In fact, I wonder if the question "when has time started" is truely meaningful...

in my opinion, "time is a dimension and mostly we use time as a reference for other dimensions and events."

if you agree with above statement then, time has no beginning for sure. for e.g. length and mass are fundamental dimension. time is also one of them, it is just used to measure physical phenomena and most fantastic fact is that it is relativistic. Hence you can say time is in fact has no beginning.

Just my thoughts... every single equation I can think of is invariant over a time shift (or "slide"), so there seems to be no grounds for really defining an "indisputable" t=0.

But dimensions do have a beginning if space (or indeed spacetime) were 'created' aka popped into existence at the instant of the Big Bang....

Chris

Your question raises an even harder question: If time did exist before the Big Bang what was before the Big Bang? If indeed time preceded the BB then there must have been an endless number of similar Big Bangs? Multiverse?

That's exactly what I was driving at Issam. There are quite interesting consequences and non linear by-ways that stem from this calculation. Doing justice to them is, inevitably, book-length ....

There is also the separate but confluent and quite interesting calculation that proves that some form of consciousness must have been present before the Big Bang for it to happen (it's by the way quite interesting that even if the Copenhagen interpretation of QM is thrown out , the role of consciousness still reappears again, in a different and unrelated way;)

Chris

Could you kindly provide the reference for Martin Bojowald's quantum calculations? Thanks.

Bojowald , Isotropic Loop Quantum Cosmology , Classical & Quantum Gravity, 19, 2002

Bojowald, Inflation from Quantum gravity, Physical Review Letters 89, 2002

Bojowald, The Semi-Classical Limits of Loop Quantum Gravity Cosmology , Class & Quant Grav, 18 2001

Bojowald Dynamical Initial Conditions in Quantum Cosmology , Phys Rev Lett 87, 2001

Also Tsujikawa et al, Loop Quantum Gravity Effects on Inflation and the CMB, Class & Quant Grav 21, 2003

See also his book: Once Before Time: A Whole Story of the Universe

Thanks a lot Chris. That should keep me busy for a while :)

Time may have preceded the big bang, however we need to concider time to have stated from zero form the big bang of our current universe.

That's the whole point - zero is a necessarily fuzzy concept, which extends to its left and to its right.

By the way, it's spelt 'consider' :-)

Yes, I see it's not the ony spelling error I made. (Sorry, my fingures are too big for the keyboard, especially late at night).

However, Roger Penrose presents a definite start of time at the Bounce in "Cycles of Time".

Actually, Penrose specifies a 'reset' of time, or a relaunch of time at the bounce, within a larger timescape spanning different cycles, so time exists before any particular bounce in Penrose's scenario as well.

Moreover - Bojowald's calculation also works in a cyclically bouncing universe, whether a brane or a black hole type bounce.

The point is that under any of these scenarios, it makes sense to speak of time *before* the BB, a view traditionally pooh-poohed (the erstwhile conventional view being that time and space begin at the BB and that it's meaningless to speak of 'before' the BB.)

Yes, I agree. "rest" is the correct way to say it.

Chris

I would like to hear your views on this:

The classical Big Bng model indicate a beginning of the universe at a Hawking-Penrose singularity, but this was mitigated by 'contemporary' Big Bng model, which opened up the posibility of an early quantum cosmological era and an inflationary dynamic allowing our universe to be but one amidst a multiplicity of possible universes within a theoretical multiverse.

However, this mitigating view was itself subsequently mitigated by the discovery of Brode, Guth and Vilinkin that every inflationary model universe(and/or multiverse) must have a beginning. Since this indicates an edge of time(prior to which there is no time), the conclusion of Borde, Guth, and Vilenkin point strongly to a creation of the universe(from no previously existing physical matter-energy). The cause of sucg a creation would have to transcend our universe(and any multiverse in which it may be situated).

Very interesting question Issam,

I have only limited time to log on to RG this week (I'm on a 'skunkworks' seminar in Brussels until Friday evening) - there are a number of answers to your question.

The short answer is: we don't know.

The long answer has many constituents.

One is Stephen Hawking's idea of edgeless time - time is finite but does not have a boundary.

Another interesting point is Paul Davies's demonstration that the multiverse (the 'ultimate' multiverse, i.e. Tegmark's category 4 multiverse) is formally equivalent to deism.

Another one is the general observation (contained within but not limited to the above observation) that deism does not bring us further, since there must be an explanation for the prior existence of a godhead, and seeking this explanation is also formally equivalent to seeking an explanation for an Ur-beginning of either the multiverse or the universe.

Another point is all the explanations that abolish time within a higher dimensional multiverse (which only shifts the problem sideways - we may stop seeking an explanation for time, but we must then seek an explanation for the existence of those other dimensions)

A further interesting point is Dieter Zeh's demonstration that a multiverse need not contain all the possible universes (thus gainsaying and eliminating both Tegmark's level 4 and the thoroughly horrid baggage of infinite near copies and exact copies of oneself across the metaverse! Tegmark even calculated that the nearest exact copy of yourself is ten to the power ten to the power 29 meters away - sorry but it's nonsense as far as I can tell - as Zeh convincingly (to my mind) shows, it's not because you have an infinity that everything - including copies of yourself - must exist within this infinity - to use a very coarse analogy, there are infinitely many natural numbers, yet the number, say, one half is not found within that infinity, and quantum effects demonstrate that even the very selfsame causes that gave rise to yourself do not give rise, anywhere in the metaverse, to further copies of yourself!)

There are many scenarios whereby universes may be born - and nothing forces universes to be begotten all in the same way. I find the 'primordial black hole' generation of universes, for instance, rather plausible and quite rather impossible, at this time at least, to gainsay.

But still this does not explain a beginning.

So the only way out of the conundrum, Issam, would be to demonstrate that pure nothingness simply cannot exist - then you'd be out of the woods, the rest would be details. An approach is through pure mathematics - Shing Tung Yau comes very close to demonstrating, for instance, that pure geometry is able to create reality out of whole cloth.

Widening the debate to beyond geometry - can we prove that abstract mathematics can create reality? Or, as a subset of the question - that apparent reality is in fact a purely mathematical construct (which is actually what Tegmark asserts.) Then there would be an equivalency between 'mathematics' and the 'Mind of God' (something that was posited a long time ago by a number of researchers)

There are many equivalent formulations of this question (e.g. can abstract mathematics not exist?) and a times a glimmer of an approach can be sensed (if something abstract does not exist, then it's not abstract, hence it not existing is a contradiction in terms and it must exist ) but they don't feel like a silver bullet, and tend to remind one of the rather unsatisfactory 'ontological proof' aka Anselm's proof.

The scenario proposed by Bojowald for a universe with no beginning is based on a theory of quantum gravity called Loop Quantum Gravity -LQG( which some uphold as a more promising way than string theory to make Einsein's theory of gravity quantum mechanical). However, what is instructive here are the comments on it by Sean Caroll a leading cosmologist:

"Bojowald uses ideas from LQG to try to resolve the initial singularity and follow the quantum state of the universe past the BB back into pre-existing universe. If you try to invent a cosmology in which you straightforwardly replace the singular Big Bang by a smooth Big Bounce continuation into a previous spacetime, you have one of two choices: either the entropy continues to decrease as we travel backwards in time through the Bang, or it changes direction and begins to increase. Sadly, neither makes any sense.

If you are imagining that the arrow of time is continuous as you travel back through the Bounce, then you are positing a very strange universe indeed on the other side. It’s one in which the infinite past has an extremely tiny entropy, which increases only very slightly as the universe collapses, so that it can come out the other side in our observed low-entropy state. That requires the state at t=-infinity state of the universe to be infinitely finely tuned, for no apparent reason. (The same holds true for the Steinhardt-Turok cyclic universe.)

On the other hand, if you imagine that the arrow of time reverses direction at the Bounce, you’ve moved your extremely-finely-tuned-for-no-good-reason condition to the Bounce itself. In models where the Big Bang is really the beginning of the universe, one could in principle imagine that some unknown law of physics makes the boundary conditions there very special, and explains the low entropy (a possibility that Roger Penrose, for example, has taken seriously). But if it’s not a boundary, why are the conditions there[ at the boundary] so special?

Someday we’ll understand how the Big Bang singularity is resolved in quantum gravity. But the real world is going to be more complicated (and more interesting) than these simple models."

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Caroll, Seun, "Against a Bounce", Discovery Mgazine,02/07/2007.

Chris,

What Caroll is referring to is that the entropy of our universe just after the BB was fantastically low. It was so low that the odds of it being so low, if it was a matter of chance, have been estimated to be a mere 1 part in 10^10^123 by Penrose. Penrose has offered as one conceivable explanation of this fantastically low entropy the idea that there might be something special about the beginning of a universe. Perhaps some as-yet-unknown law or fundamental principle of physics implies that at the beginning of a universe the entropy has to be low.

Interestingly, I had written to Sean Carroll once and he never replied (maybe I misexpressed myself - one of the challenges when writing a popular science book is to make sure you can't be misinterpreted, and if there's any breach, believe you me you'll be), whereas a lot of folks I contact with questions out of blue skies very kindly do reply - Barrow, Zeh, etc.

This being said yes, Bojowald's calculation lies within the framework of loop quantum gravity, however the validity of the calculation is not predicated on LQG. LQG is the most developed conceptual framework that entails a quantization, or rather a discreteness, of spacetime, but there are other scenarios

(By the way, the difference between quantization and discreteness is as follows: quantization applies to all dynamic processes in the universe - e.g., the quantity of movement of anything that moves - say your arm - is provably quantized - but that does not presuppose an underlying quantization of the medium - the spacetime - in which that something moves. When your move your arm, its QoM is quantized, but the spacetime within which it moves could well be non-quantized. Discrete applies to an underlying, pre existing quality of the background spacetime that would be discontinuous)

Issam, About the low entropyat the outset of this Universe, Huw Price (et al.) offers an interesting discussion of the issue.. In essence, they retain 4 valid possible explanations, three of them gainsay Penrose's approach, in essence on the grounds that his calculation is inapplicable to the conditions of the BB, and that the narrow statistical reasoning upon which this calculation is based is unreliable owing to a host of skewing parameters.

I cannot comment on Huw Price( et al.) at this instant. However, things are more difficult for models in which the BB was a bounce preceded by a phase in which the universe was collapsing. It has been argued by the particle physicists Banks and Fischer that during such a collapse the rapidly changing space-time would have excited and amplified random "quantum fluctuations" in such a way that entropy would have been driven to a very large values, rather than small ones. This makes it even more difficult to account for the fantastically low entropy just after the BB.

Ockham's razor is actually quite very suspect and should only be invoked, if at all, as a last resort in a chain of reasoning and certainly not at the beginning..

Lawrence Krauss, in his latest book 'A Universe from Nothing', gives such a spectacular example of a non-trivial case when OR totally fails, that I would consider it mostly inappropriate to serious scientific research.

Moreover, like all razors it must be handled with utmost care.

Max Tegmark, in his paper in the compendium put together by Bernard Carr, turns the 'Ockham's razor' argument of his detractors on its head and convincingly shows that in fact, his opponents' Ockham's razor arguments in fact support his position, rather than theirs.