Abstract / bottom line:
When an astrophysicist's theory of the universe turns out to be nonsense it makes "pop". The beginning of all is called a "bang", the "big bang". — This essay is about the difference.
No one can conceive where time comes from and what time really is. (Yes, it stretches the world to make differentiation possible.) But makes it any sense to come up with the idea there must be a force (an energy) that drives time to go forward?
The same situation do we have with a phenomenon that is not so present in everyday life as time is. It's the so-called "scaling of space" in the universe. (Yes, it stretches the world to make differentiation possible.)
All big voids in the universe get bigger over time. (It's the same when you say: the flow of time gets slower through big voids. [*] It's a kind of aging of time itself there – the evolution of spacetime is different outside the clusters of galaxies.)
[*] (Correction 2020-04:) It's better to say 'the flow of information gets slower through big voids'. Time itself goes actually faster.
But since there is no evidence and no possibility that time could go backwards (or scaling of space would go backwards) there is no need to say: There must be an energy that drives time forward, an energy that drives scaling of space forward.
All this is well known in science for 85 years now. Georges Lemaître postulated in 1927 the intergalactic redshift (increasing with distance) is not caused by velocity in space but by expansion of space itself.
Surprisingly in the late 20th century astrophysicists (Saul Perlmutter etc.) came up with the idea intergalactic redshift is indeed not caused by scaling of space itself but by velocity of matter in space (accelerated by "dark energy"). – Surprisingly they don't talk about scaling anymore, they talk about momentum.
What changed their mind?  Were the physicists confronted with new clues that wrecked all long existing theories? No, not at all. They simply forgot what already was known. It was a kind of relapse into dark age.
Well-educated scientists turned into (let's say it as it is) idiots.
Acceleration anyway (by gravity or by so-called "dark energy") is vectorial. Acceleration needs a direction ("dark energy" is not dark enough to make this untrue, to skip all physics).
But the vector for the forces of dark energy is always 0 , because it has no direction. The "Big Bang", the space scaling of the universe ("expansion"), the whole structure of the universe has no direction. The origin of the universe is not a location. The universe is not a sphere. There is no center of the universe (and had never been). There is no direction outwards (like an explosion would have).
– Absurdly today's nobelized cosmological theories believe so ("pea-sized early stage" of the universe, "accelerating" universe etc.). Why is it?
Well, 50 years ago someone made an illustration of the universe. It showed a point from where time and space expanded. The illustration was o.k. as long as it was understood the point origin is a metaphor for (endless?) scaling.
Today's scientists grew up with this illustration. But somehow and over time it was forgotten it's a metaphor. The astrophysicists began to take the illustration for real and literally – and became, well, idiots.
Today's astrophysicists can't answer a question like this: How can a galaxy a trillion light-years from here get there within 13.5 billion years time?
(It's simple: Because "big bang" is not a location.) 
Or: How much energy does the universe contain so it doesn't collapse but expand? Astrophysicists do not only give wrong answers ("70% dark energy"), they use the wrong methods to resolve the question – so did already Albert Einstein.
In the 1920's A. Friedmann und G. Lemaître adapted the Einstein field equations to the universe. Today it's common to assume the Einstein field equations are made for that. But they aren't. Einstein should have argued: The field equations describe gravity as geometry. There can't be "gravity of the universe" because the universe as a whole has no geometry.  
But, I guess, Einstein prefered to become famous (for the price of pushing something wrong) – just like in today's science.
Does today's science business launch disinformation systematically?  – At least that of the connection of endorsers in Oslo in 2011. Saul Perlmutter (the Nobel Prize winner in physics) just did his job of analysing supernovae. (And it can happen to draw wrong conclusions from wrong premises.)
But the Prize Committee is to blame for increasing darkness in astrophysics.
" The universe got it's momentum (of expansion) by the big bang and the expansion will slow down by gravity or speed up by dark energy."
Yes, if "big bang" were an explosion, if the origin of the universe would be a location, if the universe would have a center of mass they could bluster about momentum (!!), pea-sized universe , speed, acceleration and braking  . – But that's not the case with the universe.
Gravity or "dark energy" can't do anything about increasing distances of galaxies.
Scaling of space is more like time is defined: it goes on.
So it's funny, especially when they are wearing dress coat while performing their accelerating fairy tales.
I wrote more on the topic here:
 see my draft Not Dark Yet
 Saul Perlmutter wrote: "Most astronomers assumed [the expansion of the universe] would be slowing down because the gravity of all the combined objects in the universe would be hitting the cosmic brakes."
The Oslo Nobel Prize Committee deems this nonsense science. – Do they not know there is cosmic-scale for gravity no center of mass to gear to? (..there is no such thing as "gravity of all the combined objects in the universe"?) – Of course they know. But what they know the most is: Science is not a public service. Research is not payed for finding truth, it is payed for finding indicated and exploitable truth. It's about which kind of persons will prevail and which will disappear (i.e. it's about the global power structure).
What they absolutely not want is this final truth: a) Even if the universe would have limited mass and capacity (loop structure) there is no 'outside the universe', no border – and without a border there is no shape, without a shape there is no center and without a center (of mass) there is no 'gravity of the universe'. b) "Expansion" is the completely wrong word for what happens with the universe (expansion into what?). c) There is no point origin/ center location. – It's just scaling of the voids (ultimately scaling of the whole universe), a factor, not forced by an energy (similar to what the nature of time is).
This is a scientific proof (unfortunately a not wanted one). – It is subsidiary Einstein was wrong (he favoured the spheric bubble universe shape), the whole Friedmann–Lemaitre–Robertson–Walker metric is wrong and thousands of books and scientific papers from the last 5 generations of physicists relate to nonsense.
Sadly this applies not only to certain science foundations (who promote a special kind of scientists/ who know they give the world the runaround). – This applies internationally. The whole scientific world is trapped in dependence.
– (Added 2015-04) Why do the covers of the science magazines brim with glossy illustrations of colliding bubble universes? – Because the Einstein mafia knows how to establish a lie: a) provide impressive images (instead of valid arguments) and b) repeat it over and over again. – But can they maintain falsehood forever? (When Einstein's bubble universe would be disproved their whole house of cards could collapse.) — Well, they can, if they manage to stifle the truth for another few decades. In thirty years, in a world hermetically controlled by their global artificial intelligence they will have established Einstein's infallibility for ever.
Alan Guth said at the World Science Festival 2014: "Science is an arena of competing ideas. [.. and] the most widespread idea [is true]." In other words: Truth is where the lobbying power is. (I would call it ideological terror by the US foundations.)
 see my draft Not Dark Yet
 (Added 2013-12-14) .. And not a point in time either – that's not discussed here. Limitless 'stretching' and 'scaling' as a foundational principle makes the idea of a starting point needless.
My term 'stretching of the world ' as related to time and distances is took up half a year after my publication (this web page) by the following Scientific American article:
"In a 'Rainbow' Universe Time May Have No Beginning" (Clara Moskowitz), referring Adel Awad: Nonsingular rainbow universes.
Scientific American did reject publishing my article half a year before.
 (Added 2014-04-20) My essay The Universe as Manifestation of Sense. Part I and II, viXra.org (PDF)
 (Added 2014-05-08) An insight into today's degenerateness of parts of fundamental physics provides the case of the Illustris project.
The project purports to simulate the evolution of the cosmos/ to approve self-propelled principles of this evolution. In fact the procedure of the project does just the opposite (meaning it is -measured against its pretension- a swindle).
To vary the algorithms of the Illustris project again and again until the result looks as if it were clusters of galaxies doesn't produce any cognition about evolution.
A computer algorithm can filter and represent facts more structured and clear, it can even predict developments when all conditions are known. But the most basic property of a computer is: it puts out (in a different, calculated form) what you put in. You can't ask a computer a question and get an answer. – Before that you have to program the answer.
Illustris is an illusion, an illustration. – It is not nonsense, it produces visuals – but it does not reveal truth and cognition as experiments or measurements or thinking would do.
It's impressive how close the AREPO simulation code matches reality. But that does not proof any cosmological theories. The evil thing with the Illustris project is (besides the underlying wrong model of cosmology) it uses promoting slogans that turn academic principles upside down (and no one is bothered about it).
The project tries to make us believe to map (to depict) the behaviour of gas, of galaxies and black holes is the same as a self-propelled simulation that can reveal unknown principles of evolution (or prove established ones), that can provide cognition.
The declaration the Illustris project approves established scientific cosmological theories is imposture and simply wrong. The project does not prove, it mimics.
(I have shown above where the priority of pure imaging and of self-promotion can end up.)
But do Mark Vogelsberger, Volker Springel and the Illustris team have any other choice than to 'approve' the established science business? I guess no. The world of astrophysicists is a system of something like 'allegiance or excommunication'. (see Scientific Peer Review Is Broken, link added 2014-12)
"Even run across more than 8,000 processors, the [Illustris] simulation still took several months." – Well, Hollywood would have fired Mark Vogelsberger for such an expensive special effect.
The science business is -just as Hollywood is- a business in conformity with the U.S. system. So let's compare both shortly.
What is the only reason for Hollywood to accept that expense is no object in the production of a political thriller? – Well, these movies glorify the killing of people all over the world in the name of US interests. The best what can happen is a call from the Secretary of State: Carry on! This movie teaches the world WE ARE THE GOOD GUYS.
With this in mind, why for the MIT (or who ever payed the Illustris project) expense was no object? – Because Illustris is not only about visual illusions. It is about the illusion of producing science (and the MIT Physics Department is highly pleased with the project): This teaches the world: WE DO RESEARCH and our view of the world is the right view.
For nearly a century public funds are spent for the biggest swindle in history of science (see ) – water-level reports for the Sahara: 68.3% dark energy, 70.1%, 72.8% ..
Above I used the word 'idiots' to defend truth. But Mark Vogelsberger, Volker Springel and team are for sure not idiots. They know full well why they serve the dark forces in science business with faked proofs when they headline:
"Successful simulation lends weight to standard model of cosmology".
 (Added 2014-05-26) My essay The Universe as Manifestation of Sense. Part III and IV, viXra.org (PDF)
New Scientist rejected publishing a review of my papers  and .
 (Added 2015-04) My diction "surprisingly in the late 20th century" is polemical (unobjective if you will). Already in the 1920's and 30's the Friedmann–Lemaître–Robertson–Walker (FLRW) metric was an obliquity and its mathematicians ignored the basic nature of the universe in the parameters of theyr equations: scaling of space itself (scaling without being propelled by any force - like time goes without being propelled).
Wild excitement about finding an equation is for mathematicians much more important than the question whether the equation matches nature or not. To put it straight: they want to see their equation in the books, not necessarily truth.