Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Cosmology Has Some Big Problems

blogs.scientificamerican.com

Post reply on HN
Print view View original

41–50 of 76 posts

Re: Cosmology Has Some Big Problems

#41

Post by Koshkin »

whatshisface wrote: post #37
petschge wrote: post #34

Earlier quoted context omitted.

The fact that SOME galaxies are inconsistent with the theory of Dark Matter actually strengthens that theory. If the difference in rotation curves, the usual signature of dark matter at the galaxy level, from what GR predicts based on the mass of luminous objects was due to a modification in the way gravity works, then that should apply to ALL galaxies. Modified Newtowian Dynamics (MOND) (or the relativistic TeVeS mo…

To be clear, they're not inconsistent with the theory of dark matter, they're inconsistent with those galaxies specifically having dark matter. The author was wording that in a confusing way.

The way I understood it, they are not inconsistent with anything, they just happen to have no DM in them.

Re: Cosmology Has Some Big Problems

#42

Post by pdonis »

nabnob wrote: post #13
fhars wrote: post #8

Earlier quoted context omitted.

A more damning quote seems to be Another recent probe found galaxies inconsistent with the theory of dark matter, which posits this hypothetical substance to be everywhere. There is no reference given, but I have the suspicion that it refers to the discovery of galaxies with almost no dark matter (cf. https://news.ycombinator.com/item?id=16701248 ). That does indeed refute the assumpion that dark matter is everywhere…

Couldn't it just as easily mean that our predictions based on general relativity are wrong? Either 1) the prediction for a particular galaxy is right, and there's no "dark energy" or "dark matter" or 2) the prediction is wrong, and you add a bunch of dark energy or dark matter to make the equations work out. It's not testable at all - no matter whether predictions for a particular galaxy are right or wrong, you're st…

> Couldn't it just as easily mean that our predictions based on general relativity are wrong?

No, because general relativity with dark matter (and dark energy--the best current cosmological model we have) predicts a lot more than just galaxy rotation curves. And the whole point of the model is to explain all the things it makes predictions about, not just one.

Re: Cosmology Has Some Big Problems

#43

Post by pdonis »

astrobe_ wrote: post #30
darkmighty wrote: post #16

Earlier quoted context omitted.

It's not that simple. It's not like a) Some galaxies fit predictions, b) Some do not, and we add unknown dark matter. It's that when they do not fit predictions, there is a single parameter (the mass of dark matter) that does explain its behavior, whose model is that of a weakly interacting gas halo. So dark matter tries to fix things in a very precise way, it's not an arbitrary curve fitting exercise. Now whether th…

> Now whether there is really weakly interacting matter out there Not "out there". It's supposed to be >80% of the matter of the universe. We supposedly are eating some every day.

> It's supposed to be >80% of the matter of the universe. We supposedly are eating some every day.

It's >80% of the average density of the matter in the universe. But that's an average over all of the universe. On the scale of our planet, or of the solar system, that density is negligible--as in "30 orders of magnitude smaller than the density of the stuff you eat" negligible.

Re: Cosmology Has Some Big Problems

#44

Post by scott_s »

astro123 wrote: post #15

I'm a grad student working on cosmology, and while I agree there is a lot of work still to do (we don't know what 95% of the matter that makes up the universe is!), I disagree with a lot of what the author says. > Another recent probe found galaxies inconsistent with the theory of dark matter, which posits this hypothetical substance to be everywhere. We have found evidence of a few galaxies without DM. But most gala…

I found the authors' points about reports from the past several months disingenuous, since he's advertising a book that was clearly written and edited well before those announcements. It's valid to talk about those points in the context of arguments from the book, but leading with those points implies those arguments are a result of those points.

Re: Cosmology Has Some Big Problems

#45

Post by JackFr »

gerbilly wrote: post #18
hhs wrote: post #7

Interesting view. Near the end, it says: “The crux of today's cosmological paradigm is that in order to maintain a mathematically unified theory valid for the entire universe, we must accept that 95 percent of our cosmos is furnished by completely unknown elements and forces for which we have no empirical evidence whatsoever.” Empiricism is based on sense observations. And a lot of science has been confined to human…

One way to think of scientific instruments: like telescopes, microscopes, voltmeters is that they are extensions of our senses.

But the reality of them is that they are not extensions of our senses, so much as compressors of experience. That is they map a broader reality into our limited sensate experience. When we use them we have to remember that the map is not the territory, and we have to be especially careful about artifacts they may introduce.

Re: Cosmology Has Some Big Problems

#46

Post by AnimalMuppet »

CriticalCathed wrote: post #40
AnimalMuppet wrote: post #33

Earlier quoted context omitted.

Yes and no. Yes, we're seeing gravity waves and radio waves and gamma rays and near infrared and millimeter wave and neutrinos, and maybe a few others that I forget. We're way past human senses, or any animals. And yet I keep thinking of a deaf species that runs across Beethoven's Ninth Symphony. They can put an oscilloscope on the pressure waves in the air, so they can "see" what's going on. They can study the diffe…

Would the idea of a flatlander being incapable of comprehending 3-space be similar to what you're thinking?

Probably. The flatlander may be able to create a 3-dimensional vector space as a mathematical construct, but still has no clue what it's like to walk around in 3D.

Re: Cosmology Has Some Big Problems

#47

Post by JackFr »

AnimalMuppet wrote: post #33
jerf wrote: post #26

Earlier quoted context omitted.

When it comes to cosmology, our observations have been using light, because that's what has traveled in space. It is pretty obviously not that useful to ask a dog to sniff the sky or a homing pidgeon to navigate to Alpha Centauri, or something. We've obviously long since passed using only the visible spectrum. We also recently added gravitational wave detection to our repertoire. I seriously doubt any animals out the…

Yes and no. Yes, we're seeing gravity waves and radio waves and gamma rays and near infrared and millimeter wave and neutrinos, and maybe a few others that I forget. We're way past human senses, or any animals. And yet I keep thinking of a deaf species that runs across Beethoven's Ninth Symphony. They can put an oscilloscope on the pressure waves in the air, so they can "see" what's going on. They can study the diffe…

We also cavalierly assume that our brains are universal understanding machines. While we have no problem knowing that there are sounds we cannot hear, and light we cannot see, we're reluctant to admit that there are thoughts we cannot think. Concepts which may be true and universal, but which are literally inconceivable to us.

Re: Cosmology Has Some Big Problems

#48

Post by AnimalMuppet »

DubiousPusher wrote: post #35
AnimalMuppet wrote: post #33

Earlier quoted context omitted.

Yes and no. Yes, we're seeing gravity waves and radio waves and gamma rays and near infrared and millimeter wave and neutrinos, and maybe a few others that I forget. We're way past human senses, or any animals. And yet I keep thinking of a deaf species that runs across Beethoven's Ninth Symphony. They can put an oscilloscope on the pressure waves in the air, so they can "see" what's going on. They can study the diffe…

> Hearing Beethoven's Ninth is really different from studying the frequencies with an oscilloscope. I'm not sure that's a good analogy. When you hear Beethoven's Ninth, you're not understanding it any more than someone who drives a car is understanding the car. You can enjoy it. Which is to say, you have an appreciation for its acoustic beauty but that doesn't really have anything to do with understanding it. Perhaps…

> Which is to say, you have an appreciation for its acoustic beauty but that doesn't really have anything to do with understanding it.

Beethoven's Ninth has words (at least, the third movement does). If you speak German, you can understand them.

Re: Cosmology Has Some Big Problems

#49

Post by AnimalMuppet »

JackFr wrote: post #47
AnimalMuppet wrote: post #33

Earlier quoted context omitted.

Yes and no. Yes, we're seeing gravity waves and radio waves and gamma rays and near infrared and millimeter wave and neutrinos, and maybe a few others that I forget. We're way past human senses, or any animals. And yet I keep thinking of a deaf species that runs across Beethoven's Ninth Symphony. They can put an oscilloscope on the pressure waves in the air, so they can "see" what's going on. They can study the diffe…

We also cavalierly assume that our brains are universal understanding machines. While we have no problem knowing that there are sounds we cannot hear, and light we cannot see, we're reluctant to admit that there are thoughts we cannot think. Concepts which may be true and universal, but which are literally inconceivable to us.

But if you believe the theory of evolution, there is no reason why you should assume that. Our brains evolved to give us a good enough answer, fast enough, enough of the time to give us an edge at surviving. Any ability to truly understand what's truly going on would be purely by accident.

Re: Cosmology Has Some Big Problems

#50

Post by petschge »

whatshisface wrote: post #37
petschge wrote: post #34

Earlier quoted context omitted.

The fact that SOME galaxies are inconsistent with the theory of Dark Matter actually strengthens that theory. If the difference in rotation curves, the usual signature of dark matter at the galaxy level, from what GR predicts based on the mass of luminous objects was due to a modification in the way gravity works, then that should apply to ALL galaxies. Modified Newtowian Dynamics (MOND) (or the relativistic TeVeS mo…

To be clear, they're not inconsistent with the theory of dark matter, they're inconsistent with those galaxies specifically having dark matter. The author was wording that in a confusing way.

That's right they are inconsistent with the combination of 1.) dark matter is the correct theory and 2.) they have the usual amount of dark matter for a galaxy of that size.

Surprisingly this is does not make the theory of dark matter (#1) less likely, but actually makes it more likely. Because the alternative is #1b "gravity works differently than we think". But that would have to hold for all galaxies.

Post reply on HN