How Can a Star Be Older Than the Universe? • Hacker News Board

Re: How Can a Star Be Older Than the Universe?

#171

Post by jackgavigan » Thu, Oct 17, 2019, 11:28 AM UTC

cgrealy wrote: post #113

ShamelessC wrote: post #109

Earlier quoted context omitted.

What's the explanation?

Without giving too much away, it's not from this universe. You really should read it, it's one of the best books in the culture series and Banks was a phenomenal writer. It also contains the Affront, a species that are almost comically awful and coined the term "outside context problem": "The usual example given to illustrate an Outside Context Problem was imagining you were a tribe on a largish, fertile island; you'…

You left out the best bit!

"An Outside Context Problem was the sort of thing most civilisations encountered just one, and which they tended to encounter rather in the same way a sentence encountered a full stop."

Shades of Douglas Adams there...

Re: How Can a Star Be Older Than the Universe?

#172

Post by mannykannot » Thu, Oct 17, 2019, 11:44 AM UTC

Dylan16807 wrote: post #108

mannykannot wrote: post #91

Earlier quoted context omitted.

That actually demonstrates the opposite: despite the fact that the images themselves show the expansion of a grid in a two-dimensional space, each of the dots appears, from its perspective, to be at the center of the expansion, unless you either specify an absolute frame of reference, or (which amounts to the same thing) can identify the edge of the expanding region. Furthermore, our telescopes have only observed fro…

> That actually demonstrates the opposite: despite the fact that the images themselves show the expansion of a grid in a two-dimensional space, each of the dots appears, from its perspective, to be at the center of the expansion, unless you either specify an absolute frame of reference, or (which amounts to the same thing) can identify the edge of the expanding region. The image isn't enough to demonstrate the differ…

> The image isn't enough to demonstrate the difference...

Precisely.

> ...But the speed of light is, along with the way acceleration and redshift work.

Are you referring to the cosmological component of redshift? I suppose that, at least in principle, we could determine the cosmological component by first measuring the Hubble constant from the change in distance, over time, of those stars whose distance can be measured without any assumptions about redshift, but has that actually been done? (or some other experiment that directly calculates the cosmological component of redshift?) OP's claim is that we already have sufficient experimental results, not that we could, in principle, get them. AFAIK, experimental error in these stellar distance measurements is too large, and they are over too short a period of time, for the rate of expansion of the universe to be taken directly from them (note that the anomaly that is the subject of the article itself brings into question the accuracy of the cosmic distance ladder.)

> A chain of logic that starts with the evidence we have, goes on to what telescopes show elsewhere, and then concludes about expansion, is not circular.

It seems pretty clear that OP was thinking that if we were to observe the universe from a distant galaxy, it also would seem to be at the center of the expansion - but that would be the case in both scenarios, so it does not actually distinguish between them. If, however, OP had in mind some other set of measurements that, when combined with those from Earth, distinguish between the scenarios, then the question becomes, can you deduce what those far-away measurements would be without choosing between the two scenarios, either explicitly or implicitly? If that choice is being made, then you are begging the question. What set of measurements and calculation do you have in mind that avoid this circularity? (the above-mentioned redshift measurements are a separate issue, both because they are clearly not what OP had in mind, and because they do not depend on any calculation of what observations would be made from a distant galaxy.)

Re: How Can a Star Be Older Than the Universe?

#173

Post by brutt » Thu, Oct 17, 2019, 11:51 AM UTC

undreren wrote: post #128

devnulloverflow wrote: post #116

Earlier quoted context omitted.

Your intuition that there is an limited observable distance because is correct. But because of the expansion, that limit is bigger than "speed_of_light * age of universe" . I don't know enough General Relativity to give a solid explanation. But here is a rough over-simplification: Imagine you've spent the last 5 seconds blowing up a balloon, but there is an ant walking on that balloon, starting from a marked spot. In…

The ant and balloon analogy is really good from an intuition stand point. Great explanation! Well done :)

Except for fact than in balloon analogy, ant is moving in a direction by balloon surface, while in our case our "balloon" is perfectly still in all directions.

Imagine that balloon is not inflated at all, but ant magically moves 70mm instead of 10mm. How this is possible???

I can accept, that our part of Laniakea is expanding, but not whole Universe.

Re: How Can a Star Be Older Than the Universe?

#174

Post by Filligree » Thu, Oct 17, 2019, 12:05 PM UTC

leggomylibro wrote: post #101

lacker wrote: post #99

Earlier quoted context omitted.

There is no such thing as “read-only” in physics. If we can observe the other galaxy, that means it is affecting the particles in this galaxy. Even if it’s just a single molecule in a telescope responding to a photon that hit it from the other universe.

I guess that makes sense, thanks. I was wondering if it might have been something like the theories of what might happen much later in the universe's life, where things eventually get so far apart while still accelerating that light can't travel between them. But it couldn't be something like that if we can see the other galaxy, right?

We can see light that was emitted from it billions of years ago, before we split apart. Similarly, they can see light that ours emitted billions of years ago.

They can never be affected by anything we do now, or vice versa. This doesn't make it "read-only", though -- we can't read it either. All we're seeing is their distant past, back when we were mutually able to reach each other.

Re: How Can a Star Be Older Than the Universe?

#175

Post by kijin » Thu, Oct 17, 2019, 12:06 PM UTC

Udik wrote: post #120

kijin wrote: post #118

Earlier quoted context omitted.

No, it's observed as 93 billion light years. That's why we call it the "observable" universe. There is no meaningful spatial boundary at a point that is currently at a distance of 13.8 billion light years from us. We can detect (severely red-shifted) photons from beyond that point perfectly fine! The actual boundary is 46.5 billion light years away in both directions, hence the "observable" diameter of 93 billion. Bu…

Where I supposed that "observed as " just means how much red-shifted its light is- we don't really have any other way to measure such distances. This in turn only measures how much the space has expanded between us and the original starting point, much closer than 13 billion light years away. So we can say that we're seeing a star that is "now" 46 billion ly from us, but must have been only a few billion ly away when…

You're right. The star could not have been more than a few billion light years away when it emitted the photons that we're seeing. Otherwise, not only would the photons have not reached us yet, they will probably never reach us because the universe is expanding faster than the speed of light.

Re: How Can a Star Be Older Than the Universe?

#176

Post by Filligree » Thu, Oct 17, 2019, 12:09 PM UTC

e12e wrote: post #136

tidenly wrote: post #95

Earlier quoted context omitted.

A less brain melting way to imagine the "expansion without edge" thing is the classic balloon system. Imagine your entire universe exists across the plain of a semi-deflated balloon and then someone slowly begins to blow it up. The "space" in your 2D world is expanding, but there are no edges.

Which leads to the follow up question: if you travel far enough in one direction do you end up where you started? (well, no, not if space expands faster than c ).

If it wasn't for the expansion, and if the universe had positive curvature, then yes.

As it stands, there's not only the expansion, but the universe also seems to be flat -- meaning it's spatially infinite, not just borderless. That observation is also compatible with a very, very small curvature, but in that case the size would still be so large that it wouldn't be practically possible to circumnavigate even if it wasn't expanding.

Re: How Can a Star Be Older Than the Universe?

#177

Post by kijin » Thu, Oct 17, 2019, 12:14 PM UTC

eru wrote: post #155

spc476 wrote: post #147

Earlier quoted context omitted.

Imagine we exist in a 2D universe, but one that happens to be the surface of a sphere. Any point you pick on that surface is "at the center of the universe."

Hmm, but it looks like the universe is flat, doesn't it? (Or is it flat like a torus, so doesn't need any curvature to go loop on itself?)

Cosmologists talk about "horizons" a lot, and the analogy of standing on a sphere actually works quite well. Remember that horizons only make sense on curved surfaces. You can't see beyond the point where certain features of spacetime (black holes, expansion, or sheer distance) prevent signals from reaching you, just as you can't see beyond a mountain range or the curvature of Earth itself. Of course you'll need to extrapolate the analogy to three, four, or more dimensions, but the basic idea is the same.

Re: How Can a Star Be Older Than the Universe?

#178

Post by fragmede » Thu, Oct 17, 2019, 12:17 PM UTC

brutt wrote: post #173

undreren wrote: post #128

Earlier quoted context omitted.

The ant and balloon analogy is really good from an intuition stand point. Great explanation! Well done :)

Except for fact than in balloon analogy, ant is moving in a direction by balloon surface, while in our case our "balloon" is perfectly still in all directions. Imagine that balloon is not inflated at all, but ant magically moves 70mm instead of 10mm. How this is possible??? I can accept, that our part of Laniakea is expanding, but not whole Universe.

What'll really blow your mind is that it's expanding faster, not slower.

Re: How Can a Star Be Older Than the Universe?

#179

Post by mywacaday » Thu, Oct 17, 2019, 12:29 PM UTC

undreren wrote: post #128

devnulloverflow wrote: post #116

Earlier quoted context omitted.

Your intuition that there is an limited observable distance because is correct. But because of the expansion, that limit is bigger than "speed_of_light * age of universe" . I don't know enough General Relativity to give a solid explanation. But here is a rough over-simplification: Imagine you've spent the last 5 seconds blowing up a balloon, but there is an ant walking on that balloon, starting from a marked spot. In…

The ant and balloon analogy is really good from an intuition stand point. Great explanation! Well done :)

As light is travelling within the expanding universe does the wavelength of distant light change over long time periods or does it appear the same because our observations are expanding too(my head hurts)>

Re: How Can a Star Be Older Than the Universe?

#180

Post by madaxe_again » Thu, Oct 17, 2019, 12:37 PM UTC

brutt wrote: post #170

devnulloverflow wrote: post #116

Earlier quoted context omitted.

Your intuition that there is an limited observable distance because is correct. But because of the expansion, that limit is bigger than "speed_of_light * age of universe" . I don't know enough General Relativity to give a solid explanation. But here is a rough over-simplification: Imagine you've spent the last 5 seconds blowing up a balloon, but there is an ant walking on that balloon, starting from a marked spot. In…

Where is the waves? 13.8B years is too short to make Universe smooth again after such epic event. It's like to see green grass again just 1 second after nuke blast: mathematical formulas will need lot of magic constants for this to explain.

The universe isn’t smooth. The CMB is not uniform, and at a very large scale the universe is composed of attractors and voids, which stem from the anisotropy of the early universe.