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

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

#91

Post by pdonis » Wed, Oct 03, 2018, 4:02 PM UTC

jacobush wrote: post #48

pdonis wrote: post #28

Earlier quoted context omitted.

> I believe there wasn't any mass at first; it was all energy. It had to slow down to become mass This is partly true. During inflation, the universe was vacuum; all of the energy was in the "false vacuum" state of whatever field caused inflation (it's usually called the "inflaton" field, which IMO is a very confusing name). When inflation ended, all of that energy got transferred to the Standard Model fields, formin…

But did it end?

You mean, did inflation end? Yes, it did.

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

#92

Post by pdonis » Wed, Oct 03, 2018, 4:04 PM UTC

InclinedPlane wrote: post #20

DougWebb wrote: post #8

Earlier quoted context omitted.

I'm absolutely not an expert, but I believe there wasn't any mass at first; it was all energy. It had to slow down to become mass, at which point it was spread out enough. Or something along those lines. Another possibility is that black holes are places where lots of mass accumulates in the same place in space, while the big bang was a rapid expansion of space itself. Even if there was enough mass in one spot to be…

"Mass" is just another name for "rest-energy". A single photon cannot have mass because it has zero energy in the reference frame where it is at rest (zero net momentum). But a pair of non-parallel photons can have rest-mass, and indeed the mass of the photons inside a star is considerable, for example.

> A single photon cannot have mass because it has zero energy in the reference frame where it is at rest (zero net momentum)

I didn't catch this before, but it is not correct. There is no such thing as a frame in which a photon is at rest.

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

#93

Post by pdonis » Wed, Oct 03, 2018, 4:05 PM UTC

InclinedPlane wrote: post #49

pdonis wrote: post #27

Earlier quoted context omitted.

> "Mass" is just another name for "rest-energy". Not if you're talking about what the source of gravity (spacetime curvature) is. Massless particles still have energy and still cause spacetime curvature.

The "source" of gravity is the stress-energy tensor. "Mass" (rest-energy) is a simplification that is useful for approximations and conceptualizations.

> "Mass" (rest-energy) is a simplification that is useful for approximations and conceptualizations.

Yes, but not in a context where radiation is present, since radiation can have energy but has no rest mass (rest energy).

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

#94

Post by posterboy » Wed, Oct 03, 2018, 4:06 PM UTC

prmph wrote: post #74

One question I have: is it not possible that the big bang occurred an infinite time ago? If the expansion of the universe is accelerating with time, does it not indicate that, if we reverse the time, the deceleration is asymptotic? In that case the big bang is compatible with a universe that has always existed and did not have a beginning

The idea is interesting and probably not unique. If you try to paint a diagram of it, how would that look? It's not time that is accelerating, so you don't just draw the time axis just longer with wider gaps between the minute marks. Which leaves me guessing and remembering my old, discrete notion that time is count of change of things, so that, for sake of argument, in one unit of time, one state change happens. In that setting, it doesn't make sense to say a change was further apart than another. Actually, information decreases, thermodynamically speaking. So, tracking back infinitely, you'd have to start with an infinite amount of entropy, proportional to energy. But the theory assumes a fixed amount of energy, as far as I know.

And it's not that the lightspeed increases with time, either. Rather, so they say, space expands - which is imaged trivially by stretching the space axis. I suppose that's normalizing for time and you could just as well fix space and compress time, invariably, to arrive at your notion, with a total lack of rigour at that.

So normalizing for

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

#95

Post by mpc755 » Wed, Oct 03, 2018, 4:11 PM UTC

Dark matter is a supersolid that fills 'empty' space, strongly interacts with ordinary matter and is displaced by ordinary matter. What is referred to geometrically as curved spacetime physically exists in nature as the state of displacement of the supersolid dark matter. The state of displacement of the supersolid dark matter is gravity.

The supersolid dark matter displaced by a galaxy pushes back, causing the stars in the outer arms of the galaxy to orbit the galactic center at the rate in which they do.

Displaced supersolid dark matter is curved spacetime.

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

#96

Post by posterboy » Wed, Oct 03, 2018, 4:13 PM UTC

empath75 wrote: post #89

GarrisonPrime wrote: post #87

Isn’t “dark matter” just a way to account for an unexplained behavior? It bothers me that its existence is treated as a given, when it could be a case of mistaken assumptions. If calculations say the universe is acting as if it contained much more mass than we observe, we should also consider the possibility that our method of calculation may be flawed. I’m not claiming dark matter doesn’t exist, only that I fear the…

Dark matter is an observed phenomenon that can be explain by the existence of an additional, unknown kind of matter. I don't think scientists are unduly hung up on that explanation, though. It's merely the most likely. Plenty of scientists are looking for alternative explanations and I'm sure the rest will get on board if they find something convincing.

So far, dark matter is just the label for an incongruence in the calculation that needs to be falsified.

With regard to the article, it could still be that there are many young smaller black holes, too small or even too short lived for the methods employed here.

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

#97

Post by mattydoincode » Wed, Oct 03, 2018, 4:14 PM UTC

GarrisonPrime wrote: post #87

Isn’t “dark matter” just a way to account for an unexplained behavior? It bothers me that its existence is treated as a given, when it could be a case of mistaken assumptions. If calculations say the universe is acting as if it contained much more mass than we observe, we should also consider the possibility that our method of calculation may be flawed. I’m not claiming dark matter doesn’t exist, only that I fear the…

I think that from the optic of someone who is not a physicist (like myself!) it's natural to think "hey... we can't actually know that it's dark matter vs some other unexplained way that gravity works". And in fact there are many physicists working on trying to figure that out, but there is actually a ton of evidence that what we're seeing is in fact particles, and most physicists consider it to be the most likely current theory. I don't think anyone in the physics community treats anything about dark matter as a given either (or hawking radation for that matter). In fact part of being a true scientist is keeping an open mind.

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

#98

Post by cdumler » Wed, Oct 03, 2018, 4:21 PM UTC

GarrisonPrime wrote: post #87

Isn’t “dark matter” just a way to account for an unexplained behavior? It bothers me that its existence is treated as a given, when it could be a case of mistaken assumptions. If calculations say the universe is acting as if it contained much more mass than we observe, we should also consider the possibility that our method of calculation may be flawed. I’m not claiming dark matter doesn’t exist, only that I fear the…

Yes, Dark Matter is a statement about the premise that something appears to be operating gravitationally while have no (or perhaps so little that we don't recognize it) interaction with other matter; however, it is not without reason.

The primary focus on matter as the candidate is that if you don't believe that some form of matter is responsible, you begin having to explain how a lot of our Laws are incorrect. Dark Matter is really a problem that we have a notion how our Laws should work at the cosmic scale, yet they don't. And yet, we don't see any obvious reason why they shouldn't give how thoroughly we've tests these Laws.

For instance, if you say that our equations for gravity is off, why is it completely consistent for everything in our solar system? Why is consistent down to the most precise test for gravitational frame pulling? It seems to be only different when you talk about very large masses creating gravity on objects tugged at very long distances. Why does it just cut over to different behavior?

If you say that gravity is emergent, then how do you explain the standard model's place for the graviton? Granted, it's not testable (gravity is so weak it's beyond direct testing), but it seems to be fit right in to a spin-2 tensor where we would expect a gravity-like particle to be. Why would you expect that to not be true?

That said, Dark Matter matter candidates are getting weeded out pretty quickly, these days. No new types of particles are being discovered, and Dark Matter observatories are largely coming up empty. But, that's science: got an idea? Test it and let the results speak.

As an aside, I think one thing to remember is: just because science doesn't know the answer to something, it often knows what can't be the right answer. Look into the theory's and their criticisms.

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

#99

Post by cryptonector » Wed, Oct 03, 2018, 4:28 PM UTC

pdonis wrote: post #23

sulam wrote: post #4

This spawned a question for me that seems like it must be nonsensical but I don’t know the answer. Why did the Big Bang not just produce a Big Hole? IOW, if all the current mass of the universe was in a small volume wouldn’t it have promptly created a black hole? I’m assuming the answer has something to do with mass not actually existing in its current form until the universe expanded enough. However I don’t have any…

> 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.

I like u/Retric's comment on this too:

> A second issue is in Time 0.0001 seconds the observable universe is only a 0.0001 light seconds radius which limits how much mass could form a black hole.

That is, if you imagine all that mass-energy springing into existence in a vast universe (but with every point much closer to each other than today), there was not enough time for all that energy to notice the gravity of the surrounding mass-energy density and collapse into a black hole. Add to that the expansion rate (inflation), and the extreme uniformity of the young universe (which means gravity would have been pulling every thing in all directions rather than "inward" towards a soon-to-be-black-hole-center), and a collapse was impossible.

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

#100

Post by cryptonector » Wed, Oct 03, 2018, 4:30 PM 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.

The expansion of space is always faster than the speed of light if you pick any two points far enough apart from each other, at least as long as space is expanding and the universe is infinite. Or if the universe is finite, some time after the bang there will be points which separate faster than the speed of light from each other. These points aren't moving, of course.