrary state of the universe, specified on some Cauchy surface. We argued in Section 4 that the generic evolution of a system coupled to gravity is to dilute excitations via the expansion of spacetime. Black holes and other inhomogeneities may form, but they will eventually decay away, so that the universe approaches empty space. However, in the presence of a positive vacuum energy and an appropriate inflaton field, the resulting de Sitter phase is unstable to the spontaneous onset of inflation, instigated by the thermal fluctuations of the inflaton. If the inflaton fluctuates sufficiently high that eternal inflation can begin, it will continue forever, and new pocket universes will be brought into being in those places where the field rolls down the potential and reheats. This chain of events happens both to the past and the future of the specified Cauchy surface, leading to a statistically time-symmetric universe as portrayed in Figure 9. An arrow of time is dynamically generated in both the past and the future, as the universe continually acts to increase its entropy.
In addition to the desire to understand the origin of the arrow of time, a primary motivation for our study has been to understand the onset of inflation. As discussed in Section 3, the unitarity critique argues that a proto-inflationary patch of spacetime is much lower entropy than that of an ordinary radiation-dominated universe, and hence is less likely to arise as a random fluctuation. In our picture, the answer to this conundrum lies in the fact that the beginning of our observable Big Bang cosmology does not arise as a random choice in a large phase space of initial conditions, but rather comes via a quantum fluctuation from a very specific prior state – an empty de Sitter universe that is the natural consequence of evolution from generic initial conditions.
By taking seriously the ability of spacetime to expand and dilute degrees of freedom, we claim to have shown how an arrow of time can naturally arise dynamically in the course of the evolution from a generic boundary condition. In the classification introduced in Section 2, our proposal imagines that there do not exist any maximum-entropy equilibrium states, but rather that the entropy can increase from any starting configuration. This is not, of course, su?cient; it is also necessary to imagine that the path to increasing the entropy naturally creates regions of spacetime resembling our observable universe. In the presence of a nonzero vacuum energy and an appropriate inflaton field, we suggest that thermal fluctuations from de Sitter space into eternal inflation provide precisely the correct mechanism.
A number of other cosmological scenarios have been proposed in which the Big Bang is not a boundary to spacetime, but simply a phase through which the universe passes. These include the pre-Big-Bang scenario [94, 95], the ekpyrotic and cyclic universe scenarios [96, 97, 98], the Aguirre-Gratton scenario of eternal inflation [99], and Bojowald’s loop-quantum- gravity cosmology [100, 101]. To the best of our understanding, each of these proposals invokes special low-entropy conditions on some Cauchy surface, either asymptotically in the far past or at some moment of minimum size for the universe. In our picture, on the other hand, there is a slice of spacetime on which the entropy is minimized, but that entropy can be arbitrarily large. The Big Bang in our past is not a unique moment in the history of the universe; it is simply one of the many times that inflation spontaneously began from a background de Sitter phase, similar to the proposal of Garriga and Vilenkin [35]. Along
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with the fractal distribution of pocket universes to the far past and far future, this feature is another reminder of the importance of overcoming the Robertson-Walker intuition we naturally develop by thinking about the patch of the universe we are actually able to observe.
There are a number of points in our scenario that have yet to be perfectly understood. One obvious point is how the “initial” state is chosen. We have argued that the ultimate evolution to the past and future is essentially insensitive to the details of this state, but it is nevertheless interesting to ask what it may have been. On a more technical level, the idea of spontaneous onset of inflation (and related ideas within the paradigm of eternal inflation) deserves closer investigation. We have calculated the renormalized fluctuations of a scalar field in a fixed de Sitter background, and imagined that the back-reaction of the metric would lead to inflation if the fluctuation were sufficiently large.
A more rigorous calculation would have to involve quantum gravity from the start, at least at a semiclassical level. Finally, the issues concerning the number and evolution of degrees of freedom clearly warrant further research. Our assumption has been that the number of degrees of freedom is infinite but fixed, not increasing during inflation. We therefore require a literally infinite number of degrees of freedom to be put in their ground states during the evolution toward de Sitter, before the onset of inflation. It is important that we understand this issue more fully, although doing so may require a deep knowledge of quantum gravity.
An interesting implication of our scenario is the non-privileged role played by the Big Bang of our observable universe. As in other models of eternal inflation, the future history of spacetime takes on a fractal structure of ever-increasing volume; in addition we suggest that a similar structure is found in a time-reversed sense in the far past. The boundary conditions of the universe on ultra-large scales are unlikely to resemble simple Robertson-Walker ge- ometries in any way. Indeed, the cacophony of matter and radiation in our observable patch of universe appears in this picture as a mere byproduct of the relentless evolution of the larger spacetime toward states of higher entropy. Stars and galaxies are seen as exaptations – structures that have found a use other than that for which they were originally developed by evolution.
It is the distance between two points. I am five minutes from the mall, 25 minutes from work. I would rather be several years from work and closer to the mall.
Increasingly, and this might be evidence for both time and evolution, my attention is drawn to unusual words and words used in unusual ways. Granted, the word may be an ordinary used in its usual way and I have been misusing or misreading it all this time. But 'exaptation' is an unusual word that could find a place in the light of public discourse if we can nail down a consensual meaning.
BTW, the turnout of C/E debaters is somewhat disappointing. I am not getting a satisfactory response to my inquiry on the utility of the denial of the concept of evolution to non-scientists, or any response at all.
Thanks so much, snarks_when_bored, for this fascinating speculation! Might I point our that cosmology is essentially a philosophical pursuit? And remains so, even if taken up by physicists? A philosopher sensitive to issues of epistemology might say there are a number of undisclosed initial assumptions at work here that are taken to be valid without citing evidence for that validity, the above italics being an example.
On what basis can physics say that the early universe "was an orderly place?" I mean, here we have Hubert Yockey likening the big bang to a huge thermonuclear explosion (perhaps he meant that as a metaphor): How "orderly" is such a thing? Is this claim of an early orderly universe premised on the relic CMB -- which itself likely is some kind of a "snapshot" of early conditions at a point "frozen in time," yet which now is being distributed along with the inflationary universe to all points in spacetime? Or is this assertion made on some other basis?
To me, the biggest undisclosed assumption of the piece, however, is that of a wholly purposeless universe. Yet I do wonder about that, taking a lesson, perhaps, from the physical laws themselves.
It has been noted that almost all of the physical laws are time-reversible; moreover, they are mainly conservation laws. The huge exception is the second law of thermodynamics, which not only is time-irreversible, also not a conservation law, but rather THE law that governs change in the universe and thus makes evolution (universal, planetary, biological, etc.) possible.
If the only physical laws we had were time-reversible and conservatory, then nothing "new" could ever happen, at least not on a net basis: There would be fluctuations, but on a net basis, what there could be would only be a reflection of the inherent symmetry of the physical laws.
The second law of TD, however, is that which "breaks the symmetry," thereby allowing the realization of new forms that are more than just reshufflings of the deck of existing matter/energy seeking equilibrium in a conserved state. Plus it is the second law that gives us "the arrow of [linear] time" that is so obvious to us denizens of 4-dimensional spacetime.
Why did there have to be an arrow of time at all -- unless the universe is actually purposeful, seeking ends or goals that unfold over a time process towards a state of final completion at some unknown future time?
Is that a possibility that MUST be ruled out in advance? And if we're wholesale ruling out possibilities in advance, then is the cosmology that results really "scientific?"
Though I do concede it may be "philosophical."
As an aside, my friend Alamo-Girl has complied a rather exhaustive inventory of scientific cosmologies of just about every type imaginable. Both of us get a bit of a chuckle every now and then, in recognizing what what all members of this disparate group have in common is (1) a desire to obviate the necessity of a beginning in space and time; and (2) a desire to represent the universe as essentially lacking in purpose or meaning.
So I'm just wondering about a few issues as they might pertain to this article -- which itself is a great spur to reflection. Again, snarks, thanks so much for posting it.
The trick here is that there wasn't much to the universe, neither mass nor size at the start of the BB. Essentially all of the mass and energy came from nothing, no entropy unless there is something, until the inflation phase. Alan Guth says it actually was the free lunch there ain't no such thing as.
Traced backward in time, the ENTIRE universe must have been a MBH to begin with, so just HOW did it manage to INFLATE to it's present size?
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