Why is there a normal galaxy sitting at the edge of the Universe? • Hacker News Board

Re: Why is there a normal galaxy sitting at the edge of the Universe?

#111

Post by noizejoy » Wed, Sep 09, 2020, 7:17 PM UTC

ddevault wrote: post #102

iwonderwhy wrote: post #55

All right hackernews. Genuine question, not sure where else to ask. If galaxies and stars at the other “end” formed out of matter resulting from the big bang, just as the matter we are made of, how did we get here at the same time that that matter got there? Is it because speeds faster than light are possible, and around the big bang all matter was spread near instantly at that speed? I don't understand how matter of…

I don't fully understand your question to give a direct answer, but I think that's mostly due to a number of misconceptions you hold about the universe. So I'll just try to clear those up instead. First of all, the big bang didn't happen somewhere , it happened everywhere . There's no "center" of the universe that everything is moving away from. The universe did not start from a point, it started from a small scale f…

So will expansion accelerate enough to eventually be a stronger “force” than local gravity?

Re: Why is there a normal galaxy sitting at the edge of the Universe?

#112

Post by HideousKojima » Wed, Sep 09, 2020, 7:18 PM UTC

outworlder wrote: post #71

dumpsterdiver wrote: post #62

Earlier quoted context omitted.

I have a question: Just over 20 years ago when the accepted view was that the expansion of the universe was slowing down, would you have backed that position with as much certainty as you are backing this one? That the universe is a mere 13.7 billion years old? We cannot prove the absence of light in a place that is already so far away that it's light will never reach us. Do you agree? We can prove what we can observ…

> Just over 20 years ago when the accepted view was that the expansion of the universe was slowing down Was it ever a mainstream view? I thought it was just a conjecture, leading to the 'big crush', and was disproven long ago, way more than 2 decades ago. > The entire premise of dating our universe is based on its observed rate of expansion Also there's the cosmic microwave background radiation. We have mapped it, an…

>Unless the rate of expansion starts to slow down or we have indication that it will ever reverse, there's no big crunch. The rate of expansion is increasing

Unless space wraps around like a globe and we'll all meet at the "south pole" (with the big bang being the "north pole") or something to that effect. Which is itself wild, unfalsifiable conjecture, but there are other ways for a Big Crunch to happen other than the expansion of space slowing down or reversing

Re: Why is there a normal galaxy sitting at the edge of the Universe?

#113

Post by iwonderwhy » Wed, Sep 09, 2020, 7:20 PM UTC

ddevault wrote: post #70

zemnmez wrote: post #59

Earlier quoted context omitted.

this is because things aren’t ‘moving apart’— space itself is expanding (into what? nobody knows). it’s not that the galaxies are being propelled apart from a force in a particular place either; everything is expanding away from everything else . On the traditional balloon model, the surface of the balloon represents space itself. Those dots on the surface of an expanding balloon are not moving relative to space (the…

In case anyone is confused, I prefer to explain the expansion of space without metaphors. Space itself is getting larger, everywhere, all at once. The scale factor of the universe is getting larger. New space is constantly being "made". Every second, every kilometer in the universe grows by 2.2×10^-18 centimeters. If no other forces were in play, it'd carry everything along with it and pull everything a bit further a…

Very interesting.

So the I wonder if everything expands along with the universe, does it mean that from outside, things moving at the speed of light in our universe, are potentially moving at speeds greater than the speed of light within our universe? If i was watching from outside, a rectangle moving at the speed of light in our universe, would one of its edges be moving away from me at the speed of light + the speed of expansion? If it makes sense.

Re: Why is there a normal galaxy sitting at the edge of the Universe?

#114

Post by iwonderwhy » Wed, Sep 09, 2020, 7:23 PM UTC

at_a_remove wrote: post #73

iwonderwhy wrote: post #55

All right hackernews. Genuine question, not sure where else to ask. If galaxies and stars at the other “end” formed out of matter resulting from the big bang, just as the matter we are made of, how did we get here at the same time that that matter got there? Is it because speeds faster than light are possible, and around the big bang all matter was spread near instantly at that speed? I don't understand how matter of…

To give you specific terms to research on, in cosmology this is called the "horizon problem" or the "homogeneity problem," basically put as "Why is the stuff over there so similar to the stuff right here when they're so far away? They ought to be causally disconnected by distance!" The answer to this in the early universe is called inflation , which you can tag onto Alan Guth around 1981 or so. It involves spacetime…

Thanks for this - will read more in the two terms.

Re: Why is there a normal galaxy sitting at the edge of the Universe?

#115

Post by ddevault » Wed, Sep 09, 2020, 7:23 PM UTC

thaumasiotes wrote: post #110

maxekman wrote: post #100

Earlier quoted context omitted.

It was answered, as far as I can tell: > However, local forces like gravity and the strong and even the weak nuclear force are more than sufficient to hold everything together despite this, so the expansion cannot be locally observed.

OK, we have a meter stick. It's made of pure iron and, for whatever physical reasons, it is 1 meter long exactly. After some time, the scale factor of the universe grows, so that 1 new meter is 1.05 old meters. But, by hypothesis, as this happens , our meter stick contracts , so that it ends up being just 0.952 new meters (= 1 old meter) long. Why? Did the physical laws governing iron atoms change so that they pack m…

>And if so, why do all particles pack at exactly the same new density? Shouldn't it differ from molecule to molecule?

The dominant factor on this scale is the strong nuclear force, which governs interactions at the scale of quarks - much smaller than molecules.

Re: Why is there a normal galaxy sitting at the edge of the Universe?

#116

Post by ddevault » Wed, Sep 09, 2020, 7:25 PM UTC

noizejoy wrote: post #111

ddevault wrote: post #102

Earlier quoted context omitted.

I don't fully understand your question to give a direct answer, but I think that's mostly due to a number of misconceptions you hold about the universe. So I'll just try to clear those up instead. First of all, the big bang didn't happen somewhere , it happened everywhere . There's no "center" of the universe that everything is moving away from. The universe did not start from a point, it started from a small scale f…

So will expansion accelerate enough to eventually be a stronger “force” than local gravity?

We don't understand why expansion is occuring, so we can't say for certain. It would be a very long time in the future if that were to come to pass, and that's a whole lot of time for poorly-understood physics to happen in.

But yes, it is accellerating, and if it continues to do so, eventually it will overcome gravity. But like I said, this depends on an incomplete understanding of physics, and the current evidence suggests (inconclusively) that this outcome is unlikely. Check out dark energy if you want to learn more.

Re: Why is there a normal galaxy sitting at the edge of the Universe?

#117

Post by thaumasiotes » Wed, Sep 09, 2020, 7:28 PM UTC

ddevault wrote: post #115

thaumasiotes wrote: post #110

Earlier quoted context omitted.

OK, we have a meter stick. It's made of pure iron and, for whatever physical reasons, it is 1 meter long exactly. After some time, the scale factor of the universe grows, so that 1 new meter is 1.05 old meters. But, by hypothesis, as this happens , our meter stick contracts , so that it ends up being just 0.952 new meters (= 1 old meter) long. Why? Did the physical laws governing iron atoms change so that they pack m…

>And if so, why do all particles pack at exactly the same new density? Shouldn't it differ from molecule to molecule? The dominant factor on this scale is the strong nuclear force, which governs interactions at the scale of quarks - much smaller than molecules.

But the scale of quarks increases just as much as every other scale, doesn't it? Why did the iron rod shrink while space grew? Why are the iron atoms closer together now than they were before? Why are the protons in each iron atom closer together now than they were before?

Re: Why is there a normal galaxy sitting at the edge of the Universe?

#118

Post by weswpg » Wed, Sep 09, 2020, 7:29 PM UTC

f154hfds wrote: post #69

vkou wrote: post #22

Earlier quoted context omitted.

No, 13 billion years is the age of the universe. The observable universe is a sphere with a diameter of ~93 billion light years (And is likely to only be a subset of the entire universe).

Naive question here - is the 13bn years relative to the surface of the Earth? I assume this matters right? If we're in a bit of a gravity well wouldn't the vacuum of space experience this time a bit longer? According to [1], the ratio is about 5.56E-10, so I guess this would contribute about a year to the age of the universe, so.. not that much. I guess I answered my own question. However, this does pose an interesti…

Well, given that there is no universal clock and time is defined locally for each observer, including each particle, it would be more strictly accurate to say that "the matter which later formed our solar system has experienced 13 billion years of time" since the Big Bang

or maybe 13 billion years is from the frame of reference of the Cosmic Microwave Background? that would make more sense. I'm going to check again then edit this comment.

(i am not a physicist, not an authoritative source on this)

edit: so it turns out that the age of the universe is given from the frame of reference of the CMB rather than from Earth's frame of reference.:

> Measurements of the cosmic background radiation give the cooling time of the universe since the Big Bang

> Cosmic time, or cosmological time, is the time coordinate commonly used in the Big Bang models of physical cosmology.[1][2][3] Such time coordinate may be defined for a homogeneous, expanding universe so that the universe has the same density everywhere at each moment in time (the fact that this is possible means that the universe is, by definition, homogeneous). The clocks measuring cosmic time should move along the Hubble flow.

https://en.wikipedia.org/wiki/Cosmic_time

Re: Why is there a normal galaxy sitting at the edge of the Universe?

#119

Post by pas » Wed, Sep 09, 2020, 7:29 PM UTC

PopeDotNinja wrote: post #72

s1dev wrote: post #61

Earlier quoted context omitted.

How do you propose to power a fleet of spacecraft at 550AU? Would you be able to acquire enough RTGs? What about DSN time for the downlink?

And how would you get them out to, and stop at, 550 AU? Voyager 1 and 2 are nuclear powered with RTGs and they are respectively about 150 AU and 125 AU from the sun, and they are going really fast. The Voyager RTGs are also fairly depleted at this point. I’m guessing these spacecraft would need a faster, more efficient mode of propulsion, like a nuclear thruster or ion drive (or Epstein drive?!). One could also get a…

Sun slingshot, solar sail

https://www.youtube.com/watch?v=NQFqDKRAROI

Re: Why is there a normal galaxy sitting at the edge of the Universe?

#120

Post by pas » Wed, Sep 09, 2020, 7:32 PM UTC

trimbo wrote: post #14

tectonic wrote: post #2

Nice gravitational lens observation! We're hoping to use the same technique with the Sun to observe exoplanets by launching a fleet of small satellites to 547 AU (80 billion km / ~3 light days). The craft would sample the distorted Einstein ring around our Sun from that vantage point, then combine and reconstruct an image of the disk of an Earth-like exoplanet up to 100 light-years beyond, at a resolution of 25 kilom…

> it’d still take 25 to 30 years for them to reach a vantage point .... > mission designers would have to pick carefully because they could observe only a single target It will take 30 years to get in position but then we can only look at one target? While it sounds very neat, I'd rather fund something we can use more than once, and sooner.

The whole concept is that we should launch a fleet of them every few months. Very lightweight semi-autonomous smart lightsails with some optics.

The first fleet helps targeting the second, and all those who come after them.

https://www.youtube.com/watch?v=NQFqDKRAROI (already linked by a sibling comment, but yes, can't recommend this enough!)