Did Time Start at the Big Bang?
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Did Time Start at the Big Bang?


Thank you to LastPass for sponsoring PBS Digital Studios Our universe started with the Big Bang but only for the right definition of our universe and started for that matter. In fact, the Big Bang is probably nothing like what you were taught. A hundred years ago, we discovered the beginning of the Universe Observations of the retreating galaxies by Edwin Hubble and Vesto Slipher Combined with Einstein’s then brand-new general theory of relativity Revealed that our universe is expanding and if we reverse that expansion far enough, mathematically, purely according to Einstein’s equations It seems inevitable that all space and mass and energy should once have been compacted into an infinitesimally small point a singularity, it’s often said that the universe started with this singularity and the Big Bang is thought of as the explosive expansion that followed And before the Big Bang singularity, well, they say that there was no before because time and space simply didn’t exist Now if you think you’ve managed to get your head around this bizarre notion Then I have some bad news, that picture is wrong. And at least according to pretty much every serious physicist who studies the subject. The good news is that the truth is way cooler, at least as far as we understand it Now, before a certain crowd starts with all the scientists keep changing their minds. They don’t know anything or the Big Bang Theory is just a theory Let me be very clear, the evidence for a hot dense early universe is practically incontrovertible. The Cosmic Microwave Background is a direct line of sight to the universe as it was Only a few hundred thousand years after the hypothetical beginning of time We can see pretty much directly that all space and matter in the universe was once crunched at least a thousand times closer together There’s also the relative abundance of simple elements hydrogen and helium in particular Whose ratio is exactly what we expect if the entire universe was a dense billions of degrees nuclear furnace for the first several minutes of its existence In fact, There’s powerful evidence that we should not rewind Einstein’s equations that far, at least without introducing some very new physics For one thing there’s also convincing observational evidence that the time before around 10 to the power of negative 32 seconds Included a period of extremely rapid expansion called cosmic inflation We’ve talked about the reasons we need inflation in previous episodes and I’ll come back to it in a bit adding that initial growth spurt solves a couple of the big problems with the Big Bang Theory, but it doesn’t change the fact that Rewinding the expansion of the universe even at different speeds still leads us towards the T equals zero singularity. I’m going to come back to why we need to forget the idea of this singularity Doing so will change the way we think about cosmic inflation and about the beginning of the universe But before we kill the whole idea of the Big Bang singularity, we need to understand what we’re killing What does it really mean for all of space to be compacted into a single point? This idea is especially weird if the universe is infinite Now the universe may or may not be infinite but if we can understand this for the infinite case Then getting all of this for the finite case is baby stuff at least by comparison It’s tricky to talk about the size of an infinite universe Instead of the overall volume or radius we talk about the size of an expanding infinite universe in terms of the scale factor That’s the distance between any two points in space at some moment in time Relative to their distance at some other reference moments that reference moment is typically taken to be right now So the scale factor of the universe is currently one Several billion years ago, the scale factor was half, all points in the universe were half as far apart as they are today. So when I talk about rewinding the expansion, I mean running the clock backwards to track a shrinking scale factor. One way to do that is to keep halving the scale factor. Do that enough times and any two points, no matter how far apart they were, will end up as close together as you’d like. Do it enough times and the universe could end up as hot and dense as you like But it’ll still be infinite, spatially, the scale factor is incredibly small But an incredibly small number times infinity is still infinity Rewinding the universe this way doesn’t leave us with a singularity The singularity is when all points are not just next to each other but literally in the same spot at which point temperature and density are infinite. That last tiny step is a doozy The scale factor goes from incredibly small to zero. So the infinite universe becomes infinitesimal all points become the same point and three-dimensional space becomes zero dimensional That’s the singularity We say that it didn’t happen in any one place because a point is zero dimensional there weren’t spatial dimensions for it to happen in At the same time we say the Big Bang happened Everywhere at once because even the tiniest fraction of a second later The universe has infinite size and everywhere is expanding equally Even if the universe is not infinite then whatever space there is Comes into being at the same time from that singularity. But what happens to time at the Big Bang singularity? To get that you can’t think about the universe as having one big clock that Rewinds and then winks out of existence of the Big Bang or into existence if you’re going forward No, you have to think about time in the way Einstein Intended there is no universal clock time is relative Clocks are attached to each observer each moving frame of reference to see what time does at the Big Bang We have to trace a path through space and time back to the singularity We trace a path called a geodesic which in general relativity is the shortest path between two space-time coordinates These are the grids we use to map space-time Remember that in our rewind all points in the universe get arbitrarily close together before merging at T equals zero Well, that’s the same as saying that all geodesics in the universe converge at the Big Bang singularity In the same way all lines of longitude converge at the North Pole so each Geodesic tracks earlier and earlier times as it approaches the Big Bang infinite clocks rewinding toward zero and then they all converge and Then what well then nothing All geodesics end at the Big Bang singularity and their timelines end with them Or they start depending on how you want to think about it The point is that in the pure Einsteinian picture There is no before the Big Bang because no time line in this universe can be traced there. This is called geodesic in completeness and it also happens at the singularity in the center of a black hole all timelines end this time in the forward direction The analogy with the North Pole is a good one and Einstein himself used It lines of longitude end at the North Pole and it’s meaningless to ask what is north of the North Pole? from the pure Einsteinian point of view It’s meaningless to ask what happened before the Big Bang or after reaching the black hole Center? Okay, so I’m taking my time to explain something I already told you is wrong But it’s important because the extreme weirdness of the Big Bang singularity is part of what tells us. It’s wrong Any time you encounter a singularity in the mathematics of a physical theory you have good reason for skepticism It’s probably telling you that your physical theory is incomplete and that you push that theory too far That’s what’s happening here We used general relativity to rewind the universe, but we already know that despite its incredible successes Gr. Is an incomplete theory? At the crazy densities and temperatures of the Big Bang singularity and just after gr. Comes into terrible conflict with quantum mechanics We’ve talked about that conflict and its possible resolutions before But the upshot is that we just don’t know how the universe behaves in those conditions But we do know that pure general relativity is not a good description and so he probably shouldn’t believe its prediction that all space was compacted into a single point and that this is where Time started. Ok. So what are the alternatives? Can we really track? Geodesics and the timelines they embody through the Big Bang and out the other side If so, what do we find there? There are several possibilities and they deserve their own episodes and we’ll actually get to those soon But to whet your appetite first up cosmic inflation can offer a temporary reprieve from the singularity eternal inflation suggests that our universe appeared as a regularly expanding bubble in an unimaginably larger continuously inflating space-time in that case before the Big Bang was a period of exponential expansion that could have lasted indefinitely We’ll get to the nitty-gritty of that with its inflow tongs and bubble universes real soon There are also various cyclic universe options the first cyclic universe idea was the Big Bounce in which the Gravitational attraction of all matter in the universe was enough to cause it to re-collapse and then presumably bounce outwards again We now know that there isn’t anywhere near enough matter to do that unless we bring in string theory the Steinhardt-Turok model suggests that our universe floats in a higher dimensional space living on geometric objects called brains collisions between those brains initiate cycles of expansion of contraction Then there’s Roger Penrose Conformal cyclic cosmology it’s even weirder because it postulates the infinite future boundary of an eternally expanding universe Looks like the Big Bang of a new universe Mathematically so our heat death is someone else’s Big Bang? There are some less abstract ways to get a new universe out of an old one for example an extreme quantum fluctuation could initiate a new Big Bang given infinite time or The same amount of time could lead to all particles randomly converging back to the same spot Or maybe black holes birth new universes as in least smullins becomed universe hypothesis There’s a poetry to that last one the geodesics approaching the black hole singularity Become the geodesics emerging from the new Big Bang singularity people love cyclic and regenerating universes They appeal to our sense of narrative which might be a reason to be wary of these hypotheses Now they also appeal to our intuition for causality Things happen because prior events caused them many of our ideas Just push back the uncomfortable something from nothing moments physicists have a thing or two to say about that from quantum fluctuations from nothing – Stephen Hawking’s timeless interpretation of internal inflation that draws on the holographic principle all things we’ll discuss in the future as we travel beyond the beginning of Space-Time. 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12 Comments

  • robert charlton

    time and space are very complicated, take for example, that we can travel at the speed of light and there are twins, that are astronaughts, 1 of them stays on earth and the other goes on a trip to jupiter at the speed of light, when he comes back, the twin that stayed on earth would be older than the twin that went to jupiter.

  • Steve Openshaw

    I am an atheist, but listening to how convoluted all these different creation theories are, it makes me think that it may well just be simpler to buy that some old dude with a beard did it.

  • Chris Howe

    Was there a video that tackled the idea of Tachyons during expansion?
    My best guess. No.
    The density of the universe at the Singularity would create a difference in 'c' similar to what we see around a black hole. However, relativity brings us to a screeching halt there since it's from our perspective that 'c' is changing at all.
    Would this all come together to indicate that a sationary observer during expansion would in-fact have witnessed tachyon-level information transmission? Wouldn't this also mean that 'c' is relative to the observer and thus tachyons would be possible only from a certain perspective?

    Example: 2 starships travel conventionally at 0.75c towards eachother. The observer on starship a would witness starship b approaching at >c. viola! Tachyons??? This would also mean a new c is in order where 2c is "true" constant???

  • Daniel Baenziger

    I think Carl would have really enjoyed these videos.

    Many thanks to Matt and his team for taking part of his mantle. Cheers.

  • 420drwilldo

    If your really think about it what is time but the observation of the Decay rate of the universe so of theyer is nothing to decay then there is nothing to observe decaying

  • Alex Bordon

    I have a new gravity, time and consciousness theory that connects quantum mechanics, black holes, dark energy and gravity with existence and a universal balance, what should I do?

  • Stefano Portoghesi

    How do we know that the Universe began 13.8 billions years ago ? Surely that measurement of earthly "Time" may be valid for our human sphere of reality only but what about universal "Time" , one individual's past event in "time" may be another individual' future event in "time" .

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