[Closed] I have a question about spacetime

You need to watch interstellar another 3 times

But, if that is so, what happens to the wavelength of light if two galaxies are zooming towards each other at equal speed?

They will both see the same shift in wavelength. If there's a third galaxy involved, the two original galaxies will see a different shift in wavelength for that third galaxy if it is moving at different relative speeds for the first two.

You cannot just linearly add the velocities. For example, if A and B accelerate away from C in exactly the same direction but A accelerates twice as hard as B, when B reaches half the speed of light relative to C, A will not be travelling away from C at the speed of light. No object with mass can be accelerated to the speed of light relative to any other object.

You need to watch interstellar another 3 times

Happy to, I love that film!

However, we also know that the speed of light is a universal constant, so the velocity of those galaxies cannot be added or subtracted from the speed of light to change its frequency and therefore wavelength.

You're not changing the speed of light, you're changing the time it takes to get to you by moving the emitter - effectively changing the wavelength.

Maybe think of it like this; you and a friend are in a vacuum, your friend has a marshmallow machine gun and shoots at you. You get a stream of marshmallows hitting you at a constant frequency. If your friend starts moving away from you, that frequency will drop as it takes each subsequent marshmallow a bit longer to cover the increased distance.

I'm not a physicist, but this is my understanding. The speed of light is constant for any observer, and that can happen because time speeds up to allow for the relative movement. There is still a redshift/blueshift effect but it's adjusted by the time dilation.

Redshift is also due to the expansion of the universe. Imagine all the galaxies drawn on a huge sheet of rubber, which is stretched so that they move apart. As the sheet is stretched, the light waves are stretched and the wavelength increases, hence red.

They will both see the same shift in wavelength

But what is the shift?

1) Blue
2) Red (admittedly unlikely)
3) White (as in no shift)?

The speed of light is constant for any observer, and that can happen because time speeds up to allow for the relative movement. There is still a redshift/blueshift effect but it’s adjusted by the time dilation.

This is my understanding also

Redshift is also due to the expansion of the universe. 

This was my initial understanding, but it doesn't explain how we can see blue shift galaxies as the universe is expanding universally- none of it is contracting. Which is why your first explanation works better for me

If something is moving away from you, it is redshifted. If it's moving towards you, it's blue shifted.

It's all a hologram. Aliens are laughing at us trying to work it all out.

If something is moving away from you, it is redshifted. If it’s moving towards you, it’s blue shifted

That's what I thought. But how is this possible if time is passing at the same rate for both galaxies? Which of the three variable that I am aware of (distance, time and velocity) is giving way to allow the frequency to change? We already know it isn't velocity as c is a constant. It doesn't appear to be time as that will be passing at the same rate for both observers as they are travelling at the same speed. There's no reason for distance to change- that will also be passing at the same rate as c is constant etc. So how can the light blue shift?
If there is another variable I haven't thought of let me know 👍 Energy maybe? But how would that work?

it doesn’t explain how we can see blue shift galaxies as the universe is expanding universally

If you blow a balloon up - they are the things you use to tell everyone it's a party in case you didn't know, it gets bigger but it doesn't mean all the molecules of air in it are moving away from each other. My guess is it's something like that. but it doesn't matter how many RI lectures I watch on YouTube I generally remain happily baffled

If you blow a balloon up – they are the things you use to tell everyone it’s a party in case you didn’t know, it gets bigger but it doesn’t mean all the molecules of air in it are moving away from each other

Quite

I guess my question is how is light able to blue or red shift if it's speed- and therefore the frequency at which the waves of light hit your eyeballs- is constant. What mechanism is allowing for this?

You need to watch interstellar another 3 times

This, but while drunk

The speed at which galaxies are “moving” isn’t constant - they’re accelerating. This explains the shift as it’s caused by the Doppler effect.
(I say moving but in the case of very distant galaxies it’s actually more to do with the space between the galaxies expanding as per the balloon analogy)

However galaxies such as our one & Andromeda are, relatively, very close together so gravitational attraction is causing them to accelerate towards each other! (And ultimately they will collide together)

Daniel and Jorge Explain the Universe is a great podcast that covers this kind of stuff in a somewhat-easy-to-understand way!

If something is moving away from you, it is redshifted. If it’s moving towards you, it’s blue shifted

That’s what I thought. But how is this possible if time is passing at the same rate for both galaxies?

I think this was the question Einstein asked himself ^^^

Space is not flat, it's curved. It has deep hollows in it where stars and galaxies are, and the bits in between are stretching all the time. Because it's stretching the other galaxies are mostly moving away from us which means whilst the waves are travelling at the same speed they are taking longer to reach us. Similar to how a police siren gets lower in pitch as it moves away from you.

But how is this possible if time is passing at the same rate for both galaxies?

Within each inertial frame, time will appear to pass at the same rate. However, if you accelerate you change your inertial frame and time will pass at different rates for different inertial frames. If you accelerate way from earth, your watch will appear to run slower to someone still on earth, but you won't notice any difference with your own watch. This is a problem for things like GPS, where the speed of orbiting satellites is enough to cause a detectible difference in the speed of atomic clocks.

Frequency is proportional to the energy of the photons, not the speed/velocity.

Frequency is proportional to the energy of the photons, not the speed/velocity.

Ok, that makes more sense

My head hurts just thinking about this stuff.  then what it is with some tiny bits of stuff acting like waves in one way and particles in another?  Wavicles?  FFS make your mind up!

What if the Galaxy concerned is on a conveyor belt?

What if the Galaxy concerned is on a conveyor belt?

Then better chocolate is available.

You're confusing two different things, relativity, and the doppler effect.

Relativity says that the speed of light will always look the same (which is needed for the doppler effect to be consistent, but you get the same effect with sound waves which are less consistent so you can explain one without the other).

The Doppler effect is what makes a car go:

NNNNNNNNNEEEEEEE(high pitch)EEEEEEEEEEEEE <observer> OOOOOOOOOOWWWWWWW(low pitch)WWWWWWWWW as it passes you, but when your sat in the car it just goes EEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE (just watch some F1 with the onboard Vs pit wall cameras).

Explanation based on a fast car and a stationary observer (the one you can relate to in every day life but is harder to explain):

Each oscillation of the sound wave is being transferred to the air at the same frequency, but as it travels towards you it's transferring them into the air at different locations. Now those waves travel out from the car at a constant speed in every direction. So the ones going in front of the car have been created at the same frequency, but because the car was moving the wavelength is shorter, but they're traveling at the speed of sound (assumed constant) so they can only actually be one of those things, so the observer hears the shorter wavelength/higher frequency. Reverse that as the car passes.

Explanation based on a fast observer and a stationary generator (the one you cant really relate to in everyday life but is easier to explain):

Imagine you're on a boat at sea. The waves (which are behaving exactly like sound or light, just at a really slow speed) are traveling in one direction. You can either point your boat towards them, and it will feel really choppy and you hit a lot of waves (you observe a high frequency, perceive a short wavelength), or you turn your boat around and go with them, at this point you perceive a much lower frequency.

How that relates to relativity:
Both cases are the same, you can't say the boat is moving except it's moving relative to the sea, but for all you know you're battling against a tide. You get exactly this effect in a Channel (English or Bristol work for this example), if the swell is coming from the soutwest, and the tide is rushing out (heading west) then you get (to an observer on a boat, also affected by tides) waves closer together and it feels rough. On the other hand, a bouy monitoring the waves would see a different frequency, because it thinks it's moving relative to the water, but isn't actually moving.

This is why relativity becomes important, because you can think up a thought experiment with a wave tank on a conveyor belt and use it to demonstrate what you would see if you broke the laws of relativity. E,g if the waves speed and the tide oppose each other exactly you end up with what appears to be a stationary wave to an external observer. But a boat being dragged by the tide sees something completely different. We know that if you observe light from something you still observe it traveling at the same speed, it doesn't stop just because you change your velocity relative to the emitter or the wave.

How this relates to relativity and the speed of light:
If you know the speed of light should be constant, and you know what color the object should be (because you know it's got certain elements in it and therefore certain absorption lines) then you can calculate how fast it is moving relative to the speed of light (a 1% shift is 1% the speed of light). And that gives you it's speed relative to you.

The fact that everything in the sky looks "red" means it's all moving away from us.
The fact that everything looks redder the further away it is (i.e. it kinda looks like we're the center of the universe), proves both that the universe is expanding in every direction, and that light must travel at a constant speed relative to the observer (because we're not the center, but I'll be honest I stopped being able to follow the maths at that point which is why I went into engineering, not Physics).

Which of the three variable that I am aware of (distance, time and velocity) is giving way to allow the frequency to change?

I think the answer to that is that the relationship between those variables, in Newtonian physics, is something most of us understand. But this isn't Newtonian physics. In Special Relativity they don't behave the same way, so the frequency can change. Special Relativity is is maths and relates to the curvature of space-time; I don't pretend to understand it but I believe it's the explanation.

I don't believe in special relativity or quantum physics.  I think they are made up excuses to try to account for the gods being mischievous and winding physicists up.

what happens to the wavelength of light if two galaxies are zooming towards each other at equal speed?

Are any galaxies moving towards each other, or are they all moving away from the centre of the universe, post-Big Bang?

Are any galaxies moving towards each other

Yes. For example the Andromeda Galaxy is moving toward the Milky Way (our galaxy) and a collision is predicted in about 4.5 billion years.

Some good detailed answers there… but the marshmallow gun is my favourite.

Wavelength is effected by the changing distance between emitter and receiver. “Stretching” or “compressing” the waves (yes, I know, it isn’t, but it has that effect on what you can observe).

Are any galaxies moving towards each other, or are they all moving away from the centre of the universe, post-Big Bang?

It’s not either/or… the distance between both and the centre of the universe can be increasing, but if that’s not at the same rate, then they can be moving towards each other (while also both “moving” away from the centre of the universe, they’re not moving away, but… we can observe things that tell us they are both increasingly far from the centre).

@thisisnotaspoon, the speed of light is a fundamental constant whereas the speed of sound isn't, so the analogy doesn't quite translate

I thought the speed of light does change depending what the material it is going thru?  ( diffraction etc?) so speed of light in a vacuum is a constant but then there is no true vacuum anywhere?

I thought the speed of light does change depending what the material it is going thru? ( diffraction etc?) so speed of light in a vacuum is a constant but then there is no true vacuum anywhere?

Yes, speed of light in a vacuum. Deep space is so close to a vacuum that it doesn't matter. If you get into quantum physics, even a vacuum is a roiling cauldron of virtual particles (if I understand it correctly). However, relativity and quantum physics operate at vastly different scales and the virtual particles will just wash out as random noise, I assume. I'm sure there have been Ph.D. theses on that.

@thisisnotaspoon, the speed of light is a fundamental constant whereas the speed of sound isn’t, so the analogy doesn’t quite translate

It does and it doesn't.

The speed of sound is constant if you define the medium, it's just that in the real world "air" is far from homogenous. Ditto the speed of light is only constant if you define the medium also. As soon as you put something in the way it appears to slow down. It doesn't actually slow though, it just hits a particle, gets absorbed, then re-emitted. You can even bring "light" to a complete standstill! Well you can't, but you can create a material that at very cold temperatures will absorb the light, sore it as an excited state within the atoms, then emit it again. But visually it's been stopped, and it's an extreme example of the same process as occurs when the light goes from air to water and loses about 1/4 of it's speed.

Sounds like we're all going to end up perishing in a ball of blue flame.

Probably on a Friday, with a packed weekend ahead.

The speed of sound is constant if you define the medium

Fair point!

I think whwre I've been going wrong is it's all about the point at which the light or sound wave was propagated, rather than the speed of the waves

while also both “moving” away from the centre of the universe

The universe doesn't have a centre.

Imagine a football, but only the surface. It's two dimensional, you can only go left, right, forward and backwards, but it's not flat in the third dimension. It has a finite area but no edges, and no centre. The universe is like that but in 3 dimensions.

Drifting a bit from the OP's questions, but the scale of this puts my daughter's complaints about my roast onion farts into perspective.

The universe doesn’t have a centre.

I think whwre I’ve been going wrong is it’s all about the point at which the light or sound wave was propagated, rather than the speed of the waves

Pretty much, the sound (or waves on water) analogies only really start to fall apart when you approach the speed of those waves. e.g. a surfer paddling to catch a wave perceives such a long wavelength/low frequency that the wave stops. But that's the Doppler effect (assuming they don't catch it and the wave eventually passes them, then another and another) but it's not the same as considering relativity as you approach the speed of light.

So anyway, you know how the universe is expanding...

If the space between everything is getting larger, does that mean in relative terms, everything is getting smaller?

Or in time, will the vacuum effect of deep space stretch everything so the relative scale remains the same, just the overall volume increases...

🤔

The universe doesn’t have a centre.

Define “centre”. I’m using it to mean that it’s where all other points have come from and all are increasingly distant from. I’m not saying it’s “in the middle” of something akin to a sphere or circle. My mum is the at the centre of our family… we’re not a volumetric space. [ I’m now feeling sorry for her. ] Call it the source, or start, if you want.

Or in time, will the vacuum effect of deep space stretch everything so the relative scale remains the same, just the overall volume increases…

One of many theories on how the universe “ends”. Entropy. Everything so spread out that it has no measurable/usable energy due to its massive volume.

kelvin as I understand it every bit is moving away from every other bit so there is no centre everything moves away from

Wherever you are seems like the centre as everything is moving away from it

Well, we know that on the local scale (space and time) that isn’t true. Or we’d all be very alone. Everything is on average/eventually moving away from everything else, but there is a “centre” that everything is increasing distant from and nothing is moving towards, even temporarily or for short distance.

The speed of light is constant, but the speed that the fabric of the universe is expanding isn't limited by the speed of light.

That's why the observable universe is 13.8 billion light years in diameter. I prefer to call it the time warped universe, the one we can see, where we look back in time.

But there's also what I'd call the real time universe, which is estimated to be 46 billion light years in diameter.

Basically that point we can see 13.4 billion light years away, currently looks very different and is a different place after 13.4 billion years of time. It's not frozen in time.

Something like that anyhow.

The speed that the fabric of the universe is expanding isn't constant either. It's slower for closer objects and faster for further away objects. That's why galaxies in the local cluster can still move towards us(and will eventually merge into one big galaxy with andromeda eating up the milky way and all the near by satelite galaxys(Of which there's loads btw)), but further out ones will never get closer to us after point.

The scales are mental in size to comprehend though.

You go from galaxy to local cluster, to super cluster to your part the cosmic web and well beyond.

I'm not even sure if galaxies in the super cluster can move towards us or not, or if they've reached the threshold of the expanding fabric of space? I'm guessing there's a point where the expansion of space takes over and galaxies over a certain distance can never interact.

Even out with the cosmic web, there's vast areas of nothing that are dominated by dark matter/gravity/energy I think. And we haven't a scooby what it is. I tend to think dark matter/gravity/energy is some other dimensional construct or something. It's probably what is driving the expansion of the universe I guess. But maybe this expansion is just what it looks like to us, may be this expansion is as I say some kinda other dimensional construct, where time/space and speed don't really matter? dunno, just havering out loud really. 😆

I’m guessing there’s a point where the expansion of space takes over and galaxies over a certain distance can never interact.

Exactly. Some parts of the universe are so distant and expanding away from us so fast that light from there can never reach us. Eventually, the universe will expand so much that every single particle will be isolated from every other particle.

Define “centre”. I’m using it to mean that it’s where all other points have come from and all are increasingly distant from

Everything in the universe started in one very small point. Where is that point now? Every where, because everything was in that point.

I teach A level physics. Here is a video I made about Doppler it starts with sound and moves onto binary stars

This one’s the Big Bang and red shift

The rest of my astrophysics stuff is here.

https://youtube.com/playlist?list=PLL2NGhZ6xBiXciSfUG-SKqOVj4dQBz6Ap

PS most of these were made during lock down when despite what the press said I was quite busy

Added to my watch later list ampthill

The universe doesn’t have a centre.

That’s why the observable universe is 13.8 billion light years in diameter. I prefer to call it the time warped universe, the one we can see, where we look back in time.

But there’s also what I’d call the real time universe, which is estimated to be 46 billion light years in diameter.

I can't see how both these statements can be true. If something has a diameter it has a centre* surely?

*probably several different ones depending on how you define it see eg the centre of England/UK etc.

greyspoke
Free Member
The universe doesn’t have a centre.

That’s why the observable universe is 13.8 billion light years in diameter. I prefer to call it the time warped universe, the one we can see, where we look back in time.

But there’s also what I’d call the real time universe, which is estimated to be 46 billion light years in diameter.

I can’t see how both these statements can be true. If something has a diameter it has a centre* surely?

I actually meant radius. And we only extrapolate a 46 billion light year radius with our best guess(we don't even know how fast the universe is expanding from our perspective), that's not the end, only what we know existed in the past and likely exists today. It only has a centre in terms of our viewpoint. We are the centre in that perspective, but someone on a planet in a galaxy 5 billion light years away will see their own bubble. Doesn't mean that's where it started.

As I say there's definitely something else going on, cause time didn't exist before the big bang, it's just a consequence of the big bang, so what it all came out of isn't necessarily something that plays by our laws of physics, imo. Probably some kinda of dimension where size doesn't really mean much either I'd speculate.

look up doppler effect...

Probably some kinda of dimension where size doesn’t really mean much either I’d speculate.

Maybe I should move there.

I can’t see how both these statements can be true. If something has a diameter it has a centre* surely?

Depends on how you are defining these terms. Traditional geometry that you learned in school is called Euclidian and it only applies to flat unbounded planes like infinitely large pieces of paper.

It's a lot easier for us to think in terms of two-dimensional analogies. Going back to the football again, the surface of the football has a finite size, and if you keep going in a straight line you'll come back to where you started, after you've gone about 70cm or so. If you're an ant crawling on the surface of the football then you'd think it was about 70cm in 'size' - now does that mean the circumference? I suppose so. Now, if it has a circumference then according to school geometry it has a radius and consequently a centre. But the ant can never conceive of it (it's a two-dimensional ant). However in the case of the football, we as 3D beings know that there is a centre in the third dimension.

A four-dimensional sphere is called a hypersphere. Is the universe hyperspherical? Well it might be, if it's closed, but it can also have the same curvature and be open, which would be sort of saddle shaped (horse, not bike) if it were 3D, which it's not.

So a better statement would be "we don't know if the universe has a centre because we don't know where the edge is, or if it has an edge, or what an edge even means for a universe"?

No. My understanding is that the universe is finite, but unbounded. The exact geometry will depend on the density. As above, it started as a singularity and expanded. Every point is effectively the center with the universe expanding about it.

It doesn't relate to our everyday experiences so it doesn't seem to make sense. It can really only be understood mathematically, which is why I gave up studying physics, I realized that I would never really be able to understand it because I was too lazy to learn all the necessary maths.

universe is expanding…

And expanding.

In all of the directions it can whizz...

So is time effectively stopped at the centre of a super massive black hole? That's going to get in the way of things when the universe is trying to end, isn't it?

So is time effectively stopped at the centre of a super massive black hole? That’s going to get in the way of things when the universe is trying to end, isn’t it?

If I understand it correctly, the black holes will slowly evaporate because of Hawking radiation. This will take an incredibly long time, something like 10^100 years.

Hmmm, so @thols2 would you agree with @molgrips that if we were sufficiently dimensionally well-endowed to step outside the universe, we would be able to see the whole ball, as it were, and find the centre? Or is that one of those "might be true but it might not be like that" things?

Thanks ampthill, I could no doubt learn a lot from your videos… if I could apply myself. [ admits near total ignorance and walks away ]

I think the point is that the center doesn't exist in our 3D space. The center of a ball doesn't exist on the 2D surface and a 2D being cannot find it. From the 2D perspective, all points on the surface of the ball are effectively the center.

That really helped thols2, thanks. Pretty much what TJ said as well. [ keeps walking towards a horizon he will never reach ]

So is time effectively stopped at the centre of a super massive black hole?

Not necessarily. Time is relative remember. You might observe time as being near stationary but the people inside wouldn't. Time doesn't run the same for everyone.

Re the centre of the universe, a 4d ball would have a nice simple centre but the universe isn't necessarily ball shaped. Plenty of weird geometries in our normal world, I mean where is the centre of a torus? It has no centre inside itself, or the centre is a circle within it. Or a Mobius strip with only one side but two edges. Where's the centre of that?

greyspoke
Free Member
So a better statement would be “we don’t know if the universe has a centre because we don’t know where the edge is, or if it has an edge, or what an edge even means for a universe”?

No, because measurements tell us that every point is expanding the same way. If there was a centre it would be easy to at least locate the direction of it, but in that sense the Universe is uniform, from what ever point you measure it.

The edge of the Universe is just an other unknown quantity. Tbh when discussing this stuff we are at the edge of physics and philosophy. So the edge can be speculatively discussed a number of ways.

You know, what is a Universe?

The largest structure we know of is the Cosmic Web, but even within that, due to expansion, it's not even possible for some things to interact physically.

So you could maybe define a Universe as matter that can potentially react gravitationally?(even that's a blurred line) Should we draw the line at say the Super Cluster or our Local Group, or is everything we can see in our Universe. (and I guess over time, we'll discover/define other structures with in the Cosmic Web that are larger than Super Clusters if we already haven't)

What about the stuff beyond the Cosmic Microwave Background Radiation that we can't and likely never will see, is that in our Universe?

tbh 'our', 'a' or 'the' Universe becomes about definition at some point.

God runs electromagnetics on Monday, Wednesday, and Friday by the wave theory, and the devil runs it by quantum theory on Tuesday, Thursday, and Saturday.

Neil deGrasse Tyson subs in on Sundays to give everyone a break

No, because measurements tell us that every point is expanding the same way.

So if something with a centre was expanding evenly, how would you find the direction to the centre?

greyspoke
Free Member
No, because measurements tell us that every point is expanding the same way.

So if something with a centre was expanding evenly, how would you find the direction to the centre?

Well if you are at the centre everything would be moving away from you uniformly. If you are one of the bits that isn't at the centre it would be obvious that everything else isn't moving away uniformly and you could calculate from the velocities of different points where the explosion began.

The thing with the universe is that, at any point, everything is moving away uniformly. Therefore no centre or everywhere is the centre. So there's weird shit afoot. 😆

So there’s weird shit afoot.

Its the gods having a laugh I tell you.

No, because measurements tell us that every point is expanding the same way

The key measurement is that (in general) the speed something is moving away, measuremed by the amount of red shift, is proportional to its distance from us. The only way this is possible is if everything is expanding.

Well if you are at the centre everything would be moving away from you uniformly. If you are one of the bits that isn’t at the centre it would be obvious that everything else isn’t moving away uniformly and you could calculate from the velocities of different points where the explosion began.

I am not sure about explosions. I was thinking of the traditional analogy for this, which is a pudding of some description with currants in it, which is expanding. And not constrained by a bowl or plate, so maybe you are doing some cooking on the International Space Station and just leaving it to hover. Or maybe a loaf with seeds in it which you are leaving to prove on the ISS. Anyhow, thee dough/batter expands, the currants/seeds get farther sway from each other uniformly. The doughball/pudding has an eddge and a centre, but from a point say 1/4 the way across it, how do you tell where the centre is?

greyspoke
Free Member

I am not sure about explosions. I was thinking of the traditional analogy for this, which is a pudding of some description with currants in it, which is expanding. And not constrained by a bowl or plate, so maybe you are doing some cooking on the International Space Station and just leaving it to hover. Or maybe a loaf with seeds in it which you are leaving to prove on the ISS. Anyhow, thee dough/batter expands, the currants/seeds get farther sway from each other uniformly. The doughball/pudding has an eddge and a centre, but from a point say 1/4 the way across it, how do you tell where the centre is?

the pudding anology doesn't work, well on cause it's opaque, so you can't see the seeds, and secondly the explosion has finished by the time you see the cake, so you can't look at their velocities and directions.

We can see, stars, planets, galaxies etc, and they are still moving, so we can measure where they are going etc.

But if you could see the currants and the seeds, and they were still in their expanding state, you could calculate where the original dough ball started.

The universe doesn't work like that though. We don't understand the medium we are expanding into.

With the pudding, you are using a 3d analogy (the raisins) in a 3d object. But when considering the universe you are thinking of a 4(at least)d object in 3 dimensions. This is why the polka dots on the balloon analogy is better IMO because by comparing dots on a balloon with dots on flat paper you can see the strange effects caused by curvature in a higher dimension.

if something with a centre was expanding evenly, how would you find the direction to the centre?

We see homogeneity in the observable universe. No direction shows any difference in content. The CMB is as ancient in all directions.

If there is a central origin in 3d space (4d spacetime) it isn't within our observable universe.

As far as the centre of the observable universe... It is wherever you are making your observations.

Speculation on any structures or physics beyond the observable universe are moot. Not observable. Nothing can be observed. We can theorise but such theories are untestable. Our best theory about the larger universe is "more of the same", but we have no justification for even drawing that conclusion which, on the face of it, is an extraordinary special case.