Black holes ruled out as universe’s missing dark matter • Hacker News Board

Re: Black holes ruled out as universe’s missing dark matter

#51

Post by kryptiskt » Wed, Oct 03, 2018, 8:27 AM UTC

Filligree wrote: post #46

T-A wrote: post #16

Earlier quoted context omitted.

The early universe was ridiculously homogeneous. We can infer this with reasonable certainty from the extreme homogeneity of the CMB and from the gravitational instability of density fluctuations: as you expect, they attract more mass and accrete to form stars and galaxies. So if you start from the universe as it looks now and run time backwards, you get ridiculous homogeneity at the bang. And that's why you didn't g…

So, are we assuming the universe is infinite? Otherwise there'd be a border to that.

The Earth's surface is finite, but without a border.

A finite universe would be similar, just adding a dimension.

Re: Black holes ruled out as universe’s missing dark matter

#52

Post by ajuc » Wed, Oct 03, 2018, 8:37 AM UTC

oldandtired wrote: post #44

InclinedPlane wrote: post #22

Earlier quoted context omitted.

We have observed matter falling into a black hole. We've watched "hot spots" in orbit of a black hole in the accretion disk shine their light at us as they orbited the black hole. We've watched as the orbit got tighter and tighter and the light was dimmed by gravitational red shifting, then we've watched the light blink out and disappear as the blob of gas fell below the event horizon. The evidence for the existence…

No. Incorrect. Wildly inaccurate. We have made certain observations that are assumed to be the result of matter falling into a "Black Hole". We do not have any observations that show us this as a fact. Read the observational material very carefully. Go back over the last few decades and see how many times it is declared "we have seen x for the first time", where x is the is the same entity each time, in this particlu…

> We have made certain observations that are assumed to be the result of matter falling into a "Black Hole". We do not have any observations that show us this as a fact.

Do you believe in gravity? We've never observed it directly, always just its effects.

Re: Black holes ruled out as universe’s missing dark matter

#53

Post by HeadsUpHigh » Wed, Oct 03, 2018, 8:52 AM UTC

sulam wrote: post #33

pdonis wrote: post #23

Earlier quoted context omitted.

> if all the current mass of the universe was in a small volume wouldn’t it have promptly created a black hole? No, because it was expanding too rapidly. The usual intuitions about how much mass in how small a volume it takes to make a black hole assume that the mass is stationary, or nearly so.

This matches a description I was able to find online. What that seems to imply though is that expansion was literally faster than light. Yes? No? [edit] and it seems this is true and can be explained by the space-time matrix not being bound by light speed limits.

Yes it was. I think it's called the inflation period and is used as a possible explanation of why there are big voids in the universe.

Re: Black holes ruled out as universe’s missing dark matter

#54

Post by richardknop » Wed, Oct 03, 2018, 9:29 AM UTC

Filligree wrote: post #46

T-A wrote: post #16

Earlier quoted context omitted.

The early universe was ridiculously homogeneous. We can infer this with reasonable certainty from the extreme homogeneity of the CMB and from the gravitational instability of density fluctuations: as you expect, they attract more mass and accrete to form stars and galaxies. So if you start from the universe as it looks now and run time backwards, you get ridiculous homogeneity at the bang. And that's why you didn't g…

So, are we assuming the universe is infinite? Otherwise there'd be a border to that.

I think almost all options are currently still in play. Finite with border, finite in infinite space, infinite.

Re: Black holes ruled out as universe’s missing dark matter

#55

Post by arithma » Wed, Oct 03, 2018, 9:49 AM UTC

Filligree wrote: post #46

T-A wrote: post #16

Earlier quoted context omitted.

The early universe was ridiculously homogeneous. We can infer this with reasonable certainty from the extreme homogeneity of the CMB and from the gravitational instability of density fluctuations: as you expect, they attract more mass and accrete to form stars and galaxies. So if you start from the universe as it looks now and run time backwards, you get ridiculous homogeneity at the bang. And that's why you didn't g…

So, are we assuming the universe is infinite? Otherwise there'd be a border to that.

If it was finite, does it mean it was definitely not flat?

Re: Black holes ruled out as universe’s missing dark matter

#56

Post by SiempreViernes » Wed, Oct 03, 2018, 9:50 AM UTC

oldandtired wrote: post #44

InclinedPlane wrote: post #22

Earlier quoted context omitted.

We have observed matter falling into a black hole. We've watched "hot spots" in orbit of a black hole in the accretion disk shine their light at us as they orbited the black hole. We've watched as the orbit got tighter and tighter and the light was dimmed by gravitational red shifting, then we've watched the light blink out and disappear as the blob of gas fell below the event horizon. The evidence for the existence…

No. Incorrect. Wildly inaccurate. We have made certain observations that are assumed to be the result of matter falling into a "Black Hole". We do not have any observations that show us this as a fact. Read the observational material very carefully. Go back over the last few decades and see how many times it is declared "we have seen x for the first time", where x is the is the same entity each time, in this particlu…

You had like, a slim case before GW150914. Now you just have to either accept that black holes (in the sense of the external metric predicted by GR) do exist, or go make friends with the climate change deniers.

The science discussion about the existence is settled, as evidenced by your complete lack of any concrete objection to the interpretations. (Just saying "measuring is ambiguous, maybe we didn't understand it properly" like it is an objection is typical cranking)

Re: Black holes ruled out as universe’s missing dark matter

#57

Post by chopin » Wed, Oct 03, 2018, 9:58 AM UTC

One thing I do not understand: If a black hole in line-of-sight increases intensity, the intensity for other view points must decrease (the total number of emitted photons needs to stay constant). If black holes make up a substantial part of the dark matter, there should be (statistically) few in the line-of-sight and many more slightly off. Therefore I'd expect that this cancels out. Does anyone know how it is accounted for this?

Re: Black holes ruled out as universe’s missing dark matter

#58

Post by Fordec » Wed, Oct 03, 2018, 10:09 AM UTC

Filligree wrote: post #46

T-A wrote: post #16

Earlier quoted context omitted.

The early universe was ridiculously homogeneous. We can infer this with reasonable certainty from the extreme homogeneity of the CMB and from the gravitational instability of density fluctuations: as you expect, they attract more mass and accrete to form stars and galaxies. So if you start from the universe as it looks now and run time backwards, you get ridiculous homogeneity at the bang. And that's why you didn't g…

So, are we assuming the universe is infinite? Otherwise there'd be a border to that.

Treat the "Universe" that was created in the big bang like a firework that has just gone off. The "boundary" of the universe is just the size the explosion has expanded to for a given time. And today, 13.8B years after the big bang, our universe is 46.5 billion light-years in diameter. But just like a firework explosion, there isn't really a solid border to it that you can plant a stake or flag in.

We have no clue what is beyond the explosion itself and, for intents and purposes, that expanse is infinite by current understanding. Does that expanse contain other explosions? Don't know, there hasn't really been an observed overlap so far.

Re: Black holes ruled out as universe’s missing dark matter

#59

Post by Waterluvian » Wed, Oct 03, 2018, 10:32 AM UTC

Fordec wrote: post #58

Filligree wrote: post #46

Earlier quoted context omitted.

So, are we assuming the universe is infinite? Otherwise there'd be a border to that.

Treat the "Universe" that was created in the big bang like a firework that has just gone off. The "boundary" of the universe is just the size the explosion has expanded to for a given time. And today, 13.8B years after the big bang, our universe is 46.5 billion light-years in diameter. But just like a firework explosion, there isn't really a solid border to it that you can plant a stake or flag in. We have no clue wh…

How can the diameter be larger than 2 * the speed of light * time ?

Re: Black holes ruled out as universe’s missing dark matter

#60

Post by kamaal » Wed, Oct 03, 2018, 10:43 AM UTC

ketralnis wrote: post #38

perl4ever wrote: post #32

Earlier quoted context omitted.

"First you need to understand that the Big Bang was not an explosion of matter into space, it was an explosion of matter and space (technically space-time)." This is a type of statement I have read many, many times, but to me, it's the same as saying "wakalixes!". I've never read anything that gave me a hint of why and how there should be a difference between two objects receding from each other and "space itself" ex…

New space is being created between those two things. When they move 1 meter apart from each other through space, they'd move 1 meter back again to meet up. But when space itself is expanding between them, they've moved 1 meter apart but now to get back together they have to move 2 meters. This has some other subtler ramifications. It means that the space between the two objects can increase at faster than the speed o…

>>It means that the space between the two objects can increase at faster than the speed of light

Say they are two objects in the universe A and B. New space is created between A and B faster than the speed of light. There is a USB drive carrying a dump of wikipedia on B.

Does this imply information is now moving faster than the speed of light?