At least what we currently spend most of time studying/researching in physics right now. We can hope to expand beyond that given enough time.
There might literally not be enough time to expand beyond that, given how cosmological horizons work. Being part of the system we're trying to observe puts some nasty limits on what we can know, even in principle.
And since we know that gravity affects the rate of flow of time, wouldn’t the rate of time be enormously distorted earlier universe?
I’m not trained in any of this, so hopefully there are greater minds here who can help me understand
So there must have been a time when electromagnetism, the weak and even the strong force just didn't exist. They couldn't. So particles would just have totally ignored those forces.
We don't know if gravity is the same, but ... why wouldn't it. Though of course according to relativity gravity just wouldn't care, but that just raises a lot more questions than it answers.
What does it mean to compare ("faster") a velocity and a frequency (inverse time)?
The issue is that movement and time are fundamental to the universe, yes, in the way relativity describes but also in a totally different unknown way. In some ways particles are "just" things moving relative to one another. Which, for one thing, brings the perspective question (if you accelerated to a "real" speed, would you see a different universe? Because you would disagree with us slowpokes here on earth on what particles exist at least in some cases. But would you see an entirely different universe?)
As I understand it, that’s an approximation for Euclidean space because the area of a sphere is also proportional to the square of the radius in such a space, but it’s not true of non-Euclidean spaces like in GR because the area-radius relation is different.
IIRC, the cosmic microwave background has a gamma factor of about 1100, so the area of that shell is the same as one 1100 times closer or 1/1100^2 times the area.
> And since we know that gravity affects the rate of flow of time, wouldn’t the rate of time be enormously distorted earlier universe?
Time did indeed slow down then compared to now, although it’s not entirely obvious to me that this has any physical interpretation when it happens “everywhere”: https://youtu.be/66V4RSmDqYM
Well... yes it is, in the rigorous sense of "time" defined by general relativity. There's no "before" for a singularity. It may not be the whole story, but whatever metaphysical notion defines the "before/beyond/outside/why" that drives the big bang, it's not a place on the "time" axis of spacetime.
Similar to the singularities in black holes - everything up to a stone's throw of the event horizon is pretty well explained by GR, but as far as the horizon itself or the region beyond are concerned, there might be dragons as far as we know.
I don’t think that’s right. If we interpret Big Bang theory as claiming that there is a singularity at a finite distance into the past history of every present event, then GR can’t predict what happened at or prior to that singularity. Whether time “began” then or whether there was “more time on the other side” is a question GR alone cannot answer, not a prediction of GR
[1] https://en.m.wikipedia.org/wiki/Fuzzball_(string_theory)
How does that work for black holes? It seems like there would be a 'before' they formed in the time dimension of our universe, if not within the singularity itself.
> If time effectively halts at T0, then what possible event could occur (outside of time?) to nudge that infinitely large glob off mass into the motion we observe today?
Anthropic principle explains it. If you need a Prime Mover, you can also replace it with Anthropic principle and random chance.
For example if time halts at T0, physics breaks. Using limes when t->0 you get that v-> Infinity (or c). Slower time moves for you, faster you move in space. At that point, if nothing is there to detect it, every massless particle will be effectively everywhere, all at once. Essentially photons can have Infinite energy and just cause a random Big Bang. This is CCC like explanation.
Another answer is Hawking's North of North Pole time problem (from Universe in a Nutshell). If time halts a T0, you can't ask what's before it. It's just logical impossibility. Like what is North of North Pole? Answer is: Question is invalid. Anthropic principle implies there are however many multiple such configuration so you get a parallel or sequential universe solution.
We arise and wonder how exactly Universe exists that support life, even if we can clearly see, it's only supporting life at this moment. Our existence is a grain in the sand of a universe-sized sand clock that will be the Black Hole epoch.
The universe advanced from T0 because it had to, by definition.
If this seems like a weird cop-out, well, that’s a singularity for you.
Yes.
If you assume that all events have causes.
Our understanding of the world (aka science) is largely based on causality, and if things existed without any cause or reason, our understanding of "science" would probably be very different.
(I'm not a physicist, but this is how I understand things.)
Also, no one is "lazily tossing it aside". It comes out of the mathematics that describe the observations:
Say you find that all observations you make are perfectly described by dx/dt = 1/x with current time t1. If you follow the trajectories backwards, you find that the trajectory can not be extended back past some initial time t0 at which x(t0) = 0 as the equation becomes singular then. You are now at time t1-t0 from the initial singularity. That is the age of the universe since the big bang. Now the trajectory as it approaches t0 has some unusual properties, it moves infinitely fast, etc... These might lead you to postulate that unknown physics will actually invalidate your law as you approach t0. But there is nothing logically or epistemically _wrong_ with the law you have. The finiteness of time flows out of a causal empirical induction argument. It is not introduced ad hoc to avoid some difficulty, it just is how we find nature to be at the most conservative interpretation of the evidence.
You stop at the uncaused first cause or the unmoved mover, of course.
Think of it like a particle decaying - at some point it just happens with no trigger or internal mechanism, and now you suddenly have the decay products wizzing about with some energy and momentum that they just start with.
There is no reason to assume that everything has a cause. It is perfectly logically consistent to have things without cause in the universe.
The big bang could not create universe from nothing, it could only spread matter from a singularity into ever expanding universe (time-space).
is it even possible, to have entirety of universe matter and energy in a single point?
We don't know anything at all about the physics of infinitely small spacetimes, because we don't have a fundamental physics of spacetime at all.
GR is a descriptive approximation of the behaviour of spacetime, but says nothing about the fundamentals that generate that behaviour.
There are proposals for time stretching infinitely back, but we have almost no way of testing them.
Which is to say that there exist respected papers that outline this scenario in great detail, but there’s precious little observable evidence of a previous universal cycle.
[1] https://en.wikipedia.org/wiki/Penrose%E2%80%93Hawking_singul...
So maybe we can interpret this claim as saying that 25 billion years ago, the universe was kinda similar to what we have now, but 27 billion years ago everything was ultra compressed and gravity didn’t have a significant effect on anything. Or maybe time is like a left-open interval. Finite, but didn’t have a start. I dunno, I’m making stuff up.
However in today’s day and age that way lie no fat grants, adulating television specials, or NYT bestsellers.
If time/space-time existed before the BigBang is probably an unanswerable question (unless we are within a 4d black hole and we can listen to waves that perturbed the matter before the formation of our universe).
Why? Any sign of another Big Bang going from some other point somewhere else would indicate that "our" Big Bang might not have been the first one. It is harder to prove absence though.
Because if inflation is also correct we lost every causal connection with whatever was there before, or is so diluted that we might not be able to detect.
Another possibility could be that at the time of the BigBang the energy density was so high that everything was unified, and so when forces actually separated they "tabula rasa" anything that occupied the bubble of space-time we expanded into.
However, it could also be that the fluctuations we see in the CMB are due to perturbations of what happened before. But that possible clue is better explained by comic inflation expanding quantum fluctuations at an incredible speed that disconnected them.
That's why I said it's (probably) unanswerable. I also hope I am wrong though.
I’ll search for it but PBS Space time has some wonderful intuition building visualizations and explanations in some of their parts on models for the beginning of the universe. If I can find the ones I’m thinking of I’ll edit later, but regardless I find their background material on “wtf” re: physics can be helpful.
There's no more events to happen, and more so, no ability for more events to happen.
Thats still likely an artifact of our models though, and that when you do to something like that, that new events start happening again
Of course, people do speculate. I seem to recall some level of anisotropy in the cosmic microwave background that was a bit more than expected purely from quantum vacuum fluctuations in the pre-inflationary early universe and at least one physicist musing that it might be a perturbation from some other universe that has since lost causal connection. This, of course, makes no testable predictions, can't be falsified, isn't really science, but human intellectual curiosity goes beyond science.
that's the context in which physicists are talking
I believe it could be 'turtles all the way down' as the phrase goes. Maybe our Universe started by a collision of two or more other Universes and we got this mess called the Big Bang. This doesn't explain how those other Universes started though.
But then as mere humans, we can't conceptually grasp Infinity itself. This is not some failing of ours, it's actually convenient to not imagine infinity, as it would drive us mad. The minute you include the Infinity Symbol (∞) in a math equation, all logic starts to cease and get very wobbly.
Once upon a time I watched PBS Nova program about string theory. I remember they talked about something along these lines. It's been almost two decades since I saw it so memory is rusty. But it was something along the lines of universes existing in these big "planes" (they visualized them like these big floating membranes) that would vibrate and occasionally smack into each other. When one of these collisions would occur, there was the potential for a new plane to be formed from that collision.
Like you said, that could potentially explain where new universes come from, but not how the other "turtles" got there in the first place.
It all goes back to thermodynamics and the probabilistic understanding that entropy 'always' increases. But if entropy always increases then by reason, it must have started off at small minuscule point sometime in the past. But if energy/mass are constant, how could it have gotten to the low entropy in the first place? Given enough time, a 'once-in-a-gazillion' event actually happened. At a fundamental level, it's all mathematical guesswork.
Why not? Tons of things are, for example, entropy. I have a crackpot idea that the Big Bang itself was just an "entropy population inversion." The big bang is literally just the moment where the discontinuity occurs.
The universe will keep expanding until entropy reaches a maximum -- the "Big Freeze." Once maximum entropy is reached -- wherein all atoms in the universe are totally dispersed, everything is the same distance from everything else, and the universe is a totally uniform field -- one could say that is very orderly indeed, i.e. the theoretical minimum entropy.
I imagine that all of the matter in the universe being perfectly dispersed would quite neatly "fill" the universe. So who's to say such a situation isn't infinitely dense?
Much like the moment just before the Big Bang.
This idea goes all the way back to Plato and his Parmenidean-inspired rejection of the Pythagorean notion of the Monad as being at the centre of the universe as the Number One implies certain properties like perfection, unity, etc., and we see none of those properties in our existence. This led Plato to argue that the One existed separately from our reality, which was just an imperfect copy associated with the idea of an indefinite Dyad.
So it wasn't just that the universe was a densely packed packed ball of all the stuff we see today and it somehow spilled out or burst forward, what existed before was a monad, and all the stuff we see including space-time, the elements, and more were created at the time of the Big Bang.
Also, everything is waves
There's a 'most northern' point on the earth, and the surface is arguably continuous.
So it seems fine to have an 'earliest' point in time and still have continuous space-time.
This was Hawking's point of view. One way of thinking about special relativity is that time is at right-angles to space, and everything travels at the same 'proper' speed (c) through this 4d spacetime, just in different directions (hence why "fast" objects experience "time dilation": their 4d heading is pointed largely in a space direction, so they don't progress much in the time direction). This extends to general relativity, which describes how these directions "bend" into each other. For example, the reason objects fall due to gravity can be described as spacetime curving such that "future" points slightly "downwards" (and hence slightly less in the usual time direction; which, again, causes time dilation).
AFAIK Hawking's idea was that the curvature at the big bang was so high, that "past" is pointing completely in a spatial direction; and hence there's no such thing as "earlier" anymore. Similar to how "north" near the north pole is at right-angles to "north" at the equator.
It’s even more puzzling than that. Our current model features a period of time called the inflationary epoch which lasted between 10^-36 and 10^-33 or so seconds after the Big Bang, in which the universe expanded by a factor of at least 10^26 along each linear direction [1].
This is unfathomably rapid expansion. It makes sense mathematically but not at all intuitively. It’s totally bizarre and weird and I don’t have even close to the level of background needed to challenge it!
Matt from PBS space time covered this here.
They probably are. See Planck length.
If something preceded the big bang, it cannot be observed (by our current reckoning). Similar to things farther than cT away... Unobservable; unknowable.
There's nothing inelegant about it. C is the conversion factor between space and time.
In the paragraph above, you assume time is a primitive concept, but it is really an emergent phenomenon. The passing of time, moving by some amount of (the plank-scale limit of) spacetime interval, can happen only across connected configurations.
Think about evolution in spacetime as a combinatorial game, there can be positions where causal links are not possible, or even empty games.
The best analogous I can think of arises in the field of surreal numbers https://en.wikipedia.org/wiki/Surreal_number, with their built-in concept of generations.
It is my understanding that the density of the universe billions of years ago was radically different from the one we now observe, and since density is intrinsically tied to our perception of space and time, wouldn't it make more sense that time actually stretches infinitely the further back we go, thus nullifying the concept of a beginning?
I guess I'm having a hard time with the idea that space-time could be discontinuous.