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Comment by JKCalhoun | Hacker News Reader

Considering these 'deep space' images that capture these myriad of little galaxies so far away.... The number of galaxies that must be out there is inconceivable to me.

Spitballing.

Might there be some space/time mechanism at play whereby we're actually seeing the same handful or so of galaxies? Like maybe some lensing thing.

Or weirder, we're actually seeing right around the universe itself — as though seeing the back of your head in a mirror if you look far enough. Not a topologist, but seems a toroidal universe would have a property like this: look far enough and you see the back of your head. So perhaps the same galaxies seen from multiple angles at the same time appear to be a greater number of galaxies than there actually are.

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As well as what ISL said, if the universe is toroidial it seems like it's larger than our visible horizon. While images of an individual galaxy might be hard to tell if they are repeated, we can see enough of the structure of the universe that if our visible horizon was much larger than the real value of the universe and there was wrapping going on, we'd see it. This isn't the exact sort of video I'm looking for, but it's close enough to what I'm talking about: https://www.youtube.com/watch?v=rENyyRwxpHo Everywhere we look we see distinct structures.

It is not a bad thought, though. It may well do some sort of wrapping around, just at a larger scale than we can see. It's an open problem.

It's been over a decade since my cosmology class, so forgive any errors, but isn't there a constant that describes the curvature of the universe [1], which so far has been calculated as "flat"? That's not to say we'll ever truly know, since you need infinite precision to rule out hyperbolic or spherical geometry, but in a flat universe, how can you have toroidal geometry?

Per [2]:

> The actual value for critical density value is measured as ρcritical = 9.47×10−27 kg m−3. From these values, within experimental error, the universe seems to be flat.

[1] - https://en.m.wikipedia.org/wiki/Friedmann_equations#Density_...

[2] - https://en.m.wikipedia.org/wiki/Shape_of_the_universe

It’s actually no problem mathematically to have toroidal topology and flat geometry. (The classic Asteroids video game, where your ship can go off the top of the screen and come in at the bottom, or similarly with the left and right edges, is an example of a flat 2D space with the topology of a torus.)

Everywhere we look we see distinct structures.

This might be us being in an isotropic bubble where outside of the bubble is forever gone for us to see due to expansion of universe... Not sure how CMBR falls into that however.

Hence "it seems like it's larger than our visible horizon", "at a larger scale than we can see", and "It's an open problem".

We're talking about being toriodal or curved in a 4th spatial dimension I assume?

Watching that video will make anyone humble.

Your comment reminded me of this story where a space program director decided, on a hunch or whim, to point Hubble at a seemingly empty space for 100 hours.

The rest, as they say is history. Galaxies went from being rare things to abundant ones. Turns out the universe is teeming with them.

https://www.nationalgeographic.com/science/article/when-hubb...

It wasn't quite that dramatic a whim or lucky hunch as it's made out to be.

There was input from galaxy researchers on such an idea, and I think it was viewed as a bit of a calculated risk compared to other normal proposals to spend the telescope time on well-known targets visible with a few hours of observing. Especially when proposals outnumber available time by a factor of approx. 7, people will question why you should gamble on something unproven. Hence "director's discretionary time".

But of course it was a smart move in setting up the field for "what's the next big thing", because if you were to find something interesting, it obviously drove the call for larger and larger telescopes to study farther and farther things. Just studying the crap out of brighter nearby things would have been relatively predictable/boring by comparison. (I exaggerate a little)

There are as many galaxies in the observable universe as there are stars in our galaxy or… as there are bytes in 100 gigabytes. Within a factor of five or so.

The universe is flat as far as our best measurements go, if it is curved, it is with a much larger radius than the ~100 billion light year width of the observable universe.

It is indeed difficult to conceive but it is how it is, the universe is very big and has a whole lot of unique stuff in it.

Occasionally there is a lensing thing where we see the same galaxy twice or so, but not in any way to diminish the 10^11 other galaxies out there.

What's interesting to me is, relative to human scales, cosmic space is so much more ridiculously vast than cosmic time.

Our size as a percentage of the size of the universe is tiny, and our age as a percentage of the universe's age is still tiny. But the second number is gargantuan in comparison to the first.

It always seemed odd to me that we're so close to the apparent start of the universe. 13.8 billion years isn't that long ago, given how big the future time horizon is.

We are close to the beginning of the universe, but we are about halfway through the lifetime of our sun, which formed right around when 50% of all sunlike stars that will ever be formed, had been formed. So we're actually around average in many ways.

Oh, that's interesting! That does make more sense. Most of the sun like stars that will ever exist are formed in the early universe, so it makes more sense that's why we exist when we do.

Does that mean that roughly around the time our sun dies that many more stars will also be dying ?

And then will there simply be way less stars in the universe than there are today?

Not “way” less, because most stars are lower mass than the Sun and thus have longer lifetimes (~ trillions of years in the case of red dwarfs).

Do you have a sense of how many stars are "sunlike" compared to how many stars there are in general?

This is interesting to me because there are a lot of seemingly arbitrary ways you can try to make the size and age of the universe comprehensible, and relative to a human scale is arbitrary but at least kinda means something.

And the commenter above measured our point on a cosmic time scale by looking at where on that timeline our sun formed relative to other sunlike stars. By that metric we are kind of "in the middle." But for me the natural next question is compared to stars in general, as "sunlike stars" is maybe a bit of a texas target.

AFAIK the sun is brighter than about 90% of stars in the Milky Way. We're on the larger and faster burning side, as stars go. So there will be many stars left after the sun dies. Whether you consider this to mean that saying we're about halfway through the lifetime of sunlike stars sidesteps the question depends on whether you believe that complex, energy-intensive, intelligent life is equally viable on planets orbiting red dwarfs.

One person's idea of a "side step" is another person's "refraining from making assumptions" but point noted.

For me, in my position of refraining from associating life with particular types stars, I at least feel a bit more reassurance to think of us as in the company of stars writ large, by which measure we're not so close to the end of the cosmic span of time.

That said, I think sun-like stars is a perfectly legitimate measure to put alongside the others when casting about for intuitive ways of assessing where we stand on a cosmic timeline.

>> The number of galaxies that must be out there is inconceivable to me

Don't worry, there are many times more galaxies in the observable universe than neurons in the human brain. Nobody can conceive of it.

I don't need to be able to conceive of every individual grain to be able to make inferences about sand.

There's really only like 2 trillion galaxies in the observable universe which isn't that much more than the number of neurons.

TIL there's about as many galaxies in the observable universe as trees in the planet.

https://www.google.com/search?q=how+many+trees+are+on+earth

Yes but that’s just the observable universe. My understanding is that the observable universe is only a fraction of the total universe. However that was something I found hard to verify so I could be wrong.

We don't have an upper bound on the size of the universe. It's difficult to construct falsifiable models about things that can't be directly observed, but it may well be infinite.

That number nobody can conceive of... add one to it

People have looked for hints of this sort of thing without success so far.

https://en.m.wikipedia.org/wiki/Shape_of_the_universe#:~:tex....

Perhaps it’s all procedurally generated, and every time we build a new telescope there’s a supernatural sysadmin out there going “Oh, ffs! - We need more new hardware!”.

Surely if we do live in a simulation then the engineers just need to simulate our minds. It's easy to fake what we see then as an optimisation instead of generating a whole universe.

That assumes we're the object of the simulation rather than a byproduct. Maybe what they want to simulate is a universe and we've just happened to pop up in a small part of the simulation.

On the scale of the universe you aren’t just seeing far away objects but seeing the past. For illustrative purposes the edge of the Universe is the edge of time, where time has any meaning to us, and beyond that is inflation where light didn’t travel freely.

If somehow the Universe were as you propose and wrapping around itself so we were able to stare at the back of our heads, we would only see objects from the past to a point, and then the objects would begin to appear closer in time again until the furthest point where we would once again be looking at the present.

The universe was opaque up to about 380,000 years after the Big Bang. If the universe loops in on itself and is large enough then wouldn't we be blocked by that? And the further you go back from there the higher the density would be.

Meaning we couldn’t look straight out and see the back of our own heads. Eventually we would run into a point of the universe before time had any meaning (the opaque plasma) and there would be no looking past that back into the present.

> Might there be some space/time mechanism at play whereby we're actually seeing the same handful or so of galaxies?

Nope. Current evidence is that space is infinite. Even if that was all wrong, then its incredibly large due to inflation and our past light cone is one tiny bit of it. As time goes on we're seeing more and more of it, and parts that used to be in contact before inflation are only just coming back into contact again (their past light cones overlapping). Those are all unique galaxies. An awful lot of physics would have to be wrong for that to not be true.

"Space, is big. Really big. You just won't believe how vastly hugely mindboggingly big it is. I mean you may think it's a long way down the road to the chemist, but that's just peanuts to space."

Yes, I wonder about this also. Wouldn't need to be toroidal - could just be spherical, but would need to wrap in a 4th spatial dimension. It would explain a lot of things, like the expansion not having a clear centre, the requirement for dark matter (expansion would happen in an orthogonal dimension), background radiation (which I never quite understood) and the size of the universe.

But that was a high-school idea, and I'm sure some clever physicists and astronomers will tell me why this isn't the case.

“ Might there be some space/time mechanism at play whereby we're actually seeing the same handful or so of galaxies? Like maybe some lensing thing.”

Good question. People have been wondering this and have studied whether it is possible a la periodic boundary conditions for example.

https://aapt.scitation.org/doi/abs/10.1119/1.13499?journalCo...

Back when they were launching JWST, I kept thinking that there's a non-zero probability of it proving wrong some of our essential assumptions about the universe. Like, you know, they keep saying "galaxies that formed just X years after the big bang", but what if it turns out there was no big bang? What if our estimations of the age and/or size of the universe turn out to be wrong?

If there’s no Big Bang then the origin of the cosmic microwave background needs a new explanation: why does it exist, and why isn’t it more red or blue shifted.

The question I've always had is - what if the entire universe is not only unthinkably larger than the observable universe but also non-homogenous? In other words, what if the part of the universe that lies in our past light cone is nothing at all like the rest of the universe? The CMB and other remnants of the Big Bang could simply be remnants of something that happened in a large enough swath of the universe to cover our observable universe but still only be a tiny part of the entire universe and not represent the actual beginning. The parable of the Blind Men and the Elephant on a cosmological scale, so to speak.

Even if there wasn’t a Big Bang, let’s say the universe was infinite, we would still have the CMB from the furtherest reaching photons from the edges of the visible universe that have been stretched to microwaves right?

In other words eventually the nearest galaxies will one day become the CMB as the universe continues to expand until they are on the edge, until even they fall outside the visible universe and no photons/light outside our own galaxy will be capable of reaching us and the CMB disappears.

That's not what the CMB is. The CMB is an incredibly uniform pattern which appears to have been formed by a very hot plasma at some point in the past. It's difficult to imagine an explanation other than the Big Bang (or some kind of expansion of a very hot plasma) because the uniformity means it was almost certainly concentrated in space.

But yes, in the distant future, it will not be possible to detect the CMB anymore (and it's entirely possible that alien civilisations that evolve at that time would probably never have a model of the Big Bang because there would be no remaining evidence of it).

> That's not what the CMB is. The CMB is an incredibly uniform pattern which appears to have been formed by a very hot plasma at some point in the past.

I understood that the CMB is not photons from plasma but the first photons following recombination (when the universe cooled enough that electrons and protons formed first hydrogen atoms). In other words the universe was plasma just before the CMB.

Yes, you're quite right -- sorry for not being precise enough. However my point was that the fact that we cannot see beyond the CMB and the uniformity of the pattern both point towards a Big Bang-esque explanation.

...and the galaxies will have drifted so far apart, that nothing will appear to exist beyond their own galaxy, except dust.

The problem in an infinite universe is every possible location in the sky eventually terminates on a stellar surface - a star. So the night sky, if the universe is infinite, wouldn't be dark - it would be as bright as the sun - in fact the universe would be full of omnidirectional radiation coming from all directions at all times.

I suppose though, that if space is still expanding but the universe is infinite, this might temper it out but it doesn't seem like enough - it would have to be an expansion precisely tuned to on average send radiation to 4 kelvin so we only see a cosmic microwave background, and don't wind up being bathed in an infinite amount of whatever frequency of radiation.

Infinity is funny like that.

That’s Olber’s Paradox, of course, and it relies on three assumptions: 1. The universe is infinitely large and 2. The universe is filled with stars and 3. The universe has been this way forever.

It turns out #3 is wrong, so it’s still possible we live in an infinite universe.

(Imagine an infinite, unchanging universe filled with stars that magically popped into existence a billion years ago. You wouldn’t have a uniformly bright sky because even though every line of sight will theoretically terminate on a stellar surface, in most cases there hasn’t been enough time for light traveling along that line of sight — e.g. from a star 10 billion light years away — to reach your eyes.)

> So the night sky, if the universe is infinite, wouldn't be dark - it would be as bright as the sun

You are inferring infiniteness also translates somehow to cosmic density and luminosity. By definition of an infinite universe the vast majority will necessarily fall outside the observable universe and never be visible to us.

As to luminosity, even now when we look up and see dark patches in the sky, they are in fact are full of stars and galaxies. The most famous picture ever taken by the Hubble, the “Hubble Deep Field”, was taken by pointing towards a dark patch revealing 10s of thousand of galaxies. These 10s of thousands of stars and galaxies still appear as dark patches in our skies because they are not sufficiently luminous to appear in out sky as light without the aid of telescopes to collect their dim light.

At this point that's a quibble that loops back around to Big Bang theory - the OP was postulating we somehow discover that that theory is not correct, and my point is that if it's not - if the universe isn't bubbles of matter evaporating out of each other's spheres of influence - then you run into serious problems with why the night sky looks the way we are.

You've just looped it back around: expanding space gives finite, shrinking, observable universes. Which is just exactly the Big Bang theory.

EDIT: Dark patches in the sky though don't work with infinite light sources - at the very least you have to explain why far away ones apparently switched on in a finite time in the past. But that's the issue too - we're deliberately ignoring evidence in this discussion which points away from that - i.e. the red shift which shows us galaxies are rushing away from us.

> You've just looped it back around: expanding space gives finite, shrinking, observable universes. Which is just exactly the Big Bang theory.

There is nothing about Big Bang theory that explains the current expansion of our Universe (i.e. dark energy). In fact the Big Bang theory predicted our Universe should have stopped expanding and began collapsing on itself due to gravity (i.e. the Big Crunch), and that’s exactly why whoever can explain why the Universe is continuing to expand has a Nobel Prize waiting.

> The problem in an infinite universe is every possible location in the sky eventually terminates on a stellar surface - a star.

Isn't that only true if the universe is infinitely old as well as infinitely large?

Sure, but postulating an infinitely sized universe (full of an infinite amount of mass) does ask the question as to why the universe would not be infinitely old?

Particularly when you get into issues like entropy, which is the only real determinant of time even existing. A finite universe has a natural direction of entropy - whereas an infinite one does not, since there's an infinite amount of mass and energy and as such no possible lowest possible entropy state.

It's actually worse then that though: an infinite massed universe by definition would contain every possible configuration of that mass somewhere within it. So you and I talking right now like this, our past and future conversations would also all be somewhere else in the infinite universe happening simultaneously.

A universe with an infinite amount of mass and energy in infinite space doesn't really have any sensible notion of past, present or future - because all possible pasts, presents and future, exist at all times somewhere within it.

> A universe with an infinite amount of mass and energy in infinite space doesn't really have any sensible notion of past, present or future - because all possible pasts, presents and future, exist at all times somewhere within it.

This is actually untrue for many reasons, but one is that there are different kinds of infinities. For instance, there are an infinite amount of real numbers between 0.0 and 1.0, and none of them are pi. Just because something is infinite, doesn't mean everything is possible within that infinity. Again, there are an infinite number of integers, but none of the are pi or 0.1 or 37.5

All possible configurations was the term used. We aren't living impossible configurations of matter in our day to day reality.

> an infinite massed universe by definition would contain every possible configuration of that mass somewhere within it.

That was the statement, a universe with a 1kg blob of matter every billion light years would be infinite massed, but would not have all possible configurations.

Inside the framework of inflation this doesn't follow. You can have an infinitely large universe while still expanding and cooling over time.

If it proves there was no big bang then that's still an exceptional job well done and a huge accomplishment for science. Sure there will be more mysteries to solve, but it's not like we lose anything by proving something incorrect. It will be exciting no matter what they find!

I predict astronomers will observe and catalog all trillion observable galaxies in a century. It was probably inconceivable a century that we'd observe and catalog two billion stars in the Milky Way by now. But with improving technology we have.

You can reject some notions of repeating patterns / loops in space because it would violate Lorentz covariance. You could fire a laser in one direction and the opposite, then you know your absolute position in space.

> Might there be some space/time mechanism at play whereby we're actually seeing the same handful or so of galaxies?

That strikes you as the sort of thing nobody would have noticed throughout centuries of study?

Maybe it's a quantum observer effect and the galaxies only exist because there's an observer. Dark matter/energy is need to make things balance only because we haven't been looking in all the right places.

The universe would be an odd old place if that was true.

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